randall rc200sc

[Mars Amp Repair]

Hey folks,
I have one of these amps for service. It's totally solid state. Channel 1 (clean) works fine, but channel 2 (O.D.) squeals oscillates just by turning up either of the 2 gains pots.
The 1st gain pot when turned up past about 3/4 or so produces about a 12K sinewave & gain 2 pot produces a rounded squarewave of aobut 500hz.

Both of these actions can be changed but not eliminated by gounding the input jack board directly to other grounds on the board as well as the chassis, however when you switch the ground lift to lifted, all bets are off & back to where you started anyway.

I almost seems like the gain in the 2nd stages is just wayyyy to high.

This seems more like a design issue & the guy claims it's done it since he bought it new.

Anyone have any special connection with Randall possible service bulletins on this amp or any experience yourselves with this issue?

I've tried to snub the oscillation by placing a high value pf disc cap in the feedback path of the 2 preamp opamps to no avail.

I've tried moving around all the wires in the amp to see if it's a proximity feedback, also to no avail...

Thanx, Glen

[Enzo]

Are the gain pots feedback elements in op amp circuits? As opposed to the simple voltage divders volume controls of tube amps. Extract the pots and see if either is open.

[Mars Amp Repair]

Randall oscillation issue

Thanx Enzo,
I just received the schemo from Washburn/Randall & the pots are in the FB loops of the opamps....I haven't had a chance to get back to it.

Have you seen the pitiful warranty reimbursement that this company offers? Geeze. I think the highest they offer for power amps is like $35. Most amps it's $25/hr. What the h*$l are they thinking

thanx, glen

[Enzo]

Because the majority of their stuff is pretty cheap.

There would be little traffic from them around here for me. If I thought there would be a steady stream of it, I'd take on a line for just about any money. Every single repair that comes in is a new customer through your door. Now that he knows who youare, he will bring his other repairs in to you. How much do we spend in advertising or whatever to get a new customer. Used to be the phone book worked, not so much anymore. For me anyway. My warranty connections get me new customers all the time. I'd say an $18 repair is worth it if I get a customer out of it.

Not everyone feels that way, I;ll admit.

[Mars Amp Repair]

Randall warranty

Yeah Enzo,
I hear what you are saying. If I were only paying myself, I might consider the warranty for the free advert on the web. However, when you're trying to pay a tech a decent income there is a issue with him getting paid $25 for the same repair that other mfg's pay $50 or $60 for.

As you know sometimes the cheaper built/designed stuff can present more headaches than the better designed/built equipment.

BTW, the rep at Randall,washburn, whatever mentioned that they know their rates don't compare to others & they are trying to get the rate raised from $25 (for amps) to $35. Not much better.

glen

[Enzo]

I understand, but even paying a tech to work for you, if it brings more work in for the shop, the money you "lost" on the warranty repair is amortized over the life of the relationship with that customer - the customer you wouldn't have had otherwise.

I was not thinking so much the factory listing as the value, though it certainly is valuable, I am thinking the customer himself.

How many times have I heard the following? "Thanks for fixing my XYZ. Oh wow, you fix Peaveys/Marshalls/Fenders/PAs/recording gear/lighting/mixers/or whatever TOO??!!? Oh great, I have a lot of other stuff that needs fixing, I'll be back."

SO the repair that I got $25 for instead of $45 results in a retube on a Fender Twin, a rebuild on a GK head, genl maint on a large mixer, etc. My $20 loss is really a $400 gain.

Oh, I'll stop preaching now...