Feedback in percussion of a100 please help

[tonewheel1966]

Re: Feedback in percussion of a100 please help



Sean</P>


Have a look at the RT3 percussion hums thread on here a week back. Also the Benton Electronics site with the servicing info was a good starting point. Then the man himself diagnosed the problem first time! I know it's not exactly the same problem but it will help you to see the percussion stage and what is linked to it.</P>


Cheers</P>


Dave</P>

I am still getting feedback when I turn on the percussion. (A100) I am at a loss and have going at this thing for days. Would someone take pity on me and give me some ideas or even walk me through the troubleshooting?? (on or offlist) I would be so grateful! Here is the sound, perhaps it will help for you to hear what im dealing with. http://dl.getdropbox.com/u/129354/noise.mp3

Here are the facts.
- No feedback if I disconnect the H or J terminal
- No feedback if I disconnect all terminals
- Still feedback if I disconnect the P terminal
- No feedback if I remove V5
- only feedback when the percussion tab is fliped on.


Here is what I have done and still have feedback
- Replaced all tubes one at a time
- Replaced the entire percussion switch
- Double and triple checked the wires going from the preamp to the percussion switch and to resistor panel for a short
- Dissconected the drawbars, and preset wires so that the only thing connected to the manual are the tonewheel wires and the wire going to the matching transformer

Questions
- Could there be a problem in the preamp that would cause the feedback to occur only when the percussion tab is turned on, if so where do I start?
- Could this problem be caused by a problem in the Matching Transformer?
- Could this be occuring because I DON’T have the drawbar wires and preset keys wired to the preset key panel?

Thanks A Ton
Sean