Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => The Newcomer's Forum => Topic started by: Capt_Dunzell on October 28, 2012, 06:02:52 PM

Title: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on October 28, 2012, 06:02:52 PM
I picked this thing up for a song, very good physical condition, but not so hot in the function department. I wanted it for a project knowing it had issues. I have repaired tube amps and restore antique radios so I know some basics with point to point wiring, but not good at all with solid state beyond following instructions.

When I first got it it would just hum, an occasional blip of guitar would come through, but within a few minutes nothing.

It would switch channels when I first powered it up, but now the channel light is out, I am thinking it may be related.

Any ideas on what to look for?

I have already done the normal filter cap replacements since it quit working.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on October 29, 2012, 03:21:48 AM
Yeah, the first thing to look for is a circuit/schematic (I have already had a fair look but can't turn one up), or get from Fender; failing that some clear pix of the internals so we have some sort of idea what we are dealing with.

The "occasional blip of guitar" suggests that we have a dodgy connection somewhere, rather than dead silicon.

If this has and Fx loop or Pre Out Main In try patching these externally with a lead and see if it makes a difference.  Also check if you are getting signal out of Pre Out, and if a signal fed into Main In produces an output.

Carefully check any PCB-mounting external sockets (e.g. inputs) for dry/cracked solder joints (a very common problem with modern gear).

Given that the channel light is out, have you checked that you have all the supply voltages?

Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on October 29, 2012, 07:35:04 PM
For sure start with power supply.   Look inside for the large main filter capacitors, resolder them, yes, even if the solder looks OK.

And there will be +15 and -15 DC supplies, find them, they are zener derived from 24v rails.

I don;t have this one scanned, so contact Fender and ask them for the schematic for Stage Lead.  If you can find STudio Lead, I think they use the same board, at least for powr amp and power supply.

That blip thing makes me think you have DC offset somewhere.  Try these things:
PLug the guitar right into the POWER AMP IN jack, any sound? Turh up the reverb and rock the amap to crash the reverb springs.  Does the noise come out the speaker? Plug a CORD ONLY into the PREAMP OUT jack, right next to the power amp jack, and at the other end measure to see if there is 15v on the tip.  Do the same at the input jack.

Here, see if this schematic helps
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on October 29, 2012, 08:35:06 PM
Just found it.  :)

support.fender.com/schematics/guitar_amplifiers/Stage100_Stage100H_Stage160_schematic.pdf (http://support.fender.com/schematics/guitar_amplifiers/Stage100_Stage100H_Stage160_schematic.pdf)
Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on October 30, 2012, 02:45:26 AM
I don't think so, the Stage 100 is from 1998-2000, the Stage Lead is more from like 1982.
Title: Re: fender stage lead 212 troubleshooting
Post by: DrGonz78 on October 30, 2012, 03:30:21 AM
Yeah I almost thought the Stage 100-160 was the right schematic too...   :o

I think if I was wanting this schematic to work on this amp I would divert my attention to this web page here...

http://www.fender.com/support/articles/fender-guitar-and-bass-amplifiers-owners-manuals-and-schematics-hard-copy-archives (http://www.fender.com/support/articles/fender-guitar-and-bass-amplifiers-owners-manuals-and-schematics-hard-copy-archives)
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 01, 2012, 08:46:49 PM
Thank you for your replies.

Have replaced all the electrolytics, and non polarized foil type caps. Tested all the resistors and diodes.
Tested all the transistors, the power transistors were both bad, and replaced, the JFET was bad and replaced. I also went through all of the solder joints, a number of them were bad so I removed the solder and reset them with fresh solder. I am not sure where to put any test leads for testing this.


Now I get no hum, the channel switching is working, but I have no sound at all.

this is the schematic that I found, sorry I cant seem to get the PDF to have the correct orientation.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 03, 2012, 01:29:34 AM
I know it seems like a logical thing to do, but actually replacing a swage of components is something we generally avoid doing because it tends to introduce new problems that muddy the waters, however...

You say you found that both of the output transistors were faulty.  With the speaker disconnected, and using a limiting lamp, measure the voltage across the speaker terminals (after checking that the speaker fuse is still intact).  There will normally be a small voltage, a fraction of a volt, across these terminals.

Is this voltage close to zero, or close to one of the supply rails?

Post the voltage on each of the supply rails and across the speaker connection.


You also say you found the JFET J111 was faulty.  This is unexpected, but may be a clue.  The function of this FET is to mute the output stage until the supply voltages settle, de-thumping the output at switch on.

*If* the output voltage tested above is close to zero, try removing the FET altogether for the moment, then test with a signal into the "power amp input" J203 to see if the power amplifier has come alive at all (with a limiting lamp it may sound distorted, but any output will tell us something).

If the voltage across the speaker connection is not close to zero you still have dead silicon to find first.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 04, 2012, 03:57:00 PM
Ugh, maybe i need to throw the towel in on this one.

I havent touched it since the last post, made no changes whatsoever, but in turning it on today it popped R71, and the R77 bias pot. What the blank?? I think it has gremlins.

I am lousy with solid state.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 04, 2012, 10:38:11 PM
Capt - you're not using a limiting lamp in series with the mains, are you.  :-\  It's anti-gremlin insurance.  Please take it from the old hands, you ain't gonna get nowhere fast until you do.

The bias pot popped?  Oh dear.    :(

Take a deep breath, and back to square one.

Make up a limiting lamp with 20-40 watt globe.

Disconnect speaker.

Remove and check Q8 and all transistors to the right on the circuit, to the output pair; I make that seven transistors.  Carefully note (on paper, not memory, or take macro pix) their orientation/connection.

Obtain and fit replacement components.

Using the limiting lamp, power up and check that the output half-rail is roughly half way between the two supply rails.

Only if so, switch to a 60 watt globe and check again.

Do not apply full mains power (or connect the speaker if you value it) until you are certain that the circuit is rebalancing the half-rail to the mid point properly.  This is vital.

Take heart, we have all been where you are now, and we can lead you out of the woods, but you have to be utterly methodical and do it by the numbers, no wingin' it to see what happens - you already know.  Solid state amps can be repaired, but, as you are finding, they are unforgiving of inattention to detail compared to valve amps.
Title: Re: fender stage lead 212 troubleshooting
Post by: lurkalot on November 05, 2012, 04:18:24 AM

this is the schematic that I found, sorry I cant seem to get the PDF to have the correct orientation.


If your using a fairly new version of adobe reader, then you should be able to right click somewhere in in the middle of the drawing and then choose "Rotate Clockwise"   ;)
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 05, 2012, 05:19:22 AM
Agree and add: and if an earlier one, go to top Menu > View > Rotate View > Clockwise
Each click rotates 90 degrees so repeat twice if necessary.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 05, 2012, 09:22:00 PM
Okay, deep breath.

I have recovered from yesterday. All of the transistors are okay, I replaced R77, R79 tested unharmed.

When you say current limiting bulb, is this just a bulb with a lead off of each contact to act as a dummy load? In my head it must be as the lamp tester for old radios is a different animal unless I am not seeing it correctly.

When you say output rail, is that the terminal strip that connects to the speaker fuse? Forgive me, I am not as familiar with the terminology.

Currently I am back to where I was before, it powers up, a very very slight hum through the speakers, nothing is apparently overheating or "freaking out" so to speak, but no output.

Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 06, 2012, 12:13:48 AM
{I have tried to write carefully, so please try to read carefully, and if in doubt ask first.}

Quote from: Capt_Dunzell
All of the transistors are okay, I replaced R77, R79 tested unharmed.

Good.

Quote from: Capt_Dunzell
When you say current limiting bulb, is this just a bulb with a lead off of each contact to act as a dummy load?

NO.  A dummy load is only required when repairing a valve amp.  With solid-state amps they are serviced with the output open circuit.

A "limiting lamp" is a low wattage lamp, say 40 watts, connected in series with the mains supply to the amp to limit the amount of power that gets to it in the event that there is a fault.  And it must be an old style filament lamp, not a CFL, LED, or other fancy whatnot.  See;

Limiting lamp - please read and  digest.
http://www.ssguitar.com/index.php?topic=2093.0 (http://www.ssguitar.com/index.php?topic=2093.0)

Make up and use a 20-40 watt globe until you are certain the output half rail (see below) is centering between the two supply rails, +ve and -ve.

