Legend rock n roll 50

[Helmholtz]

Here’s what looks like with the scope on the pre out jack. Pre volume at 4 and the gain on 1.

That looks like a normal 1.2kHz signal.

[Helmholtz]

Also, this is the output with everything on 10.

Repetition rate is still 1.2kHz by the time scale of 500µs/div.

Amps don't change the frequency of signals, but may add higher oscillation frequencies.

Not clear what the readout of 24kHz refers to. Consult the scope manual.

[Helmholtz]

the pre amp out everything full out

Here's a lot of clipping involved. Lower signal level or gain settings.

[pontiacpete]

I have the full schematic now. Had to go to the computer, the pad or phone was giving me only one page. Anyway, pin 7 of the IC has a clean signal. At this point, I'm not sure which pin is which, but the signal remains the same on the blue wire out of the molex connector on the pre board and goes to the power board and the power amp in jack. Maybe if you can go back to the picture

[nosaj]

This pic corresponds to the 3rd page of the schematic

Now on the right side of schematic outside the dotted lines are the power transistors
nosaj

[nosaj]

Based on your pics the driver poweramp board is not secured(grounded with screws) The trace runs around the screw hole creating a bond point. Has it been secured with the scope signal pics?
nosaj

[nosaj]

Also you know for what its worth ME I would go ahead and replace those 3 caps especially that rubycon. This is what a 1980's amp?
nosaj

[pontiacpete]

The screws come in from under the chassis. Look at the picture of the output transistors, there are 6

[nosaj]

Look at this pic the red arrows

[pontiacpete]

Yes. There are screws right in those spots but we’re seeing the bottom of the screw

[nosaj]

Well based on the age of the amp and the low uf values and being Rubycon I would replace the 3 caps and see if your oscillations stop (if its positive feedback)
nosaj
But thats all I got

[pontiacpete]

Now that I have the schematic to the power amp, I started tracing the signals through it. I didn’t get very far though. It’s good up to the base of Q1. The pictures are of the base and the one where the wave is getting cut off is a picture of the emitter or collector(i really can’t tell from the picture, but I’m guessing the emitters of Q1 and Q2 where they connect.
The last picture there is very little wave, that’s the base of Q2.
Should I replace Q1 and 2?

[nosaj]

What are the controls at? If at max if you lower does the transistor open up? Or you could test with dvm out of circuit.
nosaj
Can't look right now but will later at scem

[nosaj]

test c3 c4 and c7 test q1 q2 out of circuit.

[pontiacpete]

Q1 and 2 test out ok. I use this device Peak atlas DCA55. It does show bad transistors most of the time but there was that one time it said one was good but when putting in a new one proved otherwise.
I had already changed all four small e-caps on the power board before the last post.