Fluke 412a  2KV Power Supply - Page 1
Forum section: electronics / repair

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 11, 2015, 11:59:48 pm
content: (I started a new thread because the other was specifically about the diode.)This capacitor is older than I am. (early 60s) Made in some place called "USA." My Peak ESR meter says 2.3 ohms, and 9 microfarads. Not bad for a 40+ year old electrolytic. (I think) However, the rubber seal is cracked, so I believe I should replace it. This is one of the "wrong turns" I made before I realized the diode was bad. I suspected this guy of being shorted.(I removed the capacitor from the paper case/label/wrapping/whatever you want to call it)

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 13, 2015, 11:02:36 am
content: I suppose you can replace it with any recent cap of the same type.I would be interested to know how well calibrated is your 412a PS. I have a 415b Fluke PS that I will soon try to fix andcalibrate, if this is possible, as I have not seen wery well what are the tuning possibilities.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 13, 2015, 12:21:39 pm
content: Quote from: JacquesBBB on March 13, 2015, 11:02:36 amWhen I get it working, I'll let you know. It was completely unregulated when I got it - the output was 3KV regardless of control settings. I was checking voltages, and heard a "pop." I quickly turned it off. When I turned it on to re-check, I had a problem with the time-delay circuit that gives the tubes time to heat up before energizing the remaining circuits. One of the resistors was smoking badly.Worse, the manual I have is for a newer version. The old version uses a thyratron tube and an RC circuit - the new version eliminates all of it and uses a commercial time delay relay.The diode I discussed in the other thread turned out to be the problem, but my time away from troubleshooting really shows. I had to draw the schematic, and I went down many dead ends. As soon as I get the schematic presentable I'll post it and see if anyone has any opinions about components.I think the capacitor above does not affect the timing, so I believe I can replace it with a 10 uf. (8 uf capacitors are not readily available)

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 14, 2015, 12:16:23 am
content: OK, it's not pretty, but this is the schematic. I didn't show the relay contacts. D1 is the diode in my other thread.C1 is the capacitor above. I don't think it contributes to the timing. I believe that is C2 and R4.I don't see any reason not to think a common 1N4007 or so would work for D1, or that I couldn't use a 10 uf capacitor for C1. The existing one measures 9 uf.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 24, 2015, 02:53:57 am
content: Had a bit of a problem. The turret board for the delay circuit is also used as a junction point for several transformer terminals. To get the board raised so I can see what was going on, and to replace the capacitor I show in the post above, I had to move the transformer as well. It's big, and heavy, and when you push it into the ceramic insulator, the iron transformer core wins.I don't think I'll be able to find a similar replacement. I'm considering simply splicing the mains wiring together and insulating using heat shrink tubing. Does anyone have any thoughts/criticism or suggestions?

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 24, 2015, 03:00:25 am
content: The news is not all bad, however. I got the delay circuit working again.The poor wiring on the lower left wasn't touched by me, but I'll have to clean that up once I get the rest of it working. You can see a blurred splice that I hadn't insulated and secured before I took the picture. The fourth turret from the bottom on the left side of the picture had two wires on it, and the resistor. I didn't have enough room to work with to get the wires on the turret, so I spliced them together and ran a small wire to the resistor. The resistor on the left overheated and smoked when the diode ceased to become a diode and caused excessive current to flow through the capacitor. (capacitor in the first post.) It's behind the board - running from the diode anode to the tube socket, where an internal jumper in the tube connects it to what I call the negative side, the other side of the transformer. (yellow wire, see schematic)The diode is a bit of an overkill, but it's a vintage "made in the United States of America" new old stock Motorola of approximately the same vintage as the power supply.I'm happy to say the delay circuit now works, so I'm back to my original problem - no regulation at all. 3.5 KV output regardless of control setting.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 30, 2015, 12:28:57 pm
content: I was incorrect above - the supply was dead despite my repair of the timer/relay circuit. I found that I had damaged a tube socket when I moved the transformer. I may just be too ham-fisted work on this thing. I still believe someone else damaged the socket as well, but I'll never prove that.Also, the white wire from the relay contact above only looks like it's soldered to the turret/green wire in the lower left of the picture above.I've ordered a new tube socket and while it's out, I suppose I should repair that ugly wiring, as I'll be able to move the transformer without worrying about the tube socket. (And I've already broken the ceramic terminal block, as described above. D'oh!)THEN I can get back to figuring out why I have no control over the output voltage.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 30, 2015, 12:51:54 pm
content: Hi,I am following your progress with interest. Could you post a general picture of the inside of your 412a ?Thanks.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 31, 2015, 12:19:41 am
content: Overall picture

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 31, 2015, 12:20:51 am
content: Two closer picturesHigh voltage capacitor and the tube socket I damaged.Precision resistors and some nice ceramic switches

author: siggi
author_rank: Regular Contributor
timestamp: March 31, 2015, 02:00:09 am
content: Wow, what beautiful construction!Sent from my iPad using Tapatalk

author: Excavatoree
author_rank: Frequent Contributor
timestamp: April 03, 2015, 01:29:19 am
content: I've been tracing the wiring, and I was incorrect in my schematic above. The delay circuit is not powered from a transformer secondary, but directly from the mains. Strangely, the diode cathode is connected to the neutral mains wire (earthed conductor) via the resistor. Seems like I'd have to draw the circuit upside down. (I won't but I wonder why they did it that way.)

author: Excavatoree
author_rank: Frequent Contributor
timestamp: October 12, 2015, 12:10:38 am
content: Sorry to post in an old thread, but I finally got the thing working.After fixing the two diodes that failed as I was evaluating the thing, and fixing the things that I broke and/or were broken when I got it, I was back to the original problem - no regulation.I'd like to say I used proper troubleshooting technique, but I didn't. I was looking at the error amplifier section and noticed that a ballast tube was open. I checked the schematic, and found this was in series with the 12V to the filaments of the tubes in the error amp. So, I ordered a new one, installed it and it's working.I don't know why the two diodes failed. I do know that it's tight working on this thing, and it's easy to do damage. (I broke off a wire from the circuit board I was working on, broke the ceramic terminal strip for the mains cord, broke/further broke a tube socket) Other than that, it was a simple failure of a ballast tube.I don't know what the specs are at the moment (I'll look them up) but it drifts a couple of tenths of a volt at 500v. (I did adjust the trimpot for least error, but it moves around a bit and gains a tenth from 500 volts to 700. I didn't get out my high voltage probe so I didn't go higher.)

author: crispy_tofu
author_rank: Super Contributor
timestamp: October 12, 2015, 02:58:46 am
content: Wow, congratulations!