Quote from: Capt_Dunzell
When you say output rail, is that the terminal strip that connects to the speaker fuse?

Yes.

Disconnect the loudspeaker if you value it.  Do not reconnect until you are certain that the output stage output is centering as above.

Please read our posts carefully, and ask before acting if you are not clear about what is being said.  The only possible alternative with solid state amps is extreme frustration, a large pile of dead (brand new) semiconductors, and an amp that still isn't going.

Termnology:

Looking at the circuit; the +ve supply rail comes from the rectifier to connection marked P5-5, nominally +55 volts in this case.

The -ve supply rail also comes from the rectifier to the connection marked P6-1, nominally -55 volts in this case.

The output half-rail passes through fuse F201, connector P7-2 to the join of R79 and R81, and some points to the left.

Under normal conditions this "half-rail" should idle half way in voltage between the supply rails, which normally means within a couple of hundred millivolts of amp ground.

It is rare that it will balance exactly to ground, but anything more than about a quarter of a volt, 250mV, is a sign of a problem.  More commonly when there is a fault this rail will go close to one of the supply rails and stay there, providing a clue as to which side, upper or lower, has dead silicon.  Shorted or open transistor junctions are by far the most common trouble in these sorts of amps.

Now;

1) make up a (safe) load limiting lamp per the link.

2) Disconnect the loudspeaker (if you don't want to end up having to replace that too).

3) power the amp up via the limiting lamp.

4) confirm that you have + and - supply rails (their voltage may be a fair bit lower than 55 volts via the lamp, but they should be the same value, e.g. +/-40 volts).

5) measure the voltage on the output half-rail, e.g at the fuse, P7-2, R79 or R81.

Post your measurements.

You remarked that you had replaced "the FET" which I'm assuming is J111 at the input to the main power amp section.  This FET performs a muting function, and if the power amp seems healthy your problem might be that it is simply being muted by this FET a) being faulty, b) wrongly replaced, or c) not getting a proper de-mute signal.

But first we must make sure the whole power amp is indeed healthy - strictly by the numbers above.


Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on November 06, 2012, 03:37:44 AM
Google the phrase "light bulb limiter" and you will see multiple examples of exactly what we mean.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 06, 2012, 07:56:04 PM
I was just confused by the terminology, I have a current limiting lamp, I have always known it as a dim bulb tester.

Okay,

I have made the measurments, speakers disconnected, current limited lamp with a 40w bulb in place.

+VE .01 vdc
-VE  .01 vdc
Half rail 0 vdc.

I am making an assumption that these are measured to the chassis?

Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 07, 2012, 01:14:41 AM
Let me guess, the 40 watt bulb lights brightly? (if not the chassis "ground" point you are using as your voltage reference point may not actually be DC ground; unlikely, but possible.)

+/-10mV is next to nothing, so it looks like you have a dead short across your supply rails, or the main rectifier is stuffed.

Since you can apparently disconnect the power supply from the output stage, do so, and see if you then get some sort of voltage on your supply rails coming out of the power supply.

If not you have a power supply fault (dead bridge rectifier?), and you need to concentrate on getting that fixed first in isolation from the power amp.

If you get some tens of volts, perhaps even the full +/55 volts, and your lamp now goes dim/out after a second or two, you have shorted transistors top and bottom in the output stage shorting the +/- rails together.  You should be able to confirm this by resistance measurement between each supply rail and the output half rail (while the supply is still disconnected, naturally)

Isolate or remove both output transistors T202 and T203 and test.  With a mere 10mV across each (if the supply is okay) I'll bet Sydney to a brick they are both shorted collector to emitter.

{A photo of the output stage and power supply physical arrangement would be helpful.}

Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 07, 2012, 07:55:09 PM
I think I need to hang my head in shame and embarassment.....

I had my meter set incorrectly last night and did not catch it, I usually use an old vaccuum tube volt meter and this one sits on the shelf :-\

P5-5 +ve +36.3
P6-1 -ve  -37.3
P7-2 +12mv

The dim bulb tested is very dim with the 40 watt bulb.

T202 and T203 test good.

I really appreciate your assistance on this!  :dbtu:

I dont have my camera with me right now, I did scan the layout page and attach it though.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 08, 2012, 11:13:29 AM
{Don't sweat it.  We've all done that.  I once spent 40 minutes ripping into a stereo before I traced it back to a blown mains fuse, and in front of some other techs too.  I have a long list of particularly stupid things I've done  :-[  ... but moving right along.}

Okay, so those voltages look quite good, particularly the fact that the half rail, is half; +12mV looks real nice.   :dbtu:

Now try feeding a line level signal (100-200mV) into the "main in" socket (from  tape, CD, MP3 player &c).  If you have a CRO look to see if you are now getting signal on the half rail.

No CRO?  Using a 0.1uF (o.n.o.) in series with your meter on the AC range, can you see AC voltage jumping about on the half rail in sympathy with the programme input?  (if you have an old analogue multimeter with an "output" socket it has a cap built in for just this sort of thing.)

Yes?  Now you are really getting somewhere, jump down to "X".

No?  Remove the FET J111 and try again.

Yes?  The problem is with the muting FET; dead, mis-connected, or its gate isn't being taken to a minus voltage (~-15V?) a second or so after power up.  Check the FET, its lead orientation, and what its gate voltage is doing.

STOP and post results.


"X" - power down, reconnect the speaker and output fuse, but leave the limiting lamp in circuit for the moment, and try again.

If you have clean output at low levels (it will distort due to the lamp if driven hard) then you are most certainly out of the woods and can whack in a larger limiting lamp and try again.

Still good?  Remove lamp, go direct power, take a deep breath (pray if you are so inclined) and give it a burl.

{my pleasure, I love a good mystery.}
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 08, 2012, 09:09:04 PM
Okay,

Fed an IPOD thourgh the main in socket, it made a little blip on the scope, hooked up the speaker and no sound. Pulled the JFET and tried again, scope went nuts, decent sound out the speakers.
JFET out of circuit tested good. Reinstalled the JFET and tested the voltage on the gate. Only .85 volts.

It is in the correct orientation.
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 08, 2012, 09:16:47 PM
How much voltage did you read on the gate pad, *without* FET?
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 08, 2012, 09:18:18 PM
oops, I didnt take a measurement without it...I will pull it again right now.
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 08, 2012, 09:44:52 PM
It's important, because we don't know whether only .85V reach that point drom somewhere else, or the Fet has some kind of problem, and is pulling it down.
Often we find measurements like this, where the same number, may mean different things.
So they must be testyed both ways.
Good luck.
And yes, we are already seeing the light at the end of the tunnel ;)
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 08, 2012, 10:04:49 PM
The reading was the same without the JFET.  .85 volts
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 08, 2012, 11:38:30 PM
Ahhh HA!  Excellent!  The de-mute signal isn't getting to the FET.  As JM says, now we really are getting somewhere.   :dbtu:


Check at the supply that the -15V supply is correct.

Yes?  Lift one end of the cap on the FET gate, C38 1uF/50V, and see if you get -15V without it.  If so, new cap (watch the inverted polarity, +ve to ground like a valve bias supply).

If not look to R61 2M2 gone open or high.

{My bet ATM is the cap is leaky/shorted.}

Nearly home now.   ;)
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 09, 2012, 09:13:39 PM
Okay,

I do not have -15V, if I am checking in the right spot, on the end of D12 and R61? I have +1.7V.

At R64 I have 42V but it looks like it should be -15V on the schematic, am I seeing that incorrectly??

Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 10, 2012, 12:10:10 AM
I'm confused.

We are down in the bottom-left corner of the (upright) circuit, just above the title word "Fender", FET J111, C32, R61 (Note!), D12.

R64 is in the top of the differential pair and indeed should have the main V+ rail on it, 40-something volts; but that isn't where you should be looking.  You need R sixty-ONE (red-red-green), clustered with the FET, C and diode on the layout, right next to input connector P4 near the middle of the board - runs between the FET Gate and the -15V supply.

