Questions about a 68' Marshall Tremolo 50 (1987T) Can it be this simple?

[ALBATROS1234]

I met an old guy from England who has a few old tube amps. The first was a Gibson GA-79RVT which he said the reverb doesn't work and his tube amp tech says the tube to fix it was impossible to find. Me having boxes and boxes of strange and interesting tubes from the 20's to present I knew better. So, I look up the schematic and it is a 7199, rare but not unobtainable, as a matter of fact I had 3 that test NOS. So, I sell him a tube and voila. Next, he tells me he has a couple of old Marshalls(one is a 63' JTM 45 with a white backplate) and one his tube tech can't fix the tremolo. I say is it an 18 watt but he says no it's a Tremolo 50/ JTM-50 1987T from the late 60's. Then he says take it home to see if I can fix it and here I am. He is right the Tremolo is not presently working on either channel, but I noticed a 4-pin jack on the backside which I imagine could be for a footswitch. Does this amp require a footswitch to make the Tremolo work? It doesn't have a switch on a pot soooo. If so, I imagine there is a way to jump it to activate the trem and make sure it actually works. Can it be that simple? Does someone sell aftermarket footswitches? I am more of a Tweed/Valco/octal preamp guy, so Marshall's are not my forte albeit I have studied the schematics and how they work. This looks like a basic cap style trem and before I plug in the soldering iron my first thought was does it need a footswitch? Please Marshall Guru's Help this poor boy from New Orleans.

[tubeswell]

You’ll have to come up with some more details (e.g., a schematic) if you expect anyone here to be able to confirm a hunch about an amp that no one else here (apart from yourself) can see.

[SEL49]

Does this amp require a footswitch to make the Tremolo work?

Yes. Look at the schematic.

[ALBATROS1234]

OK, see attached, I just figured one of the Marshall guys would know if this amp required a footswitch like a Fender BF does.

[ALBATROS1234]

Looking closer, I think what I thought was a weird plug may be a transistor, I wonder if that could be faulty? I wonder if there is a suitable replacement available. I feel like why would they not have put a switch on one of the pots? Would you just walk over and start turning knobs as you play? British people design weird switching schemes.

[SEL49]

Maybe this schematic will make it easier to understand.

     https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45tr.gif

[Willabe]

Looking closer, I think what I thought was a weird plug may be a transistor, .....

It is a transistor.

[ALBATROS1234]

Perfect, that schem does show the transistor area clearer. I thought it said 26374.

[ALBATROS1234]

Hello guys, I spent a couple hours today on the Tremolo. The tremolo itself is working in that when I put my scope on the collector I see the oscillation. When I adjust the speed knob, the speed of said oscillation increases and decreases and when I adjust the intensity knob the intensity changes BUT it is not audible.there is a shielded two conductor cable from the tremolo circuit to the 4 pin DIN plug on the back of the amp. The top and bottom pins are connected together and to the shield and the left and right side pins are connected to the collector with a black wire and the red wire from.the socket connects to a lug on the board and I assume jumps to the audio path. I say assume because none of this is shown on the schematic in a clear way imho. I jumped those two wires together to simulate a closed switch thinking it would start to work but no such luck.SEL49 stated that yes the footswitch is required but there must be something else amiss or else it should work when I jump the two conductors together. It is obvious the guy who worked on this originally changed the transistor as it appears to be non original solder joints and the type is a TR09 526 instead of a 2G374 spec'd on the schematic. Basically the schematic just shows a switch between the collector and the wiper of the intensity control so I tried  an double alligator lead directly from the collector to the wiper of the intensity pot and still nothing. Any advice from someone who has worked on this type of.amp would be helpful.

[ALBATROS1234]

I guess no one here is really into vintage Marshall JMP 50w 1987T circuits. Maybe there is a Marshall amp forum where someone has experience with these Tremolo switching circuits.

[tubeswell]

TR09 is NPN I think, and the transistor symbol on the Marshall schematic is a PNP

[glass54]

Hi Albatros,
(Sorry for jumping in late, was OS for last 10 days on Holidays with my wife and NO Computer  ).
I have had my nose into a few Marshall's over the decades, Progressively getting worse for reliability with later models ie peaked at 800/900 series!! The 2203 was highly regarded amongst some working musos I knew in late 1980s/early 1990s.
My most recent "Super Tremolo Amp Mk IV" s/n 7066, custom built with the GE KT66 O/P Tubes and mounted in 2x12 Celestion G12. Was this Eric Clapton's?? He had a custom made model 1962, supposedly a "Bass" version of model 1961.
(While in John Mayall’s Blues Breakers, Eric Clapton played a 1960 model Les Paul Standard through a 45-watt model 1962 Marshall 2×12 combo (JTM 45). The amp was stock except that the output tubes were replaced with KT66s which have a more refined mid-range and clearer top end than either EL34s or 6L6s. The amp was usually turned up full volume, even in the studio. When the engineer complained that his amp was too loud, Eric replied “That’s the way I play.”) Please Note: The speakers were changed/upgraded to suit Eric.
Marshall Industries were not able to tell the owner (here in Australia) of the history of this amp, but according to Marshall records s/n 7067 was owned by Roy Orbison and lives in a museum in USA. The Amp uses what Marshall calls "JTM45 Basic Schematic for Trem Amps Types 1961, 1962and 1987/T" This does not relate to production years as the amps were built in 1964 on, based on the Fender  Bassman but with local (English) parts. This info is debatable as far as accuracy goes BUT I'm only referring to the amp in question and I'm adding a photo to highlight your Tremolo transistor (see attached).
Now to get to your amp. Tubeswell is correct 
The transistor is a PNP Metal case Germanium device. It is a 2G374. Good luck finding data 
I have seen on some forums where someone tried a 2N4403 (common Silicon PNP device) and apparently worked 
Maybe worth trying as well as some experimentation eg up the value of 4.3k resistor on base of the transistor.
It is obvious the guy who worked on this originally changed the transistor as it appears to be non original solder joints and the type is a Basically the schematic just shows a switch between the collector and the wiper of the intensity control so I tried  an double alligator lead directly from the collector to the wiper of the intensity pot and still nothing. Any advice from someone who has worked on this type of amp would be helpful.
That should have worked assuming good transistor. I think you need an oscilloscope and Digital MM to prove transistor is functioning, then start experimenting for results in the transistor circuit. I don't have access to the amp anymore, so can't do any more measurements. From what you have described, it appears that the transistor is you roadblock. Poor advice but if you can set up DC conditions (bias) in a suitable transistor, (basically a buffer for Trem Osc), you should get a result. (later models used a FET but some of those are hard to get and you would have to experiment to adapt the device)
Sorry for the drawn out reply but keep us posted,
Kind Regards
Mirek

[ALBATROS1234]

Tubeswell, you are correct the type should be a PNP I did not check to see if the replacement was NPN I assumed the guy checked it. I have a few germanium PNP's around so perhaps this could be the issue.

[ALBATROS1234]

Mirek,
    Thanks, and I believe I have some germanium PNP types which I was using to build a fuzz pedal a few years ago. I have a transistor tester that if you plugin any transistor it will indicate the junction type. I actually did put a scope on the what I thought was the collector and I got an oscillation. The speed and intensity controls affected this oscillation as they should but when I put an alligator jumper from that point to the wiper of the intensity pot to simulate a footswitch there was still no audible tremolo although when I put the scope lead on the collector I could hear an artifact of the tremolo through the speaker which responded to the speed control but while plucking a guitar string simultaneously the audio signal was not affected.