Fun with a PI FIM-41

AM Radio discussion. Directional arrays are FUN!
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kcbooboo
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Location: East of the Mississippi

Fun with a PI FIM-41

Post by kcbooboo » Mon Nov 30, 2015 4:14 pm

A station's old FIM-41 has a very noisy CAL OSC pot. Apparently it's been that way for some 20 years. I was asked to see if I could make it better. As I recall, it was an A-B Type-J pot, 10k 2w linear, the kind that are quite well sealed with varnish. Couldn't squirt anything into it unless I took it all apart or drilled or cut a small hole in the metal cover. First problem was to remove it from the FIM. Easier said than done. I removed the knob and mounting hardware, then determined that the shaft was about 1/8 inch too long to allow me to pull the pot out through the back. So I put it all back together and said "live with it for now". That was a few years ago.

I had the chance to use the meter a few weeks ago and was annoyed by the noisy CAL OSC pot, so I looked back into it. There had to be some trick to extricating the pot. I really hated to do it, but this morning I called Potomac Instruments. A very nice receptionist answered the phone. I said "Hi. I need to talk to someone about working on a FIM-41." She said "You're working on it yourself?" I said "Yup." There was a shocking pause in her voice, then "Just a minute."

A very nice young man then came on the phone and I told him that I was attempting to replace the CAL OSC pot on an FIM-41 and before I could mention the difficulty I was having, he said "and you can't get it out, right?" "Yup. What's the secret?" So he told me that I had to remove the phenolic switching linkage, a shield, then the seven screws holding the main board to the chassis. I could then move the main board away from the front panel enough to let me get the pot out. He even told me there was a nylon washer on the bushing between the pot and the chassis that had to be there; it must be a spacer since the bushing mounts to the front panel with a lockwasher. He also told me to be very careful when putting the screws back in since a couple are obstructed by the main variable (tuning) capacitor. He was just so forthcoming with useful tidbits, it caught me by surprise. He asked me if I wanted to purchase a new pot from them. I said "I think it's a common 10k 2w pot and I can get one for under $15, but just for fun, how much does PI want to sell me one for?" "We have them for $54. They're specially made for us and take months to get." Ohhh Kaaaay, I'll stick with buying one on my own. I thanked him profusely and that ended the call.

Since I didn't have that old unit in front of me, I opened my much newer FIM-41 and looked for some markings on the pot. I could clearly see RV4NAYSD103A stamped into the cover. I found over 30 auctions on ebay and both Mouser and Digikey knew about that part number and had them for about $9; in fact Digikey had over 4,000 of them which could be bought for under $5 each in quantity. If PI bought every one, they could replace the CAL OSC pot in every FIM-21 and FIM-41 out there. I ended up buying the exact one in my unit, made by PEC in Canada, from an ebay seller in Rhode Island for under $9 shipped free.

I figured we all could use a good laugh every once in a while.

Bob M.

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davedybas
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Re: Fun with a PI FIM-41

Post by davedybas » Tue Dec 01, 2015 11:23 am

Or you could have sent it in for repair and paid the usual $1500 to get it back :D

kcbooboo
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Location: East of the Mississippi

Re: Fun with a PI FIM-41

Post by kcbooboo » Mon Dec 07, 2015 5:00 am

Follow-up: after removing a lot of screws and carefully moving the main board out of the way, I managed to get the old pot out, unsolder the wires, attach them to the new pot, mount it, put everything back together, and it actually still worked on the "A" band. I switched to the "B" band and the unit went totally dead; not even a "battery" meter movement. So I opened it up again, jiggled some wires, and now it was working again. Not sure which switch wasn't making a good connection as the A/B band knob is on a rotary switch shaft and that activates a sliding mechanism which operates five toggle switches on the main board.

But it's working now, and I discovered the 5-turn 20k wirewound "GAIN" pot is also noisy. I found some at Mouser and Digikey for $25, so I expect PI should want almost $150 for one. At least this part is out in the open and easily accessible.

Bob M.

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Bill DeFelice
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Re: Fun with a PI FIM-41

Post by Bill DeFelice » Mon Dec 07, 2015 5:57 am

@Bob: Thanks for sharing your experience with the repair. I know I don't use mine all that often but at least for these more minor issues it gives us an alternative than having to send the beast back to the mother ship.
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