'82 JCM 800 4104 Service and Mods

[physics]

Hey Physics

I wouldn't jump right to replacing caps yet; that is an easy cash grab techs do particularly when they do not know what is wrong with the amp.

The amp is a simple MV 800-style with a rather spacious chassis. If online photos are to be believed the MV is a standard type, although by that time some models used a PPI-MV with a dual pot - terrible for clean sounds.

As far as "no sound" goes, there are lots of reasons for that. The most expensive ones to rule out (easily) are an open OT or open speakers. You can check both with an ohmmeter and the amp unplugged and caps discharged. More typical reasons are cold solder connections, oxidised jack contacts, dirty pots and similar things.

If you intend to keep the amp, adding proper bias controls and meter jacks would be the first mod to do. It allows you to use alternate tube types and to mix tubes, plus easily set the bias to a safe level for any tube and hum-balance the output stage. BMK2 as our kits go, or get the parts from the usual places.

It would be handy to have a power limiting safety socket for the electrical testing

Have fun

That's a good point, and yes I've been suspicious of the diagnosis from the previous tech. There were some symptoms before it stopped working that seemed possibly more in line with a bad connection somewhere, but I don't know the exhaustive list of issues caused by bad filter caps. The issues I was seeing were that sometimes the output would get quieter, more distorted in a crackly/noisy sense, and a little intermittent before fixing itself up. At one point the amp made those sorts of sounds with the guitar unplugged while powering off. The plan is to check the things you mentioned, and then trace a signal through the amp with the MV turned down and find out where it stops. It happens on both input jacks, so it seems it's not a jack, but could be wrong. I recently found out that my guitar's wiring has issues as well, so until I fix that there is still some unknowns regarding what was the guitar and what was the amp.

As for the master volume, it's a single gang pot. The 100k resistor mod is in the queue for after I get this working again, and I'll be posting questions about that down the road (in short, the transfer function I've (mis?)calculated doesn't match the description in TUT & TUT4, or the master volume thread on this board).

I'd like to do the bias mods and stuff as well, and I have the kit back at my old place so it shouldn't be hard to get it mailed out. Power Scaling would be cool too, and useful in my apartment/non-stadium life. Despite my dislike for most Marshall sounds I've heard, this amp is quickly becoming my favorite amp I own and maybe played too. Perhaps I just don't like Celestions, and the EV's in this amp are the reason I like it. I certainly liked a friend's rig a lot more after he switched to EV's.

The PLSS is a good idea, mine is still back home. I'll have to dig up some parts at the local Ace Hardware for that.