intermittent noise/microphonic jacks?

[Jelle S.]

A little sorry my first post is a question instead of a useful contribution, but I'll try to make up later.

I've recently completed a 5E3 with bits and bobs from everywhere, but an intermittent noise resembling rumble/crackle keeps popping up infrequently. It mostly occurs when the amp or cab is touched/tapped/bumped, so I tried to recreate it by tapping around with a chopstick. The taps on the chassis were amplified loudly as popping and thumps: sounded a lot like micophony. Sometimes the tapping induces the rumbling/crackling, after which the chassis needs an encouraging whack to shut up again.

First I tried to isolate the problem by pulling tubes and V1 (12AY7) did the trick. Swapping in new tubes in V1 didn't help. Some more tapping around on the chassis indicated that the problem only occurred when the Bright/Instr volume was up; the Normal channel with the Instr volume all the way down didn't amplify the taps. So this (probably?) narrows it down to the half of the 12AY7 connected to the Instr channel.

Next I tapped the resistors, other components/hardware in the vicinity and pushed connections on the board, but nothing happened. Same for the wiring: nothing indicating a (near-)break or something. Just to keep myself busy I changed the 100k plate resistor and 820R cathode resistor, but that only solved a very slight static popping that happened sometimes. Then checked the soldering and reflowed some connections that may have been questionable if I were working for NASA, but alas...

Last thing I tried was some selective tapping on the chassis: the Instr inputs and the chassis close to them are very prone to picking up taps/bumps (no cable plugged in). As I've never had this problem before, I don't know if it's common for the shorted inputs to pick up even the slightest tap. The input jacks seem to be shorting ok, if a bit wimpy (Switchcraft 12A). If this is not normal, it would seem that an input jack is the problem, so I replaced the 1M resistor on input 1: no change. So, is it possible that the jacks are microphonic/broken or are they just picking up the vibrations that are then amplified by a faulty component/connection downstream?


Thanks for reading and suggestions are much appreciated.

[loudthud]

The contacts on the jacks can become intermittent because of oxidation and insufficient tention. You can test the connection with an ohm meter or touch a metal object such as a screw driver between the chassis and the tip contact of the jack. Also, make sure the nut is tight to the chassis.

[Jelle S.]

Thanks for the suggestion, loudthud.

Put the DMM to work on the jacks and you are right: I guess at least two jacks had oxidized to the extent that the plugs didn't short properly, even though they were brand new. Those two read anywhere from 12 to 300 ohms to sometimes infinite resistance between tip and chassis/ground; the others were not as bad.
The best part of it all was that some light sanding and then cleaning with alcohol brought the resistance down to 0.3 ohm, which is the lowest my meter can go. As far as I can tell after running the amp for some hours is that the problems are history.
In all future builds I will treat the jacks first to prevent this problem; it would have saved me several hours of work and frustration. Thanks again!