fender stage lead 212 troubleshooting

[Capt_Dunzell]

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.

[Roly]

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?

[Enzo]

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

[J M Fahey]

Just found it. 

support.fender.com/schematics/guitar_amplifiers/Stage100_Stage100H_Stage160_schematic.pdf

[Enzo]

I don't think so, the Stage 100 is from 1998-2000, the Stage Lead is more from like 1982.

[DrGonz78]

Yeah I almost thought the Stage 100-160 was the right schematic too...   

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

[Capt_Dunzell]

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.

[Roly]

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.

[Capt_Dunzell]

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.

[Roly]

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.

[lurkalot]

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"   

[J M Fahey]

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.

[Capt_Dunzell]

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.

[Roly]

{I have tried to write carefully, so please try to read carefully, and if in doubt ask first.}

All of the transistors are okay, I replaced R77, R79 tested unharmed.

Good.

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

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.

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.

[Enzo]

Google the phrase "light bulb limiter" and you will see multiple examples of exactly what we mean.