You should have -15V on both ends, if not remove (or lift one leg) of C32, a little cap electro of 1.0uF/50V, and see if you then get -15V at both ends. (remembering that your meter is basically a 1M resistor and when measuring the Gate end will drop the voltage in conjunction with the 2M2 to about one-third or about -5 volts, so a dud reading is near ground and a good reading is around -5 volts)

You say the minus-15V supply is okay, and this should be getting to one end of R61, if not you have some sort of a break in between and you're just going to have to follow the trace along with a bright light and magnifier until you find it (but I suspect you are confusing R61 with R64, or maybe I did somewhere above).

If on re-examination you find that you do have minus-15V on one end of R61 then you only have three components to look at, J111 which could have a shorted gate-to-channel, C32 gone leaky/short (which is still my hit pick, or R61 2Meg2 gone open or very high.  If the minus-15V isn't getting there then you are just going to have to track down why, that's an actual board trace fault I can't help you with remotely.

So, find R61, measure and post voltage on each end.

If you have -15 on one end and something close to ground on the other, lift C32 and retest.

If you now have -15V on both ends (see caution above, i.e. -15 and ~-5) simply replace C32 and bin the old one.

{it's 4pm here and I'm now out for a while, but I'll be back on in a few hours to see how you are getting on}

HTH
Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on November 10, 2012, 12:20:54 AM
Even though R61 and D12 are the wrong place to look, still the lack of -15 there is telling.   But forget R61.

Roly, follow from the top of R64 across and down to C47.  I think he sees the "-15" arrow from C51 and thinks it refers to the line we just followed.  That is not the case.

Look on your schematic at the power amp section, upper right of it.  See the bridge that makes +24 and -24?  D22-25?   That is where the 15v supplies come from.  look at the lower one, see R84, R86, D21?   They take -24 and turn it into -15.

Since you have +1.7 instead of -15, my thoughts are that it is not a sshorted D21, but more likely an open R84 or R86. 

R86 should have -24v on one end and -15 on the other.   If it has -24 on one end but close to zero on the other end, then see if it is hot.  If it is VERY hot, that means the D21 zener is probably shorted.  (Could also be C51 and a number of other things, but the zener is way more likely)   If that resistor is cold, it is likely open.


If R86 has -24v on one end and -15v on the other, then R84 almost has to be open.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 10, 2012, 09:39:00 AM
You're quite right.  Checking back I thought he had confirmed he had the -15V supply at the supply but not at the FET.  Now I see that this was my misunderstanding  :loco , so yes it now looks more like it is a simple loss of the whole -15V supply.  Thanks.


@Capt - there are two caps crossways at the end of the fuses; please confirm you have 24 volts across each.

On the other side of these two caps from the fuses you will find D19 and D21 and a couple of power resistors R85 and R86.  Please confirm that you find 15 volts across both D19 and D21.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 10, 2012, 02:25:22 PM
R84 is not open, it measures 500ohm, the schematic shows 10?
R85 has 19V on one side, 5.7V on the other
R86 has -19 one one side, -15V on the other
on the two caps, C52, C53 I have 19V on one side, and -19V on the other.

D19  5.7  on the line side, -.5v on  the other
D21  -.5v ob the line side, -15v on the other
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 10, 2012, 10:09:33 PM
Humm, the positive side zener way low, and the series resistor on the negative side way high.

5-6volts on the +ve zener sure ain't right.  The question is if it's the zener itself or something downstream drawing too much current.

The "500 ohms" of R84 ain't possible if it's still intact, so it looks like that's effectively open and needs replacing.  This by itself would explain the failure of the JFET to unmute the power amp.

Isolate the +/-15V supplies from their loads and get them healthy.  If the voltage across D19 remains at 5-6 volts with all load disconnected you need a new 15V zener too, but be sure to check the two 150 ohm 5 watt resistors that bring the +/- rails in for correct value as they may have had a good cook.

{I'm curious about what has happened here because it appears that both +ve and -ve supplies have failed, and it is not at all normal for two supplies to fail like this without a common cause.}
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 11, 2012, 04:16:21 PM
I replaced both of the 150ohm 1 watt resistors but they did test okay.
I also replaced R-84.
I dont have any zeners to use to replace the ones on the board, I will need to pick some up, however they do test good.
Probably a dumb question but how do I remove the load from the + and - 15 supply?
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 11, 2012, 09:59:19 PM
I was hoping that there might be a cable/connector you could just pull, but if not then you will have to lift one end of each of the 10 ohm resistors, R83 and R84, and check that you are getting the correct 15 volts across each zener.

If so the zeners are good and there is excessive load somewhere down the line; if not one (or both) of the zener voltage is off meaning you need a new 15V zener(s) of the same or larger wattage.  So check your zener voltages with the two 10 ohm resistors open before buying new ones.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 11, 2012, 10:58:46 PM
Ugh,

After replacing those parts I have:

R61 -1.5
R64 41.3

d19 -.5
d21 -.5

R85 +19
R86 -19

The 680 mfd caps +16 on one side, -16 on the other.

R84 heats up and cant be touched.n I followed the traces on the board out and can visibly see an issue.

I am going to pick up replacement zeners. This is really confusing to me.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 11, 2012, 11:11:31 PM
I meant I cant visibly see an issue, apparently I cannot spell tonight.

I have ordered the zeners.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 12, 2012, 10:41:26 AM
V @ R61 looks like we still don't have a -15V rail happening.

Can we forget about R64 please; it's on the +55V main supply rail which has nothing to do with the +/-15V supplies.  The "+15V" annotation applies ONLY to the arrow to the right, not the rail to the left.

No voltage (0.5V) across either zener D19 or D21.

The 680uF caps are on the +24V supply side.

R84 getting hot is very significant.  It's only 10 ohms so a bucketload of current must be going down there somewhere.  Are you certain you mean R84 10 ohms and not R86 150 ohms getting too hot to touch?

AGAIN - lift one end of R83 and R84 (both 10 ohms) to isolate the supplies from their load and see if that restores your +/-15V zener voltages (D19 & D21).  Fitting new zeners won't help if the lack of voltage (and obviously excessive current) is due to an external short - in fact they may still be healthy.  Lift the resistors, measure the voltage across each zener and post.
Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on November 12, 2012, 06:34:43 PM
Capt, with a hand meter you can "check" diodes on the diode test function.   That tests them as simple diodes and at about 1 volt.   What your meter will not do is test a zener for proper zener voltage.  A 15v zener that has failed down to 5 volts will still act like a diode on testing.  But that is only part of the test.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 12, 2012, 09:19:15 PM
Maybe its time I throw the towel in on this one.

I pulled on end of R83 and R84, no change on the zeners, I actually could not get a stable reading without them, the reading bounced all over.

Is there a way that I can test them since it sounds like the diode check on my meter wont do this properly?

I have ordered replacements but wont have them for a few more days.

I miss posted the hot resistor, it is R85 and R86 that are getting very hot.

Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on November 13, 2012, 12:19:08 AM
A zener has to check as a regular diode first, so it conducts only in one direction and is not shorted.  but it also must break down in reverse at its zener voltage.  The only way to test that part is to apply voltage to it.  Your circuit does that.

If you do not have 15v zeners on hand, and canot find them locally, one trick might be to move the good one to the place of the ??? one.

But really, 15v zeners are about as common a part as there is, so if you have about any local electronic repair shops - guitar amp repair, TV repair, stereo repair, CB radio repair, and I bet some others,  They are bound to have them and ought to be willing to sell you a couple.   I sell parts to anyone who walks in and needs something.  I buy 15v zeners 100 at a time.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 13, 2012, 08:26:59 AM
{You have the power amp working in good health, and now you're going to throw the towel in?  Over the preamp zener supply?}


With the outputs isolated, i.e. R83 and R84 open, and infeed resistors R85 and R86 getting hot it's a fair bet that your zeners are faulty; however you previously reported 0.5V across the zeners, now you say "I pulled one end of R83 and R84, no change on the zeners, I actually could not get a stable reading without them, the reading bounced all over."  I find this "no change" + "bounced all over" confusing and unexpected.  Typically when a zener goes down it stays down, and this sounds more like a multimeter problem, dirty contact in the range switch or something.

