Marshall DSL 100 No Reverb

[olddawg]

Now looking at the section you are at in the schematic, not the best at reading these but I see where you are. It looks like the -15V is suppsed to be coming into that board via Con2. Is this correct?

Here are the three pins of Con2 with the amp on (I am learning slowly):
Meter set at 2V

Blue Side: Keeps jumping from -.020 to .008
Blue Middle: .002
White Side: -.023 to .005

I am not sure if I am jumping ahead too far but I can not find a -15v reading at any component on this board. Combining that with these readings seems to indicate that the problem is occurring before this board?

Can you post a schematic? Divide and conquer. You need to find the -15v source. Is it a simple bridge and a pi filter with a zener regulator or do you have a transistor or monlithic IC regulator involved? I would be looking for a low value current limiting resistor or fusistor. If the +15v rail is there, the -15v rail is probably a mirror of that. Follow the +15v back to the source, then do the same for the -15v rail.

[52 Bill]

Now looking at the section you are at in the schematic, not the best at reading these but I see where you are. It looks like the -15V is suppsed to be coming into that board via Con2.

What board are you talking about? If you're looking at the schematic marked JCM2-62-02.DGM, that is the schematic for the low voltage supplies as well as the output jacks, etc.

CON2 is the ac input to the low voltage power supplies. CON4 is the output of the low voltage supply, that goes to the reverb board. On CON4 there are three pins. Pin 1 is +15vdc, Pin 2 is -15 vdc and Pin 3 is the connection for the reverb foot switch.

In an earlier post you already measured the voltages here, and you found that the -15 volt supply was missing. Then you started to check the parts of the -15 power supply. The Zener was ok, and then you replaced the filter cap.

I had asked if the -15 volts was working now and/or if there was still a short across the -15 volt bus. You didn't answer, so turn off the amp and take a diode test reading across diode ZD2. Is it still reading .004? If it does read .004 unhook CON4 does the short go away?

[TheTornOne]

Sorry Bill and olddawg, I realized I posted the wrong schem. Olddawg here is the correct schem, Bill I will get those readings to you in a moment.

[TheTornOne]

Hey Bill

ZD2 is reading .004 both directions with CON4 connected and unconnected.

I really do appreciate your help on this!

[olddawg]

Sorry Bill and olddawg, I realized I posted the wrong schem. Olddawg here is the correct schem, Bill I will get those readings to you in a moment.

This is only a partial schematic. I don't see the actual sources for the +/-15v rails. If the -15v is missing, and you don't have a short pulling it down, then something is open at the source like a rectifier, fuse, fuse resistor, or a bad CN connector.

[TheTornOne]

Here is the whole thing.

[olddawg]

Hey Bill

ZD2 is reading .004 both directions with CON4 connected and unconnected.

I really do appreciate your help on this!

Float one end of the diode. If it still reads a short it is bad.

[olddawg]

Here is the whole thing.

Do you have +15v and -15v at IC1e?

[52 Bill]

Float one end of the diode. If it still reads a short it is bad.

I think that he's done this already.

There is a six pin connector CON3 on that same board, try pulling it off and see if the short (.004) is still there.

[AmpFix]

Here are the three pins of Con2 with the amp on (I am learning slowly):
Meter set at 2V

Blue Side: Keeps jumping from -.020 to .008
Blue Middle: .002
White Side: -.023 to .005

I am not sure if I am jumping ahead too far but I can not find a -15v reading at any component on this board. Combining that with these readings seems to indicate that the problem is occurring before this board?

CON2 brings in the 22 - 0 - 22 vac supply from the power transformer. The 22vac is converted to +15vdc and -15vdc for pretty much all the ICs and the channel switching. If you have no voltage there, I'm surprised that you're getting any sound at all through the amp.



I'm just going through this myself. Both my 250ma fuses went. I'll be replacing these with in-line fuses so I don't have to special order these fuses. I hate soldered in, PCB mounted fuses! I just hope that nothing else is wrong with the amp aside from the fuses.

If you're not getting any voltage from the PT, it sounds like the PT might be toast. The tubes light up and when you're off standby you get B+?

Have you tried measuring the 22 - 0 - 22 vac right at the PT. they're the blue wires going from the PT to the tube PCB. The middle one is the 0vac ad the two outer tabs are the 22vac.

[J M Fahey]

The amp has to be on, or you'll have no voltage to measure.
Negative (black) probe to metallic chassis, positive (red) one to whatever pin you are measuring, multimeter on 20V DC scale.
Wax, besides that spilled on hot rubes and charred or gotten into "mechanicals" such as jacks, pots and switches, does not hurt regular components. In fact ceramic capacitors are dipped in wax at the factory.