Fender 212R power amp blown

[lowell]

Amp came in with loud hum. Found DC on speaker jack. Power transistors Q20/Q21 were bad, replaced them. Bulb doesn't glow brightly anymore but still +4vDC on speaker jack with bulb in series. I'm suspecting Q16/Q17. I checked Q18/19 with meter via diode test and ohmed the collector to emitters and they seem ok. Q16 Rce reads 1Mohm, Q17 Rce reads 3.5Mohm. From what I can see they both have the same surrounding components so it's odd that they read different. Q16 seems like the most likely suspect for leaking positive voltage to the speaker out. I guess my post is kind of pointless now cause I'm going to replace it with a 2n3904 and see.

Edit: I replaced Q16 and the +4vDC is still on output.

Edit: It's -4vDC, not +4vDC.

[lowell]

I also metered each of D34-D37 because I found that the anode of D35 has -2vDC. That point should be 0vDC right?

[g1]

With all the DC coupling and feedback in solid state amps it's hard to tell where the DC is coming from. That -2V on D35 could be coming from somewhere else. Have you checked all the low value resistors in the power amp section? Not sure if the Fu marking means flame proof or fusing but I would check them and the other low value ones.
Also make sure the protection circuit is ok (Q14,15).

[lowell]

Aren't Q14/15 the bias transistors? They are odd looking transistors, never seen that package before. They seem as though they control the Vbe of Q18/19. Concerning the resistors, they all check out good. I've also checked all the diodes in the power amp. I'm stuck here.

[g1]

Sorry, I meant Q16 & 17. You mentioned replacing one of them, I would try the other also (I think you originally thought the offset was pos. but it was really neg.?).
Anyway it seems the bottom side is conducting harder than the top which is giving you -DC offset. Zeners can read ok but not drop the correct voltage. You might try swapping the ones on the bottom with the ones on top and see if your offset changes from neg. to pos.

[lowell]

I swapped the zeners and nothing changed. I ordered new bias transistors Q14/15 and am gonna replace Q17 while I'm at it.

[lowell]

Ok so replaced Q14/15 and Q17 with no fix. Other thoughts?

[lowell]

Enzo, JM, Steve C, RG I'm soliciting any of your expert insight you may have on this.

[MarkusBass]

As G-one mentioned this is a difficult amp to deal with. It is DC coupled and it is very tricky. Fender tried to get some compression from the power amp. With such amps I never ever replace any parts until I know what exactly is the problem. I start with reading the schematic and I have to understand how the amp works (including every part in it). In this case you have to take a look at how the speaker is connected (in series with two 0R22 resistors andnot to the ground). AC feedback is going back to the input of the amp (U6). There is also DC feedback going back to U6 (second part), which works as a DC servo that tries to maintain 0V on the output. So you may suspect any diode or transistor on the output but I would also look at voltages on U6. What voltages are on the input and on the output. You see that the output of U6-A is connected to Q10 which together with Q9 form a differential input of the amp. If Q10 gets incorrect voltage, you get DC on the output. In similar cases I had either problems with some diodes or with DC servo. But still I suggest first to understand the schematic and then make a lot of measurements before replacing any of the comonents. For example, are all voltages on D21-24 ~0.65V? Is it the same with D25-27? What are the voltages on fusible resistors R87-90? What is the voltage on the output of U6-B and U6-A?
On the otherhand, since the output transistors were blown, I would also look at Q18-19.
Sorry for not coming up with a solution but in this case it is really important to understand the schematic.

Edit: the amp was also discussed here: http://music-electronics-forum.com/t21171/

Mark

[lowell]

Markus,
Thank you thank you for chiming in and helping out. I'll check all that and report back.