In any case opening the outputs did not result in a nice stable +/-15V, and the hot resistors suggest the zeners are shorted (and this is one way of testing them, using the circuit they are in).  Again. it's a bit odd for both of them to go down at the same time, so I can't help wondering what external thing may have caused that.

Howsoever, fit new zeners, leaving the outputs open, and establish that you have steady +/-15V supplies, then we move on from there.

{Just BTW, what country are you in?  I'm in Australia (and wondering about the time difference)}
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 13, 2012, 01:51:24 PM
Looks like he's on GMT-5 so East Canada or USA, no further West than Indiana.
2 hours from here, almost all around the World from you ;)

Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 13, 2012, 08:39:26 PM
I am in Vancouver, Washington.

I dont have any local suppliers, Radio Shack is the only one in town and they dont have much to speak of anymore. They did have some 12v zeners. The 15v Zeners should be here tomorrow or Thursday.

Probably shouldnt have but I popped the 12v zeners in because I couldnt stand not knowing. With one end of R83 and R84 pulled I got +12v and -12v on the Zeners, R85 and R86 did not heat up. I resinstalled the ends of R83 and R84 and +12v and -12v goes away on the Zeners and R85 and R86 heat back up.

Pulling the 12v zeners and will put the 15v in as soon as I get them.

Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 13, 2012, 11:08:22 PM
Don't worry, what you did is fine, you restored functionality to that part of the circuit.
So it looks like there was some Zener problem after all.
What does "goes away" mean exactly?
0V? Some other value ?
Whenever possible try to use numbers to describe things. :)
Thanks.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 14, 2012, 05:52:59 AM
Now that's looking good, but I would just like to clarify something; did you check the old zener voltages with R83 and R84 ends lifted?  Can you confirm that you were getting little or no voltage across the old zeners under those conditions?

When you say the voltage across the zeners "goes away" when you restore R83 and R84 suggests that you have something like an op-amp shorting across the rails somewhere downstream in the preamp.


In trying to make sense of a fault we always try and understand the situation that led to the failure.  Why I'm interested in the state of the old zeners is because having multiple faults, two dead zeners, and now what looks like a shorted component somewhere in the preamp, suggests that something pretty gross has happened/been done to this amp.  This does happen, but it's pretty rare - what we almost always find is that a single component failure is responsible (even 'tho other components may have been damaged as a result).

In this case the single failure we are now looking at, an op-amp or something shorting the +15V and -15V together, could explain everything so far, but not the zeners being shorted; that's a worry.  So your answers to the question above is important because they could mean the difference between hunting a simple fault, and something a bit more complicated.

Therefore it could be worth backtracking a bit, reinstalling the old zeners, lifting the R83 and R84 ends, and checking if you get +/-15V, or something else, just to clarify the situation.

I see on the circuit that there is a connector P2 which carries the +/-15V to part of the preamp.  You should locate that and  disconnect it at this stage.

Once we get +/-15V confirmed at the power supply, what we will then be doing is trying to bring the preamp up bit by bit (as far as that is possible) until we find the part that is shorting the supplies.  Are the IC's on the preamp in sockets, or soldered in?
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 14, 2012, 08:11:38 PM
The IC's are soldered in, I have sockets I can install if needed.

reinstalled the old Zeners.

R83 and R84 disconnected
Zeners are +1.5v and -.07v
R85 and R86 are cool

R83 and R84 connected
Zeners are +3.2 and -.5
R85 and R86 get hot

Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 14, 2012, 08:18:59 PM
Should I order up some of the IC's in case they are needed?
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 15, 2012, 06:58:05 AM
Quote from: Capt
R83 and R84 disconnected
Zeners are +1.5v and -.07v
R85 and R86 are cool

Okay, that seems to confirm that both zeners are indeed cactus.   :tu:

Now with R83 and R84 still disconnected I want you to measure the +/-15V rails on the preamp side, i.e. across C49 and C51 (100uF 16V) to determine if there is any voltage coming back from elsewhere (that shouldn't be). Both of these should be a zero, but it's just possible you may get a surprise and find a voltage where there should be none.

If there is, stop and post results (or you may kill the new zeners - I want to eliminate the possibility that the 24 volts supplies are somehow finding their way back to the 15 volt rails).

Do you order replacement IC's now?  I don't see any IC type numbers on the circuit but most common dual op-amps are only a dollar or so.  My normal method is to first find the fault, THEN take Berlin, however (only if there is no voltage coming back, as above)...

Restore R83 and R84 and run for a couple of minutes, while testing the IC's to see if you can find one that is  noticeably warmer than the others to the touch.  If you find one such, remove it and see if that restores the +/-15V supply rails.  If so, order a new one.

Exceptions: IC4 and IC6 - do not remove, but post results.

Since they are not marked on the circuit please list the type numbers of the IC's used.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 15, 2012, 08:51:15 PM
The 15v zeners arrived, got a pack of 100.

Fitted a pair in, have +15 and -15 as it should be with R83 and R84 disconnected.

C49 and C51 both have -.6v on them.

IC1, 2, 3, 5, and 7 are all MC4558CP

IC4 is a CD4007UBE

IC6 is a TL604CP

I went ahead and reconnected R83 and R84 and could not detect any of the IC's getting warm at all.
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 15, 2012, 09:16:30 PM
You always leave us with a little suspense ;)
"I went ahead and reconnected R83 and R84 and could not detect any of the IC's getting warm at all." .......... *and?* .............
*do* you or *don't* you still have +/-15V, with op amps connected?
You are not saying so ;)
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 15, 2012, 09:34:40 PM
Sorry, results are the same for the 15 volt supply with R83 and R84 connected.

The zener voltage dropped to +3.2, and -.5 again.

removed one leg of R83 and R84 and the zener voltage was restored to +15 and -15.

I have not removed any of the op amps at this point because none of them felt warm.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 15, 2012, 09:54:42 PM
I went ahead and ordered some 4558's so I will have them on hand. If I dont need them I will have them for something else in the future.

Again I truly appreciate all of the help on this thing, I am completely lost on it without the assistance.

 :dbtu:
Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on November 15, 2012, 11:04:38 PM
OK, so at least you have 15v rails back.  But R83,84 cause it to collapse.

Probably OK, but verify they are still 10 ohms.   And also verify R85,86 are still 150 ohms.   If they have failed way high, they could still feed the zeners alone but collapse under the normal circuit current load.

SOMETHING is loading down the 15v supplies.  ICs are the first suspects, but they ought to get warm/hot if they are doing this.  But other stuff connects to those rails.

On the schematic, lower left, see preamp out jack?  Right above it are D9, D11.  Shorted?  Also by input jack, see D1, D2.  SHorted?

I tend to think neither supply is shunting to ground, since they both are dragged down.  I tend to think then that they are shunting together.  But never overlook any possibilities.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 16, 2012, 03:06:02 AM
Quote from: Capt_Dunzell
The 15v zeners arrived, got a pack of 100.

That's just about a lifetime supply; and with the 4558's you're going to be set for some fuzzbox builds or the like.   :)


I'm glad that JM and Enzo have jumped in at this point coz I'm starting to run out of ideas.  I was worried that the 24 volt supplies to the reverb driver were finding their way back to the 15 volt rails, but the (near) zero voltage with the 15 volts supplies disconnected seems to rule that out.

{Curious point, the 10 ohm resistors are getting hot and the 150 ohm infeeds aren't.}

I agree with Enzo that this looks like a 15 volt rail-to-rail short ('tho not a dead short from the small voltage difference).

Capt - you said earlier that you had changed a number of electrolytic caps.  Did you change any on the preamp board/area, and if so could you please look at them very carefully to be certain you get them back in the right polarity?  Also check your soldering for any tiny tails that might be shorting.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 16, 2012, 09:16:01 PM
I replaced all of the electrolytics, C49, C51, C25, C47, C48, C52, C53, C54, C55 polarity is correct on the board.

