Marshall JCM2000 DSL100 Blown HT Fuse.

[gtrplayr1976]

Hey folks. I'm working on a JCM2000 DSL100 for a friend. It had no sound when I got it. HT fuse was blown. I replaced the HT fuse ,and it powered up ,but bias was crazy ( I know this is a known issue) The board is revision 4 I think. I bought a new board (revision 20) ,and new tube set. (the old quad was two sets of different tubes) Replaced the board ,and tubes. Biased just fine ,but I have no sound. I'm thinking I got some cables in the wrong places even though I wrote them down I could have easily made a mistake. I tried using the schematics I have ,but they don't seem to match the ones I KNOW I had connected right? I'm guessing I have a cable in the wrong spot ,but I need the right schematics to check them.
UPDATE: I think I found the right connection table ,and schematics. So everything seems to be connected correctly according to that ,but I still have no output only a low hum ,and when I was checking voltages on the power tube V5 pin 3 there was some noise as I was touching the pin (sounded like a relay turning off ,and on) ,and arcing to the probe tip then the HT fuse blew again? I tried to upload the schematic I'm using , but it has failed every time.
Marshall 2000 DSL_full.pdf

[The Dude]

I'd pull the power tubes and measure voltages on the sockets. You may detect something. If you are blowing the HT fuse, the problem is most likely right at the output section of the amp. Also, it might help to upload a picture of your installed board and wiring. Someone may spot a problem.

[gtrplayr1976]

I'd pull the power tubes and measure voltages on the sockets. You may detect something. If you are blowing the HT fuse, the problem is most likely right at the output section of the amp. Also, it might help to upload a picture of your installed board and wiring. Someone may spot a problem.

I have a few gut shots ,but not a good single one of it all. I'll try to get one ,and get it posted. I'll pull the tubes ,and check voltages ,but I have nothing to compare them too since there are no voltages listed on the schematics. Thanks Dude !

[Jazz P Bass]

Pin #1: NC
Pin #2: Htr
Pin #3: HV
Pin #4: Scn
Pin #5 Bias
Pin #6: NC
Pin #7: Htr
Pin #8 Grd

Heater voltage is Vac: Measure from pin #2 to #7
High Voltage& Screen 500 some Vdc Pin #3 & #4
Bias: -40 some Vdc pin #5
Ground: confirm pin #8 has zero ohms resistance to the chassis

[The Dude]

You're primarily looking for plate, screen and grid voltages and that they are reasonable- nothing horribly specific at this point. I'm assuming filament voltage is there and that tubes are lighting up, but that's another thing to check.

Edit: Jazz snuck in while I was typing.

[gtrplayr1976]

Pin #1: NC
Pin #2: Htr
Pin #3: HV
Pin #4: Scn
Pin #5 Bias
Pin #6: NC
Pin #7: Htr
Pin #8 Grd

Heater voltage is Vac: Measure from pin #2 to #7
High Voltage& Screen 500 some Vdc Pin #3 & #4
Bias: -40 some Vdc pin #5
Ground: confirm pin #8 has zero ohms resistance to the chassis

Will do. Thanks Jazz!

[gtrplayr1976]

You're primarily looking for plate, screen and grid voltages and that they are reasonable- nothing horribly specific at this point. I'm assuming filament voltage is there and that tubes are lighting up, but that's another thing to check.

Edit: Jazz snuck in while I was typing.

Yep. Filament is good. I'll try to check the rest tomorrow after all of my appointments. As far as I got today was V5 Pin 5 -43.4 , Pin 4 464 ,and pin 3 blew the fuse. (I didn't pay attention to a voltage reading.)

[g1]

Are you using slow-blow fuse for the HT?
Odd it would blow just from probing power tube plate, maybe it is setting off an oscillation.

[Helmholtz]

Oscillation when probing power tube plates can be avoided by pulling the PI tube.

[gtrplayr1976]

Are you using slow-blow fuse for the HT?
Odd it would blow just from probing power tube plate, maybe it is setting off an oscillation.

