Jbl 6260 problems

[NickH]

Hello,

I scored a 6260 the other day. I replaced all but a few of the electrolytic caps in it right away? I also put sockets in so I can upgrade the opamps too. I checked the bias following the directions I've found here on the site. I did notice that one channel was distorting quite bad. The same channel the bias pot won't change the bias no matter what I set the bias pot at. I looked at the output of that channel on my oscilloscope and it appears that a good portion of the waveform is missing. I can't tell if its extremely severe crossover distortion or only half of the phase splitter is working. When I feed it a sinewave the output is distorted on the bottom half of the waveform only. I haven't traced the signal with my scope yet though.

I'm just wondering if anyone is familiar with these issues and has any ideas for me. I was thinking an issues with a transistor in the phase splitter but the bias issues is making me think its elsewhere. If the transistor the bias pot is tied to fails, what would the symptoms be?

I was going to trace a sine wave through the amp with my o scope but my scope seems to be having a issues with its input loading. I don't want to make my problems worse by using the scope in circuit.

Any help / advice would be most graciously appreciated.


Thanks,
Nick

[NickH]

Well since I haven't had any bites on my thread here I've been slowly trying to figure my issue out on my own. Today I ran into some very strange issues. I was checking voltages on transistor on the problem channel and comparing them to the working channel. I put a volt meter on the base and emitter of q8. When I do this the problem goes away. I can increase the bias on the channel. As soon as I remove the meter leads the bias point drops back to its lowest point. I put my scope on the output and then I can increase the bias so the distortion is gone. But as soon as I remove the test lead thing go south again.

This doesn't make much since to me. But I'm still learning when it comes to solid state amp circuits. I just can't see why a meter on q8 would make things work. I'm assuming q8 is a driver for the output stage.


Does anyone have any insight into this problem I'm having?

Thanks in advance,
Nick

[NickH]

I replaced q8 and she's blowing the doors off again. Damn Skippy.


Nick

[1audiohack]

Congrats!

Barry.

[grey]

Sweet! Hopefully she brings several more years of joy. Now I know where to bring mine for service.:p

[NickH]

Ya I'm hoping it gives me many years of service. It was pretty clean otherwise. Now I just need a subwoofer amp.

Nick