Marshall JCM 2000

[keysnleslie]

This amp comes to me with the condition being "no sound". It does have a bit of output but nowhere near normal. The odd part is,there is full power thru the effects loop,and with signal generator at many points past the front end (input section) therefore I suspect the problem to be very close to the input. All components are within spec except the 4558 chip which I cannot static test. Tubes have been ruled out by replacement with new ones,as have cold solders. The amp was new in 2004 and was used sparingly.

[keysnleslie]

more info

I meant to say cold solders were ruled out on input/control board.

[Twist]

Check pins 4 and 8 of the 4558 op amp. You should see +/- 15 volts. Typically there should be no voltage on pins 1 and 7. Also make sure the input jack is soldered properly. Another common problem amps is falling onto the floor and cracking the backside of some of the pots. I've seen controls that looked fine on the outside but the little carbon trace inside the pot gets cracked. Good Luck .

[keysnleslie]

Thank you,How are the pins numbered on the 4558? I do have 15 volts positive and negative on pins at opposite corners of the chip.

[Amp Kat]

Thank you,How are the pins numbered on the 4558? I do have 15 volts positive and negative on pins at opposite corners of the chip.

Those are indeed the places where the rail voltages are (+/- 15 vdc). Where is the bias set at ? Are the power tubes all lit ? Try running a cable from the send to return jack. Take the signal out of the send jack and run it to another amps input. Full volume ? This is where a scope really comes in handy with a signal generator of some type.

[Enzo]

First do us all - or at least me - a favor and tell us the model of the amp. JCM2000 doesn't say much. Is it a TSL100, DSL100, DSL401, DSL201? WHat?

You have signal out the effects send, so the preamp is working. You have no output, so the power amp is not.

Do you have B+ on all power tube plates? B+ on ALL power tube screens? What is on all the power tube grids? Is there continuity to ground from all power tube cathodes?

Misbiased tubes still make sound. if the bias goes away it will work, but the tubes will get red hot and start to melt. If the bias goes way high it will pinch off the amp, but there is not enough voltage in the supply to put it into cutoff.

Is there signal at the grid of the phase s[plitter? At its plates? is there signal at the power tube grids? Scope these. If you have no scope, set your meter to AC volts and chase it that way.

If there is signal on the power tube plates, the amp should be working. Look on the speaker side of the output transformer then. MAke sure the wires are on the output jack board properly. FLip the speaker impedance switch around in case it has lost contact.

And of course pull the output transformer wires off and clip in another OT. It doesn't have to be an exact replacement OT, I have an old Fender Bassman one I use for this test. We don't care about impedance, we want a go/no go test. If my bassman tranny makes loud sounds and the stock one doesn't, then the stock one is bad.

RG has a transformer tester on his Geofex web site. Not to mention a tube amp debugging guide.

[thissceneisdone]

jcm 2000 dsl prob

Hey i bought a used dsl about 3-4 years ago, and to my knowledge that guy bought it new and had it for a couple years. Anyway, its bean giving me a little trouble this past year, and it has been gigged pretty hard i will admit. But my last problem leading to this starts as i replaced my power tubes with el34's, and the next time i turned my amp on i kicked the main power fuse..so i replaced it promptley. Now when i turn the standby on after letting it warm up for a couple minutes i got a little popping and a lot of heat coming from the middle pre amp tube and the third gets very bright, and also some light smoke comes from the middle tube. I also havent done much maintenance(if there is anything i could or should do besides tubes, please let me know) or havenst played it in over 4 months. I slid out the chassis, and discovered no burnt wires or solder joints...do i just need to resolder the main joints? or should i just take it in and get it revamped..anyone else had these problems or know whats up? any help please...thanx dustin

[keysnleslie]

Thank You gentlemen,Once again I am sucessful thanks to all who responded. The defective part was a plate supply resistor on the first preamp tube. I must say this amp is not very "service friendly" with the way the main board needs to be removed.