marshall jcm 2000 dsl 50 problem

[cudascott]

Hey to everyone I'm new to this site. I have a problem with my amp
I went to practice and turn on the amp with it in standby and let it warm up
as usual for a couple of minutes. plugged in took it off standby as soon as i
started playing it died! well i found a blown main fuse so I replaced it
powered it back up the fuse didn't blow but now I don't get any sound at all
out of the amp. The tubes do still glow and I have looked inside and the heater fuses are all good. I don't know how old the tubes are because I
bought the amp used about 5 months ago. And it dose have the correct rating on the main fuse. any ideas what to check out
thanks scott

[52 Bill]

... well i found a blown main fuse so I replaced it
powered it back up the fuse didn't blow but now I don't get any sound at all
out of the amp. The tubes do still glow and I have looked inside and the heater fuses are all good.

Have you checked all of the fuses? From your description, I would suspect the high tension fuse. Also, did you check the fuses with a meter? Sometimes they look fine, but are open.

[cudascott]

Hey thanks for the help,
I did check the ht fuse and the heater circuit fuses with a meter
I also tried a patch cord through the effects loop but still no sound
I did notice I could hear some sort of viberating in the output xfmr
when I struck the notes on the guitar and I could just barely hear it in the speakers. any other ideas?
thanks for the help
Scott

[52 Bill]

I did notice I could hear some sort of viberating in the output xfmr
when I struck the notes on the guitar and I could just barely hear it in the speakers.

Did you check the speakers, the speaker cable and the speaker output jack? What you just described is common when there is either no speaker hooked up or a shorted speaker or cable. Try checking all of that.

[cudascott]

I did check the speaker cab with a friends amp head and it worked fine
I also checked the cable it checked out ok also. I looked the solder joints over good on the two speaker output jacks on the output board they looked solid and the contacts on the jacks look normal and the plug fits snug.
I did check the tube plate voltages the preamp tubes were around 300v
and the power amp tubes were around 500v. Does anyone have a good
schematic on this amp head?or know where to get one
Thanks for all your help so far
Scott

[52 Bill]

I did check the tube plate voltages the preamp tubes were around 300v and the power amp tubes were around 500v. Does anyone have a good schematic on this amp head?or know where to get one

There is a whole set at http://www.drtube.com/

Have you checked the heater voltages? There is a separate supply for the pre-amp tubes V1 and V2 which is DC, the rest of the tubes run on AC. And just to be sure that you checked all of the heater fuses, there are 3 one for each output tube and one for the rest of the tubes.

[R ski]

I fixed some similar recent Marshall a few years ago. Go to schematic heaven web site, the closest document I could see is the jmc2000 100dsl, the extra output tubes would be additional, but the signal path to the power amp should be close.

So looking at the power tubes the filament are fused, seeing that is easy to check. The next step is to verify if the bias juice is conducting through the tubes, Basically pull the plug, wait then measure resistance from center tap (HL) to either H2 then H3 of the output transformer, record this number, my experience guess is approx 60 ohms give and take either way H2 or H3. When the speakers are plugged in the transformer will reveal a soft scratch when the meter probes are making contact.

Now fire up the amp and warm up and take out of stand by, using the meter on the same points you done to the resistance, now carefully probe the voltage from HL to either H2 or 3 DC around 1.5 to 3 volts. The should exists across both measurements. Using a ohms law I = E/R, the juice can be determine, the tubes should yield .025 amps 25 mA to 35 mA.

Since you mentioned a blown fuse, the most evident failure could of being in this stage.

[tim]

Doesn't this amp have dc filament voltage on V1-3? If so, it's possible that the solder joints on the bridge rectifier for that circuit have crapped out and need to be reflowed.

[cudascott]

Thanks for all the help from everyone. R ski I will check the output transformer for the resistance and voltage. I did check heater voltage
circuit to tubes and found 3.1 v.
thanks scott

[52 Bill]

If the output transformer checks ok and if the heater voltages are all ok, then it's time to inject a signal and follow it through the amp.

Is there a good signal out from the FX loop? What happens when you inject a signal into the FX loop return?