Rectifier noise in outputs.

[QSilver]

I have an SA-5300 that I've been recapping etc. Just sat down to try it out and it quite honestly blew me away! Sounds really good. Very clear sharp sound stage and deep natural bass....

Only one small problem remains... when there is no sound, I can hear the "pulses" of the rectifier diodes. I'm pretty sure its something in the power supply as when I cut the power, the amp remains till the capacitor is discharged and from the moment it is cut, the noise vanishes.

I've changed the diodes out for 4x 1N4004 that I had in the parts box but no change. I had read that it might be due to a bad or inadequate snubber capacitor? But I'm not so great with rectifier circuits at this level so could anyone shine some light on this? Maybe I need to replace or change the value of the snubber cap?

On my scope, I have noticed that a slight negative side of a sine wave is visible on the outputs and within the biasing circuit at the 50Hz level but it is a round sine wave shape. IIRC It is also present on the ground, but the unit is earthed....

Am I missing something?

 

[mister-fixit]

Are you still working on this ? If so, pull out those snubber caps and check 'em . I worked on a unit where one of the four was out of tolerace and caused a similar issue(noise.)

 

[QSilver]

Yes, I'm still working on it, currently waiting for those 5600pF caps to arrive... I've taken a photograph of the top and bottom of the main PCB and chassis - maybe we can see if the wiring is "standard" or not.

pull out those snubber caps and check 'em

Which snubber caps do you mean?

 

[mister-fixit]

I should have looked at the schematic before replying.. Looks like it has only 1 cap across the bridge rectifier. C70 0.01 uf . Maybe also check C63

 

[QSilver]

I've already replaced both of those. Replaced C70 with a new film capacitor which you can see as the red block behind the diodes on the photograph. I thought about that too... new parts arrived today so I might fit them alter and report back...