Help with Sound City LB120 fix

[DerStever]

I have a SS B120 which is running at less than 1/4 of what it should put out for sound. Cannot find where the problem is. I see dramatic loss of volume in the active control section on the scope - the input and 1st stage plate is hot but soon as you get to the first volume control it seems to lose power that doesn't get added back in the next stages. Appreciate any suggestions or help with this amp. Thanks - Steve

[gldtp99]

I've dealt with many SC 120's with the active Mk4 preamp and have been only able to actually get one back in good operating condition----- the others proved to be such an extreme frustration that I converted them over to other types of preamps----- usually Hiwatt.
Here's the last one I did:

[IMG:800:600]http://i46.photobucket.com/albums/f112/gldtp99/001.jpg[/img]

[IMG:800:600]http://i46.photobucket.com/albums/f112/gldtp99/002.jpg[/img]

It sounds like this:

http://youtu.be/tqLBIkjwlD8

Here's how far I disassembled the first one :

[IMG:800:600]http://i46.photobucket.com/albums/f112/gldtp99/SC%20120%20pre%20project/NewPics022.jpg[/img]

Here's the #3 one I did that's on ebay now----- not my auction and I haven't owned this one for several yrs:

[IMG:800:600]http://i46.photobucket.com/albums/f112/gldtp99/Hiwatted%20SC%20120-3/SC120Hiwatt3001.jpg[/img]

If you can figure out how to fix these SC Mk4 active preamps I will bow to your expertise !!! It's a big job to gut one of these things and build a Hiwatt circuit into it------ but they come out as great sounding, powerful amps---- and they have a Master Volume, too !!
Let us know what you come up with.............................gldtp99

[DerStever]

I havent given up yet but we are seeing some interesting things - for example no significant voltage on the HT fuse on either side. WHen volume is turned up the amp grabs more current from the wall socket like 4 amps then we stopped it. It sounds very distorted. When signal is inserted into the reverb in with V1-3 pulled there is no signal.

I'm going to walk thru the chain again - there is a significant loss in the circuit but not sure where it happens. Suspecting a bad cap somewhere.

[Stevem]

First off replace all the plate load resistors, then lift one end of any cap that has more than 30 volts DC on the other end of it and check the lifted end for leakage, anything more than .5 VDC and you replace it.