I have gone over the board for hours on several occasions with a bright light and magnifier and havent found any solder crosses.

With D1, D2, D9, and D11, pulled they test good on my meter. With a standard diode is this an okay test? Or is it not very valuable like the zeners?  They are 1n4448's. I dont have any of these on hand some 1N1419's that look similar in specs.

Is it worth pulling some ICs and pull some voltage tests even though they didnt get warm?

R83 and R84 and brand new, 1/2 watt and 10 ohms. 
R85 and R86 are also new, 1 watt flame proofs as that is what I had here, right at 150ohms.

I dont know if I should have done this or not but with D1, D2, D9, and D11 removed I get +3.2 and-.5 on the zeners still, would this point to the IC's even when they dont get warm?
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 17, 2012, 02:16:54 AM
Okay, good.

Testing zeners is a special case, and I think we have now covered that by replacement and testing them in-circuit; with R83 and R84 lifted we get +/-15V on the zeners.

Standard diode test for standard diodes; open circuit one way (meter +ve to cathode, banded end, overrange "1___") and between "500"mV and "700"mV the other (on diode test range).  If they test okay then they don't need to be replaced.

Now unless anybody has a better suggestion (guyz?), I'd start to pull IC's.

The short could be just about anywhere (maybe not even an IC) but I'd nominate IC2 first because one of its outputs goes to an externally accessible point, the Preamp Out socket, via R201 only 680 ohms.  The object of this resistor is to protect the IC from an external short on this socket, but it would provide little protection from somebody accidentally applying an excessive voltage to it, say from another amp output (and since the amp is 2nd hand we have to allow for all sorts of stupidity/accidents in its history).

The next I would try would be IC7 (because half is the reverb driver which is also connected to the +/-24V rails).

During this process I would not re-insert any of the IC's you remove, but put them to one side individually in little plastic bags or whatever, with a note of where each one came from.  Once a dead one is located (by the +/-15V rails coming to life on the preamp board) then that one can be replaced with a new one, and the others returned to their previous positions one-by-one, re-testing at each replacement in case more than one is faulty.  I would not pull IC4 or IC6 yet, but concentrate on the op-amps for the moment.

I hope you have a solder-sucker because de-soldering IC's is a lot easier with one (protip: running a little fresh solder onto a joint before trying to de-solder it re-fluxes the solder and makes it flow better when you suck or knock it off).

I have to warn that it is possible that you will pull all the IC's and still have a short, but we will cross that short circuit if/when we come to that point.
Title: Re: fender stage lead 212 troubleshooting
Post by: DrGonz78 on November 17, 2012, 04:46:31 AM
Sorry to interrupt into this great thread... I am really enjoying reading along following all of what is being looked at on the schematic.

I have a question that even the OP might have wondered as well... Can we take voltage tests off of pins 4 and 8 on all the op amp chips? I mean sure we can test voltage there to check but why would that not prove to be useful numbers?

I am following the idea of IC2 being lifted first and then checking IC7. I think something is telling us that +/-24V rails could have something tied into the problem too. Maybe looking at Q2 and Q3 closer(later on down the line of course) and D8 is in there too. IC7 is giving me that gut feeling even more now. Also, have we confirmed what is measured on the 24v rails? Excuse me if I missed something there, but just had to ask. Good luck on this amp and I bet you nail it soon!
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 17, 2012, 09:56:45 AM
You're not interrupting.

I have access to a millivoltmeter and yes, I would be probing the chip pins topside to see if I could get a handle on where all that current is actually going.  This could be worth trying with an ordinary DMM but I'm not all that hopeful.

No, I don't think we have confirmed the +/-24 volts supply, and that is well worth looking at at this stage, thanks for the suggestion.

I have had a niggling worry about that reverb driver for some time, and I hope your comment is confirmation and not reflecting my concern.

Something that still bothers me is that it seems that both 15 volt zeners failed, something that is in itself uncommon, and would not be caused simply by an op-amp shorting the 15 volt supplies together - there has to be some other cause.  (scratches head)

Capt - Check the +/-24 volt rails and post results.  Pulling and testing the reverb driver transistors Q2 and Q3 won't get the amp going, but if one or both turn out to be shorted it may help to explain why, and what else has failed (and you can also leave these out for the time being).

Your observations are helpful @DrGonz78, thankyou.  :dbtu:
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 17, 2012, 04:50:39 PM
On the 24v supply if I am measuring in the right spot (C52, R85 and C53, R86) I have +16 and -16. Strange as I thought earlier on I had +19 and -19?

Q-2 and Q3 are good.

IC1 pin4 -1.8v  pin8 +2.3
IC2 pin4 -1.8v  pin8 +2.3
IC3 pin4 -1.8v  pin8 +2.3
IC4 pin7 -1.8v  pin14  0
IC5 pin4 -1.8v  pin8 +2.3
IC6 pin4 -1.4v  pin8 +2.3
IC7 pin4 -1.8v  pin8 +2.3

I am going to start pulling IC's now.

Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 17, 2012, 08:11:55 PM
I removed all of the IC's and installed sockets for them.

I can only get +15v on pin 8 of 1,2,3,5,7 if IC1 and IC2 are not in the sockets, this drops pin4 to -.06v.

R85 and R86 still heat up.

I rearranged a couple of the IC's to check if they gave similar readings in different sockets, IC1 is definately bad as it tanks the voltage readings in any socket.  IC2 installed in any other socket gives the same +15v on pin8 and -.06 on pin 4 no matter what socket it is in, so it seams IC1 has failed.
Title: Re: fender stage lead 212 troubleshooting
Post by: phatt on November 18, 2012, 06:18:01 AM
If I'm reading this right,, You now have all chips socketed then with all opamps removed you should have + 15 VDC on pin 8 of all sockets and - 15VDC on pin 4 of all sockets. (except of course for IC4)
Now just keep inserting each IC until those voltages drop at which point you likely found the culprit.

Well that is how I would approach it.
You will get there it just takes time,,Phil.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 18, 2012, 07:17:50 AM
{crossed with @phatt)

At the moment it's looking like the amp has had an excessive overvoltage coming in via the Preamp Out socket, and this has caused the failure of IC2, IC1, and both zeners.

Presuming that you are still using the limiting lamp your reading of only 16 volts on the +/-24 volt rails is to be expected.


Okay, to be clear; if all IC's are out of their sockets you can now get +ve 15 volts on all pins 8 and -ve 15 volts on all pins 4, is that correct?

If so, you can start re-inserting the op-amps, replacing the two dead, IC1 and IC2, with new ones.  Do this one-by-one, being careful to insert them in the correct orientation, and check after each one that you still have the +/-15 volt supplies.

If all goes well you can then apply a signal to the input and see if it gets through to the output.  If it doesn't then use your CRO to trace the signal from the input forward to find where it stops.

If you are getting signal output then you can try refitting the reverb driver transistors Q2 and Q3 and retest.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 18, 2012, 03:24:57 PM
So I have two bad op-amps, and have also found that IC6 is bad, I lose the -15v if it is on the board.

With IC6, IC1, and IC2 out I have +15v and -15v as it should be and R85 and R86 stay cool.

Its a TL604CP, not able to find much on this except for bulk order or ebay from China....I skimmed some posts on the search that looks like this chip is an issue regularly.

I am hoping that my new op-amps show up tomorrow.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 18, 2012, 07:01:25 PM
I'm feeling quite sure now with the growing pile of dead silicon that this amp has had a serious jolt of excessive voltage into one of its external sockets at some point.

Data sheet and cross reference for the TL604 attached.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 18, 2012, 07:51:38 PM
I can add one more to the list. I reconnected P2 and tested for voltages, IC10, another op-amp, is dead as well.

Glad I ordered plenty of them.

