Fender Super Headknocker

[CaptainKirk]

I could sure use some help from you guys. I've been chasing a static problem on a Fender "Super - Amp", model 6G4-A. The symptoms are:

1. With power on, closing Standby switch makes a small "woosh" that quickly fades out.
2. After a few minutes, a random static sound appears. It's not steady - comes and goes. The volume of the static is not loud but is very noticeable. Both Volume controls are set at 0.
3. If Standby switch is opened and reclosed, swoosh reoccurs and static starts right away.

Here's what I've done:
1. Numbered the tubes from first preamp tube to the outputs as V1 - V8.
2. Found by pulling preamp tubes that pulling V5 eliminates the static.
3. Checked all power supply voltages. All within spec. for DC and AC ripple. Amp was recently recapped. All electrolytics were replaced. Power resistors looked new and resistance checked out.
4. Resoldered all eyelets on tag board.
5. Replaced V5.
6. Replaced V5 100K plate resistors, V4 56K plate resistor and V2B plate resistors with 1/2 watt metal film.
7. Found that grounding V5 grid pin 7 makes the most noticeable part of the noise go away (high frequency part). Grounding V5 grid pin 2 eliminated rest of noise.
8. Disconnected 470K resistor on V5 pin 6 then 470K resistor on V5 pin 1. Same effect as item 7.
9. Disconnected one end 250pf cap feeding V5 pin 7. Noise stopped. Replaced 250pf cap. Noise came back.
10. Replaced 0.05uf cap that couples V2B plate V5 pin 2 and 7 grids.
11. Temporarily disconnected 0.05uF coupling cap coming from V4 plate to V5 grid pin 7. Noise remained.
12. Prodded every device with a chopstick while powered. Noise didn't respond to knocking or tapping chassis.

At this point I'm out of ideas. Anybody see something I've missed?

[Enzo]

COnductive eyelet board.

[CaptainKirk]

Enzo:

Thanks for the assist. It was something I suspected along with all the other things I tried. What's the fix? I'm sure replacing the whole tag board would do it but that's a lot of work. Could I somehow bypass the problem area?

[MWJB]

Fire up the amp with all tubes installed, all volumes & tone fully clockwise - with a dry wooden chopstick press around the board and see if you can locate any particularly problematic areas (be careful, keep your palm over the upper end of the chopstick, don't want your fingers sliding down & hitting a heater wire). Try lifting the boards apart as well...see what effect that has.

Sometimes this problem can be localised and a rubber grommet around a board mounting screw under the noisy area, to put some space between the boards, can solve it, as can pulling an unused tube in the afflicted area, like V1 normal preamp? Otherwise fitting a non-conductive sheet of material between the circuit board and the insulating board might help?

If you are getting significant voltage leak across the top board from eyelet to eyelet, and eliminating problem areas of the circuit is not feasible - then replacing the board is your only option and the ideal solution at the end of the day.

[52 Bill]

You could try heating the board with a hair dryer to dry it out.

[Enzo]

I have successfully cleaned eyelet boards with a heat gun and contact spray. Not at the same time.

I discovered my first conductive eyelet board once while puzzling over a circuit. I rested my ohm meter probes against the board surface and noticed I got a reading.

[Sir Cuitous]

Enzo, for my burgeoning education, what is it that gets dirty on the board that can be cleaned this way? I have a Vibrolux Reverb with the same symptom that I have checked over and over.

[Enzo]

It is the board itself. Pick a spot somewhere and cram your probe points into the material surface a couple millimeters apart. If you get a reading, the thing is conductive.

A waxy coating can build up on the surface, and that absorbs moisture from the air. Or so the theory goes as I get it. The result is conductive. Not like tin foil or steel wool, but at 400v, it doesn't take a lot to make a small noise signal or create other trouble.

A search here ought to find some discussions of it. I also just googled "fender eyelet conductive" and after seeing this thread there were a couple discussions of it on other boards.

[Vaughan]

I'll just throw in a small suggestion - in our workshop, when we get a leaky pcb, we spray with isopropyl alcohol. allow to stand for a couple of minutes, then dry with hair dryer (an essential tool on my bench). The alcohol displaces the moisture. Obviously this won't work if the board has started to carbonise due to overheating or arcing.

[Sir Cuitous]

Thanks for the info. I was familiar with the phenomonon, but not the underlying cause. I'm a "why" freak I guess.

[JoeM]

Are there certain years of manufacture of Fenders that were more prone to the conductive board problem than others? Or is it more due to the environmental conditions the amps are in, i.e. humid locations etc.?

[52 Bill]

Are there certain years of manufacture of Fenders that were more prone to the conductive board problem than others? Or is it more due to the environmental conditions the amps are in, i.e. humid locations etc.?

I've heard some say that there are specific years that were worse than others, but I haven't ever seen a real pattern develop. It's really not that common of a problem. It may have something to do with the specific run of the material itself.

For a number of years Fender either laquered or wax dipped the stuffed boards before installation into the chassis.

I've repaired amps that have been floating in floodwater for a couple of days and not had any problems once the boards have been cleaned and dried.

[Bruce / Mission Amps]

I've heard some say that there are specific years that were worse than others, but I haven't ever seen a real pattern develop. It's really not that common of a problem. ....
I've repaired amps that have been floating in floodwater for a couple of days and not had any problems once the boards have been cleaned and dried.

Me too... no obvious patterns although I would never rule it out.
With respect to a vintage Fender amp developing any issues with a bad board, ...I've only run into this conductive board issue two or three times in many years of working on this stuff.
However, I've found that an old amp like this is more likely to have a dirty or ill fitting preamp socket, oxidized ground connection of the input jacks to the brass plate, a broken wire from the eyelet board to the brass plate or an oxidized solder joint on the eyelet board or the brass plate.

[Gtr_tech]

Tried replacing the standby switch? They don't last forever....'specially since they're not rated for the application.

[CaptainKirk]

In answer to the question about replacing the Standby switch, no I haven't. Would that cause the intermittent static? I'm thinking that if the static disappears when V5 is removed, the problem is not a general power supply issue.

Can anyone give me more detail about the cleaning process?
1. Do I need to remove components or just clean around them? Focus on the eyelet area?
2. Do I need to clean the bottom of the component board and also the chassis insulator board?
3. Can I get at the bottom of the component board without removing all the tube socket wiring?
4. Should I scrub with a soft brush (plastic, brass?)? How much?

I'm a detail guy ya knowhttp://music-electronics-forum.com/images/smilies/smile.gif