Fender Acoustasonic sfx shorted? Ugh!

[g1]

TP73 is the + and TP75 is the - , so maybe you got them reversed?
The bias voltages are derived from the 'string', composed of Q17, 15, and 19.
In my experience it's usually Q17 or Q19. When Q15 is bad it's usually blown up good, with damage to other components in the string. But it could be many other things, even a bad solder connection somewhere.
Q15 is for thermal tracking on the heatsink, so you should expect changes when hitting it with heat or cold.

[Indyryder02]

Ok, that makes sense.

[Indyryder02]

Truly hate this amp! It’s become my nemesis!

i have the following readings on these test points if this gives anyone a clue..
I have more parts on order..

TP 69 31.9 VDC These are a bit low
TP 71 -31.5 VDC but in the right directions at least
TP 73 -8.1VDC / 0MVAC Why so much dc? And Where’s my positive voltage? Where’s my 200mvac?
TP 75 -10.6VDC / 0MVAC. Why so much dc? Where’s my 200mvac?

thanks in advance for any and all help!!

[g1]

The DC voltages for test points are in the boxes, AC signal voltages are in the ovals. The notes area of the schematics tell what signal to apply where, and what settings for the controls when doing the AC signal voltage checks.
The DC readings must be done first with no signal. Once they are all correct you can move on to the AC checks if required.

Are you on a lamp limiter?
If TP69 & 71 are wrong then TP73 & 75 will be wrong.

[Indyryder02]

Ah, duh. Of course I wouldn’t have ac without signal.
And not using a limiter for the tests.
thanks

[Indyryder02]

Ok. Made more progress.

With everything back in place. Including a replacement
Q15 Q17 and Q19 (too hastily I know) and all new outputs for both channels.
(was afraid my mixing of new and old wasn’t helping)

Using the dim bulb tester and just enough voltage to get some ballpark readings. (About 50-60v)

I can see everything is fine with the new outputs on the unaffected amp. (Q34 Q36 side) I’m getting positive voltage around 22v expectedly lower with variac and bulb limiter.. but in the right direction and ballpark. ) and I get my 1.15 or so of + and - at TP 72 and 74.

in trying to better understand how this bias circuit works..
(voltage divider Bias correct?)

I don’t want to pull the board and swap anything else, getting closer I feel and want to understand what was wrong and why.

so, on the affected or troubled amp side (Q33 Q35)

things are in the general correct order around TP 69 and 73
again about 22 v and about 1v at 73 (little lower than other amp and spec.)
That 1V seems to go across Q15 to Q19 where I should be seeing -1.15.

the resistors 251 253 and 255 all spec out.
Q15 Q17 and Q19 are new.. and perhaps the problem..
I have around 2.5 v at R261 and 263 And 0 on the good amp at R260 and R262.

I know my problem has to be in this area.. but I want to know if there’s any way to further pinpoint it instead of just swapping components.

thank you for any and all help in further understanding this!

[g1]

R261/R263 junction is the output. You have a positive voltage there so either the top side outputs are conducting too hard or the lower side outputs are not conducting enough.
If there is no voltage across Q15, maybe Q19 isn't turning on.
What are E,B, and C voltages for Q19? And for reference, what are they for Q20?

[Indyryder02]

Q19 EBC. -22.30 • -21.93 • 1.01
Q20 EBC. -21.93 • -21.22 • -1.29.
Thanks

[Indyryder02]

I have a flir camera handy.. interestingly.. the affected channel is making no heat.. Q19 and 17 are cold..
whereas we can see the good amp is lit up!
Q20 and 18 are show quite bright as well as all the outputs.
Neat for sure.

[Indyryder02]

EBC for 17 and 18 In case this is also useful

17= 19.57 • 19.22 • .292
18= 19.34 • 19.00 • .025

[g1]

I missed that you are using both the bulb, and the variac. It should really be one or the other.
It could just be that one channel isn't coming up as fast as the other channel and may stabilize as voltage increases.
You are running at lower voltage in post 55 as 51 (TP73 is Q17 collector). To do comparisons, you need to have a consistent level at the supply rails every time.

Q19 has only .37V between B & E, Q20 has .7V there. See if that Q19 B-E voltage increases from there as you raise the variac. If it does, at some point it should start conducting and the DC on the output should start moving toward zero.

[Indyryder02]

Raised voltage , but did not see a change in 19. Just higher values. But still about .37 B-E

And yes.. I’d only ever used a Variac with amp meter till this amp! Haha! The bulb is mostly for an accidental bump of the probe into the wrong thing etc. prevent shorts. Since I’ve been through enough of these outputs haha!

Ive never found tube amps to be this touchy!

However it seems to be working fine for the good amp side. I get all the appropriate values I’m looking for, and not on the bad side.

[g1]

The string of resistors, Q13, and diodes, from top to bottom should have current through it. That current develops a voltage across R243 and D31 which sets the base voltage for Q19.
Q13 gets it's base voltage via the drop across R239 and D25 via conduction of Q9.
So have a look at Q9 and Q13 and compare to the good side.

[Indyryder02]

Bce q13 16.2 -10.60 22.96.
Q14 23.21 -9.74 22.78
q9. 1.24. 21.97 1.56
q10 .249 21.77 .135

[g1]

Q13 is PNP. The base can only be .7V more negative than the emitter if it is good. So if those voltages are correct (Q13 b 16.2V; e 22.96), Q13 must be bad.
But Q14 voltages don't make sense that way either, so I wonder if you got the pinout mixed up?
Maybe double check those.