Fender Stage 100 DSP blowing output transistors...

[lolvera]

Hi guys,

I'm fixing a Fender Stage 100 DSP that keeps blowing outputs, I think I have isolated the problem to the power amp driver stage, measured about 9V DC at TP31 and TP32 (which are the output transistor bases). So far I have in-circuit tested Q10, Q11, Q12 and Q13 and they seem to be OK. I removed U7, installed a socket and powered the amp with U7 and OTs removed, then re-checked TP31 and TP32 and I got the correct value about 1VDC and -1VDC respectively.

After installing a fresh op-amp on U7's socket and powering the amp on I measured U7-A's output which drives (Q10 and Q11) and it slowly latched to about 13 VDC. While doing some other measurements in the circuit, I discovered that U7-A's output sometimes latches to -13 VDC. This made me think that maybe C49 was open and I just bypassed it with a jumper, measured again and I got +/- 1VDC at TP31 and TP32 respectively.

I have searched the forum for similar problems but haven't found any, I checked the PCB for cracked solder joints but it doesn't seem to the cause.

Any help will be appreciated. Thanks in advance.


Stage 100-160 DSP.pdf

[g1]

There was a similar problem recently, in that case the DC levels would not "get right" with the output transistors removed.
Could be the same problem here?

[J M Fahey]

1)there's no way that you can have +9V and -9V at Q14/16/18 and Q15/17/19 , because their bases can't be more than 4 diodes (around 2500/2800mV) away.
Check that the ballast/emitter resistors are not open (R100/101/104/105/108/109)
It's not that common that their bases are open, but not impossible either.
Also some track might have cracked or vaporized. Check that.
2) U7's outputs drifting big way and latching on either rail *may* mean that it's simply trying to do its job, which it can't.
I still distrust transistors and resistors mentioned on point (1).
3) C49 *is* open ... for DC .... ; do not bypass it.
4)

with U7 and OTs removed,

This may be useful to check something, fine, but recovering proper +/-1V *without* those important parts does not count as success.
I repeat that I still distrust the output stage "muscle".
5) Q10/11 are bias transistors, one for each polarity . If they are open, U7 can't drive OTs; if shorted weird DC things can happen. The least of which would be crossover distortion, but now you have bigger problems to solve.
6) Q12/13 are short protection transistors.
Momentarily put them out of the picture by lifting one end of D21 and D22.
7) I *hope* all this time you were working with a lamp bulb limiter and no speaker connected.
Good luck.

[lolvera]

Hi J M, thanks for your help...

Before doing all measurements I disconnected the speaker, and removed all OTs. Since then I have not installed fresh OTs.

1)there's no way that you can have +9V and -9V at Q14/16/18 and Q15/17/19 , because their bases can't be more than 4 diodes (around 2500/2800mV) away.
Check that the ballast/emitter resistors are not open (R100/101/104/105/108/109)
It's not that common that their bases are open, but not impossible either.

I measured +9VDC at all OTs bases but since OTs were removed I referred to this points as TP31 and TP32 (as per schematic) this occurs when U7 latches to +13VDC which happens most of the time, as I mentioned I got this measure without OTs installed.

[lolvera]

1)there's no way that you can have +9V and -9V at Q14/16/18 and Q15/17/19 , because their bases can't be more than 4 diodes (around 2500/2800mV) away.
Check that the ballast/emitter resistors are not open (R100/101/104/105/108/109)
It's not that common that their bases are open, but not impossible either.
Also some track might have cracked or vaporized. Check that.
2) U7's outputs drifting big way and latching on either rail *may* mean that it's simply trying to do its job, which it can't.
I still distrust transistors and resistors mentioned on point (1).
3) C49 *is* open ... for DC .... ; do not bypass it.
4)
This may be useful to check something, fine, but recovering proper +/-1V *without* those important parts does not count as success.
I repeat that I still distrust the output stage "muscle".
5) Q10/11 are bias transistors, one for each polarity . If they are open, U7 can't drive OTs; if shorted weird DC things can happen. The least of which would be crossover distortion, but now you have bigger problems to solve.
6) Q12/13 are short protection transistors.
Momentarily put them out of the picture by lifting one end of D21 and D22.
7) I *hope* all this time you were working with a lamp bulb limiter and no speaker connected.
Good luck.

Hi J M,

I disconnected one end of D21 and D22 as you suggested. Checked all ballast resistors they are OK. I'm still working without OTs installed, here are my measurements:

U7A's output pin -13.65 VDC.
U7A's inverting pin 0 VDC.
U7A's non-inverting pin -1.24 VDC.

Voltage at TP31 (Positive rail OT's base test point) -7.48 VDC.
Voltage at TP32 (Negative rail OT's base test point) -9.73 VDC.

There are no shorts between Q10's nor Q11's pins.