Heathkit IM-4120 Frequency/Period Counter
Circuit Type: Solid State - TTL, MOS, and ECL
Prices
1979 Catalog - Page 64
Kit - $299.95
Assembled - $480.00
Excerpt:
Two separate inputs are provided to accommodate signal parameters encountered at various frequencies. The 110 MHz amplifier consists of a protected high impedance FET/Bipolar pair and a 3-stage IC amplifier. The 250 MHz amplifier is a 3-stage wide band, transistor amplifier with a 50-ohm input impedance.
Three types of integrated circuit logic are used in this instrument. TTL and MOS logic use 0 volts for logic low and 5 volts for a logic high level. ECL logic uses a -1.6 volt for a logic low and -.8 volt for a logic high level.
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January 10, 2021
I found this on ebay and decided it would be a good addition to my test gear collection. Normally I'd go for something with Nixie tubes or vacuum tubes but I've found that unless it was made by HP or Tektronics, it'd be too expensive to repair and maintain. This is the first "old" solid state piece I have and there may be more later as needs dictate.
The unit doesn't appear in the 1968 Heathkit catalog or the 1976 edition so it must be a very short lived run. Most of the finds are 2410's from the '80s. This one uses decade counters with internally or externally driven oscillator. It can also be used as a signal generator. All terminations are 50 ohms impedance on BNC connectors. Jeff Tranter did a YouTube review and article in his book on the later IM-2410 and IM-2420 pieces.
For my part in this I'm only going to replace the four electrolytic capacitors then run diagnostics. I found a callsign and date written on the inside of the bottom case cover. The unit might work just fine considering the previous owner, now a SK, appeared to have worked on it in 2013 by the date he wrote, I'm hoping this will be a quick preventative repair and return to the IG-102 for a checkup as to why it's not functioning any longer.
That's it for this session.
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January 16, 2021
This seems to have become a really short project. The capacitors were the only issue. Calibrating it was simple and after that tested the IG-102 discovering it was stable all along. The homebrew frequency counter was the faulty part.
That's it for this repair. Can't wait to get into some alignments!
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