Peavy has a replacment circuit that plugs into the TL604CP socket since it looks like this IC is out of production, has anyone tried it?  Its cheaper than buying the IC and I wouldnt have to wait for it to ship in from China. http://shop.magicparts.com/PV99598160
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 19, 2012, 05:16:56 AM
{That cross reference turned out to be singularly useless when I looked at it - teach me to take it at face value  xP }

@Capt_Dunzell - the TL604 is a CMOS switch, effectively a solid-state relay for switching signals, and if it's out of production and somebody is making a substitute then you are in luck; yes, go for it (as you don't seem to have much option, and it looks reasonably priced).

For the time being you can simply use a wire link to activate whichever channel you wish, or even put in a small SPDT channel selection switch during testing until the substitute board arrives.

Yet another dead op-amp?  Hooo boy, this one certainly has had a serious dose of electric cancer go through it, maybe even the tail end of a lightning strike or similar, however you do seem to be grinding it down into submission now - you have the main amp and power supplies back up, and the preamps now looks like just a matter of persisting.

I don't suppose we will ever know exactly what happened to it, but I think this is one I won't forget for a while.  When you finally pass the finish line and can plug a guitar into it and make a loud noise I think you will be well justified shouting yourself a libation and a little victory dance.
Title: Re: fender stage lead 212 troubleshooting
Post by: DrGonz78 on November 19, 2012, 06:16:27 AM
Makes you wonder what happened to that amp!! LOL  :-[  :trouble

Good job troubleshooting ya guys!! I look forward to the end result of a guitar passing signal through the amp soon.

Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on November 19, 2012, 04:51:30 PM
Yes Peavey makes a workaround for the non-available TL604.  They have been making this thing for a long time.  Yes, it works.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 19, 2012, 08:04:55 PM
Well no delivery of my replacement op-amps today....so much for any more progress tonight.  :(

Thanks for letting me know that the Peavy part works Enzo, its on order.  :dbtu:

Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 20, 2012, 07:56:31 PM
Op-amps are here and installed....I have +15v and -15v as it should be on every one.   :tu:

The replacement for the TL604 wont be here for another few days.

That said, if I jumper for the channel switch temporarily, which pins would I use?

It looks to me like I can use pins 3 and 4 jumpered to either pin 6 or pin 7 if I understand the schematic correctly.

Can someone confirm this for me so that I dont break something?  :dbtu:
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 20, 2012, 09:23:39 PM
Capt_Dunzell - pins 3 and 4 jumpered to either pin 6 or pin 7   :dbtu:

3 and 4 appear to be the pole or output, while 6 (Channel 1, clean) and 7 (channel 2, effected) are the inputs selected.

+/-15 volts everywhere it's supposed to be?  Great, more real progress.   :dbtu:
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 20, 2012, 10:05:19 PM
Okay, jumpered and started up on channel two with the limiting lamp.....bad hum and no sound through the input...did I move too fast?

Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on November 21, 2012, 01:44:36 AM
Um, I am old, senile, and forgetful.   Clearly you have replaced a bad op amp or two and solved the loading on your 15v rails.  Great news.

Did we actually determine that the TL604 was actually defective?  Or are we just replacing it along with all the other ICs?  I mean if you stick it in the socket where it was, does it load the rails back down?   If it works, it works.  And if it is dead, then you can jumper and wait for replacements.

Now you have a loud hum, and the amp will not pass signal, right?   Pull your jumpers out of the TL604 socket.   That will cut the circuit more or less in half.  Hum remain or go away?

Assuming nothing, plug something into the power amp in jack to disconnect the whole preamp.  Hum go away?

Turn reverb dowwn, any difference?  For that matter, shake the amp to crash the reverb springs, does that come out the speaker?

The key to solving any problem is to isolate the problem.


If pulling your jumpers stops the hum, then it is coming from before the 604 spot.  You had jumpered to channel 2?  What happens if you jumper to channel 1 instead?  And whatever channel hums, do ANY of the controls affect that hum in ANY way?
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 21, 2012, 05:43:26 PM
...also...

This hum could be due to running via the limiting lamp on reduced voltage.  I think you said the 24 volts supplies were down to only 16 volts which leaves the 15 volt zeners with almost no headroom to regulate.

I would try moving up to the next highest wattage of limiting lamp.

Check that the output half rail is still very close to ground.

Measure the "24 volt" rails and post measurement.

Has the increase in supply voltage reduced the amount of hum?

Can you now inject a signal into the input and trace it along the signal chain using your CRO?
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 22, 2012, 02:31:38 PM
The TL604 is definately bad as I loose -15v when it in in the circuit.

I inserted a higher wattage bulb, was on a 40w, now a 60w, hum still present on both channels, and with jumpers in or out of the TL604 socket.

The amp is not passing a signal, no noise from the reverb tank, the hum increases on channel 1 and channel 2 with the volume and control knobs.

Q9 is heating up, warm, not hot.

I tried running my ipod into the power amp input, there is still sound as before, however now its not clean like it was before when I tried this. Did fixing the 15v supply change something with this?

I will try some signal tracing later this evening.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 22, 2012, 09:34:32 PM
The sensitivity of the hum to the front-end controls is a good sign.   :tu:

Disconnect the speaker and check the idle voltage coming out of the power amp is still very close to nothing (i.e. that the output stage is still rebalancing the half-rail); if it is, reconnect speaker.

You should also check the voltage across the emitter power resistors R79 and R81 0.22r to work out the idle current through the output pair.  This should be somewhere around 20-40mA, that is you should be getting;

E = I * R

0.04 * 0.22 = 0.0088, say no more than about 10mV.

If it is *considerably* more than this then you will need to note the current position of the bias pot R75, then carefully readjust it for about this idle current ('tho I don't expect you will need to do this).  If it is around this value or less, leave it alone.

Important question; is the FET J111 in or out of circuit at the moment?  (this is the only way I can see that the 15 volt supplies could have any effect on the output stage).

If J111 is out, does plugging your iPod into Main Amp In get rid of the hum?

For the moment turn the reverb control down to minimum (does that make any change?), strap IC pin4 to pin6 so we select the clean channel, Ch1, and we will concentrate on trying to get that up first, then move on to the more complex channel 2.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 23, 2012, 04:22:20 PM
You guys have the patience of a saint to be helping me through this.  :)

J111 is in the circuit

I have 17mv across R79 and R81


The reverb control has no effect.

The hum goes away when I plug in to the power amp input jack.

P5-5 47v
P6-1 -47v
P7-2  18mv


I pulled J111 out and plugged the ipod into the power amp input and the went away and I have a clean tone. The hum is also gone with J111 out.

No sound goes through the normal input at this point.



Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on November 23, 2012, 06:19:47 PM
When you had the 15v rails missing, not much can work.  By repairing the 15v rails, other existing problems are now able to reveal themselves.   It is quite common to repair a power supply problem only to find something else needing repair too.

You just verified the power amp works.

I am going to guess the volume controls only affected the hum when the TL604 jumpers were in place, and not with them out?

Did we establish that you get no signal passing AND you get hum on BOTH channels?  (selected one at a time by jumper)

If so that really points to the first stage.

I don't know if we did this already, but plug a cord into the regular input, then measure for DC voltage on the free end of the cord.    There should be none.   hat is a cheap way to check for shorted D1, D2, wwhich if bad can inject ripple hum into the signal path and squash your real signal.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 23, 2012, 07:06:12 PM
Ladies and Gentlemen we have a working amplifier!

J111 is back in the circuit, playing through an ipod both channels are now working and no hum!

There was another bad op-amp even though it wasnt pulling the 15v rails down. and a bad connection in P2 that I discovered by bumping it...I eliminated P2 after being unsuccessful at cleaning the plug ends and ran wires directly to the board.

still have to switch channels with a jumper as the replacement for the tl604 isnt here yet.

So, the two on board pots, bias, and offset, do they require any additional adjustment?
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 23, 2012, 07:08:39 PM
I am still running on the limiting lamp, I should be able to up the bulb wattage to double check everything and then pull the lamp out and give it a go?
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 23, 2012, 07:53:16 PM
Pulled the limiting lamp....eveything is working great...no hum, clean tone....trying a guitar in place of the ipod next.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 23, 2012, 09:28:39 PM
no limiting lamp, guitar for the input, everything is working....ran for 30 minutes and all is holding.
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 24, 2012, 12:56:59 AM
Quote
Ladies and Gentlemen we have a working amplifier!
:tu:
Quote
no limiting lamp, guitar for the input, everything is working....ran for 30 minutes and all is holding.
<3)

Cool, way to go.

