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

[cluster]

Hello all,

I have a 1984 JMC800 2203 amp that is blowing the CT fuse off the PT. (see pictures). The fuse blows as soon as I turn the power on. NOT the standby. Standby is off. But as soon as I turn the AC on to the PT the CT 500ma fuse pops (F6CenterTap). Funny enough, the amp runs with this fuse blown. Tubes are on, etc. My plates are at about 445VDC but my bias is low. The highest I can get my negative bias is -39ma (makes me suspicious). Not sure if this is a bad cap issuer or what??? On the picture you can see where I have remove the fuse (empty slot at F6). The yellow wire runs from the PT to one end of the fuse (F6) and then out the other side to a 50/50 cap via another yellow wire. Then from that cap a red wire runs to the standby switch. That cap is also connected to another 50/50 cap next to it where a black wire that is connected to the ground then runs back to the fuse board and connects to one end of a diode (D5) which then connects to a fuse T1A, which then connects to F5HT. Any insight as to what might be causing the fuse to blow?? Thanks

PS: on the attached schematics I don't see a fuse on the secondary CT. However, clearly this 1984 version does.

[nevetslab]

The power supply wiring in this schematic looks the same as in my JCM 800 1992 diagram, again no fuse in the centertap. It could well be that there is a difference in charging current of the two series-connected buss caps. The C/T forces each cap to be at the same potential, much like ballast resistors we see on stacked supply caps. What happens if you run teh centertap to the junction of the two caps? I've actually never seen this version of the JCM 800. Without the centertap there at the junction of th tw ecaps, you don't have the assurance of them equally sharing the charging current thru the caps. What voltage do you see at the center, without the C/T connected? 1/2 of the 445VDC?

[Pedro Vecino]

The amplifier can work without the CT fuse because that connection is only to balance the voltage between the electrolytic pair. Make sure the CT fuse is slow type.
I have not seen that fuse plate on a JCM 800. I know some countries have had speecial requirements regarding electrical safety. If I remember correctly, Sweden, although it is possible that in Norway or Finland too.

[cluster]

The amplifier can work without the CT fuse because that connection is only to balance the voltage between the electrolytic pair. Make sure the CT fuse is slow type.
I have not seen that fuse plate on a JCM 800. I know some countries have had speecial requirements regarding electrical safety. If I remember correctly, Sweden, although it is possible that in Norway or Finland too.

would it be dangerous to jumper that fuse and run the amp that way? or will it just blow another fuse one on of the HT windings??

[cluster]

Ht1 ~ 190vdc
ht2~ 190vdc
ct ~ 189vdc

[g1]

Sounds like this is a CSA version. Does it only have 4 and 8 ohm settings on the rear switch, no 16 ohm?
You didn't answer if you were using a slow-blow?
I see the original values screened on the fuse board have been updated, the 2 HT fuses changed from T500mA to T1A, I would try that for the center-tap fuse, or jumper and see what your center tap DC voltage goes to. Worst case it should blow one of the HT fuses like you were wondering.
Once you get the center-tap connected and are not blowing fuses, you can get to the bias issue.

Most of the other CSA versions didn't fuse the center-tap, like this example:

[cluster]

Test the voltage across the cap and got 0. Figure the cap is shorted and thats what caused the fuse to blow.

[g1]

You said earlier there was 445VDC at the plates.
And 189V at the CT.
So how can you have zero volts now, and where?

[nevetslab]

That makes sense, based on the voltages you reported on post # 3. . The caps two individual cells are in parallel. And, assuming their voltage rating is 500V, you probably could run this with just the one cap...you'd need to ground the bottom of the good cap. At last until you order and receive the replacement cap...perhaps a new pair of them, just to be safe.

[Chuck H]

Test the voltage across the cap and got 0. Figure the cap is shorted and thats what caused the fuse to blow.

If it's the bottom cap on the totem I would say that's a good possibility. So with no fuse in the CT the amp would operate pretty much as normal, but with a 100uf filter going to a short to ground instead of the 50uf series filter. I would replace both caps and perhaps the others if they are of similar age or show any bulging or leakage. Don't forget the bias supply caps.

This condition shouldn't cause the bias supply to have a lowered range, so that's a different issue.

[Chuck H]

You said earlier there was 445VDC at the plates.
And 189V at the CT.
So how can you have zero volts now, and where?

I think that was at the fuse socket, no fuse in place, so, reading from the CT as unconnected.

[The Dude]

........This condition shouldn't cause the bias supply to have a lowered range, so that's a different issue.

If I read right and understand correctly, bias is 39mA, which is fine, IMO. I'll give you that the statement "The highest I can get my negative bias is -39ma" is confusing. Some clarification might be in order.

[Chuck H]

Yes. Confusing. But I figured if it peaks at -39V then that's probably too limited. And if the most current you can get the tubes to conduct at idle is 39mA then that's also limited. So I think something must be wrong with the bias supply (or the tubes!?!) in either case if the number 39 is a limit on voltage OR current I was planning to get to the bottom of the actual meaning if it came up

[cluster]

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.

[cluster]

yes, it was the bottom cap. It was open (no voltage going across it at idle). I replaced it with an old Daley cap I had lying around and all worked well. Played the amp full blast for 20 minutes and a fuse went again. Don't know which one. Probably the other cap died as well. Too late to take t apart again to figure it out. Do it tomorrow and report back. Need to order new F+T caps. It's only an 1984 but I guess if I replace one, I should do them all. New set of power tubes and slow blow fusesas well. Anyone recommend https://www.tubedepot.com/ for parts?? On a side note, would you guys recommend replacing those red lego signal caps with sozos?? While I'm doing this work, though I could make some improvements to the tone. thanks for the help. will report back with some updates.