[J M Fahey]

My personal opinion, which as usually is politically incorrect, is that this amp is stupid, silly, needlessly complicated, a mess, you name it.
There must be a reason for the "do not repair" notice it carries.
I always found strange to have such warning attached to a 100W combo, maybe would understand it on a 10W "free with your guitar" one.
Now I know.
End of rant.
If I had it on my bench, I would lobotomize it, meaning I would pull about 60% of kludgy components , and leave only the regular, classic, differential input, Lin type, quasi complementary amplifier that lives within.
I must warn you that the time it will take will not compen$$$ate you or its owner.
Now if you consider already lost, and take it as a "fun" project, maybe it will do.
So, in increasing degree of difficulty, the possibilities are:
1) Plan A: It pays to check for major transistor blowouts, plus maybe a couple resistors. If you change those, and it lives, wrap it in shiny paper and drop it at your friend's. Pocket the $$$ justly earnt.We are already beyond that , I guess.
2) Plan B: Seeing the amp mess, I would just kludge a new power amp board (probably from a kit), connect it to appropriate points (speaker, PSU, preamp out, etc.) and call it a day.
Unfortunately, without seeing it, I guess *everything* lives in a huge PCB which occupies all of the chassis interior, so, no free space for new boards.
3) Plan 9 from Outer Space (google it) : pull crazy parts , turn it into a regular amp, and at worst repair it.
Problem: "it will not compress like the original one"
Solution: "the original one does not compress either"
Think about it .
NOTE: very probably somebody else is well acquainted with it and can suggest a less radical solution.
4) Plan Z: being a combo with a thin chassis on top and a lot of space below it, *maybe* you can mount a 100W Kit amplifier on the bottom (external side) of the chassis, and route necessary wires through a couple holes. Plan B notes still apply.

[lowell]

JM,
Well that's no good. I remember you said the same thing about a Randal RG that I was working on. It had some bad protection circuitry design flaws. Anyway where would I get such a "kit" from?

Markus,
The vdrops across D21-23 are all different. However the vdrops across D25-27 are all apprx .5v.

D21: fluctuates from .5-.9v
D22: .1v
D23: .02v

Would you suspect bad diodes or Q9? Maybe Q12? I would think that the Q9's Vc should be 1v less than Q10's Vc, right? My reasoning is that the drop across D22-23 should be about 1v. Maybe I'm wrong and D28 plays a part here.

[Enzo]

Like everything else on the DO NOT REPAIR list, it is inexpensive. Has nothing to do with repairability or likelihood of failure. They simply don;t want to pay their warranty centers $50 to fix it when they can just replace it for less.

Lowell, how much do those diodes cost? 10 cents tops, right? Don't worry about figuring out if they are bad, just replace them. How much bench time do we want to spend wondering about 10 cent parts?

If 22 and 23 are shorted, then Q9 is suspect, and yes I'd feel better with a new Q12 too. I see no parallel paths that could make 22 and 23 appear shorted.

Does D28 measure short? If so, replace it. As to 28 involved with 22,23, I see no place on the far end of D28 that could get back to 22,23 to mess with them.

[J M Fahey]

It's interesting to see how different Countries/Realities call for absolutely different answers to the exact same *technical* problem.
In the U.S. it's straight replacement of a perceived as cheap amp; here it's ... "WOW !! .... it's a ... FENDER!!".
Couple that to the fact that there is *no* replacement available, neither "official" parts and the situation starts to darken.
Customs officials couldn't care less about Fender replacement/repair policies, for them a 2nd. amp is *another* amp, and must go through Customs, pay taxes, etc. as much as the first one.
That's why shop owners often call me and tell me "just repair this bugger anyway you can, my in house technicians can't cope with it and the kid's father has promised to destroy the shop if we don't solve his problem"
Not that people is specially violent there, but after 3 months delays people start to get angry.
Oh well.

[MarkusBass]

My personal opinion, which as usually is politically incorrect, is that this amp is stupid, silly, needlessly complicated, a mess, you name it.

I'd say you should think twice. The amp is so complicated (is it?) because Fender wanted to simmulate valve-sound with transistors. The resistors in series with the speaker provide negative feedback, which depends on the level of the output signal (isn't a compression?), and the transistors in the middle of the schematic provide variable gain also depending on the level of the signal (they smooth high level signal). It's the musicians who pay for the amp and not the technicians that try to fix it. Without musicians the amp wouldn't be on the market. And if you say that this is a complex amp, what would you say about Markbass amps (all SMD components), or the latest amps from Peavey or QSC? They shouldn't exist because you consider them to be too complex?
I suggest that you look at the schematic once again and try to understand why it is more complex than average Peavey from the 70-ties. We have 2010 now .

Mark