[sdiy] paia 4700 power supply troubles

[rkmoore at memphis.edu]

Hello,

I'm still struggling to get these 4700 systems fully functional.  I've 
had problems getting the filters to work and there's a pretty 
significant hum through the output.  At first I thought that the hum 
was just due to grounding issues, but now that I've looked at the power 
supplies it seems that the troubles may be there.  

I looked at the output of the supplies suspecting abnormal ripple and 
found a 20VAC component.  This seems really odd to me, but I also 
noticed that there were no resistors in series with the primary 
filtering capacitors.  I would think that they would be necessary to 
smooth out the voltage.  The capacitors are new (I replaced the old 
ones) and the diodes check good.  Is this the way the output of this 
power supply should be?

I checked the supplies loaded and unloaded and got similar results.  
The modular system seems to be functioning fine except for the filters 
and the possibly abnormal hum.

Richard