Marshall DSL 100 No Reverb

[TheTornOne]

Let me clarify that. I am wondering if the amp needs to be powered on to take the IC readings.

[bluesfreak]

+/- 15Vdc are supplied via CON3 to the reverb circuit and yes, you should have powered up the Amp for that (Standby can be on). As far as I can see on the schematics F1/F2 are the fuses responsible for 15Vdc and they are 250mA types, remove them if possible for testing. The fuses are on PCB JCM2-62-00, be careful, there are several F1/F2 fuses in this Amp!

[52 Bill]

Yes the amp does need to be plugged in and turned on, so be very careful working on it.

Attach the black lead to the metal chassis for your ground connection. First check the +&-15 volt power supplies. Then check the inputs and output pins of the reverb circuit ICs. You should find little or no voltages on any of the input or output pins.

[pontiacpete]

I believe the amp should be powered on when taking these measurements and ground the black volt meter probe to the chassis.

[TheTornOne]

Here are the readings from CON3 from the end that plugs into the reverb board with my meter set on 20V.

Top Blue Cable: 1.39
Middle Blue Cable: -.36
White: 13.11

I removed and tested F1 and F2 on PCB JCM2-60-00 and they have checked out ok.

Here should be the correct readings with the negative probe grounded to the chaise, the amp on, and my meter set at 20V:

LM348
Pin1: 1.57
Pin2: 1.58
Pin3: Nothing
Pin4: 13.05
Pin5: 1.57
Pin6: 1.58
Pin7: 1.58
Pin8: 1.58
Pin9: 1.58
Pin10: 1.56
Pin11: -.36
Pin12: 1.56
Pin13: 1.58
Pin14: 1.58

M5201:
Pin1: 10.51
Pin2: Nothing
Pin3: 12.46
Pin4: -.35
Pin5: 12.46
Pin6: 11.81
Pin7: .01
Pin8: 13.03

Thoughts?

[52 Bill]

Here are the readings from CON3 from the end that plugs into the reverb board with my meter set on 20V.

Top Blue Cable: 1.39
Middle Blue Cable: -.36
White: 13.11

Thoughts?

You are missing the -15 Vdc supply here.

The middle pin should have -15 volts. Follow the cable back to the other end and see if the voltage is present there.

[TheTornOne]

Pulled the cable from the output board, and got the same reading when taken from the center post of the connector on the board. -.36

[52 Bill]

Pulled the cable from the output board, and got the same reading when taken from the center post of the connector on the board. -.36

You'll need to troubleshoot the -15 power supply.
(Do all of these tests with the amp off and unplugged.)

You said that the 250mA fuses F1 and F2 are ok, right?

Next check diodes D3, D4, D5, D6, and the Zener Diode ZD2.

Also check the 270 ohm resistor R23 as well as caps C18, C19, and C20.

I assume that you know how to meter test the diodes. As for the caps, just be sure that they are not shorted or showing a low resistance.

[TheTornOne]

My mistake, I just realized I was looking at the glass fuses on board JCM2-60-00 not the soldered in fuses (F1 and F2) on board JCM2-62-00. I have never seen this type of fuse before.

This board is pretty crusted with wax but scraping through I got a measurement of .6 on the 200 Ohms setting (I do not have a continuity tester on this DMM).

Some of the diodes are reading both directions which means the are shorted right?
Diode setting on DMM
D3 - Both directions: .249
D4 - Direction 1 (Positive probe on anode, negative on cathode): .525
Direction 2 (reverse): .975
D5 - Direction 1: .526
Direction 2: .976
D6 - Both directions: .250
ZD2 - Both directions: .004

The resistor is reading 267Ohms.

Not sure how to tell if the caps are shorted.

I can tell this area has been worked on quite a bit in the past. A bunch of the caps are hot glued to the board in addition to being soldered. Not only is there a lot of wax in this area still but some of the traces are looking beat up. There is what looks like rust under some of the wax where the caps are soldered to the board. I have tried to take pictures but I can not get any detail out of them.

[52 Bill]

Some of the diodes are reading both directions which means the are shorted right?
Diode setting on DMM
D3 - Both directions: .249
D4 - Direction 1 (Positive probe on anode, negative on cathode): .525
Direction 2 (reverse): .975
D5 - Direction 1: .526
Direction 2: .976
D6 - Both directions: .250
ZD2 - Both directions: .004

It looks like the Zener diode ZD2 is shorted. Try unsoldering one side and retesting it out of circuit. If the diode is bad, it will still read .004, if there is a short somewhere else it will read high in one direction and low in the other.

If the Zener tests bad replace it, if it tests ok, then I'd pull out C20 and check it for a short circuit.

The other diodes may be testing odd, because of the short on the output.

[TheTornOne]

Hello, spent two weeks cleaning the wax out of my amp. Now back to the original issue.

I pulled one end of the two zeners and here are the readings:

ZD1 .621 one way, nothing the other way
ZD2 .627 one way, nothing the other way

Upon further inspection of the area C21 doesn't look so hot, looks like the top has swollen out a bit. Should I just go ahead and swap out C20 and C21 for new caps? I guess I am wondering if something else might be the culprit that is making these go bad and if I swap them they will just go bad again in the future.

[52 Bill]

I pulled one end of the two zeners and here are the readings:

ZD1 .621 one way, nothing the other way
ZD2 .627 one way, nothing the other way

Then it seems that the Zeners are ok.

Pull out C20 and test it with your ohm meter. Does it read either a low resistance or dead short? If it does read shorted, then replace it, if it doesn't then follow the entire -15 volt supply line looking for a shorted component or a shorted trace.

[TheTornOne]

Pulled and replaced both C20 and C21 caps, C20 looked like it was in bad shape, but still no reverb. As far as following the -15 volt supply line, I am unsure what direction I am going and what exactly I am looking for. I certainly do not see any traces that are shorted. Unfortunately my knowledge of knowing how certain components are supposed to read with my meter is pretty basic.

[52 Bill]

Pulled and replaced both C20 and C21 caps, C20 looked like it was in bad shape, but still no reverb. As far as following the -15 volt supply line, I am unsure what direction I am going and what exactly I am looking for.

Think about what you are trying to do here.

We've found that the -15 volt supply was not working. Either the negative power supply has a problem or the circuit has a problem that is pulling down the -15 volts.

You checked the Zener and the limiting resistor and you have now replaced the filter cap. Do you have -15 again?

What is the voltage on the input side of the limiting resistor?

With the power off, if you read the resistance from ground to the -15 volt bus are you still getting a short?

[TheTornOne]

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?