Stumped by Marshall DSL40CR standby switch issue

[Duncan]

Hey guys,

I’ve been working on a Marshall DSL40CR that came in with some output issues (which I determined was a burned up dropping resistor which has been replaced).

For those of you not familiar with this amp, the standby switch is a three position switch that goes low-standby-high.

For some reason, the middle position does not put the amp into silent standby anymore. I’ve tried disconnecting the switch and cleaning the contacts (also with contact cleaner inside and actuated) and the issue persisted

I’ve confirmed with Marshall that the switch is wired properly, and I’ve worked through the schematic and found nothing amiss.

Does anyone have any ideas as to why this is happening? I’ve attached a schematic that Marshall sent to me for this amp (well, for the 100 W, but it also applies to this model).

Thank you in advance.

[sluckey]

For some reason, the middle position does not put the amp into silent standby anymore.

Does the voltage on the plates (pin 3) of the EL34s go to zero when the switch is in standby position?

[Duncan]

For some reason, the middle position does not put the amp into silent standby anymore.

Does the voltage on the plates (pin 3) of the EL34s go to zero when the switch is in standby position?

No it doesn’t. Voltage in the “high” mode is 440v, “low” mode is 161, and in “standby” it’s sitting around 315.

[sluckey]

How many lugs are on this switch? If it's a SPDT with center OFF (three lugs) as shown on the schematic then the switch is faulty or wired incorrectly. Show us a pic of the actual switch.

[Duncan]

How many lugs are on this switch? If it's a SPDT with center OFF (three lugs) as shown on the schematic then the switch is faulty or wired incorrectly. Show us a pic of the actual switch.

Here is the Marshall switch:

https://i.imgur.com/Mu5Hftk.jpg
https://i.imgur.com/1eEis5w.jpg

And here is the replacement I have (which will require me to combine the lower two banks onto one side of the switch, as it is a DPDT):
https://i.imgur.com/10MntRn.jpg

This is the model of switch:
https://www.digikey.ca/en/products/detail/bulgin/C1570AABB/4319306

[JPK]

Slucky please don't mind my amateur attempt here. I like troubleshooting but no where near you level. So...if you disconnected the switch and there was still voltage then it's not likely the switch right? Something down stream is shorting out some HV+ to the circuit. A failed trace or a component? Assuming nobody made a change to the wiring.

[Duncan]

Slucky please don't mind my amateur attempt here. I like troubleshooting but no where near you level. So...if you disconnected the switch and there was still voltage then it's not likely the switch right? Something down stream is shorting out some HV+ to the circuit. A failed trace or a component? Assuming nobody made a change to the wiring.

To be fair, I've not attempted firing up the amp without the standby switch connected. I've heard from a (relatively) local Marshall warranty shop that the switches in these amps are prone to failure.

[sluckey]

I'm sure you took a pic of that switch before you disconnected any wires.

[Duncan]

I'm sure you took a pic of that switch before you disconnected any wires.

I did, yes. Connections are consistent.

I’ve also confirmed the connections with Marshall (attached)

[JPK]

Slucky please don't mind my amateur attempt here. I like troubleshooting but no where near you level. So...if you disconnected the switch and there was still voltage then it's not likely the switch right? Something down stream is shorting out some HV+ to the circuit. A failed trace or a component? Assuming nobody made a change to the wiring.

To be fair, I've not attempted firing up the amp without the standby switch connected. I've heard from a (relatively) local Marshall warranty shop that the switches in these amps are prone to failure.

Ah ok, I misread that. You took the switch out to clean it, not to see if voltage is still there with it disconnected. Hopefully your new switch is the ticket.

[idontknowyou]

Any update on this?
Im encountering the exact same problem on a DSL40CR. Slightly lower HT on the plates and screens whem the switch is in "standby." Got a replacement switch and the problem persisted. The schematic has me stumped, I can't even see a possible path for voltage to be present there when the switch is off!

[WimWalther]

Any update on this?
Im encountering the exact same problem on a DSL40CR. Slightly lower HT on the plates and screens whem the switch is in "standby." Got a replacement switch and the problem persisted. The schematic has me stumped, I can't even see a possible path for voltage to be present there when the switch is off!

The schematic is somewhat unintuitive, and it seems that the standby switch isn't drawn properly.

From what I can tell, it's a 3-position switch (SP3T) with two "on" positions and one "off". It's this 3rd (off) position that isn't shown.

In any case, the "on" positions select either the upper (higher volts) or lower (lower volts) B+ power supply. In the unmarked (center?) position, neither supply is selected and the amp is in standby.

There should be no B+ other than residual in standby. If there is still steady B+ in standby, high or low volts, the switch is bad.

[idontknowyou]

Yeah, I tried a replacement switch and the problem persists. Both the original switch and the replacement are DPDT on-off-on switches with 6 terminals. Both tested fine with a multimeter out of circuit.

I agree there should be no voltage present when the switch is in the middle off position. It seems like one half of the switch is switching between two different power supplies for the HT. If we have to assume the switch is working properly, is there any other path for voltage to be present at the power tubes?

[WimWalther]

Yeah, I tried a replacement switch and the problem persists. Both the original switch and the replacement are DPDT on-off-on switches with 6 terminals. Both tested fine with a multimeter out of circuit.

Ok, so they're double-poled switches. Are both sets of poles in use? All 6 terminals?

If we have to assume the switch is working properly, is there any other path for voltage to be present at the power tubes?

FWIW, I can't presently see any other current path.

Of course, the schematic could be in error, or I could be misreading it - either are possible.

And you're sure that it's not just residual? That is, the amp remains playable in "standby"?

[mresistor]

Ok, so they're double-poled switches. Are both sets of poles in use? All 6 terminals?




Wim in Reply # 4 he has provided pictures of the switches which should answer your question above.

[idontknowyou]

Yeah, the amp is remaining playable and passing signal even when in standby. Sitting in the 300 volt range, same problem the OP had.

The second set of poles seems to be switching a ground connection for relay 2 and IC4? It also has paths to LT1, low1, and low2, which i dont see in that page of the schematic and I assume are going to the preamp.

I also don't see any other possible path for HT voltage and am utterly stumped.

[WimWalther]

Next step is to determine which B+ supply is sourcing the mystery current. Check for voltage drops across the resistors in each supply.

If a supply is sourcing current, you should be able to tell by the drop(s).