Fender Solid State Stage 100 Amp huge buzz Problem... help needed ...

[Tubetec]

Its been a while since I worked on one of these ,
From my memory the problem lay in what seemed like a tie down resistor somewhere in the PSU side .

If you replaced the output transistors , some of the driver transistors might also be damaged , that often happens .

 

[Chandra]

The transistors you replaced may be having different gain.
R110, R111, C61 are part of the Zobel network to suppress HF oscillations.
If you seen them glow, means definite HF oscillations present.
Check C49 and C54. You may have to increase one of those capacitance values.
The hum you hear is because of diode rectification properties of the transistors.

Have you left any wire loose or changed the positions like tied to something else as dressing?

Regards.

 

[la-2a]

@Chandra. at the moment only 2 darlingtons are built in to narrow things down a bit. R110,111 only glows when I touch pin 2 on the U7 (4560) with the measuring tip of the oscillator, but then the hum also goes away. The circuit board is installed in the chassis as otherwise, no cable changes. What could be the problem: I don't have the original 2SD1857 / 2SB1236 but similar types A1306/C3298. the hum is about 1,2V AC on speaker output will test the capacitors. many thanks for your help

 

[la-2a]

Well I somehow got rid of the hum... had re-soldered all relevant soldering points and increased the capacitor values as suggested... Then I soldered new Darlingtons in again and the hum was back...then removed the new ones and everything is quiet... .the 0.47ohm resistors are measured and they are ok the Darlingtons were unused new......I know that with 6 Darlingtons it hums a little more but that is much louder..

 

[industrialarts]

I went back thru my tech log because I know I worked on one of these that caused me to tear my hair out - but it was about 10 years ago. This is a weird amp - as mentioned above - plus I am not sure what the second half of U7 does exactly - it has something to do with changing the gain or the eq of the op amp stage when a second cab is plugged in.

In any event, my notes say something about replacing Q18 and some small signal diodes in the amp driver sections (D19-D24?). I remember my problems was the same or similar. Again, the grounds seem to be suspect, those resistors get hot because there is a lot of current going thru them

I would replace the TL072, the jfet Q20, the fet driver Q21 just to rule them out. Also, C61 just to rule it out. I don't remember if the schematic is correct about the parts that aren't stuffed, I seem to remember that as wrong.

Does it work Ok without a load? I always tshoot solid state amps with no load until I am sure that it works, then variac it with a load

 

[la-2a]

Surprisingly, the amplifier works with 2 Darlingtons in the power amplifier...some distortion, little power but no hum. as soon as I used the other 4 and I tried it with brand new ones, it buzzes. I'll implement your suggestion and see what happens. Thanks alot

 

[scondon]

I have a Stage 185 that had a huge angry buzz that I eventually traced to the effects loop. Check the jacks, pot and switch. IIRC in my amp the issue was with the little 3way level switch in the effects loop.