This is from a Sansui 7070 receiver...one of the opamps was dead, and rather than search for a replacement, I built a +/-18V zener regulator and a couple of small adapter boards, and used a pair of OPA604's instead. This brought the amp back to life, and it sounds great.
However... , any movement of a tone control creates horrible scratching or thumping. The bass control in particular nearly bottoms a woofer when moved through its range. The output of the opamp is fine, and does not change with tone control movement (as it shouldn't, of course), but of course the collector of the transistor after the pots jumps nicely when a control is adjusted. I can see the collector tied to one side of each pot, so there is about 10V on the collector, and of course that voltage is across each pot as well.
How in the hell was this ever supposed to work?? Of course, I never had a chance to listen to the amp before I started troubleshooting and found the dead opamp....but with all that voltage on the pots how could this thing have ever been quiet when adjusting tone? Something is wrong, and I can't see how it could relate to the new opamps....and I can't see how to remedy it either...
However... , any movement of a tone control creates horrible scratching or thumping. The bass control in particular nearly bottoms a woofer when moved through its range. The output of the opamp is fine, and does not change with tone control movement (as it shouldn't, of course), but of course the collector of the transistor after the pots jumps nicely when a control is adjusted. I can see the collector tied to one side of each pot, so there is about 10V on the collector, and of course that voltage is across each pot as well.
How in the hell was this ever supposed to work?? Of course, I never had a chance to listen to the amp before I started troubleshooting and found the dead opamp....but with all that voltage on the pots how could this thing have ever been quiet when adjusting tone? Something is wrong, and I can't see how it could relate to the new opamps....and I can't see how to remedy it either...
Attachments
BTW, the supplies for the opamps feed the opamps only...no connection to the transistor/tone controls other than sharing the ground.
I still don't see how this design ever worked right...I must be missing something...😕 😕
I still don't see how this design ever worked right...I must be missing something...😕 😕
The potentiometers are AC coupled as they should be to avoid scratches by changing the DC bias of the transistor when you turn the controls. In your case it seems that C07, C23, C08 or C26 is leaking. Do you have scratching noise on both channels or just one?
Steven
Steven
Thanks for the reply Steven. C07 and C08 are new caps. The others on the bases of the transistors are old ones, but it seems very strange that both channels are behaving exactly the same. However, it is no big deal to replace the caps on the transistor bases, but my bet is that it will not solve the problem.
What you are saying is that...as long as the wiper is cap coupled, it's perfectly fine to have a bunch of DC on the pot. I've just never seen this done before. It also looks to me that the bottom one (bass I believe) is DC coupled.
I need sleep...but when I get up I'll pop in a new cap on the transistor base and see what happens.
What you are saying is that...as long as the wiper is cap coupled, it's perfectly fine to have a bunch of DC on the pot. I've just never seen this done before. It also looks to me that the bottom one (bass I believe) is DC coupled.
I need sleep...but when I get up I'll pop in a new cap on the transistor base and see what happens.
EchoWars said:
Right. You should not have different DC voltages on both sides of the potentiometer, however. In this case all the potentiometers are at a DC level determined by the collector of the transistor; this is the only DC path, all other paths have a capacitor in series. Seems to be OK. The DC voltage doesn't need to be at ground potential, as long as it is constant and the same on all contacts of the potentiometer.
Steven
If only one of the opamps is dead, you could still replace one opa604 with it.
See what happens.
I'm familiar with this problem in repairing such kind of amps.
Only I have a bad day to remember the cure
/Hugo - thinking...thinking 🙂
See what happens.
I'm familiar with this problem in repairing such kind of amps.
Only I have a bad day to remember the cure
/Hugo - thinking...thinking 🙂
Steven, you were right. Both caps on the bases of the transistors were leaking like a bastard. Replaced them and all is silent when adjusting tone now.
I had just refused to believe that both caps went bad. One I could have easily accepted, but two? Anyway, thanks for insisting that this was the cause...
I had just refused to believe that both caps went bad. One I could have easily accepted, but two? Anyway, thanks for insisting that this was the cause...
I would go and get all the electrolytics out if I'm fixing an amp over 5 yrs...they get leaky real fast...wonder if there's a cure for it..like longer life caps??
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