I think the fuse I was using was 1A medium blow? I have new 1A slow-blow fuses.
That's what I was thinking. I've never even had a noise from probing power tubes before ,but this one on anode was bad. Screen grid ,and control grid were fine. I had a slight hum on the amp ,and it seems this was affecting it as well. Was showing about 1.3 VDC on the meter with just the ground connected?

[gtrplayr1976]

Oscillation when probing power tube plates can be avoided by pulling the PI tube.

Thank you. I'm going to pull the power tubes first ,and check voltages to see if anything looks funky then I will try pulling the PI ,and checking again.

[gtrplayr1976]

Pin #1: NC
Pin #2: Htr
Pin #3: HV
Pin #4: Scn
Pin #5 Bias
Pin #6: NC
Pin #7: Htr
Pin #8 Grd

Heater voltage is Vac: Measure from pin #2 to #7
High Voltage& Screen 500 some Vdc Pin #3 & #4
Bias: -40 some Vdc pin #5
Ground: confirm pin #8 has zero ohms resistance to the chassis

I pulled the power tubes to check voltages. I STILL had a loud arc on the anode of the power tube ,so I shut down to look at other possible issues. I also checked the resistance from pin 8 to ground ,and they all read 1.3 ohms.

[Helmholtz]

I pulled the power tubes to check voltages. I STILL had a loud arc on the anode of the power tube.

What? Power tubes pulled but still arcing?? And where?
No tubes no anodes.

I also checked the resistance from pin 8 to ground ,and they all read 1.3 ohms

Makes sense as there are 1R cathode resistors.

[gtrplayr1976]

I'd pull the power tubes and measure voltages on the sockets. You may detect something. If you are blowing the HT fuse, the problem is most likely right at the output section of the amp. Also, it might help to upload a picture of your installed board and wiring. Someone may spot a problem.

I pulled the power tubes ,and STILL got the loud arc from pin 3 HV of the power tube socket. I pulled all of the boards today to look for bad solder joints ,and such. Everything looked pretty good. I made a couple of small improvements to the rear board that I got from Lyle Caldwell. I DID find one bad cap on the fuse board C2. Should be .022uf ,and was reading .04nf. I also did some simple tests on the OT. Something strange I noticed. If I checked resistance from any of the OT secondaries to ground they all showed near 0 resistance ,but if I disconnected the leads the OT terminals to ground show open ,as do the leads to the board? That doesn't make sense to me. I also checked the primary side ,and they were all open to ground. I checked resistance between the primary leads blue (CT) to each of the other leads was around 16.4 ohms. The two together were 33.4 ohms. The secondaries to common were all about the same .1-.2 ohms ,and 0 between each of the wires. I also checked the transformer by putting around 10 volts AC into each wire of the secondary ,and reading the result on the primary. They were Red 52.9 VAC , Orange 73 VAC ,and Green 103.3 VAC. Shouldn't these be a lot higher?

[gtrplayr1976]

I pulled the power tubes ,and STILL got the loud arc from pin 3 HV of the power tube socket. I pulled all of the boards today to look for bad solder joints ,and such. Everything looked pretty good. I made a couple of small improvements to the rear board that I got from Lyle Caldwell. I DID find one bad cap on the fuse board C2. Should be .022uf ,and was reading .04nf. I also did some simple tests on the OT. Something strange I noticed. If I checked resistance from any of the OT secondaries to ground they all showed near 0 resistance ,but if I disconnected the leads the OT terminals to ground show open ,as do the leads to the board? That doesn't make sense to me. I also checked the primary side ,and they were all open to ground. I checked resistance between the primary leads blue (CT) to each of the other leads was around 16.4 ohms. The two together were 33.4 ohms. The secondaries to common were all about the same .1-.2 ohms ,and 0 between each of the wires. I also checked the transformer by putting around 10 volts AC into each wire of the secondary ,and reading the result on the primary. They were Red 52.9 VAC , Orange 73 VAC ,and Green 103.3 VAC. Shouldn't these be a lot higher? First photo shows the C2 capacitor I replaced. The second are the small mods I made that I saw on one of Lyle Caldwell's videos.