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Hey guys,
I have a MusicMan 112 RD 65. Hybrid amp, solidstate preamp and Tube drive uses EL34/6CA7. It's from the mid 70's and everything you've ever heard about these amps is true. It's a LOUD beast, tons of headroom, but you gotta get your grit from a stompbox. Anyway.... lately its been a little hissy and then it intermittently just quits, I mean she's lite up and tubes are still glowing but absolutely nothing out of the speakers. Sometimes a slight tap on the cabinet and she's back on, like a trace is broke or a solder joint is bad but I really think it's time for a cap job, with the hissing and all.
Can caps intermittently fail?
Either way it's a starting place...I've been lurking the MM amp forum but the moderators haven't answered my registery yet... and I know we have some tube amp guys here. So in advance, THANX!

Sounds to me more like a dry joint, especially if you can tap the cab and the sound comes back on.

I once repaired a Carlsboro amp and found that the insulation of the speaker wire had been 'soldered' [ie, melted by a soldering iron] to the jack instead of the actual speaker wire - and that was how it came out of the factory !

Good luck hunting down your problem. Let us know how you get on.

After lot off working years and the vibrations of the cab it's possible that ICs and sockets need cleaning.

Yea, after all these years anything that can oxidise probably has so you need to clean every contact you can get at.

If it has an effects loop clean the jack contacts, they are a regular oxidisation point.

I have never known of capacitors being intermittent. They are good or bad.

I am not a fan of changing them for changing sake. I they are leaking or bulging (tops, cans, or rubber bungs) then definitely change them. After that I usually check their value and if it within spec then leave them be. YMMV of course.

I've ran a half a can of contact cleaner thru it... all the pots, all the IC sockets, cleaned the jacks... haven't changed any caps - That was all the cats on the MusicMan amp forum - seem to think it's important. But I'll tell you - visually this thing is immaculately clean inside. I'll be firing her up soon while she's open so I can look and listen to the circuit as it heats up ... maybe I'll see what's up.

Gently wiggle the Valves ,, use a cloth or thick paper towel or something as they are hot.

If that bring it back to life then let it cool down then remove and insert the Valves a few times.
You can even scrape the pins if need be.
Or use electrical cleaner as well.
If not Valve sockets then Strong Light (Preferably bright daylight) and a magnifying glass and start inspecting joins.
Hairline cracks in stressed solder points are common.

As has been said EF loops is a very common failure and even cleaning may not fix it.
A sure fire way to establish if efx could be an issue is simply plug in a loop lead which bypasses the internal switch.
Phil.

If cleaning and inspecting joins does not help, go to the next level with plan B.
Prepare DVM and find what is your chassis number of amp. If my memory
still working MM 112RD-65 has chassis number 2165RD and here is schematic.

I've worked on these before, they are built really well. Do the chopstick check, it's way more effective than contact cleaning. Sometimes it just comes down to the one lead being loose. Also, do the power tubes dim out when the sound cuts out or stay lit? It probably wouldn't hurt to do a cap change, you'll be able to get a better look at things as well.

Thanx for the schematic... I have it already. Ernie Ball has them all archived. haven't had a chance to fire it back up after cleaning, but I did a visual and like I said it's all good, I have a BRIGHT light and magnifier on my bench (helps my old eyes) I don't use the effects loops but cleaning the contacts on the jacks couldn't have hurt any. MM has an odd way of implementing a loop, uses 2 stereo jacks out for 2 different levels and each is an in/out using a "stereo to 2 mono jacks" cord... weird. Since it's a SS preamp anyway I just push the front end with my pedal board. Been sourcing caps for the two big electrolytes may change them just because she's old... first the test run and check to see about the intermittent problem... oh yeah new JJ 6CA7 tubes and no issues there. When I replaced them last year it was a big improvement over the old sylvanias...

Well, I ordered all the caps today from mouser... guess I'll try reflowing all the solder joints too. Tap tap here, tap tap there, all the chopsticks did was make me hungry... So I pigged out today at a chinese buffet... The tubes do NOT get dimmer when it cuts out... I suppose the output tranny could be having an intermittent open in one of the wires... grr. I know enough to not be scared of working on it but I ain't no tech.
I did build a big ass banjo out of a floor tom last week...

deltafred wrote: I have never known of capacitors being intermittent. They are good or bad.

I have had larger electrolytic caps "dry out" with age, and sometimes they develop intermittant internal shorts. They're usually supply smoothing capacitors, so cause intermittant fuse-blowing!

The cats on the MusicMan Amp forum suggest recapping all the electrolytes for the same reason, drying out. They say a bad cap can take out a transformer, so that's why I'm worried. So a bit at a time, and if I have to replace one or both transformers... so be it. I love this amp that much. It plays well with pedals, having a solid state preamp, and it'll push another cabinet.