Marshall JCM2000 DSL100 High Current Draw

[misterc57]

Picked up a non working JCM2000. Seller said it does not turn on.

Discovered missing mains fuse, and both fuse holders needed to be reseated and soldered with cold joints.

Test time. Standby in standby position. Dial up the variac and the current draw jumps immediately at just a few volts of input.

Looking at the schematic I am taking readings across primary and secondary of the PT and OT all in circuit. Have the desired low resistance all over. No open readings.

Checked the 8 diodes on the main board in circuit, all read good. Cannot find a board layout.

Schematic shows the heaters connect at W7 and W8 on the main board. W8 has no wire connected to it. Thought that was strange but in watching a repair video on this same amp I also saw no connection at W8. W8 in picture is at the top center above the purple and gray wires.

Suggestions?

Thank you! MC


Marshall-JCM2000-100W-DSL100-Schematic.pdf

[Helmholtz]

Looking at the schematic I am taking readings across primary and secondary of the PT and OT all in circuit. Have the desired low resistance all over. No open readings.

That test won't reveal shorted turns.

Schematic shows the heaters connect at W7 and W8 on the main board. W8 has no wire connected to it. Thought that was strange but in watching a repair video on this same amp I also saw no connection at W8. W8 in picture is at the top center above the purple and gray wires.

Are you sure there is not another W8 on the board with the mains socket? Identify both heater wires and see where they connect.


Try to be methodical.

Pull tubes, any change regarding mains current?

To test the PT, disconnect all secondary wires (make sure they can't short) - any change?
If not, the PT is the culprit.
If yes, reconnect one secondary winding after the other to see which causes high current draw.

BTW, the 2 black filter caps are bulging. Means they are probably leaky.

[The Dude]

......BTW, the 2 black filter caps are bulging. Means they are probably leaky.

Possibly so, yes. I've also seen where just the plastic top cover looks bulged, but the cap is fine. If suspect, I'll cut the plastic cap off so you can see the top of the aluminum cap.

[Helmholtz]

Possibly so, yes. I've also seen where just the plastic top cover looks bulged, but the cap is fine. If suspect, I'll cut the plastic cap off so you can see the top of the aluminum cap.

Ok, possibly not probably.

They should be tested anyway.

Have you ever seen new caps with bulged plastic tops?

[Valvehead]

I would pull the speaker out pcb and the main pcb..its fairly easy , just draw a diagram of where all the connectors go . .jcm 2000 series are notorious for cracked joints . Im not saying thats your issue but its worth doing

[The Dude]

Ok, possibly not probably.

They should be tested anyway.

Have you ever seen new caps with bulged plastic tops?

Yes. I agree they should be tested and inspected further and, yes- I have seen relatively new caps with the plastic tops (only) bulged.

[g1]

Are you sure there is not another W8 on the board with the mains socket? Identify both heater wires and see where they connect.

Looks to me like one of those brown wires near the IEC socket is going to a W8 ?

I have also seen good caps with bulging plastic tops, but aluminum is not bulging.

[g1]

Schematic shows the heaters connect at W7 and W8 on the main board. W8 has no wire connected to it. Thought that was strange but in watching a repair video on this same amp I also saw no connection at W8. W8 in picture is at the top center above the purple and gray wires.

You must use the correct revision schematic. Attached is a different version that uses different numbers for the designations. Find the right revision for your amp.

[misterc57]

Looks to me like one of those brown wires near the IEC socket is going to a W8 ?

Yes there is a W8 on the small PSU board. I was trying to find W8 on the main board. However, I did not realize there were different schematics for different versions.

Tomorrow I will try to determine which board versions I have and find the proper schematics. As well as more analysis from feedback so far.

Thank you

[nevetslab]

I would pull the speaker out pcb and the main pcb..its fairly easy , just draw a diagram of where all the connectors go . .jcm 2000 series are notorious for cracked joints . Im not saying thats your issue but its worth doing

Assuming you verify the power xfmr is ok (after disconnecting the secondary wires & tubes disconnected.....the rear panel PCB over time is a traditional collection of solder joint fractures. I also find that on the bias PCB, though to get at that, you do have to lift out the main PCB, which, of course, is a PITA. When I get back to the shop in the morning, I'll post a chart that lists all of the jumper connections (to/from). Their schematics do make it crazy to navigate all those pages.

[misterc57]

After disconnecting tubes and the secondary wires, no change, still high current draw.

The following readings were taken with the primary wires lifted off the board,

The primary red to red is open. I believe this is the 117 V mains connect, W1 and W6.
The red to brown has continuity.
The red to blue has continuity.
The brown to blue is open.

[Helmholtz]

After disconnecting tubes and the secondary wires, no change, still high current draw.

The following readings were taken with the primary wires lifted off the board,

The primary red to red is open. I believe this is the 117 V mains connect, W1 and W6.

If your primary actually was open, mains current would be zero.

The primary consists of 2 separate windings which are wired in series for 230VAC and in parallel for 117VAC.
W1 and W6 connect to different windings and will read open if the primary wires are not connected to the board.
So connect to board and measure between W3 and W8.

Also make sure you have the wiring for 117V. W4 and W5 must not be bridged.

[misterc57]

If your primary actually was open, mains current would be zero.

The primary consists of 2 separate windings which are wired in series for 230VAC and in parallel for 117VAC.
W1 and W6 connect to different windings and will read open if the primary wires are not connected to the board.
So connect to board and measure between W3 and W8.

Also make sure you have the wiring for 117V. W4 and W5 must not be bridged.

All primary wires are reconnected to board.

W3 to W8 = 1.10 ohms
W4 to W5 = 0.1 ohms

So W4 and W5 are bridged. C1 reads a short in circuit.

[Delta362]

Do you have wires connected to W4 and W5? If not, I believe the board traces link those two. When the PT is fed from 230V, one wire from the top winding would go to W4 and one wire from the bottom winding would go to W5. For a 115V application, the top winding wire would connect to W1 (instead of W4) and the bottom winding wire would connect to W6 (instead of W5). There would then be no wires connecting to W4 or W5 in a 115V application.

Does C1 still read a short if W1, 2, 3, 4, 5, 6, 8 and 9 all have no wires connected to them?

[misterc57]

Do you have wires connected to W4 and W5? If not, I believe the board traces link those two. When the PT is fed from 230V, one wire from the top winding would go to W4 and one wire from the bottom winding would go to W5. For a 115V application, the top winding wire would connect to W1 (instead of W4) and the bottom winding wire would connect to W6 (instead of W5). There would then be no wires connecting to W4 or W5 in a 115V application.

Does C1 still read a short if W1, 2, 3, 4, 5, 6, 8 and 9 all have no wires connected to them?

W4 and W5 have no wires connected. The wires are connected to W1 and W6. See my original post picture.

I disconnect W1, 2, 3, 4, 5, 6, 8 and 9, also 7 and 13. Check C1 again. No longer shorted. However W4 and W5 still read 0.1 ohms between them.