Marshall JCM800 2203 (1984) Blowing CT fuse off the PT. Help.

[The Dude]

sorry. i meant the highest bias I can get is about 40ma. Maybe it's my tubes that need replacing but that is all I can get with the bias adjustment pot.

So, what is the range of available bias voltage?

[g1]

The schematic shows the different values of resistors that are used in the bias circuit, depending on whether it is set up for EL34 or 6550.
If it has the 6550 values, the overall bias range will be colder.
The one shown at the right of the bias diode will probably have the biggest impact on the range.

[Chuck H]

Figure the cap is shorted

If it's the bottom cap on the totem I would say that's a good possibility.

yes, it was the bottom cap. It was open.

Open is different from shorted. I think we need clarification. Because I responded to "shorted". None of what I said applies if the cap was open and we can't help help until we know. It's in question (at least for me) at this time.

[cluster]

Open is different from shorted. I think we need clarification. Because I responded to "shorted". None of what I said applies if the cap was open and we can't help help until we know. It's in question (at least for me) at this time.

I believe open, for when I test the voltage to ground on the two lugs tied together (to one end of the HT secondary) , I get the 452VDC but then when I test the voltage (to ground) on the corresponding negative single lug (the one that that connects to the second 50/50 as well as the fuse (no fuse in place)) I get zero volts. That tells me that no voltage is being passed across the cap. Is that mean open? I tested the screen caps and they pass voltage across. Those seem to be working.

[cluster]

The schematic shows the different values of resistors that are used in the bias circuit, depending on whether it is set up for EL34 or 6550.
If it has the 6550 values, the overall bias range will be colder.
The one shown at the right of the bias diode will probably have the biggest impact on the range.

This amp uses EL34s. Maybe at one point it had 6550s but no-one bothered to change the resistor.

[Chuck H]

...when I test the voltage to ground on the two lugs tied together (to one end of the HT secondary) , I get the 452VDC

This condition doesn't occur schematically. As far as I know there aren't two capacitor connections at either end of the HV secondary winding. The caps are in series with one caps + connected to HV, then that caps - connected to the other caps + at the HV wind CT, then the second caps - connected to ground. I don't see anywhere where there are "two lugs tied together (to one end of the HT secondary)" Unless you simply mean any connections and not specifically capacitor lugs?

...when I test the voltage (to ground) on the corresponding negative single lug (the one that that connects to the second 50/50 as well as the fuse (no fuse in place)) I get zero volts. That tells me that no voltage is being passed across the cap.

That tells me that ALL voltage is being passed across that cap. If there are zero volts at one end (ground) and zero volts at the other then the ends are shorted together, right?

If that cap were open and not shorted the amp wouldn't operate as normal. It would have considerable ripple on the output. Since this isn't reported I think the cap must be shorted. A shorted cap would also explain the blowing CT fuse. I don't think an open cap there would. You could always measure resistance across the suspected cap section lugs.

[Chuck H]

This amp uses EL34s. Maybe at one point it had 6550s but no-one bothered to change the resistor.

A good possibility.

[cluster]

This condition doesn't occur schematically. As far as I know there aren't two capacitor connections at either end of the HV secondary winding. The caps are in series with one caps + connected to HV, then that caps - connected to the other caps + at the HV wind CT, then the second caps - connected to ground. I don't see anywhere where there are "two lugs tied together (to one end of the HT secondary)" Unless you simply mean any connections and not specifically capacitor lugs?



That tells me that ALL voltage is being passed across that cap. If there are zero volts at one end (ground) and zero volts at the other then the ends are shorted together, right?

If that cap were open and not shorted the amp wouldn't operate as normal. It would have considerable ripple on the output. Since this isn't reported I think the cap must be shorted. A shorted cap would also explain the blowing CT fuse. I don't think an open cap there would. You could always measure resistance across the suspected cap section lugs.

Here is a photo with text to explain. Let me know if that helps. Maybe it is a short and I'm just not looking at it right.

[nevetslab]

I believe open, for when I test the voltage to ground on the two lugs tied together (to one end of the HT secondary) , I get the 452VDC but then when I test the voltage (to ground) on the corresponding negative single lug (the one that that connects to the second 50/50 as well as the fuse (no fuse in place)) I get zero volts. That tells me that no voltage is being passed across the cap. Is that mean open? I tested the screen caps and they pass voltage across. Those seem to be working.

That second cap is shorted, NOT open.

[g1]

Chuck: each 100uF shown on schem. is 2 sections of 50/50 can paralleled. So that's what he meant about the lugs tied together.

This amp uses EL34s. Maybe at one point it had 6550s but no-one bothered to change the resistor.

So does it have the 6550 resistor values or the ones for EL34's? Unless you owned the amp since new, anyone could have changed the resistors at some point. You need to verify what is there now. Lots of metal guys liked to change them over to the 6550 "US" versions.

[cluster]

Chuck: each 100uF shown on schem. is 2 sections of 50/50 can paralleled. So that's what he meant about the lugs tied together.



So does it have the 6550 resistor values or the ones for EL34's? Unless you owned the amp since new, anyone could have changed the resistors at some point. You need to verify what is there now. Lots of metal guys liked to change them over to the 6550 "US" versions.

Thanks for that. I'll check.