JCM800 Micro input wiring problem

pelayostyle

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Hi everyone. I just got done building my JCM800 Micro EF80 and have a small issue with the input wiring. When the guitar is plugged into the Low input, its dead quiet and plays fine. When the guitar is plugged into the High input, I get a high-pitched whine, like oscillation.

I followed version v3.8a of the wiring guide on Rob's site except for the input wiring.

I'm using the following diagram for the input wiring.
InputJackWiring.jpg


And this is how I have it wired.
JCM800MicroInput.jpg


Could someone help me confirm that I have the input wiring right or wrong?


This is wiring diagram from Rob's site.
Screenshot 2022-05-17 152035.png


Bonus pics of the amp.
JCM800MicroTop.jpg


JCM800MicroBot.jpg
 
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dan40

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You went with Cliff jacks and Rob's drawing uses Switchcraft jacks. It's hard to tell if you wired the Cliff jacks correctly from the picture you posted. Did you look at an actual JCM 800 layout that uses Cliff jacks when you wired them? If not, there are several layouts online that use them such as the Triode Electronics layout and the Ceriatone layouts.

If you are willing, there is a quick test that I'm hoping you will try. Please unsolder the nfb wire that connects to the output jack and runs back to the board. Just unsolder it from the output jack and place a piece of tape over the bare end. Now try playing through the high input jack and see if the high pitch squeal still happens.

Edit: I'm sorry, I missed the pic you posted of the Cliff jack wiring. If you followed that correctly then you should be fine. Please try lifting the nfb wire from the output jack and let us know if that helps.
 
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dan40

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If disconnecting the nfb wire doesn't help, the oscillation that you are hearing may be caused by the wires leading to the preamp tube sockets. You have many of them suspended in the air and crossing at various points. This could be creating the oscillation that you are hearing when playing through the cascaded channel. Take a look at some pics of actual JCM 800's to get an idea of how the wires need to be routed to the sockets and pay careful attention to how each one crosses the other.
 

pelayostyle

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Thanks for the replies, guys. I have an update for today. I removed the NFB wire and still have the same issue. Yes, I can hear the guitar playing over the squeal. I did relocate pin 1 wire closer to the chassis and shortened the path to the pin 2 68k resistor. Is there anything else you guys see that I can improve upon? Perhaps pin 8 wire should run closer to the chassis too?

UPDATE #2
Plugged in my guitar and played on Low Input for about 10 minutes and all was well and good. Decided to switch over to High Input again and got the squeal/oscillation. I switched the amp off and immediately heard a local radio station playing for a few seconds!! Now I'm kind of convinced its my lead dress that's causing my problem.

UPDATE #3
Replaced V1 with a low microphonic and well matched Gold Lion ECC83 off my phono preamp. Still getting oscillation. =(
Web capture_18-5-2022_12825_photos.google.com.jpeg
 
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dan40

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One tip I can give is to move the wire leading from the board to the presence pot. You have it laying across one of the wires leading to the tonestack and this wire should be separate from any of those. The wires leading to the v1 socket look ok but the ones connecting to v2 and v3 really should be shortened and separated from each other. One or two of the wires from each socket are crossing paths with each other so try to keep the wiring to each socket separate. Pay special attention to the pin 6 plate wire on v2 and the pin 1 plate wire on v3 that are crossing each other.
 

pelayostyle

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I fixed it!! And I feel like an idiot for not catching it sooner. When I heard the RF leaking through the amp I started thinking about grounds. Specifically, the chassis acting as a cage to block RF from coming in. Then I thought about the how I deviated from the original diagram and used Cliff jacks instead of Switchcraft. Then I stumbled upon the Thor layout of the JCM800 which does use Cliff jacks and there it was.... I forgot the chassis ground at the input.

Patched it up and now the amp sounds great. I'm sure cleaning up the input wiring helped things make it even quieter.

Web capture_18-5-2022_15956_robrobinette.com.jpeg


Final picture before I closed it up.
IMG_7215.jpg
 

robrob

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I'm glad you found the problem. I was going to say the oscillation was probably being triggered by the twice as strong input signal from the Hi input jack, not the input jack wiring.
 




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