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Kepco Model 400 Power Supply

January 19, 2012

This was donated to me by my friend, John Dahlman. It gets a lot of use.

All the paper/wax capacitors needed replacing. Also, one phase of the negative supply winding is open. I simply created a hybrid bridge off one phase using two diodes together with the existing rectifier, obviously the available current is reduced however, it has worked perfectly since I did this. The other fault was that the high limit pot was open, since it is used as a variable resistor, I simply re-connected it to the other end. The other modification I made was to include a 20 second thermal B+ delay. This provides enough time for the B+ error amplifier to warm up so that if it is turned on with the B+ output switch closed, the full unregulated B+ of 800V won’t appear at the terminals. My Lambda Model 71 has this feature. Here is the underside, the delay relay is located top right:

It is a cheaper design than the Lambda which has a variac that controls the B+ transformer in step with the desired output voltage thereby limiting the power dissipation in the series pass tubes if you require full current at low voltage. This one has four 5881 series pass tubes to deliver 150mA at up to 500V. I haven’t done the math but I imagine that if you want 50V at 150mA, the 5881s will be hard-pressed, not to mention run way beyond their plate voltage rating! I haven’t experienced any problem running at low voltage but I have never drawn much current at low voltage either. One reason why I use it as much as or maybe more than the Lambda 71, is that the Lambda buzzes which I understand is “normal”. Normal or not, it is not peaceful and I value peace in my environment hugely.
Here is my hand-drawn outline schematic, it is does not include every feature of the circuit.


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  1. Christian R. Fandt permalink

    Hi Richard,

    I got one of those TS-452C/U sig gens too. I have not found a manual anywhere yet, so thanks very much for your article on it! You’ve told me more than I was able to heretofore discover myself. It’s not a hot issue to checkout and restore as the lower freq. range is way too high for my need. But its an interestingly nice piece to have.

    I’ve also got one of those KEPCO 400 power supplies you wrote about in another article. It’s in my (rather long) restoration queue to be electronically restored. Perfect unit for checking out and restoring tube type auto radios that I repair for folks -plus home radios too.

    Sometime back (at least 5-8 years?) I called KEPCO and got a manual made from one of their originals on file. They made a real nice bound manual for around 25 bucks, I think I recall. I have not had a chance to scan it yet as my scanner is acting ornery, but would you like a copy too? Or even a paper copy if I can find an 11 x 17 copier around the plant (it’s just over a million sq. ft building)?

    I see yours is s/n A-3520. Mine is A-4449 and the manual is for A-3128 with some rather minor circuit changes they’ve well documented. My unit is likely very close to it, yours perhaps too.

    Thanks again,
    -Chris Fandt, Jamestown, NY

    • Hi Chris.

      Yes, the TS-452 is a very interesting piece.
      On the Kepco, I would like a copy of the manual, thanks for the offer. If you would like to reply to this with your email, I will email you with my address and delete your reply so that your email does not become public.


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  3. Eric S. permalink

    Does anyone have a copy of the Kepco 400 manual / schematic? I’ve been trying to get mine working again. The C- works fine, but the B+ just goes out of control to full voltage.

    • The schematic I drew out and included in my article is sufficient to repair this PSU If you understand how a series voltage regulator works. Simply, the series tube(s) is a cathode follower that is controlled by an inverting error amplifier that samples the output Bplus. As the Bplus tries to rise, the error amp turns on harder, pulling the CF grid down and with it, the cathode, and vice versa.
      Other than a shorted series tube, the reason why the B plus goes to max is failure of the error amp to conduct resulting in the series control grids going high. This is why I incorporated a delay. On first turn on, the series tubes conduct ahead of the error amp and the B plus goes high then comes under control, a nasty characteristic and is why better quality units have a delay. It may simply be that the error amp tube is dead. Is the OA3 striking? If not the error amp is not conducting. Maybe the low limit preset is open, the high was open with my unit. The lower regulator provides the minus 260V reference rail and must also be working. In your case this seems to be ok,
      Note, I didn’t show the other 3 series tubes, just the control grid connections because I’m lazy and they are simply quadrupled triode connected series pass tubes.

  4. Eric S. permalink

    A few new capacitors and a fresh 0A3 tube made things right. The adjustment of the H and L pots is a bit touchy. It does regulate now. Thank you.

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