F-100 MFD pickups all of a sudden feeding back

[msintros]

I hadn't plugged my F-100 in for a couple months and the first time I did, as soon as I switched over to the distortion channel on the amp, started getting uncontrollable feedback. I was playing at the same volume and settings as I always have. I tried a different guitar and it was normal, so it is definitely the guitar and not the amp. The cable and metal parts on the guitar also seem really microphonic (tapping on them comes out very clearly from the amp, even when not on the distortion channel).

A week or so ago my cat kinda knocked the guitar off of its stand. It didn't sustain any obvious damage, but now I'm wondering if that caused this to happen. Does anyone have any suggestions? I could open it up and have a look but I don't know what I'd be looking for.

[Craig]

I hadn't plugged my F-100 in for a couple months and the first time I did, as soon as I switched over to the distortion channel on the amp, started getting uncontrollable feedback. I was playing at the same volume and settings as I always have. I tried a different guitar and it was normal, so it is definitely the guitar and not the amp. The cable and metal parts on the guitar also seem really microphonic (tapping on them comes out very clearly from the amp, even when not on the distortion channel).

A week or so ago my cat kinda knocked the guitar off of its stand. It didn't sustain any obvious damage, but now I'm wondering if that caused this to happen. Does anyone have any suggestions? I could open it up and have a look but I don't know what I'd be looking for.

Check out the G&L Knowledgebase for posts on microphonic/squeal pickups. There are several with some suggestions to resolve it.

Let us know once you get it resolved and how it was resolved.

Hope this helps.

[msintros]

Thanks, I took it apart today to have a look. Unfortunately it doesn't look like it's a loose plate in the pickups - those both look secure and didn't see anything that looked weird in there. I also took out the electronics to check those out and don't see anything out of the ordinary.

If it is doing it on either pickup selector position, does that decrease the likelihood that it is the pickups themselves?

[Craig]

Thanks, I took it apart today to have a look. Unfortunately it doesn't look like it's a loose plate in the pickups - those both look secure and didn't see anything that looked weird in there. I also took out the electronics to check those out and don't see anything out of the ordinary.

If it is doing it on either pickup selector position, does that decrease the likelihood that it is the pickups themselves?

Have you looked under the pickup covers to see how the wax potting looks? If this guitar is a Leo-era F-100, the wax potting is likely dried up or evaporated.
You should consider having them re-wax potted. Reference post: My MFD went massively microphonic! Suggestions?.

Hope this helps.

[msintros]

This is no doubt a stupid question but how do you take the covers off? It looks like mine don't even have "covers", they look like they're constructed of just a solid piece of plastic which has magnets glued or otherwise affixed inside. I've never taken apart a pickup before but it doesn't look like there is an easy way of taking separating the plastic part from the other parts. Do you just pry them out? I'm worried about damaging them.

A few other things I've noticed:

- When I flip the middle switch (I think this is phase reversal?) I get a very loud buzzing in addition to the feedback, as though the guitar is not grounded at all. I've looked at the electronics and there do not appear to be any loose joints or anything. Is it possible a pot or switch has been damaged?

- The ground plate inside that cavity is loose. If I pull it out there looks like glue or wax that used to hold it in place. I have got out all the old pieces with no change to the sound. I am not sure the best way to fix this back in place - it's hard to tell what it was that was holding it down, but it looks like some kind of dark colored glue. I'm not sure if this would account for the problem or not. I kinda temporarily taped it down just to keep it from floating around totally freely in there with no effect on the sound.

[Elwood]

This is no doubt a stupid question but how do you take the covers off? It looks like mine don't even have "covers", they look like they're constructed of just a solid piece of plastic which has magnets glued or otherwise affixed inside. I've never taken apart a pickup before but it doesn't look like there is an easy way of taking separating the plastic part from the other parts. Do you just pry them out? I'm worried about damaging them.

Here's a shot of some empty covers...maybe a light hot air treatment with a hair dryer would get them going.
Be Careful, Go Slow!

[Craig]

This is no doubt a stupid question but how do you take the covers off? It looks like mine don't even have "covers", they look like they're constructed of just a solid piece of plastic which has magnets glued or otherwise affixed inside. I've never taken apart a pickup before but it doesn't look like there is an easy way of taking separating the plastic part from the other parts. Do you just pry them out? I'm worried about damaging them.

A few other things I've noticed:

- When I flip the middle switch (I think this is phase reversal?) I get a very loud buzzing in addition to the feedback, as though the guitar is not grounded at all. I've looked at the electronics and there do not appear to be any loose joints or anything. Is it possible a pot or switch has been damaged?

- The ground plate inside that cavity is loose. If I pull it out there looks like glue or wax that used to hold it in place. I have got out all the old pieces with no change to the sound. I am not sure the best way to fix this back in place - it's hard to tell what it was that was holding it down, but it looks like some kind of dark colored glue. I'm not sure if this would account for the problem or not. I kinda temporarily taped it down just to keep it from floating around totally freely in there with no effect on the sound.

Here's a photo of my F-100:



Each pickup cover has three screws, so remove them and you should be able to lift out each pickup.
As Elwood said, try using a hair blower if the covers don't lift off easily. Reference post: Pup's covers swap.

Regarding the brass ground plate: Make sure there are four gound wires soldered to it and insure the ground wire from the plate to the bridge is secured at the bridge.
See this Album in our Gallery for wiring diagrams: Album: F-100 Wiring Schematics and Picture Diagrams.

Please post photos of your guitar and wiring under the control plate. Having more eyes checking your wiring might be helpful.

[KenC]

That's a gorgeous F-100!

The chance of both pickups going microphonic at the exact same time, after 35 years, seems pretty slim to me. I would look more closely at the grounding connections (as Craig mentioned), and also make sure none of the solder joints on the controls have deteriorated. I recently had one bad solder joint on an '86 ASAT cause problems in all pickup combinations.

Ken

[msintros]

I can't see anything wrong with the wiring but I agree just from a basic troubleshooting standpoint that it doesn't seem like the pickups would both go microphonic all of a sudden. I looked at taking them apart but it seems the inards are pretty well glued in there (if I kinda pry at the side of the plastic I can see the glue kinda stretching apart in the gap there. It would probably be quite a pain to take them apart and I don't want to do that if it's not likely to be the pickups.

Some new things I noticed:

- The buzz coming out of the guitar changes when I touch any metal part on it. This probably was like that before to some extent but I feel like it is a lot more exaggerated now.

- If I touch the bottom row of poles on either pickup, it buzzes more, but not the top row which does nothing if I touch them. I don't know if this helps at all in trying to figure out where the electronics fault could be

- I only see two lines attached to the brass ground plate: 1 coming from the bridge (which I took off to see if it is attached and it is) and 1 coming from the rest of the electronics. There are no lines that should be going there but are detached. So this looks quite a bit different from other wiring diagrams I've seen.