Mark IIC+ Hum Problem

[BPski]

OK, so I have a MK IIC+ with an issue. In the Lead channel the amp has a prominent hum even at low volume. Oddly enough it goes away when I use the Pull Shift on the Bass pot. Could it be a bad cathode bypass cap?

Thanks!!

 

[Boogiebabies]

Does it hum with no guitar plugged in?

Does it Hum with the Reverb on?

 

[BPski]

It hums with the guitar in or out.
It also hums with the Reverb on or off.

Pulling the Bass pot (Pull Shift) makes it go away.

And it's only the Lead channel.

 

[Boogiebabies]

The pull deep works on the cathode cap of the V2B. The lead circuit drops in there before the Pull Deep and the lead circuit as well as the loop. Try jumping the loop first to see if it goes away. If not, I would check every resistor to ground before I went swapping cathode caps. Remember that the lead circuit is pulled in from LDR #4. They have been know to be noisy,pop or just die, but I would leave that for last. The chances of a decoupling cap are rare, but can happen. Look at the orange drop and see if the dip is still shiny and the leads are still sealed to the cap. This is more often seen on the PI circuit due to its proximity to the power tube heat. You can also get hum from a bad Vol. 1. The possibility of the hum going away may be due to the pull deep circuit is putting a different load on the tone stack and the the Vol 1. pot. I also had a SUS-Mount ground give me a fit once. On these there is a braided cable stapled to the cabinet can get loose and lay in the circuit near the PI or Cap board.

 

[BPski]

Thank you very kindly. I will try looking at this over the weekend and see what happens. I will also check out the braided cable on the shock-mount.

I will post results in a day or so.

Thanks again!!!!!!

 

[BPski]

Oh, one other thing. While the amp is humming with the Pull Shift on the Bass pot pushed in (in Lead mode) the notes sound like bad filtering. Sort of a dirty note along with the good one. I'm sure you know what that sounds like. Not very musical.

Thanks again!!!!!!

 

[Boogiebabies]

Now it could be a bad filter cap. You can test these by clip leading them over the one that you think is bad. Do not discount the one on the preamp board. It could be a cold solder joint or one that has finally busted loose after all these years. I would check the bias filter caps and bias while I was at it. Also, the two 100 ohm resistors below the speaker outs should be checked. They cannot be measured in series, but when measured they do give a reading of 50 ohms to ground in parallel.
When I get an old amp I blueprint as much as possible. That way I know where to look if there is trouble later. I go as far as all preamp plate voltages, heaters, power amp plate voltage and bias at each power tube.
I also test any resistor to ground. So far I have only seen screen grid resistors and the 100K on the bottom of the preamp board off the 30uf 500V cap for the main preamp plate supply changed the most. It was originally a 1/4 watt and was mainly upgraded to a 1/2 watt. My hardwood has this upgrade and of all things, the 15K off the pull bright was replaced. Mike B. did the work in 1992.

Sounds like a typical Boogie repair checklist is in order.

 

[BPski]

OK, I had a few minutes and opened it up to take a look around, read some resistors and check things out. To make a long story short I was talking about the Pull Shift on the Bass pot which is switching a cap and resistor on V1a. I may not have explained that clearly. It turned out to be a BAD TUBE in V1. I want to kill myself! Anyway, the amp sounds excellent now. Ever since I got the amp I thought it didn't sound right but since this is the only C+ I've ever owned (but played through two briefly) I thought it was partially the way a C+ sounds but just a little tired. I put brand new tubes in when I got it so didn't think a new one out of the box could be the problem. I should know better.

Anyway, thank you very much for your input!!!

I'm rockin' now!!!