All of a sudden when I turn the bass knob is has no effect on the tone...the knob is attached firmly and it rotates freely
I bias all my amps, but have never encountered this before
what should I check for?
The amp has recently had a PTP board installed, along with the Lee Jackson Volume 2 mod as well as his Tube Buffered FX loop, it was all working fine, I had a nice bias at 39...then one day i noticed a significant loss of volume, this was traced back to a power tube moving around in it's socket...this has since been fixed
Thanks In Advance

This message has been edited. Last edited by: Shwing,

I'd take it to a tech.

Shwing;

This could be anything but sounds likely to be a bad solder con. (a leg of the bass pot broke free maybe)especially if its had a mod recently. Or, a component gone open perhaps. Hard to say without seeing it.

Did someone do the mods for you ?(eg. PTP board, Jackson Vol and Tube buffer)
If so, have you talked to them ?

the gentelmen who installed all the mods has since moved, it sounded incredible until the bass issue, also the presence knob emits a low bassy scratching when it turned, what should I do about that..I have Deoxit ect
thanks

Schwing;

Try Deoxit on the presence pot. If that doesn't do it, you probably have soldering issues. The legs like to pull off of the PCBoards if they're
directly soldered.

ampdoc

deoxit didn't work, I also noticed the presence control has no effect on tone as well :-(

these are all new components, brand new ptp board ect...

here is a page on the build of my amp:
http://s169.photobucket.com/albums/u217/Ivabiggun/JCM%20800%20fx%20loop/

here are the boards he makes:
http://stores.ebay.com/HurlesSound-Amplification

Shwing;

It's too bad this guy moved away.
Did he leave you with any schematic documentation ?

It sounds like something failed on his board.

Now you've got to pay another tech to "sift thru" and either repair or undo the job.

finally! I nailed the problem down, a solder connection has broken
I took a bunch of pic's of the amp so you can check it out

link to pictures of the Marshall :
http://www.divshare.com/download/5722033-5a0

It is a 1986 Marshall JCM800 8104 50 watt chassis with the following mods
PTP board with Sozo's
Lee Jackson Marshall Mod Volume 2 Mod
Lee Jackson Marshall Tube Buffered Effects loop
25/50 watt Mod
BIAS points installed at the back with lockable adjuster

NOW I have a bigger problem
I guess one of the kids was playing with my multi meter, I didn't check and it was set to ACV750!
I leave it at DCV200m
I herd a pop and I might have seen a flash, not sure though

Now the amp constantly blows the Mains T3A fuse
Where should I start looking?
I'm ready for the bad news....sigh :-(

Thanks Again

This message has been edited. Last edited by: Shwing,

Shwing;

I'm not sure that having a meter at the wrong setting would damage anything but the meter.
Did you happen to short anything in the amp with a probe?

Anyhow, if she's blowing main fuses, I think the 2204 is the one with the discrete rectifier diodes (instead of a bridge rectifier) off of the secondary of the power transformer. It's likely that one or more of these diodes are shorted.