[Milsurplus] The FR-38 Saga, and GF-11 diodes

Mon Dec 4 14:50:02 EST 2017

The stages on the most part are binary flip flops that have diodes from two different DC busses that are routed to the plate circuts. One bust is +70 volts and the other +90 volts. The common B+ bus runs at +210 volts. Others are in the grid circuit and run to the -70 volt bus.
Like you say they all appear to be used for forcing a tube to go into the high or low state and latching them there.
Capacitance would be a huge issue, using a 10X oscilloscope probe is often enough capacitance to cause a stage not to work correctly coupling capacitors are in the 22 pF and 27pF so all this stuff is real high impedance in nature also. Nothing like solid state electronics.
The GF-11 is the only type diode used in the counter, that along with a mysterious big blue block that appears to be a full wave bridge that’s across the incoming AC bus and feeds the DC blower motor.
Another interesting foot note is the manual tells how to build a leakage current test set up that uses 4.5 volts, 100K resistor and a 50Ua meter to read leakage current. They have a chart that gives a Go/No Go value for the leakage current and instruct that you have to elevate the temperature of the diodes to get an accurate reading and this is the preferred method for testing and not to use an Ohm meter because of the differences in voltage used. I did not do this and just relied on looking at resistance with an analog meter on the 1 K scale and that worked for me. May build up the test set later but then back to the same problem of needing replacements.

Ray F/KA3EKH

From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Bob kb8tq
Sent: Monday, December 04, 2017 1:11 PM
To: Jim Whartenby <antqradio at sbcglobal.net>
Cc: Milsurplus <Milsurplus at mailman.qth.net>
Subject: Re: [Milsurplus] The FR-38 Saga, and GF-11 diodes

Hi

Let’s back up a bit.

Most of this stuff is doing basic logic with diodes. Put maybe four diodes to the grid of a tube. Each one goes
back to a plate circuit of another stage. Pull any one of them “low” and the tube turns off (if set up one way) or
pull any one of them “high” and the tube turns on (if set up the other way). A twenty volt grid swing is
likely plenty to turn a typical triode on or off.

If the cathode of the tube is roughly at ground, the grid will be running from -10 or -20 to around zero volts.
The preceding stage output is padded / biased down to be in this range. None of the diodes (in normal operation) see
a lot of voltage. The grid bias *is* often high impedance. Leakage (or capacitive feedthrough) is a problem. They
would probably go to higher voltage rated parts to get lower leakage.

Once you have silicon, leakage becomes much less of an issue. A big junction with silicon still has a lot of capacitance.
That’s one of the things that makes a diode slow. There are a few other things as well.

Are all circuits done the same way? Of course not. Without the schematic, there’s not much of a way to *know*
what HP did in this case. They didn’t have a lot of “prior art” to look at when this counter was designed. Solid state
diode logic was very much a new and scary technique back in the late 1940’s.

Best to dig into the schematic and do some scope measurements. See what the diodes are getting hit with and
pick something according to what the circuit really is doing. If it’s always in the < 30V range, go with a > 60V rated
part. If it actually does see more than 100V, by all means, don’t use anything with less than a 200V rating. Keeping
down the rating (if possible) gives you a bigger selection of fast diodes to pick between ….

Bob

On Dec 4, 2017, at 11:49 AM, Jim Whartenby <antqradio at sbcglobal.net<mailto:antqradio at sbcglobal.net>> wrote:

Ray
the 1N914 has a 75 Vr rating.  See if you can find the 1N645 which has a 225 Vr rating, I would think that this would work well with tube circuits and voltages.
Jim

________________________________
From: Ray Fantini <RAFANTINI at salisbury.edu<mailto:RAFANTINI at salisbury.edu>>
To: "mrca at mailman.qth.net<mailto:mrca at mailman.qth.net>" <mrca at mailman.qth.net<mailto:mrca at mailman.qth.net>>; Milsurplus <Milsurplus at mailman.qth.net<mailto:Milsurplus at mailman.qth.net>>
Sent: Monday, December 4, 2017 8:00 AM
Subject: [Milsurplus] The FR-38 Saga, and GF-11 diodes

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.

Ray F/KA3EKH
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