G-K RB400 On w/no sound

[dezmoduo]

First, Hello to everyone. Its my first post. I have a couple of SS amps, My Tech 21 TM-60, an older Yamaha G-112, An even older Peavey Renown 2-12, a previous and deceased and missed, GK-SEL-2100 (RIP, sniff)
  Somehow I've wound up with my sons RB400, which I took out to a BBQ/jam over the weekend. I set up and powered on but there was no sound. Pilot light lit, speaker plugged in, (swapped cable JIC), but no sound.
  As I inspected the head for any obvious problems, the top of the amp was very warm (not hot) to the touch, even after only a short time. The heat sink was also fairly warm but not hot, considering the under 2 minute run time.
  I traced back the amp usage over the last 3 months and I took it out one other time to power a floor monitor.  The monitor didn't work and I blamed it on the sound guy or the send cable. I'll bet the amp was failing then also.
  Yesterday I got to ask my son if he'd had any problem and he didn't recall, BUT, (big BUTT), there might have been one time last year he was running some music through the amp for a party and there were "Sprinkles" and he covered the amp with a plastic tarp. Possible red flag there I think.
  What might be the initial trouble shooting points on this amp? Output transistors, power supply, (amp lights up). Is there a fuse on the output?
I'm going to rummage up an schematic and get prepared in the mean time.
I've got above beginner skills, not expert. I've built a 2-6L6 tube amp from scratch, (no kit, sourced parts), but the well of knowledge is quite deep and I'm just dipping a ladle in.

Thanks for any and all info,
Dez
NYC

[J M Fahey]

Hot but mute amp usually means shorted speaker cable.

*So far* the amp is not dead, and if you quickly understand what's happening and turn it off, usually no big harm is done.
Otherwise.....

Start by building a lamp bulb limiter and googling the schematic.

Asking for it at GK usually brings back some .pdf too.

Post it here.

[Roly]

Hi Dez, welcome.

For people with "tooob" experience who aren't too sure of transistors, I've written a page here on S.S. amp repair.

[dezmoduo]

Very nice page Roly. I've sent off an email to G-K. I have a bulb limiter from firing up my recent amp project.

Dez

[dezmoduo]

I've gotten a response from G-K sales and IT coordinator requesting a serial number which I sent back already. I think I'll see some .pdf's from this. I'll host them as needed.
Meanwhile, I took a spray can of cleaner/lubricant and hit all the inputs and outputs, the send and return as well. Bam !! She's back online.
So I'm going to call it a little moisture and unuse, plus storage in a basement area, creating light to moderate corrosion on any or all of the jacks.
Its been idling for 20 minutes and the top is cool to the touch.

Beer is in order.

Gary

I will post the schematic as soon as it arrives.

[dezmoduo]

Posted:
http://www.ssguitar.com/index.php?topic=3028.0

[Kaz Kylheku]

Do not use lubricant on electrical contacts!

A lubricant is a film of some hydrocarbon, silicone or fluorocarbon.

What do these materials have in common? They don't conduct!!!

When you put a lubricant film between metal contacts, you're creating a capacitor.  If the contact was so dirty that it didn't work, the lubricant may improve it, but the result won't be ideal.

The best contact is made by smoothly polished metal surfaces that are free of any dirt, film or oxidation. The smooth polishing maximizes the contact patch area, and freedom from nonconductive material and oxidation ensures that the contact points have near zero resistance, and are ohmic (distortion free).

Use metal polishing paste on a cotton swab to polish the contacts inside the jack: only a small quantity worked into the swab, so as not to leave a build-up in there. Follow up with a clean cotton swab loaded with rubbing alcohol to remove the paste, and give it a final rub with a dry swab.   

(I use toothpaste, with great results.  When I run my tongue over the jack contacts, they have that fresh-from-the-dentist feeling, and none of my gear has ever had a cavity!)

[J M Fahey]

Do not use lubricant on electrical contacts!

A lubricant is a film of some hydrocarbon, silicone or fluorocarbon.

What do these materials have in common? They don't conduct!!!

That's *exactly* the point, we don't want randomly conducting stuff there.

When you put a lubricant film between metal contacts, you're creating a capacitor.

If and only if they do not touch each other .... which is not the case.   

If the contact was so dirty that it didn't work, the lubricant may improve it, but the result won't be ideal.

The lubricant plus the light solvent plus other additives present wet and soften big way whatever grime is present, so it's much easier to clean.

The best contact is made by smoothly polished metal surfaces that are free of any dirt, film or oxidation. The smooth polishing maximizes the contact patch area, and freedom from nonconductive material and oxidation ensures that the contact points have near zero resistance, and are ohmic (distortion free).

Nice.
The contact cleaner does the chemical part of that job.

