Please help troubleshoot '83 MKII-b

[RustyChops]

'83 MKII b w/reverb 60watt
This amp sat basically idle for many years (15 years or more). I recently replaced the tubes.
The problem is that the amp will suddenly "swell" to full volume. The OD channel has little or no gain. When switched into the OD channel the amp volume will suddenly swell up to full volume and then back down again. The overall sweet tone of the rig has gone away and is now harsh.
I'll have to take it to to a tech for repair but was just curious if anyone has some input into this problem.
Thanks,
-rc

 

[gonzo]

sounds like it needs a recap.

 

[Old BF Shred]

Sure sounds like a tube....I know you say they were replaced, but defects are possible.....

If the swell is very pronounced it's probably not a power tube as all would have to fail to make a large difference in the volume when this happens.

Likely one of the small preamp tubes.
Does it happen in either channel? with or without reverb?
Try swapping out one at a time, starting with the first (v1, closest to input jack).

But! Gonzo has a point 8) ........FIlter capacitors can fail with age and non-use...with no charge on the plates the dielectric (insulation) can break down....corrosion can set in....they won't always hold a charge.

But either way, getting a spare 12ax7 tube to swap (if you haven't yet) is cheaper than paying a tech just to look at it as you'll end up paying a minimum bench charge.

Good luck

 

[Boogiebabies]

The volume swell is most likely a dirty pot in the signal chain.

 

[RustyChops]

Thanks for quick replies...I'll check out the suggestions given here. I've some spare 12ax7 tubes. I guess I should've mentioned that the amp did this before I replaced the tubes and that prompted the replacements.

My thoughts were the caps look like the likely suspects and perhaps secondly dirty pots...
It scares me to take this sweet lil amp to a local but I guess its not brain surgery ...

thanks again !!!

Great board by the way!

 

[Boogiebabies]

My past experience with filter caps:

A- The amp will blow fuses immediately
B- The amp will hum very badly
C- The caps will explode, burn and stink you out of the house
D- They work and the problem is elsewhere

I would wager that even if you sent the amp back to Mesa, there would be an 80% chance Mike B. would leave the original caps in.
Do yourself a favor and go to Guitar Center. In the Pro Audio section they have Caig De-Oxit and Fader Lube. Clean them, Lube them and roll with it.
If it fixes the problem, then a cap job should be done when you are ready. If your in the NYC area, Stokes here on the forum has recapped a ton of
old Mesa's and we can vouch that he is a pro.

 

[stokes]

Thanks for the nod,BB.Your problem doesnt sound like it is a common cap problem,but I wouldnt recommend turning an amp that has sat for 15 years on without bringing the voltage up slowly with a variac.You have already done it and they didnt blow,so far so good.Try the suggestions here,and if that dont work change the caps.Although it isnt a "common" symptom of a weak,dry old cap,the caps are the heart and soul of your power supply and if the power supply is fluctuating due to old caps leaking,it could possibly be the cause.Even if you can cure the current symptoms,I can guarantee a cap job will improve your amps response and feel.

 

[Restless Rocks]

BB and Stokes speaketh the truth. While you're cleaning those control pots don't forget to go ahead and clean the FX LOOP JACKS as well. A very well known cause of aberant volume behavior in vintage Boogies.

 

[RustyChops]

Wow...I really appreciate the response here...thank you!!!

A bit of clarity - I would turn the amp on every now and then when I got the urge and play it until I got bored with myself...maybe a few times a year so it didn't actually sit dormant for the whole 15years.
I'm going to try the remedies that are suggested here and hopefully that'll work. I had checked the effects loop already. The pots are not scratchy just ineffective. I'm going to clean the ol girl up and see what's what!
So again I thank each of you for your insight..I'll let you know how things turn out...