Marshall 100w JMP (2203) - output switch

[Sir Cuitous]

Question about the speaker impedance switch on this amp. Forgive my lack of experience with these amps, but the impedance switch looks more like a socket than a switch. Is this switch selected with some kind of shunt or jumper? Any general opinons on the playability of this amp?

[drewl]

The old ones were just a jumper or "shunt" encased in a plastic handle.
Don't know when they switched to a switch...think it was a little before the JCM800's or in the late 70's.

[Sir Cuitous]

I believe this amp is dated 1979. The amp was given to me and I was told it does not work. I don't have a speaker to test it yet, but it passes visual inspection. It may be as simple as connecting that switch. I suppose I can use some solid wire to test it and ultimately replace the receptacle with a toggle. It's too much power for my needs, but it may sound good.

[Enzo]

You can still buy "Marshall impedance selector plugs" from various sources. it is a jumper wire in a plastic knob. There is a square window hole in it that exposes one of the impedance numbers on the female part. So the proper humber is off to teh side of the actual jumper position.



That is from AES at www.tubesandmore.com for $8.95.

Without the plug, there is no speaker connectin from the OT to the speaker jacks. DO NOT play the amp this way.

A piece of bare 12ga solid copper wire as used in house wiring will usually work. Bend a short piece to fit.

[Sir Cuitous]

Thanks for the info. I will get the selector, but I will most likely test it first with some wire.

[Alex R]

- sometimes people soldered them up behind the socket so it was permanently 8 ohm or whatever. Not a bad idea as it means you don't get problems with bad contacts on the plug, though of course it ties you to the one impedance.

If it ever made any sound in its current condition then a soldered-up selector is what you likely have.

[drewl]

Yeah I lost one for my '73 years ago.....got the replacement.
Back before I worked on amps I took a piece of buss wire and wrapped it around the switch and secured it to the mounting screws....

Anyway, if this amp is too much power for you I'll take it off your hands

[Sir Cuitous]

Finally....

OK, so I finally tested it out. It has Awesome tone. It was simply that the previous owner did not have the speaker selector and did not realize it was missing. The volume pot is a little scratchy and the imput jacks need to be replaced, but otherwise it's perfectly functional and fun to play. It's still more power than I can use, but I already have buyers lined up. It's amazing what people will just give away.

[BStringThumper]

Hi,
I too have the old style impedance selector on a JMP 2203. I was hoping someone could confirm the wiring and orientation of the plug. My O.T. has the black wire going to the 4 ohm lug, the yellow wire to the 8 ohm lug, and green to the 16 ohm lug. The purple NFB wire is connected to the 8 ohm. When I push in the impedance connector I noticed that the connections being made are the center hole (the one for the output) the other connection being the 16 ohm hole, the square window is showing 8 ohms. I thought center to 16 ohm would be 16 ohm????
Thanks for any help with this.

[Tom Phillips]

Hi,
I too have the old style impedance selector on a JMP 2203. I was hoping someone could confirm the wiring and orientation of the plug. My O.T. has the black wire going to the 4 ohm lug, the yellow wire to the 8 ohm lug, and green to the 16 ohm lug. The purple NFB wire is connected to the 8 ohm. When I push in the impedance connector I noticed that the connections being made are the center hole (the one for the output) the other connection being the 16 ohm hole, the square window is showing 8 ohms. I thought center to 16 ohm would be 16 ohm????
Thanks for any help with this.

The push on plug can be confusing because the view window is offset from the connections. You have listed the OT secondary wire colors correctly for each impedance tap. Is there any indication that the wiring to your impedance selector has been messed with? If not then check again carefully. With "8" showing in the window of the selector plug you should indeed have zero Ohms continuity between the speaker jack and the yellow wire.
Regards,
Tom

[BStringThumper]

Hi Tom,

Thank you for the response. I had read on other posts that the yellow wire is the 8 ohm wire. Looking at the impedance switch without the plug it says 8 ohm. The yellow wire was connected there. When I insert the plug with the 8 ohm showing in the window, it is making contact with the green 16 ohm lug and the center output. I don't have the O.T. wires connected at this time, but doing a simple continuity test shows that there is no connection to the 8 ohm lug from center output, there is continuity from the 16 ohm. It's hard to tell if the wires have been moved around, but if they were, and not marked, someone could have just put them on the lug that says 8 ohm when it should go to the spot that says 16 ohm. That would make the window correct. Wow, that seems confusing to explain. Did that make sense to you? I will see if I can find some pictures online. Thanks.

[Tom Phillips]

I understand what you are saying.
Sounds like someone did some re-wiring without understanding how the plug in switch was intended to operate.

[BStringThumper]

Hi Tom,

I agree. I bought this amp not working, the transformers were cooked. So I am looking everything over closely. I will wire it with the window corresponding to the correct ohm, but was hoping that someone could confirm this.

Thanks.

[Tom Phillips]

Consider it confirmed.
One additional point to add is that the secondary colors of the 50W and the 100W Marshall output transformers differ. I don't know why they did this but the colors listed above are for the 2203 100W amp.
Tom

[BStringThumper]

Thank you. Yes, it's a 1976 2203 100 watt. The output transformer was a Dagnall C2373. Brown, black, yellow, green, on the secondary side. I wonder if the impedance switch was wired wrong from the factory and that's what burnt out the O.T. over time?
Have you ever tried to rewind a transformer? I've got both the original transformers. I can see the burnt windings on the P.T.. Probably not worth the time, but would be great to save them and build another 100 watter.