Heathkit IM-25 (also IM-16): Things I Have Learned About Restoring These To Operation

**** See addendum at the end of this description
PDF files for the battery eliminator design that works with the IM-25 and also with the IM-16 may be downloaded here: www.serpentwebs...
Over the last few years, I have restored several of the Heathkit IM-25 solid-state VOM's, as well as a couple of the stripped-down IM-16 version. They were all ones that I bought on eBay, and they all appeared to have once been operational and showed evidence of working for a long time, yet they all exhibited the same kinds of failures by the time I obtained them.
In this video, I show the many tips, tricks and other insights I have gained from all these restorations. Hopefully these will be useful to others trying to resurrect other IM-25's (and perhaps some IM-16's).
****** Addendum 4-16-23
In this video, I glossed over one kind of failure that can cause problems with this meter. While I DID mention the second FET which is part of the AC Attenuator circuit, there are a handful of small electrolytic capacitors which I did not mention. One of them is connected to the wiper of the DC Bias potentiometer, and if this capacitor is too leaky, either it will be impossible to Zero the meter, or the Zeroing will be drifty, and this might also be one cause of the meter always pegging hard over to the right. Another couple such capacitors are part of the AC Bias potentiometer sub-circuit, and if one or both of these is excessively leaky, it can be impossible to properly obtain a 6V measurement that the Heathkit manual's calibration procedure calls for. Unfortunately, some of these capacitors are located in places that are very difficult (but not impossible) to reach for replacement. Personally, if their leakiness is not too bad, and I can still calibrate the IM-25 properly, I tend to leave then as-is. But if I were to be restoring one of these for actual use, I would certainly expend the extra effort and cussing to replace them.

[@davidlane9379]

I recently purchased an IM-25 and then found your excellent Videos on it's repair and restoration. Thank You so much for doing this, they were a great help in bringing mine back to life. Just a couple of my findings: 1.) After doing the rework/repair per your instructions mine was still pegging the meter all the way to the right. After much testing I found that C14 was very leaky and would not allow the DC Bias to properly set the bias on Q7. A new 10uF at C14 repaired this problem. 2.) I was unable to set the AC Bias to 6V across the 47K resistor(4.25V max) Replaced the remainder of the 10uF caps in the AC Attenuator and easily set the voltage to 6V. It is now Calibrated and ready to GO!

[@youtuuba]

Yes, I briefly (and probably inadequately) touched on this issue in this specific video, when I mentioned that I found it too difficult to get to some of the switch-mounted components that are sort of behind a nest of other wires and such, and that sometimes I just don't completely restore then in that case, and just let my buyer(s) know that there is still an issue. You are correct that, along with the second FET (used as a buffer in the AC Attenuator circuit) there are a few 10uF electrolytic caps that are hard to get at for replacement. Luckily, on most of the ones I have restored, those caps have still be at least good enough to not really require replacement. On the IM-25 in my own collection, I DID replace ALL of the capacitors because I wanted it to be as close to perfect as possible. I should have highlighted this more thoroughly in this video.

[@michaelkpeterson]

Thank you for this excellent series on restoring the IM-25. I just cracked the lid on the one I built 50+ years ago, and it is in the sad condition you predicted. Is it possible that the circuit board artwork and BOM is available for building your 3V/1.35V battery eliminator board? You've done an outstanding job on these IM-25's.

[@youtuuba]

michaelpeterson, I have just revised the descriptions on my IM-25 & IM-16 related videos to include a link to a download page where two battery eliminator files may be obtained. The software I use for my home-made PCB's goes not generate Gerber files, so to use the PCB artwork PDF, you will need to use one of the direct or photo resist methods to make your own board.

[@michaelkpeterson]

@@youtuuba Thanks a million for the PCB files. I'll definitely be using them!

[@michaelkpeterson]

@@youtuuba I've discovered that the 2N4304 FET is basically unobtainium. The only source I've found is InterFET, and their minimum order is 500+ units. Not exactly what I was looking for. Mouser lists them as out of stock, and their source is InterFET. Anyone looking to restore an IM-25 should probably be aware of this.

[@youtuuba]

@@michaelkpeterson , I recently bought some from Quest in California. Have you tried octopart.com? It usually finds several sources for even very rare old parts. I also have had excellent luck finding NOS semiconductors from Jotrin Electronics in Shezhen, China. Also, the way Heathkit used this FET in their meters, it should be easily substituted with any number of similar general purpose FETs.

[@johnkemas7344]

I found that the 2N4304 FET can be replaced with a NTE133 FET unit. NTE unitas are 25 V vs 30 v.

[@tomcroley7084]

The IM-25 seems to have many problem issues, so I wonder what makes it worth all the trouble when there are plenty of other meters to restore. Is there something unique about it? You do a superb restoration and I enjoy your vids v much!

[@youtuuba]

Tomcroley, the IM-25 is a classic meter from that period, which makes it collectible, and thus worth restoring. I don't think anyone who buys or owns one of these today does so with the intention of actually using it, but it is less gratifying to collect broken junk than it is working artifacts.

[@lelandrogers1078]

I built the GC-1197 Digital Grandfather Clock about 35 years ago. I loved it, but the bulbs burned out too quickly and eventually I sold it because I couldn't find the bulbs. I also built the training device to learn machine language programming, but didn't keep it up. My brain isn't wired for programming computers. 😁

[@johnkemas7344]

Excellent tutorial! $5.00-9.00 per FET is absurd pricing!! Sign of the times -gouge, gouge gouge!!

[@youtuuba]

Johnkemas, I don't know why you would assume that a NOS obsolete FET costing a high price is "price gouging" or especially indicative of our times. It is just plain old economics "supply and demand", and has been a "thing" as long as humans have traded things for other things.

[@johnkemas7344]

Still a great video, well done my friend. I don't disagree on the NOS 2N4304. But the NTE 133 is a current repelacement device, very similar specs. Same as guys asking $30.00 -$100 for Heathkit meters like the IM-25 and IM-18 another common for sale items that are usually non-functional, sold for parts or repair, beat up and in many cases badly built kits with crap solder joints, and bad construction that originally cost about $45 40-50 years ago. Like you said, you just never know till you open it up!. No body forces anybody to buy these units at that asking price. If you want one bad enough you'll pay the price and take the chance. I buy instruments for use, not collectability. I have a near mint IM-16 I bought at a hamfest several years ago in near mint condition because I like the big meter and 11 meg ohm input. It was on the "Make an Offer Table" . I offered $10.00 and they gladly accepted. Certainly a good deal by any standard, but I could test it before I bought it and it required minimal restoration. Is what it is! @@youtuuba