Marshall JCM2000 DSL100 Blown HT Fuse.

[The Dude]

The OT secondary windings resistance is very low. It will be a near short reading on a DVM. That is normal. Connected to the circuit, you are reading through the winding to ground. Did you measure power tube voltages yet? If so, please post your readings.

[gtrplayr1976]

The OT secondary windings resistance is very low. It will be a near short reading on a DVM. That is normal. Connected to the circuit, you are reading through the winding to ground. Did you measure power tube voltages yet? If so, please post your readings.

I WAS getting power tube voltages until I got to the anode (pin 3) of the first tube ,and that loud pop ,and arc came through the speaker so I stopped. That was with tubes out. I was afraid of doing damage to the new board. I'll try again tomorrow with the speaker hooked to my Weber attenuator ,so I can do it without hearing that loud pop.

[The Dude]

If you have the output tubes removed, you don't need a load.

[Helmholtz]

I WAS getting power tube voltages until I got to the anode (pin 3) of the first tube ,and that loud pop ,and arc came through the speaker so I stopped. That was with tubes out. .

I don't understand what you're measuring.

Anodes are integral parts of the tubes. So with all 4 power tubes out there are no anodes.

Are you measuring at empty sockets?

[gtrplayr1976]

I don't understand what you're measuring.

Anodes are integral parts of the tubes. So with all 4 power tubes out there are no anodes.

Are you measuring at empty sockets?

Sorry yes the empty socket. Pin 3 where the anode WOULD be.

[gtrplayr1976]

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



Makes sense as there are 1R cathode resistors.

Still making a loud pop ,and arcs at pin 3 with power tubes out ,but didn't blow the HT this time. That cap I replaced on the fuse board may have been the cause of the HT blowing?

[Helmholtz]

If you have 16.4R between each pin 3 and OT CT, there's no need to measure pin 3 voltages.
With tubes out pin 3 voltages will be identical to B+ and with tubes in a couple of volts lower.

What is your B+ measured at the OT CT?

[g1]

That cap I replaced on the fuse board may have been the cause of the HT blowing?

Possibly. If the cap was shorting before it went open it may have done it. The schematics don't really show where it goes, but your picture shows it definitely connects to the fuse.

[gtrplayr1976]

The OT secondary windings resistance is very low. It will be a near short reading on a DVM. That is normal. Connected to the circuit, you are reading through the winding to ground. Did you measure power tube voltages yet? If so, please post your readings.

Here are the voltages I got on the power tube sockets with power tubes ,and preamp tubes removed. All tubes were pretty close to the same reading. With preamp tubes in the voltages dropped a little. 526VDC on pin 3. Never got the arcing today either ,but also the bias at the test points was really low. about 15mA per side. I had it set at 70 initially ,and now it won't bias past 30ma per side with pots maxed? Something must have failed last time it blew that fuse? Tried another quad of tubes as well. Same result.
Pin #1: NC =0
Pin #2: Htr = 3.77 VAC
Pin #3: HV = 545 VDC
Pin #4: Scn = 535 VDC
Pin #5 Bias = -48 VDC
Pin #6: NC
Pin #7: Htr = 3.77 VAC
Pin #8 Grd = 0

[gtrplayr1976]

If you have 16.4R between each pin 3 and OT CT, there's no need to measure pin 3 voltages.
With tubes out pin 3 voltages will be identical to B+ and with tubes in a couple of volts lower.

What is your B+ measured at the OT CT?

I actually got the measurement at pin 3 with all preamp tubes installed. 526 VDC.

[gtrplayr1976]

Possibly. If the cap was shorting before it went open it may have done it. The schematics don't really show where it goes, but your picture shows it definitely connects to the fuse.

Here is the part of the schematic that shows the C2 cap.

[gtrplayr1976]

I got all the tube voltages checked with the preamp tubes in, and no arcing today ,but when I installed the power tubes I couldn't get the bias up to where it should be. When I first biased it after installing the new board I had it set at 70mA per side. Now it is at around 15mA. I even tried another quad of tubes ,but same result. Something must have fried ,but I didn't see any indication. Nothing burned.

[gtrplayr1976]

This is a noise I have as well. Increases as I increased the bias adjustment.

[The Dude]

Your output tube (socket voltages) look reasonable. Do you have a scope? If so, I'd insert a test tone and follow it through the amp to see where signal is getting lost.

[g1]

when I installed the power tubes I couldn't get the bias up to where it should be. When I first biased it after installing the new board I had it set at 70mA per side. Now it is at around 15mA.

In the video it looks like you have the meter set to measure current? You should be measuring milli-volts DC at those test points.