Now, *who* was wanting to throw the towel just a few posts back ?  :lmao:
Title: Re: fender stage lead 212 troubleshooting
Post by: lurkalot on November 24, 2012, 04:11:54 AM

Ladies and Gentlemen we have a working amplifier!


Excellent news, well done Capt_Dunzell.  :dbtu: 
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 24, 2012, 06:37:58 AM
Quote from: Capt_Dunzell
Ladies and Gentlemen we have a working amplifier!

Meh. {shrug}   8|


But no, seriously, very well done Capt, that's great to hear, and good onya for sticking with it.   :dbtu:

Okay, so you've still got one IC to go, but you are effectively out of the woods now.

Should you adjust the bias/idle current?  Generally I leave that alone unless I have good reason.  Run the amp on idle keeping an occasional finger on the heatsink.  It should get a bit warm, just enough so you know it's on, but not hot, and hold that same "warm" for an hour or two; if so, leave it alone.  If it starts to get "hot" while idle then you should try turning the bias current down a bit.


This has been one of the really nasty ones.  It thankfully isn't often that you get an amp that has suffered some gross insult like having the mains connected to an input or hit by lightning, but when you do the result looks something like this one.

It is uncommon for more than one part to fail at a time, and when that happens you can generally see how, for example, a shorted output led to a shorted output transistor led to burnt emitter power resistors and perhaps a dead rectifier, maybe even a melted power transformer, but while such cascade failures do happen, they are (thankfully) not all that common.

I hope that you can now see that all the components you replaced at first didn't actually come near the real problem, and had the potential to actually introduce new faults.  My policy is to do as little as possible until I have a handle on what is going on/has happened, then try to move with "surgical precision"; not only finding and double checking that a part is faulty, but trying to understand why, what might have been the cause of the failure, because (particularly with DC coupled solid-state output stages) some obviously dead component such as a blown fuse or shorted output transistor, may only be a symptom of the real basic cause (e.g. wires twisted together inside the plug of a speaker lead - and I've had this one several times).

In this case I'll surmise that somebody connected the output of another amp to this one, let's just guess mistakenly to the Preamp Output.  Not getting the desired result they turned up the first amp and hit the guitar hard.  Even a 50 watt amp is capable of around 35 volts peak, and this would have caused the final preamp IC to fail, thereby placing the excessive voltage on both the + & - 15 volt rails, leading rapidly to other IC's failing and finally the +/-15 volt zeners.

Once the -ve rail had failed the power amp would not unmute due to J111 staying on.

In a commercial repair shop this kind of cascade failure might be BER - Beyond Economic Repair, and I think you can now see why.  In any case it's the sort of repair you come to dread because even after you have found and replaced all the obviously dead silicon there may remain a working but damaged op-amp just waiting to flip out and take your reputation as a repairer with it.  So don't be too surprised if in a few days, weeks or months it suddenly rolls on its back and plays dead again.

BUT - by now you and this amp should be old adversaries, you should know the innards like few guitarists ever get to know their amps, and it should contain little or no fear for you now - the next time it plays up you will have it up on the bench with its guts hanging out in no time, and the bit between your teeth, taking no nosense.  :trouble

I also hope that this whole thread stands as an object lesson in how to move step by step, stage by stage, through fault-finding, about isolating what seems to be wrong and proving it, bringing that part up, then moving on to the next problem.  And this amp is atypical in having so many faults at once; it's more normal to find just a single failed op-amp or power supply zener through old age or excessive heat, not a whole raft of dead stuff.  I haven't been keeping score, but you must have quite a pile of dead stuff on your bench by now.

As I said, a particularly nasty repair, and I think you are justified in feeling rather proud of yourself for having stuck it to the end.  Enjoy.  :dbtu:
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 24, 2012, 02:47:30 PM
Ran on an idle for a few hours this morning, heatsinks warm, not hot.

The peavy module arrived, they had two different ones, I got the less expensive one, but It doesnt seem like it should work, the pins dont make sense when I look at it.

When it is plugged in it turns channel two on and off, but when channel two is off it doesnt activate channel one.

In looking closer at the board there is no connection on pin 6 which is what should go for channel one..maybe I should have gotten the other module.

Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 24, 2012, 06:35:36 PM
I think the unconnected pin is #3 or #7 , definitely not #6 (don't know which way it should be pointing).
Try to plug the module the other way.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 24, 2012, 06:54:49 PM
That may be, but it wont plug in to the socket in the correct orientation to make it pin 3 or pin 7.

Pin 7 is channel 2 though, so maybe it must be pin three.
Title: Re: fender stage lead 212 troubleshooting
Post by: DrGonz78 on November 25, 2012, 06:49:38 AM
Looking at the datasheet pin 1 is ground...

Pin2 is marked as "A" on the data sheet, it ties over to the base of Q1 J175 and I think you should look at the transistor there to makes sure it is sound. Perhaps is was Zapped out of the circuit too!

Pin3 and Pin4 are tied together which is S1 and S2 tied to Pin 6 and Pin7 S1/S2. This is the switch.

So that leaves out pin5 and 8 as voltage carriers.

Looking at the picture pin1 and pin2 on the chip PV replacement (refer to picture) look like these ones to me, but not quite sure of this as a fact. Just a hunch after looking at it and studying it's layout. So, one channel is shorting to ground maybe due to the Q1 malfunction. To me it would seem that sure the pin out could be different on the replacement it is for a 604 chip but not DPST. Maybe this is only for SPST type switch and needs some modifications? Not sure I know exactly what I am saying here... Maybe even typing something silly will cause someone more knowledgeable to explain what could be going wrong here...?

Also, the more expensive other module you saw on that site is discontinued and really was the kit version to build what you have now. So, that is not reason why this won't work.

(http://www.fileden.com/files/2012/8/8/3334901/tl604cpPin12.jpg)

http://www.ti.com/lit/ds/slas042/slas042.pdf (http://www.ti.com/lit/ds/slas042/slas042.pdf)

Edit: Too funny a double posting... Also, you said on the main board there was nothing tracing anything to pin6? Explain which pins are what according to the picture... Also, show a link to the other module you thought it could have been. I might even have DPST thing wrong and it is as Juan said a "SPDT" type circuit needed. These things always confuse me at first... Or rather as Juan really put it... "Two independently driven SPST switches" Now that the solution here...

 

Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 25, 2012, 06:54:04 AM
Are you using a Fender made or a Peavey replacement module?
Because I can be quite sure that Peavey won't issue anything non-functional *but* maybe they use the TL604 in a different way than Fender did.
So the module not necessarily works somewhere else.
Say one of them *always* uses them as an SPDT, both halves driven from the same pin, and only 3 "switch legs" needed, while the other uses them as 2 independently driven SPST switches (4 legs needed).
2 mutually incompatible modules might be built , emulating either one or the other behaviour.
A *full* TL604 emulation can of course be made ... but it will take much more than 3 Fets and a couple resistors and diodes.
Let me think about it, *maybe* I can cook up some solution ... but don't hold your breath ;)

EDIT: Oooops!! , simulposting !!
But anyway DrGonz and I are hinting to the same problem: the replacement module meant for a Peavey won't necessarily work there.
PS: why don't you lift the module circuit and post it here?
Title: Re: fender stage lead 212 troubleshooting
Post by: DrGonz78 on November 25, 2012, 09:22:38 AM
My final thought is that 3 FETS will not get the job done here on this replacement circuit. However, I have a question as to whether J175 Q1 plays into the that role of making the logic of the switch come to life?
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 25, 2012, 11:29:13 AM
I think I have a working solution here, involving a single PFet (a J175 will do fine) and a 10K to 22K resistor, but I'm leaving to a friend's "asado" (http://1.bp.blogspot.com/-f669-sUYYsU/TcgfbIpiGaI/AAAAAAAAAZ4/vt3NrYbJqIg/s1600/100_0150.jpg), but when I get back, after adding a couple pounds of charcoal grilled beef and a couple bottles of wine to my waistline, hope will have some neurone left to draw a schematic.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 25, 2012, 05:05:30 PM
I think I have a working solution here, involving a single PFet (a J175 will do fine) and a 10K to 22K resistor, but I'm leaving to a friend's "asado" (http://1.bp.blogspot.com/-f669-sUYYsU/TcgfbIpiGaI/AAAAAAAAAZ4/vt3NrYbJqIg/s1600/100_0150.jpg), but when I get back, after adding a couple pounds of charcoal grilled beef and a couple bottles of wine to my waistline, hope will have some neurone left to draw a schematic.

