Hawk HR-101 Hum

[smilan]

Hi,
I have a Hawa HR-101 Japanese spring reverb.
The output is quite noisy. There's a 108 Hz hum (the mains frequency here is 50Hz).
So recacp the PSU seems to be the first thing to do.
Since I can't find schematics for this device I draw the PSU.
My question is how much can I increase the value of the filter caps?
I can find 1300uF caps that will be fitted in the physical dimensions to the 220uF old caps.
This is the right thing to do to reduce the 100 Hz hum in this PSU?

 

[Voyager10]

First thing - if C2/C3/C4 in the schematic really are 25V-rated, that's too low (20V AC peaks at 28V, and for long life you need safety margin beyond that). Recapping with 35V-rated ones is a wise precaution, but 470uF-1000uF should be plenty.

I would assume that the 20V rail is the one used for 'critical' low-signal-level circuitry, and it's C6 (rather than C3/C4) which is important here. Replacing this with, say 220uF would certainly reduce ripple on the 20V rail.

The other place to look for hum is the 'pickup' end of the reverb tank, which will be a magnet+coil arrangement like a dynamic microphone, and very good at picking up stray magnetic fields. You can try temporarily taking the pickup out of circuit and shorting out the input it's connected to. If the hum disappears you'll have to try screening the pickup better (or moving/rotating the mains transformer).

 

[smilan]

UPDATE:
I replaced all the 220/25 with 1000/35 and all the 47 and 100 / 25V with 220/35V and now it's dead quiet