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Thread: FR-38B/U Frequency Counter

  1. #1
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    Default FR-38B/U Frequency Counter

    I have started working on a late fifties military FR-38B/U Digital Frequency Counter. This is the military version of the HP-524 and is a first generation vacuum tube frequency counter that uses vertical column displays and was one of the first units to provide real time digital display of frequency and time events. The development of high speed switching diodes in the late fifties along with around eighty tubes makes this possible.
    Although itís not a computer it dose use tubes to gate, count and divide its time base that all form the basics of early digital circuitry and so I am assuming it may also be of interest to others on this page.
    So far I have the power supplies, time base and the counting circuts up and running. But I am still working on the input stage that uses a tube amplifier that drives a Schmidt Trigger that feeds the main gate to the counters. It will work at lower frequencies but stops above five or six Megahertz. The input circuit is supposed to work out to ten megahertz. Also have an anomaly in one of the time base dividers that has to be resolved.
    Working with vacuum tube digital circuts is a true art and nothing like the solid-state devices, there are both positive and negative high voltages at +210 and -190 VDC high temperatures and a whole host of dangers that are not there when working with transistors or integrated circuts. Also have found that a lot of the tubes were not ideally suited for digital operation and suffer a higher failure rate, some of the circuts use tubes that look like a common 12AT7 but have different number and were manufactured just for digital service and long periods of on time. So needless to say this has been a real education so far and will be planning to post a YouTube video when I get everything working.
    One thing I have learned so far is to develop a massive respect for the engineers who designed and built the first generation computers and the difficulties they had to go thru.


    Ray F/AKA Qbus
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  2. #2
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    Ok so working on this FR38B/U frequency counter and running into an issue. They use lots of early Germanium diodes to trigger and provide faster switching times in most of the vacuum tube stages. They have a part number of 212-G11A but I have not been able to find any information on them. Was thinking of replacing a lot of the week performing ones with 1N914 switching diodes but am a little concerned about voltage rating but have to assume that the late fifties first generation G11 has to be a lower voltage ratting then a modern silicone diode?
    There is a panel that originally had a dozen diodes as spares in the unit but someone stole or used those years ago. They must have been a high failure rate device.

  3. #3
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    Better to use Schottky diodes, rather than standard silicon general-purpose ones. Forward bandgap will be about the same as a Ge diode, but inverse current leakage will be very much better.

    Very often, Schottky diodes are sold on eBay as "germanium" ones. While fraudulent, it's hard to tell the difference in-circuit--but you can see the difference if the diode has a glass envelope.

    The other reason that you want to use something that's close to the forward voltage drop of the original is that early diodes were used in a lot of equipment as "gates". Basically, you apply a low-voltage signal to the diode, which initially doesn't conduct, then apply a forward bias to it and the signal comes through.
    Last edited by Chuck(G); December 2nd, 2017 at 09:53 AM.

  4. #4
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    Ended up using just some generic 1N914 diodes that I had laying around the shop. Had only two original diodes that were way under the minimum reverse resistance per the manual specification of 75K Most of the Germanium diodes if not all have some leakage and would be good to replace most of them at some point in the future but the replacement of the two leaky ones got my gate trigger MMV up and running. The next problem was the decade divider that divides the signal from the time base was only dividing by two. That’s mounted on a sub assembly and is removable so after removing, checking the tubes and cleaning it up somehow it magically started working again. Suspect that there may be a bad 0.01 cathode bypass that’s causing it to run as a MMV and not a flip flop and may go ahead and change that while it is out.
    With the gate circuts now all working and after completing a time base calibration late last night I was able to do a successful self-test of the 100KC and 10 MC internal reference oscillators. All of this was with the input section removed but have seen it generate output so going to assume that there are no issues with that. The next step will be putting everything back together and all the covers on and seeing if it still works then.
    Working on vacuum tube digital electronics has been a learning experience and armed with a good dual trace scope and service manual not as daunting a task as I assumed it would be when starting but have to say that without proper documentation and the excellent service manual that not only included schematics, waveforms and most important theory of operations would not have been able to do it. Also thanks to everyone who helped out with the diode question. Think what I will do on that front is maybe next year at Dayton will stop of at Midwest Electronic Surplus and see what they have in a bunch of identical small signal switching diodes and try the dreaded shotgun approach to try to keep all the diodes the same type.

  5. #5

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    Which 12AT7 variant does your counter use? 6201, 5965 or 6414? If you get stuck for spares let me know.

  6. #6
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    Will look at the tubes and see what there number is, don’t remember off the top of my head but know it’s a four digit military number. So far none of the tubes have been an issue. Biggest problems have been recalibrating all the time base dividers and getting the faulty decade divider that fires the gate to work reliably.

    Did a video of the FR-38B/U Digital Frequency counter yesterday and just put it up on YouTube at:

    https://www.youtube.com/watch?v=3ka1wwvex5o

    Just about all the issues have been resolved, got all the time base chains working. The gates working at the proper time and most of the bugs worked out. Still have some jumps up and down in frequency but attribute this to poor performance to many of the diodes that have not aged well in the dividers. Did replace a couple of the worst offenders in the gate trigger MMV but think an entire new set would be in order being noticed that after the counter has been on for several hours and gets real hot instability increases. Also have one bad neon bulb in one of the decade counters and will have to find a replacement for that. And if all that’s not enough found that the little temperature gauge for the crystal oven has died and short of finding another unit as a parts donor don’t expect to find one of them.
    Been a fun project, don’t know what’s next on the list.

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