Fairchild 792A pulse gen refurb - Page 1
Forum section: electronics / repair

author: cheeseit
author_rank: Regular Contributor
timestamp: July 21, 2016, 08:15:13 pm
content: Refurbishment only, I hope.I bought this good looking vintage 792A puls gen for €10 as "powers on, untested" which I think could be a nice score? It does power on and seems to work, at least somewhat, but the dials and verniers are very noisy so it is a bit hard to tell yet. At one point I did get a steady pulse out of it at around 4.4MHz.I can't find much on it so does anyone have a service manual or schematic? Does anyone know anything about this generator, specs and age would be interesting?Front looks nice but BNC's are black. Mains connector on back does not look original but is nicely done.Top side, tinned traces only.Bottom side, nice and shiny gold plating I presume? I measured the thickness of traces to about 0.19mm, several times in different places - that is around 5.5oz copper plus gold, right?The rotary switch contacts are oxidized and dirty and it looks a bit worse than the image below shows. I'm guessing that they are possibly fragile so how should I go about cleaning them? I have Kontakt Chemie Kontakt 60, 61 and WL but I don't like it flying everywhere and desoldering and taking them out feels a bit risky. The vernier pots are encapsulated in metal and without any good holes for thorough cleaning, I think, so I'm guessing I might have to take the switch/pot assembly out and disassemble anyway?I have never seen components like the dotted red one or that black one, what are they? The color code makes me think that it could be a capacitor but the arrow, a polarized cap perhaps? The black one has three pins I believe, In, Out and Gnd.And this, above the cap? Measured in circuit there is no continuity or capacitance but 1.715K Ohm across and in diodemode 1.0062V one way and 1.4585V the other way.Rail voltages are written on the board so I measured them after warming up for half an hour. A bit high but nothing alarming, I think? Resistors in the supply that have drifted over the years?RailMeasured7.5V8.11V0-20V0-22.14V15V16.34VHelp and suggestions on how to proceed are greatly appreciated.

author: bktemp
author_rank: Super Contributor
timestamp: July 21, 2016, 08:24:19 pm
content: Quote from: cheeseit on July 21, 2016, 08:15:13 pmIt's a 6-dot mica capacitor:http://www.radioremembered.org/capcode.htm

author: Vgkid
author_rank: Super Contributor
timestamp: July 21, 2016, 08:28:26 pm
content: I cant help you with the repair , but...red cap is a micamold mica capacitor.black rectangle, I don't know. Thought it was a pot, but it isn't.Silver thin thing, variable capacitor.i was beat to the reply...

author: Andy Watson
author_rank: Super Contributor
timestamp: July 21, 2016, 09:38:04 pm
content: I think the black package is BelF (Bel Fuse) delay line. Are there any other markings on it?

author: cheeseit
author_rank: Regular Contributor
timestamp: July 22, 2016, 02:05:58 pm
content: Thanks all. Apparently I did not switch that variable capacitor out of circuit and it measures 180nF now that I did.@Andy Watson: Sounds reasonable, not that I know anything about a circuit like this but it makes sense with my limited knowledge. No other markings than the ones that are somewhat visible in the first picture but here is one from the top.Took the delay switch assembly out and disassembled the pot and I don't think I've ever seen a pot like this. Is it safe to clean it with the Kontakt series mentioned earlier? I have canned isopropyl as well as canned and compressed air available. The switching part of the pot is unused and in fine condition but the pot is very noisy and feels crusty when turned.

author: cheeseit
author_rank: Regular Contributor
timestamp: July 22, 2016, 06:01:19 pm
content: I measured capacitors and resistors on the delay switch assembly and those Gudeman 2% foil? capacitors are very close to spec, like 1µF @ 1.015uF and 100nF @ 101.68nF. No ESR measurements since I lack the means to do so but they seem healthy I guess. The resistors are 5% and have drifted up almost 10% so is it advisable to replace them with new ones now that I have the assembly out, or should I just leave them as they are? If they are to be replaced, should I then use carbon or is 1% metal film fine?I realize that every resistor in the unit has probably drifted so it might not matter much in the end anyway. But, I guess that they are dividers? so they could affect the performance/accuracy?