Randall rg120 buzz

[dmartn149]

I just got a really good deal on a Randall rg120 Commander II, but it has a hum, or a buzz. It sounds like the buzz that some fluorescent lights make. It's volume follows the master volume control, and it pulses with the tremolo. The channel volumes don't affect the buzz. Also the eq controls act funny. When they are turned to 0 they turn their portion of the signal off. In other words, if they are all turned fully counter clockwise there is no sound. If I turn the bass control up I get volume, but very bassy. If I turn up the mid I get volume in the midrange. Same with the treble. with all eqs on (above 0) it sounds great, except for the buzz. Is the eq supposed to work like this, or is this maybe related to the buzz?
I don't have a schem. and don't know where to get one.
Anyone have an idea about how to fix the buzz?
Thanks

[J M Fahey]

The tone controls are fine, that's what Fender type controls do.
As of the buzz, I don't have the schematic either , post a couple pictures, it will probably be similar to some other one.
Randalls basically come in two "flavors": RG80/100 type Metal machines and a more classic "Twin type" one.
Both usually have Fet gain stages.

[dmartn149]

Thanks JM. This one is like a twin. It's a 1976 model. Two non switching channels, tremolo on ch2. It has a good clean sound (except for the buzz) It will distort if I turn the channel vol. up, but it's not a good kind of distortion. (at least to my ears)
I'm hoping someone here has had this exact problem and knows exactly what to do. (I'm hoping to win the lottery too )

[J M Fahey]

Look what the cat brought:

and

Good luck.

[dmartn149]

Thanks again JM! Now I guess I need to check voltages against those given on the schematic. The only control on the amp that has any effect on the buzz is the master volume, so I'm thinking that the problem should be in the area between the eq. and master vol.
I've never done this before, and I don't have an electronics education, but I have done some tinkering(built a chip amp, effect kit, cap job on tube amp, replace pots, etc.) Trouble shooting on a circuit board is new to me. So please don't get too frustrated with my ignorance. And thanks very much for what ever guidance you can give me.

[dmartn149]

Since there aren't that many components between the eq and the master vol, I thought I might use the shotgun approach and just replace them all. So as I'm tracing the circuit, and comparing it to the schem. I found that Q4 which should be an MPSA 13 is another TIS 58. Could this be the problem?

[J M Fahey]

Not likely.
By sheer chance, *that* point is the one where one or the other type may work about the same.
Do not change parts at random.
Try to get a scope.
If you can't, I have been experimenting with PC software based scopes because my own died 4 days ago ; as a backup I downloaded and tested a few programs and I found a couple quite interesting.
The problem is that you just can't plug your PC Mic or Line input straight into any amp because you will burn it at once.
I'm experimenting and designing a safe interface, will post it here when ready.
Scopes are really *much* more useful than what people usually imagines.
Stay tuned.

[dmartn149]

Bummer, I was hoping I was almost done. Okay, so the tone controls don't affect the hum, and if I run a line from pre out on this amp to the effects return on my other amp I get buzzing in it too. So what if I plug a cable into the effects return on my good amp and use the other end to pick up the signal at points between the tone pots and the pre out. That should tell me where the buzz starts. Right?
I don't know where to get a scope, and don't know how to use one anyway, though I guess I could learn (if I knew where to get one)

[J M Fahey]

The next best piece of equipment, lacking a scope, is a signal tracer.
What you suggest may work, connecting a later (buzz-less) stage to different points of earlier ones to check where the buzz appears.
To have independence I would build an LM386 simple amplifier on a piece of perfboard, power it with a 9V battery as to be independent , drive a couple earphones with it and connect the input to a pair of crocodile clips which will allow me to "sniff" noise at different points.
It should take no more than 1 or 2 hours to build.
Check this as a reference:
http://www.vellemanusa.com/us/enu/product/view/?id=350589
http://www.vellemanusa.com/downloads/0/illustrated/illustrated_assembly_manual_k7000.pdf
You can order that inexpensive kit (U$20) or just build the tracer part (LM386) on perfboard.
Very useful.

[phatt]

Maybe C45 could be dying? (point (B) on the PSU.

Check for AC voltage at point (B) should be close to no AC there.
If there is try replacing the Cap.

Also switch off the reverb (if Fitted) and see if the noise goes away.

Note: Preamp output is not really a preamp out,, it should be labeled as a Line out from main amp.
Phil.

[dmartn149]

Actually mine is not exactly like the schem that J.M. so generously provided. My pre out comes right off the master vol. through a 1k resistor.
The reverb control has no effect on the buzz. The only control that does is the master vol., though the buzz does pulsate with the tremolo.

[phatt]

Hi Again,
            Are you aware that a lot of Amps do actually hum?
In fact some are so bad I can't understand why the kids buy them,, maybe they are just not familiar with gear.

If the hum is anbearable to the point you can't play then Filter caps could be in need of replacement.

In General,
The mains hum from a big wattage Amp in a bedroom can be quite disconcerting but at a live Gig you won't even hear it in amongst the drums 'n Bass.

Also,,,With all those fans screaming at you it won't be a problem. 0:)

Phil.

[dmartn149]

It did occur to me that it might be normal for this amp, but it is pretty loud, and my RG80 is pretty quiet. Yes I do realize they are different amps. If I can't find anything wrong with it I may just learn to accept as it is.

[JHow]

I have an RG80 that has a bit of power-amp hum (I talked about it on another board).  After fiddling with it (I still have bit more fiddling I plan to do) I am maybe coming to the same conclusion about accepting some hum.

[dmartn149]

So what if I plug a cable into the effects return on my good amp and use the other end to pick up the signal at points between the tone pots and the pre out. That should tell me where the buzz starts. Right?

Okay, That didn't work. I got some very loud (scary) humming, and was afraid to continue. I did notice that one of the big filter caps looks like it's been hot. Where is the best (cheapest) place to buy replacements.