Fender 212R power amp blown

[lowell]

It's a 3ohm load. 2 6 ohm speakers... at least thats what they have labeled. No offset with a guitar plugged in.

[Enzo]

If the 4v is present on the speaker, I;d want to get rid of it, myself.

So it sounds like the mute circuit is upsetting the power amp in some way.

[lowell]

Yes indeed. I think I have some opamps that I may sub in for U6 as well as check U6 surrounding components. I may start with checking the supplies for U6 just to see what's up there.

[Enzo]

And what does U6 have to do with the muting circuit?

Q6,7 turning off the base of Q8 is the muting circuit.

[lowell]

I guess I was thinking that because RG said,

"I think that U6-A is a DC servo to keep the output at 0Vdc; I don't know exactly, because I don't like servos and don't use them in my amps, so I've self-limited my knowledge about them. I'd have to so some actual thinking (always hard and painful!) to suss that out. However, it is *very* clear that DC offset issues with the U6-A circuit and failures of the various capacitors in the feedback path or with U6-B would cause DC offsets."

[Enzo]

Yes, no argument there. But doesn;t your offset go away when a guitar is plugged in? Isn;t the offset only present when the amp is muted? How would U6 know if the amp were muted or not?

ANy number of things can cause DC on the output, we have to chose which ones might be involved in any given case. U6 may be involved in making the amp go to the DC offset, but it is probably trying to correct an imbalance caused elsewhere. Perhaps without that action by U6, the amp would be at -34v instead of -4v.

Then again, I may be wrong.

[MarkusBass]

I have few remarks:

1. We are not sure whether this offset is "by design", or there is still a problem with the amp. Why not check another amp (whether it has this problem)? I assume that Enzo does not have similar amp on his bench but maybe you could ask someone else who has this amp to make few simple measurements?

2. I assume that Lowell is from the States. Why not give Fender a call and ask them about the offset? Are phone calls expensive in the States?

3. As everyone (hopefully) understands; when the amp is muted, the Q8 transistor is switched off and there should be no current flowing through Q9 and Q10. The DC servo's voltage is connected to the base of Q10. So when the amp is muted, DC servo is not working. So you cannot expect that disconnected DC servo will correct the offset.

4. Still incorrect voltage on U6-A output can result in incorrect behaviour of the amp. But instead of cutting pins and replacing the IC, I would just measure voltages on U6.

5. I think it was stated that Q9 is not conducting but you never said this about Q10. Can you check this (simply by measuring voltage drop on R86)? What makes me wonder is that you said that D25-27 are all 0.5V (so they are conducting). How can they conduct if Q9 and Q10 are switched off (which means that Q11 is also switched off). This can be checked with just a few measurements but somehow is very difficult to get them from you, Lowell. I asked about some voltages and you instead of providing the voltages, you said: "I understand how the transistors are biased". You are not helping but you want us to help you. Is it fair?

I think that this offset is either by design (which can be verified with another amp or by speaking to Fender support), or some part starting from Q11 to the right on the schematic has failed. This again can be easily verified with few measurements. But to do it, you have to understand how the amp works. I would start looking at D25-27 - which voltage causes that they conduct when the amp is muted.

Mark

[lowell]

I apologize if I'm forgetting some measurements. Here's where I'm at.

DC offset is there with a load and amp both muted and unmuted.
DC offset is gone with and without a load when unmuted.

With amp muted:
Q11
Vb: 40v
Ve: 41v
Vc: -38v

Q10
Vb: 8mv
Ve: 150mv
Vc: 40v

Seems good to me. Q11 is on because it's biased with a Vbe of 1v via R84 and D24 is it not? Doesn't seem that the CCS or muting circuitry have any control of Q11.

[MarkusBass]

DC offset is there with a load and amp both muted and unmuted.
DC offset is gone with and without a load when unmuted.

Lowell,

Plese read slowly these two sentences - they cannot both be true. Yo wrote that the offset is there with a load when the amp is unmuted, and then that the offset is gone with a load when the amp is unmuted .
Have you tried calling Fender support?

With amp muted:
Q11
Vb: 40v
Ve: 41v
Vc: -38v

Q10
Vb: 8mv
Ve: 150mv
Vc: 40v

Seems good to me. Q11 is on because it's biased with a Vbe of 1v via R84 and D24 is it not? Doesn't seem that the CCS or muting circuitry have any control of Q11.

In case of transistors it makes sense to specify additionally Vbe measured separately.
Here, again understanding the circuit is very important. I see that Q11 is on (so D25-27 are also on) but in this case I would immediately check where does 1V (Vbe of Q11) come from. Is Q10 on or not? How is it possible that Vb = 8mV and Ve = 150mV? Is it measured relative to ground? Isn't it -150mV (minus)?

Mark

[Steve Conner]

DC offset is there with a load and amp both muted and unmuted.
DC offset is gone with and without a load when unmuted.

Are you sure? These two statements contradict each other in the case of the amp being loaded and unmuted. So one of them must be false. You probably think I'm being OCD, but little details like this matter immensely in troubleshooting.

[R.G.]

