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.)
Sounds like you have a solid plan so far. Check any connectors that could be inside the amp as well.
It can be quite surprising what a good cleaning can do for an amp. I picked up a really nice Peavey Bandit once for dirt cheap that had little/no output & crackly sounds when I tried it out. I had a hunch it just needed cleaning...and that's all it did need. Once cleaned, it had nice, full output.
You have the proper cleaning fluid, right? De-Oxit or similar?
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.
Yep.
Quote from: Bear on August 03, 2014, 11:56:36 AM
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.
Tick.
Tick.
Tick.
Works for me. ;)
Be sure to give the input socket(s) solder joints an extra close scrue if they solder directly onto a PCB.
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?
This is a bit of a line ball and a personal choice. Generally I don't replace components without good reason. It also depends on your skill level because the more you do, the more you could mess up. We had an example of someone who re-capped a s.s. amp and managed to get one of the replacement main caps in backwards.
From your few posts it sounds like you have some idea of what you are doing, so if you don't have bulging ends, excessive hum, or "motorboating" low frequency instability, it really comes down to your personal feeling, that you might feel more comfortable on stage knowing your amp has been fully re-capped with new electros.
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.)
hi Bear,
I doubt they would be glued on,,Use a heat gun or hair drier to heat the knobs then carefully remove.
Yes you have to be careful as the knob can pull the whole shaft out of the pot resulting in destruction of the internals of the pot.
Phil.
Quote from: BearThe Deoxit treatment took care of most of the problems (and got my Rat into top shape, to boot).
We do not endorse the use of contact cleaner on pets.
(http://my10online.com/wp-content/uploads/2011/09/rat.jpg)
Just because knobs are not normally glued on doesn't mean that somebody later didn't use glue because it was a loose fit - you find all sorts of "fixes" hidden under knobs that were loose.
Assuming it doesn't have a grub screw, I insert a small flat screwdriver under each side and very gently rock the knob side-to-side easing it up. As
phatt says, if you get too enthusiastic you can pull the shaft right out of the pot, or bust the knob.
One of these can help pull knobs with no damage. :)
http://www.stewmac.com/Luthier_Tools/Tools_by_Job/Maintenance/Knob_and_Bushing_Puller.html (http://www.stewmac.com/Luthier_Tools/Tools_by_Job/Maintenance/Knob_and_Bushing_Puller.html)
Could be fabricated with a little ingenuity.
Or the cheaper version... I have this one. It works ok.
http://www.stewmac.com/Luthier_Tools/Tools_by_Job/Maintenance/Knob_Puller.html (http://www.stewmac.com/Luthier_Tools/Tools_by_Job/Maintenance/Knob_Puller.html)
Bear, I picked up one of these a while back, and had to wedge toothpicks under the knobs to get it to play. Do to the age, I replaced all the pots, and the jacks, (it's already apart, why not). I also replaced the 30 yo filter electrolytics, and added an aux output jack. The switch was also intermittant, so it too was replaced. I enclosed the rear to gain a little more bottom from the 10" speaker. All parts are availlable from CE, or AES.
What a great little amp!
Quote from: Bill Moore on August 13, 2014, 08:26:23 PM
Bear, I picked up one of these a while back, and had to wedge toothpicks under the knobs to get it to play. Do to the age, I replaced all the pots, and the jacks, (it's already apart, why not). I also replaced the 30 yo filter electrolytics, and added an aux output jack. The switch was also intermittant, so it too was replaced. I enclosed the rear to gain a little more bottom from the 10" speaker. All parts are availlable from CE, or AES.
What a great little amp!
Bill,
Was the aux output a speaker out or an alternate line out? I was gonna move from the hardwired speaker to a jack and plug anyway--I suppose it's no more trouble to add an aux speaker jack, too.
And definitely liking it already. In fact I've been scouring craigslist for more cheap ones. Makes sense because I want to be able to use the individual delay tap outputs in my Akai Headrush.
Bear, I just paralled the speaker output. (It really sounds great through a 15" JBL)! It uses T03 output transistors, and decent heatsinks, and a knowledgable tech said there should be no problem with a 4 ohm load.
I plan on building a matching speaker cab, and have a Marshall "stack".