Mark IIc+ problem (loud buzz/hum) - Need help from experts!

[Enc3f4L0]

Hello,

I've been the proud owner of an upgraded IIc+ for a few months now.
A Simul-class + REV + EQ head. Amazing amp, really!

Everything was fine up until a week now.
It's "randomly" busting into this LOUD buzz/hum mode now.

The guitar signal is completely cut, and all that's heard is noise (imagine a guitar cable plugged to a loud amp, unplugged from the guitar, with your fingers touching the contacts). When this happens the power tubes start glowing Blue (I know it's normal with loud signals, but this is one of the most intense blue I've seen them glow). I immediately turn the amp to stand-by when this happens.

If I turn it on again the problem can still be there or not. The first time it happened, I turned off the amp and a few hours later turned it on again and the issue was gone.

Sometimes I'm playing and it'll click in for a couple of seconds and fix itself before I even get to the stand-by switch.

At this point, after having called Mesa Boogie customer service 3 times, I have already swapped out all power tubes, and all preamp tubes, including V5 (driver).
Yesterday when I swapped out V4 the problem was gone, so I thought I had found the culprit, but a few hours later there it was again, so it was a coincidence after all...

Customer Service couldn't offer any other trouble-shooting tips for me. They said it was strange that this wasn't a tube problem, because it sure seemed like it. They also asked me if the Reverb was set to 0 (which it was), because sometimes the Reverb is to blame for noises...

Yesterday the noise also came on a bit differently... Instead of a LOUD BUZZ (like an unplugged guitar cable) with no guitar signal, my guitar dropped in volume a bit and a HUM (much lower in volume than the buzz) came on. I noticed the "humming note" was something in between B and Bb, if that's of any use.
The hum volume was not affected by turning up or down Master 1 (although my guitar signal still was).

I left it on humming for maybe a minute or 2 and started smelling something burning, so I turned it on to stand-by... When I switched it back on again, LOUD BUZZ, no guitar signal.

When the amp is buzzing, it doesn't matter what channel it's on, if the reverb is on or off, if the EQ is in or out, or even if the guitar cable is plugged to the input or not. Only thing that stops it is turning the amp to stand-by.

I tried replicating the problem when the amp is functional by slightly messing with the input jack, the on/off/standby switches, the push/pull pots and even hitting the amp on a few different angles hoping that it's a bad contact or something, but couldn't get it to trigger the problem. It is TOTALLY random, the **** thing has a will of its own.

I could take it to a tech but I wanted to atleast be able to replicate the loud buzz. I'd hate to come into a tech's shop and look like a moron after we turn in on and play for hours without any issues.

Any suggestions?

 

[zodiac272]

nice amp... you may want to get it serviced at mesa in petaluma. chances are it hasn't been serviced in 26 years. mike b. will have it like new when he's finished. totally worth the shipping and 6 week wait to get it back

scott

 

[Enc3f4L0]

Forgot to mention... I live in Brazil, so sending it to MESA to get it serviced is not as simple/cheap.

It was serviced by Mike B. on Feb. 2010, so it hasn't been that long. He cleaned the pots, changed caps, a bunch of resistors and put in new tube sockets.

 

[exbest182]

Maybe just give Mike B. a call, he loves to talk old Marks and he might be able to give you some insight, however, the end result will probably be him telling you to send it in.... but you never know... I actually had an upgrade that was getting too hot and wrecking tubes - Mike B. said someone prior to me owning it had biased it way to hot and it was killing my power tubes, and he even mentioned may caused some issue with my power tube sockets... I dunno man, without being able to replicate it, it's a tough call.

 

[mark2boogie]

Random problems can come from tubes, as you swapped them without any improvement, it means that there is a bad contact somewhere in the amp - a hidden plague... Here are some tips based from my nearly 30-years servicing experience that I would suggest, if you feel confident in your patience and your tube electronics skills, of course, otherwise... Stay wise and refer to qualified personnel.

At first, let's locate where the random problem borns :

1- plug a shorted jack in the POWER AMP IN. Is the random problem still alive ?
- YES = the problem is located in the power section : final amp or PSU --> see 2-
- NO = the problem is in the preamp section and it's supply --> see 3-

2- remove the phase splitter tube. Is the random problem still alive ?
- YES = the problem is around the power tubes circuit / bias circuit / feeding circuit / HV circuit.
- NO = the phase splitter circuit is the culprit (resistor, cap, contact on the socket, any...).

3- remove all the preamp tubes except the phase splitter, and remove the shorted jack. Is the random problem still alive ?
- YES = it's preamp PSU problem, or the problem is located in the last preamp stage circuit before the phase splitter.
- NO = see 4-

4- reinstall preamp tubes one by one, going from the last one before splitter stage to the first one at the input. When the random problem reappears, you've located the preamp circuit section which is faulty.

Secondly, you will have to open your amp, examine, smell, take measurements, use a wooden stick to gently displace elements or press de-ci-de-là, in order to point/to trigger/to tackle the problem - usually where the previous steps have guided you, but not solely : bad contacts can be very painful hidden surprising fantaisists...

Good Luck and Patience !

A+!

 

[Enc3f4L0]

Thanks for the help guys.

I took it to a tech and left it there for a couple of weeks for him to check out.

Since I was suspecting the Standby switch to be triggering the short, he kinda went from there...

He told me the switch was "carbonized" (that's the term he used for how much burnt/rusted it was).

He bought a replacement switch, which turned out to be too big to fit in the amp, so the solution was to clean the original one back to a half decent condition.

He replaced the burnt fuse and the amp turned on fine (I had to replace one of the power tubes aswell, cuz it died, probably during the fixing process). He left it on for a few hours and the problem seemed to have gone away, so I brought the amp home.

It might be too soon to say it's fixed, but so far so good. I played it for an hour tonight and not a sign of buzz/hum.
Let's hope it stays that way cuz I have an album to record. *Fingers Crossed*