1986 400RB problem, help

[Alexdeth89]

I just got an old 400rb in good cosmetic condition. The problem is, sometimes it turns on and sometimes it doesn't.
When the amp turns on, I can play for like 5 to 10 minutes and it sounds great actually, but that's it, suddenly the amp dies again.
I checked inside and everything looks normal, the fuse is ok, the power chord is properly attached as well as the sink. Any ideas?

 

[Dave W]

I'm hardly an amp tech, but it sounds like 2 issues. A faulty power switch and maybe a heat related problem?

 

[tjh]

Try running a patch cable from the effects send to the effects return jacks. One of the first things to try on these if experiencing things out of the norm ... then if no luck, solder joints seem to be one of the next things to check from my experiences with them ...

 

[RoadRanger]

I've been told these have a tendency to catch fire so if I was you I'd send it in for repair before anything more serious happens. If you just got it can you take it back?

 

[will33]

Try the effects loop jumper as mentioned first.

If you're a little handy, you can unhook the power switch, taking it out of the circuit, and wire straight through. It'll turn on when you plug it in and shut off when you unplug it. If it works normally that way...bad power switch.

If you have a multimeter, you can also check the voltage at the feeds coming out of the power supply when it works and when it shuts down to verify things are good that far. Also, being that's 28 years old, look at the power supply capacitors (the big ones) for any signs of bulging or leaking oil.

Can also use the line out/effects loop and a second amp to isolate the problem to the pre or power sections.

Few not-too-involved things to try. If they don't provide any results, maybe tech time.

 

[okcrum]

I've been told these have a tendency to catch fire so if I was you I'd send it in for repair before anything more serious happens. If you just got it can you take it back?

Never seen one catch fire even when the output devices fail. Unless catching fire means blowing the fuse. Clean the jacks, check the power switch with an ohmmeter. The power supply is simple; no shutdown circuitry in there except on the low voltage. I assume when it doesn't turn on the power light is not lit green. Do you ever see it light red? If not, it's very likely the power switch.

 

[will33]

Never seen one catch fire either, and I ran mine only a notch or two away from balls to the wall for 4-hour gigs (with hourly set breaks) purposely for a long time. It still runs good as new.

 

[Bill Whitehurst]

Open it up, find the cable from the pre amp board to the main board.
Clean and maybe tighten the connectors.
They can get loose and or dirty and become intermittent.

 

[johnson79]

I bought mine used and it cut out and made all kinds of noise. It worked fine for a minute and then went all wonky. Cleaning all of the jacks and pots fixed the issue. I'd try that first.

 

[khangurumc]

I send mine to my tech last week ,he will change the filter cap and clean it up.Those old amps need this after years of services.And it worth it.They sound so good...