Marshall JCM2000 Fault

[Ant]

Hi Everyone,

I'm new to the site and in the search of some help.

I'm not familiar with working on amp repairs (televisions is my normal scene) but a good friend of mine has asked me to have a look at his amp for him and I'm kinda obliged to do what I can - who knows I might even take it up as a hobby!

The amp is Marshall JCM2000 DSL of Sept '03 vintage.

The problem is that the amp powers up ok, tubes are heating up etc. When the volume is turned up you can hear the hiss level go up on the speaker cab but there is no input amplification when the guitar is plugged in i.e. the guitar can't be heard at all.

Things to note are that my friend has replaced all the tubes (power tubes have been replaced by EL45's and I'm not sure what was originally installed but the fault existed with the previous tubes anyway) and he has also replaced the output transformer after talking to some repair shop guy who said that the fault sounded like a Tx problem.

As I said before, I'm not familiar with working on amps so if anyone could give me a starting point to check basics I would be very grateful.

P.S. I have already removed the amp main board and checked for dry joints etc.

[Amp Kat]

Well as I've said on this BBS several times one of the best things to do is isolate the problem to either preamp or poweramp and then go from there. If you take the signal out of the send to another amp and it works it's in the power amp section and you can disregard the preamp. Since you have the old transformer out why don't you do a simple resistance check on it to see if the old one really is bad. There are three wires a red, white and either brown or yellow I can't quite remeber which color but that's the primary side. If you take your ohm meter and read across these wires you should get around 16.5 ohms on two of these and from one wire to the other two will be around 35 ohms (center tap). If you get that the tranny is good and it wasn't the problem. I've had lots of trouble with the WB008 rectifier that goes from power tubes to rectifier to DC for the preamp heaters on the 12AX7's I'd change that no matter what. When you get the board back in up and running check pin 3 of the power tubes for 500 volts dc then check pin 5 for the negative grid voltage around -48 vdc. If still no sound you might need a signal generator and scope and start following the signal and see where it disappears. Shouldn't be to hard for a TV tech as I have repaired both and tv's are much harder than amps ! and more dangerous,

[Enzo]

SInce you can hear hiss, and it is controllable, then I have to think most of the amp is working. Without looking it up, on a lot of Marshall amps, the amp is muted without something plugged into the front. So in some cases, you have to have a plug if some sort into the front input even to apply a signal to the effects return.

By the way, JCM2000 is not very specific. It means a series of models. It is about like "Chevrolet." Even DSL is not specific. DSL100? DSL401? DSL201? I am assuming DSL100 for now.

EL45? Don't know them. EL34? EL84?

MAke sure the MUTE button is not pressed in.

In any case, I would plug something into the front and then connect a signal to the effects return jack. This more or less inputs right to the power amp - you are testing the back half of the amp.

Do you have the drawings? I think Larry has them online at Schematic Heaven.

First thing I'd do would be check at the input. Follow the traces back from the input jack. Furst cap it comes to - is it broken free at one end?

Are ALL the tubes lighting? Make sure the small ones on the end closest to the input jack are lit. MArshalls are famous for loss of solder contact on the bridge rectifier for preamp heaters. The end couple of tubes will be dark.

On the 12AX7 tubes, pins 2 and 7 are the grids. With the amp operating and something plugged in front, touching those grids with a small screwdriever or even most of the time just a meter probe - especially if the other probe is not grounded - will inject a hum signal you can easily hear out the speaker. That is a quick go/no go.

Apply a signal to the front, and trace it with a scope, tube to tube. No scope? AC voltmeeter.

[Ant]

Thanks for the help guys. I'll get a look when I get back this evening and keep you posted on what I find.