The Heathkit IM-4100 Frequency Counter

In this video I look at an example of the Heathkit IM-4100 digital frequency counter including its history, features, a demo of it operating, and the restoration of this unit.

[@rancebradley9141]

I realize this is an old video but I was very happy you went to the trouble to create and post it. Thank you.

[@RobManegold]

Jeff Tranter - Thank you for the link to the Manual. I am donating a IM-4100 to my local high school along with my IM-1212 Nixie tube digital multimeter, and ET-3100 breadboard with a ton of parts.

[@PremMenon]

Thanks for the detailed and informative video(s), Jeff; checked out the Heathkit IM-2410 and IM-2420 video as well. I got an IM-4100 as a package deal with some parts for a linear a few years back. After all these years, your video encouraged me to pull it out of cold-storage and bring it back to life. Bridge-rectifier diodes were burnt (common-problem ?) and many wires were flying-leads. The OM who built mine has done a great job; has included a divide by 10 prescaler on a well (homebrew) made FR4 PCB - piggy-back mounted in the space near the trafo. The sensitivity is better than spec'd @ -30dBm in the MHz range - upto about 90MHz and then drops-off to around -10dBm at 250MHz. A little 'deaf' close to 290MHz @ around 0dBm. The sensitivity is apparently better because of the 1M i/p Z against my Marconi 2022D's 50 Ohm o/p Z. Over all, a great instrument for its time. BTW, he's mounted a prescaler in/out switch on the front panel with professional lettering/labeling. I replaced the burnt diodes with 4x 1N5401 - by drilling larger holes. Not sure why Heath used 1A diodes when the spec'd current was already 1.25A. With the prescaler even the 3A diodes become pretty hot. All the ICs appear to be from the original build.This instrument adds the vintage touch to my vintage shack (mostly 80's era rigs). BTW, mine has the original bail-stand/handle. Wish I had a digital-copy of the complete manual. Thanks again, and 73. Prem - va3uma.

[@W5KCM]

Hi Jeff. Thanks for doing this video. I picked up my IM-4100 at a local hamfest a couple years ago for $10. It works great with a small wire plugged into the front BNC and checking the frequency of my Johnson Ranger. Hamfest fleamarkets are great places to buy older test equipment at bargain prices.
73, Randy, w5kcm

[@wa4aos]

Another FIRST CLASS Video from Jeff T..
Oh, his Test Equipment is also EXCELLENT ! ! ! !
Tnx es 73
Glenn WA4AOS

[@dennisf4194]

Do you still have the manual? There are no full manuals available online. Thanks!

[@jefftranter]

There is a full manual available at www.mods.dk

[@dennisf4194]

@@jefftranter Thanks for the quick reply, Got it! - 73 W4ZOR

[@8xlaxx]

Could you please tell me the difference between the IM-4100 and the SM-4100 counters? Thanks

[@jefftranter]

+8xlaxx The SM-4100 was the factory assembled and tested version and the IM-4100 was a kit. Other than that they are the same.

[@bubakhungaricus3969]

Thanks for the informative video. Would you kindly make a recommendation for replacement 7490 chips to boost the speed? There's a blizzard of ICs out there and I'm mostly a vacuum-state electronics fiddler.

[@jefftranter]

I had been thinking that replacing the 7490s with a faster logic family might increase the range of the counter. In would require some experimentation to see if that was true. The 74F family is faster but I did some investigation and it seems that a 74F90 was never offered. I've only found 74LS90 which is lower power but not faster than a 7490. I think you would have to use a different counter IC and that would require modifying the circuit significantly. If you simply want a faster counter, you can buy kits on eBay for a 50 MHz frequency counter for about $5 and an assembled 65 MHz counter for under $10. I even see a 1.1 GHz counter on eBay for $13.