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View Full Version : Prosonic Hum Fix per Fender Tech Notes??


red_cahill
05-10-2011, 07:21 PM
Fender Tech Notes: TN 97-6 states: "Audible hum when no signal is applied(more apparent on normal channel)"

It's basically a small wire from V6 pin 7 to the PCB and it needs to be replaced with a shielded one. Simple enough. However the tech notes are missing the the next page with the illustration so I'm a little stumped. Where do the shields go exactly? On the end to be attached to the PCB the shield should be attached to "reverb jack ground"-- I assume that's one of the RCA jack's two posts(tip or sleeve) if so which one? Also I don't know what to do with the shield on the other end that's connected to pin 7. Do you cut it off or does it connect somewhere?

BTW-- This amp is early LO serial. Production "FL". I take it that means December 1995. Blue jewel lamp. Yes, blue not green. Only one cap in the bias circuit has been changed. I'm a little hesitant to touch the amp at all, but a shielded wire? Not a biggie, I just don't want to mess it up. I've looked and looked and can't find this shielded wire subject. Help is appreciated.

WaltC
05-10-2011, 08:09 PM
when using a shielded signal wire, only one end of the shield should be grounded. (tie that one to the reverb jack ground per the instructions I guess) The reverb jack ground is the sleeve, not the tip.

On the other end, what I do is, strip the outer insulation and cut the shield back as close to the trimmed insulation as possible, then put a piece of shrink wrap over the outer insulation and overlap just a touch on the insulation of the center wire. Strip the end of the center wire and solder it to v6P7.

this is pretty much standard procedure for all shielded signal wires with the exception of the location for the ground. In most cases I'm running a shielded cable to a pot and I just solder the shield to the ground on the pot and treat the other end as I described above.

red_cahill
05-12-2011, 10:48 AM
Thanks for clearing that up for me.

If anyone's interested, I didn't notice any difference. The slight hum I still hear may have something to do with the noisy reverb circuit which I'm just about to address as soon as the parts come in. I'll update with the results.