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Another preamp DIY investigation

James

James

Lord of the Erg\o/s
I've landed a vintage Pioneer C-21 preamp. The unit powers up and plays music just fine. But there is a rather loud mains-related buzz and hum coming through the left channel that seems independent of the level or source. The C-21 must be at least 20 years old and does not look as if it had been serviced. My intention is to find a service manual for it, and give it a good going over where I'm able. The circuit looks simple enough, and my first guess is that the electrolytic caps desperately need replacement. Would dry electrolytics contribute to this buzzing? What other bits typically need replacement as they age? I presume resistors, diodes and transistors should be fine as they are?

James
 
126 views and not a single response?

Anyhow, with Google as my friend I have located a C-21 restoration project. Unfortunately, it's in Japanese. With on-line translation sites, I'd figured out that the OEM electrolytics have been replaced by Nichicon MUSE caps.

Are these any good? I seem to recall that Nichicon is affiliated with Roderstein and the latter is mention with a degree of affection here. Views, anyone?

The restoration project also replaces a bunch of transistors, but I'm given to the understanding that transistors don't wear out like caps do. Why on earth would you replace a transistor unless it's broken?

James
 
Nichicon are fine.
It sounds like the caps -- you might as well change them. UNLESS there's (also?) a bad mechanical connection s/where & the noise rejection is off...
 
there is a rather loud mains-related buzz and hum coming through the left channel that seems independent of the level or source.
Click to expand...
That might not be the PSU alone, but rather a local decoupling or bootstrap cap (depending on circuit design) that has died. After 20+ years replacing te main PSU caps will be a good idea, but fixing this hum will require also changing any electrolytic caps on the amp PCB.

In fact I'd do that first, working my way through the affected channel from input to output to find the suspect, then replace teh same art on the other channel, before re-capping the PSU - which might mask the problem at idle.

Transistor failure would likely result in non-function in one channel, or the output being DC at one rail or the other- very obvious, in other words.
 
After loking at the pics it appears that the PSU might have seperate regs for each channel. Check voltages before and after these first, to verify things are the same side to side, and that there is at least 3-4v drop across the regs themselves. It could also be that a rectifier/diode has died, dropping the raw PSU enough on one rail that the reg 'drops-out' and feeding hum through on one rail.

This might help on general fault-finding, too:
http://sound.westhost.com/articles/troubleshoot2.htm
 
martin clark said:
That might not be the PSU alone, but rather a local decoupling or bootstrap cap (depending on circuit design) that has died. After 20+ years replacing te main PSU caps will be a good idea, but fixing this hum will require also changing any electrolytic caps on the amp PCB.
Click to expand...
I intend to replace all electrolytic caps, but what are those flat orange coloured discs that were replaced on the project?

c21crhazusi.jpg


Transistor failure would likely result in non-function in one channel, or the output being DC at one rail or the other- very obvious, in other words.
Click to expand...
Ouch!

James
 
martin clark said:
After loking at the pics it appears that the PSU might have seperate regs for each channel. Check voltages before and after these first, to verify things are the same side to side, and that there is at least 3-4v drop across the regs themselves. It could also be that a rectifier/diode has died, dropping the raw PSU enough on one rail that the reg 'drops-out' and feeding hum through on one rail.

This might help on general fault-finding, too:
http://sound.westhost.com/articles/troubleshoot2.htm
Click to expand...
Thanks for the additional hints, Martin.

Is it generally a good idea to replace diodes/rectifiers? How can I tell a suitable replacement?

James
 
The flat orange discs are ceramic types. Leave them for now - while usually bad for musical purposes, they are reliable.
 
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