Hi Guys, I was wondering if someone here could shed some light on a situation I am having with an amplifier. Ive been trying to fix a keyboard amplifier (Carlsbro Colt 120), that would take a while to start working and when it did would only give a really quiet signal. So decided to replace the MJ802's because everything on the power side seemed to be working up to that point. Now, after replacing them I am still getting a quiet signal but with a high pitched hum, that I think sounds like the transformer.

After doing some prodding around with a multimeter I noticed that if I place the red lead on the Tl072 pin 4 (there are 4 tl072's in total) and the black lead to ground it completely removes the hum from the signal, but when I take the probe off it goes back to humming. Does anyone have any suggestions as to what is going on here, or what other components I could try replacing.

As always any help would be greatly appreciated.

Thank you

 

Hi Guys, I was wondering if someone here could shed some light on a situation I am having with an amplifier. Ive been trying to fix a keyboard amplifier (Carlsbro Colt 120), that would take a while to start working and when it did would only give a really quiet signal. So decided to replace the MJ802's because everything on the power side seemed to be working up to that point. Now, after replacing them I am still getting a quiet signal but with a high pitched hum, that I think sounds like the transformer.

After doing some prodding around with a multimeter I noticed that if I place the red lead on the Tl072 pin 4 (there are 4 tl072's in total) and the black lead to ground it completely removes the hum from the signal, but when I take the probe off it goes back to humming. Does anyone have any suggestions as to what is going on here, or what other components I could try replacing.

As always any help would be greatly appreciated.

Thank you

Schematic?

Since pin 4 on the TL072 is for the negative power supply, (-12 or -15v). Maybe an internal compensation capacitor has failed on one of those TL072s, they are dirt cheap so replace them freely.

Otherwise, you may have corrosion in your PCB causing poor conductivity.

and the issue might be your PNP power transistor (you changed the NPN output transistor).

 

Maybe an internal compensation capacitor has failed

I have also experienced preposterous results from probing a high impedance op-amp circuit and traced it to a capacitance problem. I can't remember exactly what was wrong with the circuit right now. It was an audio application and IIRC, it would burst into oscillation when I measured it. That made no sense at all until I figured out the real problem.

 

Schematic?

Since pin 4 on the TL072 is for the negative power supply, (-12 or -15v). Maybe an internal compensation capacitor has failed on one of those TL072s, they are dirt cheap so replace them freely.

Otherwise, you may have corrosion in your PCB causing poor conductivity.

and the issue might be your PNP power transistor (you changed the NPN output transistor).

Sorry, schematic attached. Thanks for the reply, I will have a go at replacing the caps around the board see if that changes anything. Its funny that you mention corrosion though as the board does have a bit of corrosion, so ill have a look at the connections to.
Could you have a look at the schematic and let me know which PNP transistor you are talking about, as there is quite a few on there. I could replace those to then.

Thanks