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CL find: Heathkit IM-4100 frequency counter

Discussion in '"Boat Anchor" & Classic Equipment' started by KJ6ZOL, Nov 20, 2021.

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  1. KJ6ZOL

    KJ6ZOL Ham Member QRZ Page

    Sometimes you have to get creative with search terms on Craigslist to unearth good deals. Searching for "vintage radio" on SF Bay CL brought up an ad labeled "vintage radio transistor tester". It had several pieces of old test equipment: the mentioned Heathkit FET/transistor tester, as well as an old Eico VTVM, a 1960s benchtop DC voltmeter, and a Heathkit IM-4100 frequency counter. Old Heathkit frequency counters are hard to find, so I called the guy up and set a time to meet.

    Yesterday I drove to Berkeley to meet the guy. He is a Black blues musician whose grandfather was a ham for decades. It was unusual, but not unheard of, for a Black person to be into electronics and hamming, and CA was more liberal and open to stuff like that. The guy said that his grandpa left behind a bunch of radios and other detritus of a life in electronics, and though he'd sold the radios he admitted that he had zero clue what all the benchtop test equipment was for. I got the counter for $20.


    It appears to work well. Looks like the original owner cared for his stuff.


    The back. The owner had chopped off the ground peg on the AC plug, apparently he lived in a very old house that lacked modern AC plugs. I will have to replace the cord.


    What the heck is this 12vDC plug? Another Heathkit semi-proprietary interface?


    The jacks too. Not quite sure what these are.

    So, I will have to hit Google and see if I can find a copy of the manual. As I said, the AC cord needs replacing due to the ground peg being chopped off.
  2. WA1GXC

    WA1GXC Ham Member QRZ Page

    Not proprietary. This appears to be a standard 3-pin "Molex"-brand (or compatible) connector used for low voltages. Used to be readily available
    at Radio Shack.

    Partial unit manual available on the WorldWide InterTube.

    The Molex is for field use, unit can run on external 12.6V DC to the top + pin. Botton "GND" is negative/common. Center pin has no connection.

  3. W9GB

    W9GB Ham Member QRZ Page

    Heathkit introduced the IM-4100 in 1975 (as I started college)
    This frequency counter was so popular, that Motorola contracted (likely with prescaler) a version (Motorola S-1357A) from this Heathkit design for the Motorola Land-Mobile dealers.
    im-4100_1975 catalog.jpg
    Heathkit IM-4100 partial manual
    NOTE: 1976 Service bulletin, for the +5 Volt Regulator circuit.
    Your IM-4100 version MAY not have these changes.
    RESPECT, the era of the technology (then quite New) This is 1975 … not 2021.
    Input for an External Time Base Reference (10 MHz common: GPSDO or TCXO today)
    Shown in Figure 6 of the IM-4100 Manual.

    Molex 0.062” 3-position nylon interconnects (created 50 years ago),
    were found in every Radio Shack retail store since its introduction in 1970s.

    The 0.093” versions were commonly used in amateur radio equipment and
    internal wiring connections for millions of appliances (then US-made).
    Last edited: Nov 20, 2021
  4. W9GB

    W9GB Ham Member QRZ Page

    What is the Difference between SPT-1 and SPT-2 Flat wire electrical cords ?

    SPT is an abbreviation for Stranded, Parallel, Thermoplastic, a designation found on most flat wire. This abbreviation is always followed by a number denoting the insulation thickness. Despite what many believe, insulation thickness is the only difference between SPT-1 and SPT-2 wire.
    The image above shows a side by side comparison of an SPT-1 wire and an SPT-2 wire.
    As you can see, the insulation around SPT-1 wire is 0.03 inches thick.
    SPT-2 wire is noticeably thicker, with a thickness of 0.045 inches.
    The overall Width of the flat cable (~ 0.31”) is normally the same.

    Which Should You Choose?

    Ideally, MEASURE the Old Power Cord you are replacing, and strain relief bushing #.
    The choice between using SPT-1 or SPT-2 wire often comes down to personal preference.

    Many consumers choose SPT-1 wire because it’s the most popular, easiest to find,
    and tends to be a little less expensive. Others prefer SPT-2 wire because of the thicker insulation.
    Once you do make your choice, be sure that the Stain Relief Bushing on your radio or test equipment has the same SPT rating. Heyco product numbers for flat cable, link below.
    Otherwise, they may not fit on the replacement cord or wire.

    HEYCO Strain Relief Bushing
    for Straight Thru Flat Cables

    They have three version (SPT-x) of light-duty power cords in 18-3 size.
    However, SPT-3 is wider (0.38”) versus SPT-1 or SPT-2 (0.31”).
    Six (6) foot lengths are shown below, BUT Eight (8) foot lengths are available.

