Matchless HC-30 problem - my tech is stumped

[sfarnell]

I'd appreciate any help you experts can give. I have a '97 Matchless HC-30. It started life out as a DC-30, but I bought a head cab for it.

It sounds great until the volume drops almost completely off while it's being played. I replaced all of the tubes but that did not help. I took it to a local tech who said that he could not even get the amp to do what it was doing when I played it. So, I took it home and the first time I fired it up the same thing happened, the volume dropped off after five minutes of use. I took it back to the tech who has had it for three weeks. He finally got it to lose volume but he is still stumped as to the cause of the problem. He thinks it's in the power supply. He also says that the problem did not arise unless the chassis was in the head cab. He could not reproduce the problem with the chassis removed from the head cab. Why would that be? I'm picking up the amp today and am wondering what to do next. Any suggestions as to what may be wrong with the amp and what I can do about it? The amp is a real jewel when healthy so I don't want to give up on it.

Thanks for any help you guys can give.

[skogm]

Heat? Weird grounding issue? Head bolts?

[Jujo]

I've experienced a similar problem and my buddy heard it to. I think mine was caused by or related to switching the amp between 15w/30w without putting it on standby first. If I flick the wattage switch without going to standby, it would play fine for less than 1 minute and then drop out- not totally silent but pretty darn quiet. Flip it back the other way and the same dropout would occur. I get out of the loop by going to standby and flicking the wattage switch. Haven't had the problem since I started going to standby before switching the wattage.

I get ghost notes on mine when I turn it up loud and hit it with OD pedals. Jamison sent me some new coupling caps (power section) but I haven't installed them yet. The problems could be related.

[Blue Strat]

A power supply problem should be easy to figure out IF he can get the low volume condition to happen.

With the chassis out of the head, there may be less flexing of the chassis which could mean a bad chassis ground connection. I'd be looking at these.

Could also be a parasitic oscillation which would show up as mucho power tube current draw in the low volume mode.

[mark norwine]

I own several fluke meters (87's) that can "record" min - max - avg. They've been a godsend in such situations.

Hook them up to suspected areas, plug in a dummy load, put a signal into the front end & walk away.

If I suspected the B+, I'd monitor a few critical areas. That's where I'd begin

[donnyjaguar]

Does it do it when plugged directly into the amp?

[sfarnell]

Does it do it when plugged directly into the amp?

Yes, that's the only time that I'm able to notice it.

[Pete Cage]

Just for kicks - take a short 1/4" cable and jumper from the effects send jack to the effects receive jack. See if that makes the problem go away.

[jezzzz2003]

Just for kicks - take a short 1/4" cable and jumper from the effects send jack to the effects receive jack. See if that makes the problem go away.

Goosd advice, also the problem could also be a transformer, this happened to me with a Marshall once intermittently.

[samdjr74]

Years ago I had a KMD head, it started doing this after about 3 months. It sounded fine then about 5 minutes in the power would drop until no volume at all. I changed out the power tubes, no luck, had the PT checked it, seemed fine. The problem turned out that the amp was overheating and shutting down. I had a fan installed in the head and the problem never came back.

[EL34]

Just for kicks - take a short 1/4" cable and jumper from the effects send jack to the effects receive jack. See if that makes the problem go away.

Pete - what problem does this address? Is this common with C30's?

[rockon1]

Pete - what problem does this address? Is this common with C30's?


Yes-common with Peaveys! The contacts in the FX loop jacks must close and make good connections when nothing is plugged into them. They get "dirty" and can cause intermitant lose of volume as the signal chain is interupted. Bob

[Pete Cage]

Yeah, what he said. I've seen it on a number of DC/HC-30 amps. They use one of the more complex Switchcraft jacks in those spots, and the switching contacts get dirty or sprung or corroded. I don't think it helps that they've been bathing in the acetic acid fumes outgassing from all of that RTV inside the chassis, either...

If you can't find the exact jacks to replace them, you can make up a short piece of 90-degree George L and just bypass the loop with a loop (so to speak.)

Of course, it could be something else, but this is easy to try.

My $0.02,
Pete

[EL34]

thanks Pete and rockon1!