JCM800 2205 impedance wiring question

[rickmccl]

Hi all, I'm looking for advice on this impedance selection switch. Complaint reported was weak output when selecting 16ohms on the impedance switch and using 16-ohm cabinet. I am a neophyte in tube amps and I have not been able to figure out these wires. It has the flat rotating selector switch, not the plug, not the domed selector.


The OT was previously replaced with a Hammond, so the wire colors are different from the schematic and other pictures I've found; fortunately it is labeled:


The Orange 8-ohm wire, it is connected to the terminal which is jumpered to the opposite side; this is where the black output wire is attached.
There is one terminal to the right of the orange that has old solder on it. I am _suspecting_ this is where orange used to be, before it became the new black ...

Wiring:


As you know it is nigh impossible to use the multimeter to suss out the switch due to the transformer carrying the current to all the other jacks.
What does this look like, wired for 8-ohms all the time? Selecting 4 or 16 is going to be pretty hairy, isn't it? Is it a 6-8-12 ohm selector now?

Is this some JCM800 mod I haven't yet read about? The previous techs had also wired the power switch backwards so that it worked, but did not light the indicator; it seems anything is possible in here. I may have to remove the switch for closer examination, maybe it is broken and this is was a workaround? Do these switches come apart for cleaning?

Big pic of the internals, you'll notice a couple more mods I haven't tried to figure out yet: https://goo.gl/photos/NzBdihY1Ui2bVkPz6

thanks in advance for your advice!
Rick

[The Dude]

If you want to verify the switch and connections, just temporarily unhook the transformer wires and check continuity on the switch- both for working and for proper wiring.

[rickmccl]

Obviously that's what I will have to do if no one with any actual experience on the topic finds my note. Being such a tight fit, I hate to take the time to disturb it if I might avoid doing so.

[Tom Phillips]

Obviously that's what I will have to do if no one with any actual experience on the topic finds my note. Being such a tight fit, I hate to take the time to disturb it if I might avoid doing so.

Well...I have actual experience wiring these switches (but I'm not in the same location as my notes) so I used my my actual experience looking things up. See attached diagram.

I'll assume that you didn't purposely mean the tone of your post but ya did sound like Marie (The mother on the sitcom Everyone Likes Raymond).
I might have more info to add later about terminals #4, 5 & 6 but need to wait until I have my bench notes rather than doing more google now.

Since you don't know if your switch is even good you may need to disconnect it anyway to test.

Edit: See post #7 in this thread for detailed information about the switch style the OP asked about.

[The Dude]

I found that diagram also, Tom. I didn't post it because I think his switch is different looking at the picture. Maybe not so.

[Tom Phillips]

Yep. But there must be diagrams of other switch versions out there.

[Tom Phillips]

Here is the most relevant photo I have showing an impedance select switch in a Marshall Model 2204 made in 1980. One just needs to reverse engineer the connections by noting the stock Marshall wiring & wire colors.


Edit: Added photo of notes about the switch characteristics for this style (flat coin slot with 8 solder tabs mounted around the back edge of the switch body / no center tab) switch. The switch body is the same as the one used for line voltage select. Only the labeling changes. The switch is held together by an external snap ring in the center of the back side which can be removed to disassemble the parts for inspection / cleaning.

Cheers,
Tom

[g1]

Sorry to ask, but have you verified the complaint by checking power output into 4,8, and 16 ohm loads?

edit: never mind, looks like it will have to be straightened out first anyway.

[rickmccl]

>sound like Marie (The mother on the sitcom Everyone Likes Raymond).
Went right over my head, sorry Tom...

I was annoyed because I thought that the Dude had told me to go figure it out myself.
My apologies to him and the rest of the forum. Good thing I toned down my first response.

After thinking about Tom's comment that I didn't know if the switch was good, I realized the Dude is telling me the switch is likely to truly be broken; the observed wiring change would have been a bodge to keep it working, and the way forward is that the switch needs to be removed and inspected rather than just moving wires around. I do have snap-ring pliers, that is a very encouraging tip about the switch.

Tom, thank you very much for your notes, this will absolutely help diagnose.

Thanks to all for the responses, helps fill the time between chances to work on the amp. I can update when I've been able to get back to it.

[rickmccl]

Update - after discussing, the owner has decided to keep it as-is for now.
Thanks again for the assistance.