Marshall 5275 Combo Amp - boost gain 1M pot with pull switch

[TomCarlos]

MEF members....

I have a Marshall 5275 Combo Amp (AKA Marshall 75 Reverb) that has a bad pot, VR1 - the boost gain 1M pot with pull switch. The shaft has lots of play and when you touch it, you can hear an electric connection issue. I don't how to describe that. Unlike other pots, this one has the pull switch attached to the back. I'm not sure how these pots are constructed and whether or not they can be disassembled and repaired. So before I pull this from the board, I wanted your opinion.

I see a couple potential replacements out there, one from Amp Repair Parts (Part #P-VM24-A1M-PS). It looks like a fit. It shows a Marshall part number POTP-00026 but I do have have a parts list for this amp. The schematic shows a Log pot for VR1; I am not sure if the replacements are Audio or Log. That may not be a big deal.

Has anyone repaired or replaced one of these pots?

Thanks, Tom

[g1]

I take the A1M to mean it's 1Meg log. Looks like that should work.
The one at Full Compass listed as POTP-00026 looks like a sub that you would have to run wires from the lugs to the board.

[J M Fahey]

Problem is not so much the pot itself, but the long threaded neck which allows it to be placed some 5 or 6 mm further than front panel than normal, and which is not an over the counter component.
You might ask Marshall, or try do disassemble it and insert a new carbon track or use a regular pull switch pot, attached to front panel, and run wires as suggested by G1.

*Mmmaaaayyyybbbbeee* somebody sells a short adapter, female thread at one end to take a regular pot ,male thread at the other to take a nut and be fixed to chassis.
Itīs a very inexpensive piece if done in a revolving turret lathe or a modern automatic one out of brass (or even aluminum) hexagonal bar, but minimum order would be 500 or 1000 of them ... which can be filled in a couple hours.

There were tens of thousands of "long neck pots" Marshall amps so definitely there is a niche market there; Iīd search around a little.

[TomCarlos]

Thanks Gents....

I saw the pot at Full Compass. It looks similar to the one at Amp Repair Parts except for the legs. And yes JMF, you are correct. The female threads on this pot are different compared to the others.

I will run the options by my friend and see what he wants to do. Unless anyone else has ideas, I will follow up with a post and photo of the fix.

[g1]

Looks to me like the one at Amp Repair Parts may have the long bushing. They claim the length & diameter of bushing are both 9.5mm.
How long is the bushing of the original?

[TomCarlos]

If I could only hold my ruler steady.... I do have some plastic calipers. Maybe time to get a nice one.

It looks like the Bushing length on the installed pot is 15/32", 11.9mm.

The difference of 2.4mm is ~ .094 inches (3/32 of an inch).

The Bushing diameter looks like 10/32", 7.9mm.

I am wondering if the description of that pot (on the Amp Repairs site) is wrong.

[TomCarlos]

BTW - according to Owners Manual ....

Gain Pot (Boost Section) - Controls the amount of Drive and degree of Overdrive required. When the control is pulled forward, the Master EQ is switched in to allow greater tonal flexibility).

Footswitch (RTS) - (1) Reverb On/Off, (2) Boost Pedal Input

But "Boost Pedal" Input really means Channel Switching? Oh those Englishmen ...

This is very different compared to a Peavey or Fender where you can switch channels with either a Footswitch or Button. So at first, I was thinking the switch part of the pot was for Channel Switching and could thus be eliminated because we could do that with the footswitch - nope!!

[J M Fahey]

Plan B:

If itīs absolutely impossible to get a suitablepot, split its job in two:

1) get an A1M pot that fits panel hole.

2) add wire stubs to reach PCB holes

3) install a small toggle switch like those used in guitars which are quite non intrusive and do not look bad at all, just drill a 1/4" hole near that pot for it, and wire needed contacts to proper PCB holes.

Volume function which is the main one will be restored; gain boost function is available at will; it does not look bad or out ofplace.

Tell customer: "that is an 80īs amp; Marshall used a specially made pot, it has been unavailable for decades and not even Marshall can find one for you, at any price" ... which to boot is absolutely true.

Also:"amplifier will work perfectly as new, with no hitch,and the micro switch does actually look cool"

[TomCarlos]

JMF - I believe that is the BEST alternative.

The other option here is to ask my friend, do you always keep that control pushed In or Out? If that is the case, we abandon the switch.

I was also looking at a CTS pot but the reviews on it are horrific. I don't want to install crap parts that will soon fail.

Thanks!!!

[g1]

BTW - according to Owners Manual ....

Gain Pot (Boost Section) - Controls the amount of Drive and degree of Overdrive required. When the control is pulled forward, the Master EQ is switched in to allow greater tonal flexibility).

Footswitch (RTS) - (1) Reverb On/Off, (2) Boost Pedal Input

But "Boost Pedal" Input really means Channel Switching? Oh those Englishmen ...

The pot switch is shown at the bottom of the tone stack. Looks to me that when you engage the boost ch. via footswitch, the EQ section is disabled. Pulling the pot switch out gives the EQ back.

As you say, it should be easy to hardwire for whichever way he normally uses it. I bet he never pulls out the switch.
I think people who do use the pull-switch would find you have to adjust the EQ depending on which channel you choose via footswitch.
Even seems to me people complained about that when these were new, but could be poor recollection.