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

Mon Dec 4 13:57:00 EST 2017

You're right Bob, without a schematic it is just guessing but we both seem to like solving puzzles.
When I looked at the photos on Nick's site, the diode color bands looked to me like the color code for the 1N451,  Seems to fit what Ray was saying and the Vr would handle tube circuit voltages while the circuits powered up or down.
Ray has now indicated that the 1N914 works but the PIV is below 100 volts; that is why I thought that the 1N645 would be a better choice with a PIV over 200 volts.  

I have a few of the 1N645 diodes but when I looked up sources, the price was pretty high.  Using Mouser's filters I just found the BAV21 with: 200 PIV, 250 mA If and a junction capacitance of only 1.5 pF, pretty slick.

Best price is still eBay @ $3 for 50 with free shipping but from Singapore.  Mouser is $7.10 for 50 but then there is the added shipping.  The cost of convenience!
Best of luck on the project Ray, keep us posted,Jim

      From: Bob kb8tq <kb8tq at n1k.org>
 To: Jim Whartenby <antqradio at sbcglobal.net> 
Cc: Milsurplus <Milsurplus at mailman.qth.net>
 Sent: Monday, December 4, 2017 12:10 PM
 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) orpull 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) seea lot of voltage. The grid bias *is* often high impedance. Leakage (or capacitive feedthrough) is a problem. Theywould 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 andpick something according to what the circuit really is doing. If it’s always in the < 30V range, go with a > 60V ratedpart. If it actually does see more than 100V, by all means, don’t use anything with less than a 200V rating. Keepingdown 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> wrote:
Raythe 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>
To: "mrca at mailman.qth.net" <mrca at mailman.qth.net>; Milsurplus <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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