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Pioneer SA-9900 volume pot DC noise

quadmatrix82

quadmatrix82

AK Subscriber
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Hi all, I’ve recently got up and running a lovely Pioneer SA-9900 amp, all the electrolytic caps are replaced and known to fail transistors but I’m getting a strange fault with the volume pot which is noisy through the speakers when adjusting on both channels - sounds like DC and the woofers pulse in and out as the pot is moved.

I’ve cleaned the pot with faderlube and contact cleaner but I’m sure it’s not a cleaning issue

I’ve replaced the tantalum caps on the input board with Nichicon UKL and checked for any bad joints etc

All of the transistors were replaced on the control amp board awg-031 with ksc3503 /ksa1381 and ksc1845/ksa992

Can confirm the fault exists locally to the pioneer having tried using it as a pre only and paired with another power amp

Any suggestions what else I can check to rectify this?
 

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If you are using a KSC3503D for Q11,12 on the AWG-031 Control Amplifier, you need to compensate for it's low gain which causes a negative voltage to appear on the emitter allowing DC to get through C15,16 to the volume control. The easiest way around it would be to install C15,16 opposite to the polarity that is indicated on the PCB. Another option would be to change R41,42 from 560K to 180K ohms. This resistor change corrects the operating voltages for Q11,12 so that the emitter voltage will now be positive. A third option would be to use a KSC2383 instead of the KSC3503D. Either way, you should probably replace C15,16 in case they were damaged by the reverse polarity voltage.
 
LesE said:
If you are using a KSC3503D for Q11,12 on the AWG-031 Control Amplifier, you need to compensate for it's low gain which causes a negative voltage to appear on the emitter allowing DC to get through C15,16 to the volume control. The easiest way around it would be to install C15,16 opposite to the polarity that is indicated on the PCB. Another option would be to change R41,42 from 560K to 180K ohms. This resistor change corrects the operating voltages for Q11,12 so that the emitter voltage will now be positive. A third option would be to use a KSC2383 instead of the KSC3503D. Either way, you should probably replace C15,16 in case they were damaged by the reverse polarity voltage.
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Thanks LesE this is great to know, I have some ksc2383 on hand and spare caps so I will try these and report back
 
LesE said:
Just to clarify, if you use the KSC2383, you do not need to flip the caps. Just install new caps with polarity that matches the PCB silkscreen.
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Thankyou for your help I swapped out the transistors to KSC2383 and caps and works great now! For fun I tested both the caps and one of them had gone high in uf and esr so glad I replaced them
 
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