Randall RG 120 (1976) recap : ADVICE NEEDED !

[inbearsuits]

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 It's been serviced once (as mentionned by the seller) : some 7 watts resistors have been replaced by newer cement ones, because one of them had obviously burned (nasty burn mark on the PCB).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.

[J M Fahey]

Using 20% "smaller" caps will not change much, definitely nothing you can hear, so don't worry about that.
The amp is still well filtered.
FWIW a similar power Valvestate and many others in the same league use only 2200uF, go figure, and *that* getting close to the edge.
2 more considerations:
1) 6000uF is not a normalized value, while (adequate) 4700uF is widely available.
*Sometimes* you can get 6800uF .
2) electrolytic caps have wide tolerance anyway, often as high as "-20% +50%" so your "4700" might very well have *more* actual capacitance than the specified "6000".
3) don't replace them "just to match the schematic" but if you will recap anyway, Mouser may have 6800uFx50V ones.

[inbearsuits]

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 ?

[Roly]

I've been using this amp for some time now without any problem

"If it ain't fixed, don't broke it."   :cheesy:

[J M Fahey]

About the tremolo pot, I don't have the schematic before me, but I seem to remember that it has a linear pot but would benefit from an anti-log or "C" curve type.
Which by the way are quite hard to come by.
However, since it's not a control in permanent use (such as volume) but more of the "set and forget" type, it's not that important.

[g1]

  The amp is from 1976, are you looking at a schematic that old?  The values used would likely have changed since then, also the caps may have been replaced since '76.
  As far as the tremolo pot, maybe it just needs a cleaning?

P.S.  You said it was an "import type" amp, I thought in '76 the were "made in USA" and marked as such on the rear of the chassis?

[inbearsuits]

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 !

[Roly]

are you looking at a schematic that old?

Yeah, the old drafting style is a bit hard to read...



...but we old geezers get by.   :lmao:

It's a bit too hissy

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

Circuit posted here;
http://www.ssguitar.com/index.php?topic=2117.0

Does the hiss change with;
- the vol/gain controls?
- the tone controls?
- the reverb control?
- the master volume control?

[inbearsuits]

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...

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

I agree.

[Roly]

I can't say for the other old hands here, but I replace caps at a tiny fraction of the rate they get suspected here.  They do cause problems, but not nearly as often as people seem to think.

Do you have access to an oscilloscope?

Channel 2 appears to have a switchpot for the treble control that adds extra boost; does this also increase hiss?

If you want to try replacing anything, rather than randomly replacing caps which may be perfectly serviceable (and almost never cause hiss anyway), then I'd target the two first stage FET's.  The TIS58 is an N-channel JFET and appears to be functionally equivalent to MPF120, 2N3819, 2N5459, BF245, J305, and quite a few others.  These cost between one and two dollars each in one off.

Googling 2N5484 and equivalent or cross reference yields the following results:
2N3819
2N5457
2N5486
2SK104
2SK168
2SK17
2SK37
2SK49
2SK55
BF410A/B
ECG312
FE5484
J305
J5103
K305-18
LS3819
LS5103
LS5246
LS5484
MPF102 (MPF-102)
TIS58

Just be careful of the leadouts, they can differ a bit, so you will need to download the datasheets for the TIS58, and whatever N-JFET you can get at your local electronics store.

Hum in the reverb sound like a different problem, and I have encountered a few reverb springlines that have lost their ground connection through age and oxidation.  I'd turn the reverb control down for the moment, we can come back to that, and concentrate on reducing the overall hiss first.

We don't (yet) have a board layout, so start at each input socket and trace back to the first transistor you come to in each channel, and this should be marked "TIS58" ('tho you may need a bright light and magnifier to read it).

[inbearsuits]

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...