I didnt think I could be hungry again after dinner but looking at this got me hungry, hope was great!

It is a peavey module, the other module advertised is the kit form with a different chip.
The module I ordered did not come with a data sheet and I cant seem to locate on online so far.

Q1 is not on the mail board, it wasnt there at all when I opened it up, I saw a post somewhere that this was some sort of optional mute function?

Did fender have their own verison of a replacement? I couldnt seem to find one unless I have some search issues.
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 26, 2012, 05:03:05 AM
Ah yes, it's "barby" season here too.  {drool}  ;)
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 26, 2012, 09:57:08 AM
Ok, the boa is back after having swallowed a cow ... or so I feel.

I think you can solve your problem this way, at least that's what I would do If I had it on my bench.

Being in Argentina I wouldn't even *try* to find a TL604, at any price.
This is not a general purpose TL604 replacement by any means (well, even Peavey's isn't either ;) ) but something which will solve your particular problem, in a Stage Lead (or the similar Studio Lead) amp.

(http://img62.imageshack.us/img62/2655/tl604fender.gif)

The red dot I call C.V. , Control Voltage, is where voltage can take any of 2 values, enabling one channel or the other.
CV is fed through a divider from +15V : R39(820 ohms)/R41(682 ohms)+Led and can be grounded by panel or foot switch.

When open, it must sit around 8 volts; when closed it's 0V , of course.
Digital guys would probably call the 8 Volts a "logical 1" or a "high" and 0V a "logical 0" (duh !!) or a "low".

Now back to the Music World.

I know that on "low" (CV=0 volts) Q1 is a closed switch and grounds (mutes) signal at R9, meaning it mutes Ch2 (dirty), and think it must connect TL604 Pin6 , which is Channel 1 (clean) to Pins3+4 (output); at the same time it disconnects Pin7 (Ch2) from Pins3+4 , further muting it.

I forgot: Ch2 Led also turns OFF.

CV= High means the exact opposite: Led ON, Q1 open (Ch2 unmuted), Pins3/4 connected to Pin7 and disconnected from Pin6.

Argentine solution:

1) you say Q1 was not installed (a proper working TL604 means it's not *that* necessary).
OK, now we need it, get a J175 and solder it where it belongs.

2) get another J175 (let's call it Qx)and solder it to the TL604 pads, under the board (or you might plug it in the socket, but I doubt the thin Fet legs making good contact on those old worn sockets).
Source: Pin6
Drain: Pins3+4
Gate: Pin2 (control voltage in)

3) get a 10K to 22K 1/4W resistor (or even a smaller 1/8W one) and solder it from Pin7 to Pin3+4

4) enjoy.

Explanation:
I'm a dyed in the wool Functionalist, and for me, if it flies like a duck, walks like a duck, and quacks like a duck, it *is* a duck. ;)

Or in other words, if it mimics original Function close enough, it *is* enough.

Let's "test" it.

a) Foot/Panel switch closed. C.V. Low.
Led OFF
Q1 ON (Ch2 muted)
"Qx" ON (Ch1 enabled)
Note: I added the extra 10K to 22K resistor because Ch2 *still* is connected to Pin3+4, and setting R37 (Master) to 0 would mute also Ch1 ; the separation resistor I added takes care of that.
So: Ch1 ON, Ch2 OFF  :)

b) Foot/Panel switch open. C.V. High.
Led ON
Q1 OFF (Ch2 enabled)
"Qx" OFF (Ch1 muted)
So: Ch1 OFF, Ch2 ON  :)
Title: Re: fender stage lead 212 troubleshooting
Post by: Roly on November 26, 2012, 07:18:43 PM
A duck is a duck ... until you encounter a Lyrebird, ground dweller and superb mimic, not only of other birds but machines, camera shutters, human coughing, &c&c.

http://www.youtube.com/watch?v=WeQjkQpeJwY (http://www.youtube.com/watch?v=WeQjkQpeJwY)
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 26, 2012, 09:39:48 PM
I can do that, I will need to order some J175's the only FET that they had in the local shop was an MPF102.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 28, 2012, 07:00:28 PM
Hoping the J175s arrive by tomorrow or Thursday, in the meantime an original Peavey adapter kit with the chip arrived today. Is it worth assembling to try or would the results be the same as the module that I already have?
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 28, 2012, 09:26:22 PM
Assembled the Peavy replacement kit and gave it a go....works perfectly! <3)

I think I am finally done thanks to everyones help!

Going to reinstall the chassis into the cabinet and give it some hours over the next few days.

Thank you everyone for all of your help, there is no way I would have been able to get this taken care of without it!!  You guys are the greatest!  :dbtu:
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 28, 2012, 11:12:36 PM
Cool. Post its schematic for future reference.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 29, 2012, 04:19:27 PM
It didnt have a schematic with it, but here is an image of the board and parts list.

 :)
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 29, 2012, 11:06:56 PM
Thanks.
Doesn't it come with some kind of instructions?
Maybe you can EMail Peavey, thank and congratulate them on their successful solution, and politely ask them for the schematic, because you want to understand it, not just mechanically solder.
Title: Re: fender stage lead 212 troubleshooting
Post by: DrGonz78 on November 30, 2012, 04:43:49 AM
Also, in addition to asking for the schematic which I think I will try to ask them myself too. Where did you purchase this replacement kit? I mean I saw on that first site that there were no more kits available... That would be nice for others to know where you bought this... a link...

I definitely interested in the schematic to this board and anyone here too. You did Great work on this amp Captain. Could not do it without the great techs here (they know who they are) but you also played into the factor of the amp repair being successful too! Yup standing on the shoulder of others should feel great no matter how high you rise.
Title: Re: fender stage lead 212 troubleshooting
Post by: Capt_Dunzell on November 30, 2012, 01:43:08 PM
I attached the  instructions along with a picture of the kit in the previous post but for some reason it doesnt show up unless you click the link for the pdf.

I am reattaching the pdf file, it has the instructions, parts list, as well as some pictures of the circuit.

I ordered the kit from Antique Electronic Supply, they say that they still have them in stock.

http://www.tubesandmore.com/products/P-QTL604-AK

 :)

Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 30, 2012, 01:59:23 PM
Thanks a lot.
Title: Re: fender stage lead 212 troubleshooting
Post by: joecool85 on November 30, 2012, 03:37:52 PM
I'm surprised Peavey even supplies such a kit, many manufacturers would tell you "tough, it's old, get a new amp".
Title: Re: fender stage lead 212 troubleshooting
Post by: J M Fahey on November 30, 2012, 05:55:05 PM
Peavey is Peavey  :tu:
No kidding.
Title: Re: fender stage lead 212 troubleshooting
Post by: Enzo on November 30, 2012, 07:24:55 PM
Joecool, that is EXACTLY why Peavey is such a great company.  If you call most companies for a part, and they no longer have it, you are done.  If you call PV for a speaker or a transformer for some 30 year old amp, they probably don;t have one, but they do have equivalent parts for current amps, and they will recommend something.

They used certain OTA chips in their products, in SOME cases you can use another type in its place.  PV will tell you that and supply the alternative.

The TL604 was used in TONS of PV amps from the 1980s era.   They COULD have said get lost, but they are such a customer friendly company they designed a litle workaround that connects directly in place of the TL604 on the board.

And unlike most other factories, PV will sell their parts directly to anyone who calls and places an order.  You don;t have to FIND these kits, you can buy them from PV.  Or from any PV parts seller, like AES.