OK. Traced through the circuit. Let's step around the mute/unmute issue. Lowell, can you put a temporary jumper across collector-emitter of Q7? This will make the power amp unmuted no matter what is plugged or not plugge into the input jack, and therefore remove the mute/plug issue entirely. The amp has to operate correctly when unmuted to be fixed, so this step removes whether the mute circuit is operating from any consideration.

Then you can get on with fixing the offset in the power amp without the muting issues. Once the power amp is fixed, you can come back to the mute, and if that's the actual cause, fix the mute circuit.

[lowell]

My bad. Whoops!

DC offset is there with and without a load and amp muted.
DC offset is gone with and without a load when amp unmuted.

RG, the offset is gone when the amp is unmuted so there's nothing to troubleshoot when the mute is not active.

Markus let me double check Q10 and other voltages from earlier.

[lowell]

In case of transistors it makes sense to specify additionally Vbe measured separately.
Here, again understanding the circuit is very important. I see that Q11 is on (so D25-27 are also on) but in this case I would immediately check where does 1V (Vbe of Q11) come from. Is Q10 on or not? How is it possible that Vb = 8mV and Ve = 150mV? Is it measured relative to ground? Isn't it -150mV (minus)?

Mark

With amp muted: (added decimals to readings)
Q11
Vbe: -.5v
Vb:40.2v
Ve:40.8v

Q11 is a pnp so a negative Vbe of -.5v means it's on right? Which explains why D25-27 have voltage drops across them.

Q10
Vbe:-280mv

Guess Q10 is off.

U6
+/- 16v good on pins 4 and 8
no DCv on any other pins except for 500mv on pin 7

[MarkusBass]

Q11 is a pnp so a negative Vbe of -.5v means it's on right? Which explains why D25-27 have voltage drops across them.

Lowell,

I'm not asking you whether Q11 is on. We know that it is on for at least a week when you specified the voltage on R87 = 20 mV. I'm asking you to check why it is on.
Again, few remarks:
1. When the amp is muted, the negative DC feedback is not working. So it cannot correct the offset.
2. Due to reason that you haven't found yet, Q11 is conducting.
3. Diodes D25-27 are conducting because Q11 is conducting.
4. Q13 (connected to -V rail) is conducting because diodes D25-27 are conducting.
5. You get negative offset on the output because Q13 is conducting and Q12 is not. You can tell this because there is 0V on R88. I assume that when you unmute the amp everything is OK. It means that most probably R88 and Q12 are also OK.

So the main question is still: why Q11 is conducting. But please don't tell me that it has 0.5V drop on Vbe. You have to find out why there is voltage drop on R84 and D24.
If you look at the schemaitic, you can see that Q11 and Q12 are connected in almost exactly the same way: a resistor from emitter to +V rail, a diode and a resistor from base to +V rail and their bases are connected to Q9, Q10, which both should be off when the amp is muted. So why they are working differently?
I suggest that you measure voltages on R85, R86 (I'm asking about this for the third time) and possibly replace Q10. If this does not help, I would replace Q11. If this also does not help, I'd say that this is by design and you should call Fender support and ask about it.

Mark

[R.G.]

RG, the offset is gone when the amp is unmuted so there's nothing to troubleshoot when the mute is not active.

OK. Then the muting is causing the offset some way. Markusbass is correct in looking at Q11.

My cut is that when mute happens, Q8, Q9, Q10, Q11, Q12, and Q13 should all turn off. Q8 is the emitter current for Q9 and Q10. Bang, they're off. Only collector-base leakage should flow, and that should be nano-amperes if they're not damaged. Q11 should turn off because there is zero current going through Q10 collector. Q13 should turn off because there is zero current flowing through Q11 to turn its base on. Q12 should turn off because there is zero current flowing through Q9 collector to turn it on. DC balance of a sort should be maintained by the "matching" of the drops across the string of R91, 92, 94, 95, 96, 97,99, 100.

The output stage is "biased", such as it is, by Q14 and Q15 in an odd double Vbe-multiplier. I suspect that you'll find that the junction of R92/D32 and R99/D33 moves four volts down from their balanced voltages when the amp is muted.

So there are two places to look. Either (a) the Q8/9/10/11/12/13 things are not fully turning off and leaving some offset they feed into the bias places, or the oddly complex bias setup string is damaged and it goes to an offset. Could be both I guess.

If it were me, I'd measure the resistor values in those areas, and frankly, simply replace the transistors as well as putting a base-emitter resistor on Q11/Q12/Q13. This can be big, maybe 100K to 1M; it's just there to pull nA of leakage under the Vbe threshold when imponderables happen like the base being opened by the mute circuit. This might cure a leaky transistor in this mess, but replacement would be better.

It is also possible although I think more unlikely that either Q14 or Q15 are funny, and either leaking or not operating properly when the feed currents from Q12/Q13 go open.

It's hard to use a voltmeter on things like this because even the input impedance of a simple meter is low enough to affect the voltages on a thing like a transistor collector being held off.

I'm also deeply suspicious of those resistors marked "FU47" and "FU470". I'm hoping that means "fusable" instead of some other connotations I can think of. If they are fusable, they may also be non-obviously damaged. They're worth checking.