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Pioneer SA-900 restoration

spaghetti

spaghetti

Active Member
I have this Pioneer SA-900 that needed some care. I got with one channel not working, there was a shorted driver transistor, easy fix, but the problems are not finished there.

It's an early model that mounts 2SD218 outputs, while the later model mounts 2SC1115. The difference between the two models, besides the outputs, is a few resistors as per this picture from another Audiokarma user: https://audiokarma.org/forums/index.php?attachments/sa900output-jpg.206783/

I was having problems regulating the DC balance, I was not able to go higher than 36V, so I decided to upgrade the resistors to the new design. I can now adjust it properly. Not sure if this was a good move, but I thought that there was a good reason if Pioneer decided to change those resistors.

But I still have a couple of problems. The bias value on one channel is a bit strange: at powerup it's about 15mV, then it drops rather quickly down to 1.8mV, then it slowly goes up to about 11mV. The power amp board was recapped as well. This channel, except for the caps and the changed resistors, is 100% original.

The second issue is with the high tones controls. When rotating them, at each step, I hear a high pitch pop in the corresponding channel. If I rotate the pots back soon enough, the pop does not happen. But if I wait for a while, it does.

Any suggestion about what could be the culprit?

Thanks,
Andrea
 
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The reason for the high pitch pops is the fact you are switching capacitors.
So it is a result of the design and nothing is at fault.
They charge or discharge as they like, because of static, airflow, or whatever, if connected to nothing else/floating.
While this amp is VERY quiet once restored, it is the only thing annoying me.
Maybe once I try to bypass them somehow using 10 megohm resistors or even higher if I have them, to make sure there is no different voltage level at the moment of switching the tone control.

The bias thing , I have no idea, exept from probably some component unstable (like the STV3 sensing diode or some driver transistor). Maybe "cold spray" can help identify.
 
@gslikker oh, thanks for the information. I wonder why they made it that way, looks like a design flaw to me.

Regarding the bias, I'll try to swap the parts you suggested with the other channel and see how it goes.
 
I had problem setting bias and DC balance after recapping my 2SD218 version of SA900. I ended up increasing resistance values of VR1~4, a solution found on the internet. VR1 and 2 were 50K, and now are 100K. VR3 and 4 were 50R, and now are 100R.
 
RobinTW said:
I had problem setting bias and DC balance after recapping my 2SD218 version of SA900. I ended up increasing resistance values of VR1~4, a solution found on the internet. VR1 and 2 were 50K, and now are 100K. VR3 and 4 were 50R, and now are 100R.
Click to expand...

That's what I did too. I had two SA-900s and both needed the trimmers increased.
 
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