FIM-41 Schematics?

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DelD
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FIM-41 Schematics?

Post by DelD » Mon Sep 15, 2014 4:23 pm

Does anyone have schematics for and FIM-41? Mine has developed a weird Cal Osc problem thanks to a nudge off my hood :( .

Thanks!
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Deep Thought
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Re: FIM-41 Schematics?

Post by Deep Thought » Mon Sep 15, 2014 4:55 pm

That'll probably be something wrong with the rotary switch wafer stack and PI will want four figures to repair it... :shock:

I am pretty sure they've never published the complete diagram.
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Re: FIM-41 Schematics?

Post by RGORJANCE » Mon Sep 15, 2014 5:01 pm

This may be way off base, but I have seen two of these critters do really weird things with the cal osc that turned out to be cold solder joints. Could be the whack alongside the 41's head loosened something up.

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Re: FIM-41 Schematics?

Post by kcbooboo » Tue Sep 16, 2014 2:46 am

They DID publish the entire schematic for both the FIM-21 and FIM-41 in the original manuals. Being PI, they only want $50 for each one, and I don't think they're even up-to-date to match the last version they made. For obvious reasons they didn't post these on their web site; why give something away when you can charge for it.

The FIM-41's band switch consists of a rotary switch with an attached slide lever that actuates five multi-pole double-throw toggle switches on the main board that select the appropriate tuned circuit components. I believe the same board is used on the FIM-21 but they hard-wire the not-installed switches for the A-band and probably don't populate the board with parts for the B-band. I've only ever seen an FIM-41 so I can't tell what else may be diferent.

Bob M.
Last edited by kcbooboo on Sat Sep 20, 2014 5:29 am, edited 2 times in total.

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Re: FIM-41 Schematics?

Post by Aetheradio » Wed Sep 17, 2014 3:41 am

had a problem where the cal would peak on the tuned frequency normally, but when cal switched off to do the reading the unit would be off the station by about 20 kHz at each attempt.

We have inherited two of these and the other unit in worse shape but useful in that someone had helpfully pencilled the purpose of each trimmer / coil inside.

I first thought the osc coil was mechanically jumping as it could be corrected by a touch on the tuning slug but it seemed more touchy than that - more like a dry solder joint within or on the coil, and the fault would reappear before one could get the unit back in the box. This was the last coil at the edge of the board and could be got to with removing a shield can and the bandswitch slider and removing the board a bit. Resoldering those coil terminals did the trick everything worked perfectly after that which is fortunate as I would otherwise have been attempting to swap out the coil - not an easy task.

I've searched the site for manuals but that doesnt seem an option - best of luck

Ralph

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Re: FIM-41 Schematics?

Post by Bill DeFelice » Wed Sep 17, 2014 5:26 am

Deep Thought wrote:That'll probably be something wrong with the rotary switch wafer stack and PI will want four figures to repair it... :shock:
When I sent mine in a couple of years ago it was mid three-digits for the switch, the rest was labor.
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Re: FIM-41 Schematics?

Post by W7SVJ » Sun Sep 28, 2014 11:03 am

I do have a FIM-41 manual with schematic. I could copy the page.

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Re: FIM-41 Schematics?

Post by Radio Ranger » Thu Oct 16, 2014 12:00 pm

I also have a problem with the calibration oscillator in my FIM-41.

I posted a question on another chat board and found out that often the only problem is an intermittent connection in one or more of the socket mounted IC's on the printed circuit board, and that re-inserting the IC's is likely to solve the problem. I haven't had time to check to see if this is my issue or not, but if the offer of a schematic diagram for the FIM-41 is still good, I'd love to have a copy.

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Re: FIM-41 Schematics?

Post by kcbooboo » Thu Oct 16, 2014 12:58 pm

There are four socketed ICs on the main board of the FIM-41. Be very careful that you draw which way each one is plugged in, because they aren't all inserted in the same direction. Unplug one at a time, use a toothbrush and 91% or higher isopropyl alcohol to clean the sockets and the IC pins, then reinsert. Apparently this common fix is something PI is incapable of, or unwilling to do when they find any material built up on the IC pins or sockets. Their solution is to replace the sockets and ICs and charge you big bucks. I refused their solution and did it myself in 10 minutes.

Bob M.

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Re: FIM-41 Schematics?

Post by Radio Ranger » Fri Oct 17, 2014 9:49 am

Thanks to kcbooboo for the additional information.

Well, mixed results with cleaning/resocketing the IC's---the cal oscillator now functions normally most of the time, but of course the real problem is with the A/B bandswitch.
Since I can measure MW harmonics with a spectrum analyzer and a Chris Scott notch filter and calibrated loop antenna I'm tempted to just jumper over all the damned toggle switches and rotary switch and turn the meter into an FIM-21. Unless any of you can pinpoint to me which of the band switches are the likely culprit?

The only clue I have about WHICH switch might be bad is that occasionally the cal oscillator doesn't work on the MW position, but rotating the bandswitch a couple of times makes it work normally.

Steve B

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Re: FIM-41 Schematics?

Post by kcbooboo » Fri Oct 17, 2014 2:36 pm

As you've probably noticed, the BAND switch turns a rotary switch (I think for metering only), which you can squirt, and it activates a bunch of double-throw toggle switches on the main board. I'd try squirting some contact cleaner into the switches where the toggles enter, with the unit oriented so the cleaner goes down into the guts, then actuate each switch a dozen times to hopefully clean their contacts a bit. That phenolic actuating rail comes off very easily and you need to remove it to access the ICs.

Other than that, you'd have to determine what part of the unit is not switching. They switch the front end tuning coils, receiver oscillator, and calibrating oscillator to cover both bands. Remember it was a FIM-21 before they added the "B" band so all those switches are just changing the coverage range from 540-1600 kHz up to 1600-5000 kHz. The FIM-21 schematic is essentially identical except for the switches and "B" band components.

I suppose when the unit malfunctions, you could attempt to pull the main board out enough to access the switch's soldered leads and perhaps use an ohmmeter to figure out which one(s) aren't closing. I've never had my main board out and I don't know what's involved in doing so, but there's no other way to access the solder side of the board so that would be your last resort.

Bob M.

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Re: FIM-41 Schematics?

Post by Radio Ranger » Fri Oct 17, 2014 3:40 pm

I removed the phenolic board that drives the toggle switches from a cam on the bandswitch so that I could wiggle each of the toggles while I watched the front panel meter, but none of them seemed to change the readings, so then I did what you mentioned and put a shot of De-Oxit into each of those switches, plus into the rotary switch and after about 30 excercizes of each switch things seem to all be operating normally, both A and B bands. I looked at what it would take to remove the whole receiver board and decided not to go there, and apparently didn't need to. One of the Site Safe engineers lives in the area, so I'll contact him to check calibration against his FIM-4100. Thanks to all for your ideas.

Steve B

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Re: FIM-41 Schematics?

Post by kcbooboo » Fri Oct 17, 2014 4:18 pm

Well, that's a good sign. Nice to have a 4100 to compare to.

I was about to edit my last post and suggest that you could set the unit to the "B" band, remove the phenolic actuator, turn the unit on, and toggle each switch individually to find the one that's giving you problems. But the DeOxit at least got you out of a jam for the moment. The switch may go bad again.

I don't know how much has to be unsoldered to get the board out. The wiring harness might be long enough to allow extraction but I'm not sure about the various tuning coils and caps on the board. Maybe they come out with the board, in which case it's just 4 or so screws in the corners that hold it in place.

Bob M.

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