'73 JMP 50w Input II - guitar noise

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wgstech
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'73 JMP 50w Input II - guitar noise

Post by wgstech » Tue Apr 10, 2018 11:05 am

I've recently rebuilt a JMP that had previously been modded to the point of being unrecognizable. All new electrolytic caps, new ARS filter caps to take care of a pervasive hum, a few out of spec resistors, a UF5408 diode in front of the bridge rectifier, and added a PPIMV.

When using either of the main inputs, the amp performs beautifully. On the Input II side, I'm getting noise from the guitar's volume pots and pickup switch... Loud pops with the pickup selector, and static noise as I roll the volume pots back and forth.

Does this sound familiar to anyone? After a few days of searching, I haven't been able to find anything remotely similar to the problem I'm having. I'd appreciate any help I can get, thanks!

jarrodthebobo
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Re: '73 JMP 50w Input II - guitar noise

Post by jarrodthebobo » Thu Apr 12, 2018 1:25 pm

Sounds like you're getting DC on that input. You my have a leaky coupling cap or something. I'm not entirely knowledgable on the signal path of the JMP, but if it's anything like that JCM800 that follows it, I'd assume that input 2 bypasses the first gain stage? Perhaps the coupling cap in that following stage is leaky?
"I hear you knocking but you can't come in-
I know you've been drinking Gin." -Louis Jordan

danman
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Re: '73 JMP 50w Input II - guitar noise

Post by danman » Thu Apr 12, 2018 7:39 pm

Have you tried a new tube in v1? Sometimes a bad tube will put DC voltage on the input. Also make sure the 1meg resistor on that input has a solid connection on both ends.

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neikeel
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Re: '73 JMP 50w Input II - guitar noise

Post by neikeel » Fri Apr 13, 2018 11:10 am

I agree that both the above bits of advice are important.
Check the switching jack contacts are all clean and that the grounds are clean and tight.
If you have access to a scope then you can pinpoint where the extraneous noise is being injected.
Neil

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