Use metal polishing paste on a cotton swab to polish the contacts inside the jack: only a small quantity worked into the swab, so as not to leave a build-up in there.

Nice, but often impossible to do.
If you spread typical leaf contacts so much so as to have space for a cotton swab, they'll probably never make good contact again, if any.

Follow up with a clean cotton swab loaded with rubbing alcohol to remove the paste, and give it a final rub with a dry swab.

See above.

(I use toothpaste, with great results.  When I run my tongue over the jack contacts, they have that fresh-from-the-dentist feeling, and none of my gear has ever had a cavity!)

You must be joking.

[Enzo]

Lube in a pot or fader is not meant to get between the contact surfaces.  It is assumed they touch.  The lube is there to assist smooth movement, and to also loosen any goo that might get in the works.

Think of a car engine, covered in a thick grime.   hard to wipe off with a finger.  Now douse an area of said engine with fresh oil, and smear it around.  It loosens the old thick goo, and you can now easily wipe an area clear with a fingertip.    In a pot nice fresh lube allows the little wiper feelers to slide along easily pushing aside any bits of debris.


"Lubricating contact cleaner" was always a staple in electronic service.  Try squirting plain old Deoxit into a 60mm fader on a mixer.   It now may have good electrical contact, but the sliding motion is sticky.  Now squirt in some Caig faderlube, and it smooths right out.

[dezmoduo]

I understand the point on lube creating a coating. I was taken by the "Cleaner" part of the label. Since the condition has been remedied, and the amp will get some use and stored in a nicer environment, I could shoot a blast of electrical cleaner, not a lot, but perhaps to clear off and residuals.
I was going to buy some Deoxit online and the supplier couldn't tell me which of his products would be best suited for my needs, I lieu of his customer service, I went to radio shack to get abused and spend too much but I had something in my hands immediately

[Roly]

I have a very long distance suspicion of cans of "instant technician", particularly when there are dozens of different formulations.

"I gave it a spray of CRC"
"Which one?"

I had a long running nightmare saga with a radio station where people would treat every problem by giving it a spray of CRC.  The stuff they actually had was a moisture dispersant and contained a light fraction that caused every cap in the desks to swell up, and a light oil that was literally running around the base of the desks.

- silicon sprays, etc, are an absolute no-no around electrical contacts.

- dry alcohols such as isopropyl (tape head cleaner), methylated spirit, (or Russian vodka  ), all make good contact cleaners.

- the edge of a newspaper makes an excellent mechanical contact scrubber; rough enough to clean, not rough enough (or thick enough) to damage.

[Kaz Kylheku]

The "medicinal ingredient" in Deoxit is oleic acid: a fatty acid found in vegetable oils such as olive oil.

The same stuff is also used in some traditional metal polishes. I think Brasso contains it, or may at one point have.

It cuts through oxidation, perhaps not so differently from the action of solder flux.

I've read some claims by hobbyists that you can make your own homebrew Deoxit equivalent by cutting some oleic acid in a petroleum distillate: camp stove fuel like Coleman or lighter fluid. I haven't tried it myself, yet, but it sounds promising.

The brand name DeOxit is priced like it's some of elixir of immortality made from the extract of the flower of a rare herb that only thrives in one single tiny forest somewhere in Nepal, and blooms only for one week, once every 23 years.

[Kaz Kylheku]

See here: http://www.dialcover.com/components.html

Scroll down to "CEASE & DESIST" CONTACT CLEANER

xP

[Enzo]

You can pay twice the real price by getting it at Radio Shack.   Or you can pay $7-8 a can by getting Deoxit from a regular supplier.   I run a commercial repair shop, and one can lasts me a very long time.


One can make some sort of ersatz Deoxit in the bathtub, but getting it into the aerosol can is trickier.  I can direct my spray up into a pot.  Much harder using an eye dropper.   And to say Deoxit is jus a can of oil is like saying a prime rib dinner is just a slice of cow.

[phatt]

I have a very long distance suspicion of cans of "instant technician", particularly when there are dozens of different formulations.

"I gave it a spray of CRC"
"Which one?"

I had a long running nightmare saga with a radio station where people would treat every problem by giving it a spray of CRC.  The stuff they actually had was a moisture dispersant and contained a light fraction that caused every cap in the desks to swell up, and a light oil that was literally running around the base of the desks.

- silicon sprays, etc, are an absolute no-no around electrical contacts.

- dry alcohols such as isopropyl (tape head cleaner), methylated spirit, (or Russian vodka  ), all make good contact cleaners.

- the edge of a newspaper makes an excellent mechanical contact scrubber; rough enough to clean, not rough enough (or thick enough) to damage.

And I suppose the bottle of Vodka is head cleaner?   Yeah right,, Which head,, the one on the bench or the one on your shoulders.    :lmao:

Phil.