    Part number: 70355K85 , Price: $ 4.14
    Black, 0.31" wide, PVC Plastic insulation, SPT-1
    6 foot length, Temperature range: 0° to 140°
    UL Listed, CSA Certified

    Part number: 7025K21 , Price $ 7.17
    Black, 0.31" wide, PVC Plastic insulation, SPT-2
    6 foot length, Temperature range: 0° to 140°
    UL Listed, CSA Certified
    Last edited: Nov 20, 2021
  5. KJ6ZOL

    KJ6ZOL Ham Member QRZ Page

    Thanks y'all, the connector didn't look like any Molex I was familiar with (showing my age as a young'un) so I wasn't sure. I will make sure to write this down if/when I need a matching jack.

    Also, yeah I keep forgetting about time base inputs. I need to go digging and RTFM, obviously.

    I was wondering where to get a good replacement, now thanks to W9GB I know. I will also have to replace the strain relief bushing, and get the proper bushing manipulator tool. The ground plug was sawed off most likely because the original owner apparently lived in a run down Victorian in north Oakland, probably due to redlining and other Jim Crow laws that forced Blacks (and Asians in California) into the oldest, most decrepit, least desirable housing. The house likely only had 2 prong power outlets. I've seen this sort of thing on occasion when a person who is faced with a 3 prong plug and a 2 prong wall outlet takes the path of least resistance to solve the problem.

    Edit: Line cord is a SPT-1. Looked at it a little more closely. It's a three cable SPT-1. Duh.
    Last edited: Nov 20, 2021
  6. W9WQA

    W9WQA Ham Member QRZ Page

    yrs ago they about gave away the heath 512 mhz counters
  7. W9GB

    W9GB Ham Member QRZ Page

    Electrical portable manufacturers (kitchen appliances, hand tools, etc). were required to install safety ground plugs (NEMA 5-15P) beginning in 1962 or 1965.
    ALL of the Weller TCP-1 solder stations, introduced in 1965 had NEMA 5-15P plugs.

    I have Frequently found a Sawed-Off ground pin on older 1960s/1970s acquired bench tools.
    When I restore these, I replace the cord set or install replacement plug.

    NEMA 1-15R (120 V) receptacles were commonly used for home construction until 1970.
    These receptacles (but now polarized) are still available for repair work to older home wiring.

    The 1970 NEC required NEMA 5-15R receptacles in new residential construction in the United States and Canada. Commercial construction began usage in 1962 and generally adopted by 1965.

    Since January 1, 1974, all new power outlets (everywhere in North America) were required to have a safety ground connection, using grounded receptacles (typically 5-15R or 5-20R) that accept both grounded and non-grounded plugs. Copper and copper alloy shortages, due to the Vietnam War, delayed adaptation (1965-1974, local building codes) in many areas of North America.
    I grew-up in a Midwestern rural home (Cape Cod) built in 1920, just outside the county seat.
    The home’s original electrical wiring was 2-wire (Knob-n-Tube) with a 60 amp main breaker, originally used largely for lighting. The main breaker panel was a Rats Nest of sub-panel additions using cartridge and screw-in fuses. In late 1960s, I would discover one sub-panel that jumped around the main home breaker (violation of any electrical or building code).

    Receptacles were NEMA 1-15, and many of the light switches were original 1920 push button with pearl inlay (and ceramic body for insulation).

    Over the next 50 years several additions and changes were made (especially after WW2),
    largely for a 1950 dormer addition and kitchen remodel.
    In 1970, with a major addition to this house, new 100 amp main breaker (and service) and NEMA 5-15R receptacles in the addition, kitchen, and select outlets added (not part of original knob tube wiring).
  8. W9GB

    W9GB Ham Member QRZ Page

    The Bushing tool is a good approach, especially if you are doing a large number of repairs.
    For the past 50 years, I have used Quality Needle nose pliers with electric tape over the needle jaws (to protect the rubber/plastic strain relief bushing) to remove/install.
    IF you just need ONE, you can order a SAMPLE, from Heyco !!

    HEYCO 1872 : SPT-1 Strain Relief Bushing for Straight Thru Flat Cables
    Double Check strain relief mounting hole (Size, Round, D, or Double-D) on rear metal panel

    HEYCO Strain Relief Bushing for Straight Thru Flat Cables

    McMaster Carr
    Part number: 70355K85 , Price: $ 4.14
    Black, 0.31" wide, PVC Plastic insulation, SPT-1
    6 foot length, Temperature range: 0° to 140°
    UL Listed, CSA Certified
    Last edited: Nov 21, 2021

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