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fender stage lead 212 troubleshooting

Started by Capt_Dunzell, October 28, 2012, 06:02:52 PM

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Capt_Dunzell

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?


Roly

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.}

If you say theory and practice don't agree you haven't applied enough theory.

Capt_Dunzell

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.

Roly

{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.}
If you say theory and practice don't agree you haven't applied enough theory.

Capt_Dunzell

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.

J M Fahey

How much voltage did you read on the gate pad, *without* FET?

Capt_Dunzell

oops, I didnt take a measurement without it...I will pull it again right now.

J M Fahey

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 ;)

Capt_Dunzell

The reading was the same without the JFET.  .85 volts

Roly

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.   ;)
If you say theory and practice don't agree you haven't applied enough theory.

Capt_Dunzell

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??


Roly

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
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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.

Roly

#28
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.
If you say theory and practice don't agree you haven't applied enough theory.

Capt_Dunzell

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