Bare Knuckle Pickups Forum

Forum Ringside => Tech => Topic started by: TomW on August 18, 2010, 10:18:14 PM

Title: JCM800 Problem
Post by: TomW on August 18, 2010, 10:18:14 PM

Hi guys,

The high input on my jcm800 4104 seems to have stopped working. Has anyone got any ideas of what could have caused this? The low input still seems to be working fine. I also just changed the preamp valve in the V1 position, but I don't see how that could have caused the high input to stop working.

Any ideas?

Title: Re: JCM800 Problem
Post by: Twinfan on August 18, 2010, 10:25:44 PM

Broken input jack or solder joint?  Should be a cheap and easy fix?

Can you put the old valve back in to check?

Title: Re: JCM800 Problem
Post by: gwEm on August 18, 2010, 10:59:23 PM

i haven't looked at the circuit in a while, but i seem to recall the high input has an extra gain stage from V1. A dodgy valve could cause this. twinfan makes good suggestions, also look at the valve holder.

Title: Re: JCM800 Problem
Post by: martinw on August 19, 2010, 10:06:34 AM

The High signal on those doubles back through the switched part of the Low input jack. As hardly anyone ever uses the Low jack, the contacts of that switched part get dirty and break down. Try using some contact cleaner and inserting a jack plug a few times in the Low input.
If not, and a new valve doesn't fix it, you'll need to have a look inside and break out a meter.  :)

Title: Re: JCM800 Problem
Post by: TomW on August 19, 2010, 11:14:50 AM

Cheers for the suggestions. I'll have a go at them later today and see what happens. Thanks