Assistance replacing an impedance selector on Vintage Marshall JCM 800

[sohosteve]

Hi
I wonder if someone could help me with the wiring of a replacement Impedance selector.
I can handle a soldering iron, solder sucker etc and have carried out some fairly complex mods on my valve amps using graphic directions though I am NOT an electronic engineer and cannot even read a schematic diagram.
I have attached a photo of the old selector (still wired) and the brand new replacement.

The new one has terminals 1-6 plus the earth in the centre.
Could someone please just tell me which colour cable (green, purple, yellow & black) goes to which of the 1-6 terminals and if I need to add a jumper cable from earth to one of the numbered lugs (with the black cable)

The old selector does not have any numbers on the terminals...

Please bare in mind that the chassis is sitting upside down when looking at the wiring of the old switch!

Thanks

[J M Fahey]

Not sure yet, can you please post a picture of the other side?

Is any/both the old pull out-turn-reseat type or a rotary switch type?

There's been many variations across the years.

Another doubt: you mention "ground" but both old and new show 4 wires: 4/8/16 transformer taps plus the wire going to output jacks "hot" terminal.
What am I missing?

Do you have a link to the new selector switch page?

Thanks.

[sohosteve]

Thanks for your kind reply

Here are the requested shots (I've also added them inline to my original post)



They are both switches - neither are pull out/push in type

My mistake regarding 'ground'.
As mentioned I am not an electronic engineer but assumed that one was ground.
Sorry if I've confused the issue here!

Here is the link to the replacement switch Marshall Rotary Ohm selector

Hope that helps clarify

Thanks

[g1]

You will need to determine what the color code is to the old switch. I don't see any that match in the schematics, and I don't trust the 2nd hand info out there, it's easy enough to check.
Set your meter to ohms. Make sure there are no speakers connected to output jacks.
Black wire is the output terminal. Connect your black meter probe to it, and leave it there for all the switch test. Set switch to 4 ohms. Put the other probe to each of the 3 color wires at the switch. Which color gives a zero ohm reading on the meter? That is the 4 ohm tap. Set switch to 8ohms and repeat. Then set switch to 16 and repeat.
Please post results for others who may need it in the future..
Now look at the pic you posted for the P-H1700. Black wire connects to "P". Pins 1,2, & 3 go to the colors you determined with your test.

[drewl]

100w or 50?
The color codes are different between the two.
Okay, four tubes-100w.
Green-16
Black-4 with violet feedback wire
Yellow-8.
The picture in The Marshall book matches your picture.
No DI fitted.
The color codes usually are correct, and there will not be a ground connection on the switch, that goes to the output jacks.
Tin your wires with solder and don't use too much heat so you don't damage or melt the new switch.

[g1]

Okay, four tubes-100w.
Green-16
Black-4 with violet feedback wire
Yellow-8.

So which color goes to the speaker jacks then? I don't think the violet is NFB in this case, I think it comes off the OT.

Sohosteve, which wire goes to the output jacks? Where does the violet wire go? More pics would help.

[drewl]

The switch common or center lug goes to the output jacks hot or signal.
The feedback wire should be easy to find as it goes to the PC board.

That green wire looks like it has a resistor for the DI, does it have DI?

[g1]

drewl, I was asking you which color you think is the common on the original switch in his picture.

[drewl]

Whichever one is connected to the output jacks, you I can't see it but it should be the black wire with the lead running up to the other half of the switch.
The Marshall book has a hand drawn picture that matches your picture.

on the bottom left to right:
yellow -8 ohm
output - which is also connected to top half of swtch- 3rd from left
Black -4ohm with violet wire for feedback (except model 1992)
Open-
Green -16ohm on top right

[drewl]

Is this a 1992 or Canadian model?

If so it may not have a 16ohm tap since it is a combo!

That makes sense.
It has two 16 ohm speakers in parallel for 8 ohms.
With another 8 ohm cab you would select 4 ohms

You would never select 16 ohms under normal circumstances.

[g1]

Whichever one is connected to the output jacks, you I can't see it but it should be the black wire with the lead running up to the other half of the switch.

I agree with this, which is why the black can not also be the 4ohm tap.

This is the picture with the new style switch I found on the web. I though it was correct but wanted him to verify that green is 16ohm tap and purple (violet) is 4ohm tap.

[drewl]

This isn't rocket science.
They may have used a black wire to connect the switch common to the output jacks.
But the black wire coming from the output trans should be the 4 ohm tap.

[sohosteve]

Firstly, a huge thank-you to you all for pitching in here.

Secondly a huge apology as I have just noticed that (as 'drewl' pointed out) there is a second Black wire that I'd missed as it was hidden underneath and connects up with the violet(purple in my description) wire.
And I didn't realise that I'd missed it in my description but it is a 100 watt 4211 combo.

To answer drewl's other questions
1. it DOES have a DI output
2. It is a UK model - I'm in London England :-)
3. It has 4, 8 & 16 ohm selection choices as standard - some people would bypass the combo speakers and use a different cab I guess, though I'd think the main reason that it's there is so that Marshall could save money using all the same components and chassis for the head or combo..!

All the wires go directly to the underside of the PCB (as shown in the new photo added on this reply) with the ONE exception of the solitary Black wire (which has the bridge soldered across two terminals) which goes directly to the speaker output jacks.
So that is obviously the output signal.

Finally, huge thanks to g1 for your suggestion.
However this drew a total blank (very bizarre meter readings compared to those expected) probably due to the fact that the switch is intermittent at best - which is why it's in need of replacement.

Anyway, hopefully my new info will help nail this down for once and for all.

I suspect that drewl's post regarding the drawing from the Marshall book may have hit the nail squarely on the head.

If that is the case do I need to add a bridge across two terminals (as on the original switch on the Output wire) or is this negated by the new design of the replacement switch?

Thanks

[sohosteve]

Well, impatience got the better of me and I decided to trust in drewl's wiring found in the Marshall book and....

It works!

OK, Star Prize goes to drewl for finding the correct wiring for the new impedance selector which I can now confirm should be wired as follows

Terminal 1 (4 ohm) - Violet & Black leads
Terminal 2 (8 ohm) - Yellow lead
Terminal 3 (16 ohm) - Green lead

Center terminal 'P' - black lead that goes to the speaker outputs.

And just to confirm this is for a 1985 Marshall JCM 800 4211 100 Watt Combo which is exactly the same as the JCM 800 2210 100 Watt Head version.


As something of a footnote to this I feel that I should point out that THE most annoying and time consuming pain in the butt was provided by the moron who decided the gauge of the bolt holes on the replacement part.
They were FAR too small for the bolts used to attach the original to the chassis!
Consequently I spent the worst part of an hour slowly wearing away the inside of the metal inserts with the smallest blade on my penknife.
Got there eventually (after re-sharpening it twice) but Jeez! Don't those fools check those kinds of things?
Evidently not...

Just to close, a huge, nay, HUGE THANK-YOU to all you good folks out there who pitched in and assisted me in my hour of need.
You guys ROCK!
And now, due to your help, so again, do I :-)

[drewl]

Tell us the model first next time.

I had a 2210 head, the 4210 50w 1x12 combo and even the 5210 solid state version.