Brian and Roly, thanks for the guidance.  Thankfully, easy enough going so far.

The Deoxit treatment took care of most of the problems (and got my Rat into top shape, to boot).  Inputs and the headphone/line out are working fine, I've got gain and volume in the sorts of proportions I expect, and things are mostly working as I would expect.  Except for the gain pot.  Only conducting predictably all or most of the way up.  I can get by with that for the moment, but a replacement will be part of my next parts order.  But sounds quite nice for the $40 I've got in for it and the Deoxit.

Annoying thing is only the gain knob comes off easily so far, so I haven't gotten to removing the board, as I will have to do for the pot replacement.  Does anyone know if Marshall was gluing on their knobs in this era?  (This one's an 85.) 

If I can get the board out, I'll to the electrolytics, too, to see if that will help on the hiss level.  I've done some pedal builds and mods, some mods on my Pathfinder, soldered cables, guitar wiring stuff, so I'm not too worried about working on the board.  The PCB looks like the same quality as the JCM amps of the same era with good layout and wide traces, much more friendly then some of the stuff on the Pathfinder.  (The amp is built simply but very solid, with a nicely built chassis.  I get why guys like to use them as donors for tube amp projects.)

Quote from: Roly on August 04, 2014, 10:34:38 AM
Be sure to give the input socket(s) solder joints an extra close scrue if they solder directly onto a PCB.

They do.  Jacks and pots are all board mounted, but the jacks look like the biggest potential for stress.  New off-board jacks were gonna be my first plug-n-pray in my sequence.

Does the conventional wisdom I've learned apply, that it's a good idea to replace the electrolytics whether needed or not in a 25-30 year old amp?

I was just doing my homework on cleaners.  Deoxit should work for both jacks and pots, right?  Long overdue to get some--my Rat has had scratchy pots for too long.

Hi, all.  I'm gonna be the total newb by joining just to ask for help.  (Though I've lurked for a while and downloaded Teemuk's book a while back.)

Picked up a Marshall Lead 12 in "needs repair" condition.  Powers up, plays, passes audio, all jacks work, output works, all pots seem to do something.

The issues I'm having are low output, low gain, and lots of static with either input and as the knobs turn.  I haven't removed the board from the chassis yet, but component side seems clean, with no obvious issues on the electrolytic caps.

What would your order of operation be in my shoes?

My thought is first try a good clean of input jacks and pots, and a check of the board to see if the traces and the solder joints are holding up.  After that, I guess my next moves are checking that the pots aren't shot, but after that, I'm basically in replace-and-pray.

(Hoping I can get this guy rocking next to my LED-mod'ed Pathfinder 15R for a bedroom-level My Bloody Valentine rig.)