Before replacing anything I took a good long look inside the amp and studied the ground scheme.

I've rarely seen so many ground loops in a stock amp. The PCB ground strip was connected to the chassis in probably 4 different places, and the filtering section was both connected to the chassis by way of a tag strip and to the ground strip on the PCB (itself well connected to the chassis as mentioned before...). I've started by eliminating those and the amp has quieted down a bit. Not entirely and the reverb is still humming and hissing, but it helped with the overall noise.

I've also noticed that two of the input jack sockets (the "low" inputs) were of the non-shorting type. I've plugged into one of the hi inputs and jumpered the "low" one to ground and, lo and behold, the amp got a bit quieter too. So I'll replace those by shorting jack sockets.

It's getting better...

Quote from: Roly on April 10, 2013, 06:02:47 AM
Does the hiss change with;
- the vol/gain controls?
- the tone controls?
- the reverb control?
- the master volume control?

By all of these. The reverb, adding insult to injury, also adds hum.

It's a big amp with a lot of components. I'm usually down for testing caps and replacing things one by one, but I feel a bit overwhelmed by the amount of work that finding leaky caps would take and I'm sometimes tempted to just replace each and every one with new quality equivalents...

Quote from: Roly on April 10, 2013, 06:02:47 AM

That will most likely be noise from the first/early preamp stage(s).

I agree.

All's not fine otherwise I probably wouldn't touch it. It's a bit too hissy (water running type sound, maybe a leaky cap involved...), the reverb's noisy too, and the electrolytic caps in it date back to 1976... I know it's not supposed to be a 'dead silent' amp by any means, but I think it could be quieter and it would be safer to recap it anyway.

I see what you mean about a rev-log pot fixing it but not being totally necessary...  I'm just curious whether it's a defect or it's how the pot acts from stock. I don't think spraying the pot will help but it won't hurt either. The schematic I have is the '77 Randall-issued schem that's floating around the internet, so it should be right.

Thanks for chiming in, guys !

Thank you ! That's pretty much what I thought, since I've been using this amp for some time now without any problem (plus those were in there from the get go).

I've another question, that's more directed towards owner of RG120's : the tremolo depth knob only works between 7 and 10, before that it doesn't do anything. Before 7, nothing, at 7 the tremolo effect is barely noticeable, and it becomes more pronounced if I push it past 8. Is it a normal, though lackluster, feature of this amp, or is there something wrong with mine ?

Hello,

New guy here, in need of some help to recap an old Randall Commander II (RG 120). I hope one of you fine fellows will be able to provide some insights.

I've opened the amplifier and looked in. It's been serviced once (as mentionned by the seller) : some 7 watts resistors have been replaced by newer cement ones, because one of them had obviously burned (nasty burn mark on the PCB). All the caps seem to be original (with the expected amount of oxydation on their leads and solder joints).

So I was a bit puzzled when I noticed the two large filter caps have lower values than indicated on the original schematic : they should be 6000 mfd / 50 VDC but they're 4500 mfd ?!

Do you think that I should replace them with equivalent caps or switch to 6000 mfd as showed on the schem ?

Note that it's an Import-type amp. I don't think this is really relevant, but my knowledge in electronics has its limits, so I'd rather be careful than sorry.