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Overhauling High voltage PSU

Discussion in 'Troubleshooting and Repair' started by Proschuno, Feb 15, 2014.

  1. Proschuno

    Proschuno

    87
    1
    Aug 1, 2011
    So I"m overhauling a 50 year old home-builtPSU in serious need of refitting. Using two pass tubes of them has a busted filament and the other was replaced about a year ago. Should I worry about getting a matched pair or just get a single tube?

    Also, the circuit uses two oil filled caps that I would like to replace for good measure (along with all the other ones in the schematic). Seeing that these on ebay are quite expensive along with any other electrolytic alternative, I'm wondering if I couldn't simply tie two 450V caps in series as shown for the +B supply? This would be much cheaper than getting another oil cap or similarly rated electrolytic ($US 80 on Mouser).. I notice one of the oil caps' leads is tied before the choke, so I wonder why so?
    Proschuno

    And for the missing piece: The OA2 tubes, along with the 1st and 3rd leads of the pot, are in parallel with the 30MF caps, and the cathode of the error amp tube is connected to the wiper of the pot.

    And BTW, taking apart the PSU I found the schematic; notice the date…. it's literally as old as my mother lol :))

    The pass tubes are 1619 (or VT164), the error amp is 6sj7, the rectifier for +B is 5v4g, and the 2nd one is 6x4.
     

    Attached Files:

  2. duke37

    duke37

    5,201
    713
    Jan 9, 2011
    The pass tubes should be matched if you are going to run the PSU to maximum. You should find out why the filament went open circuit.

    Why replace the oil insulated capacitors?

    If you put electrolytic capacitors in series, they will need parallel resistors to equalise voltages.

    30MF seems a wee bit large, I would try 30µF !

    The 5R4 feeds into a 4µF capacitor, this should not be increased or the current pulses in the rectifier could be too high
     
  3. Proschuno

    Proschuno

    87
    1
    Aug 1, 2011
    Sorry, i meant microfarads!

    About the tube; considering it was dated 1986, I think that might explain it. I suppose the oil caps are fine, but I will replace the electrolytics for good measure (these are cheap anyway; $1.50 per cap at the rated voltage, that's fine for me.).

    I have no real way to measure capacitance, but when I took one of the oil caps out and placed 12V across it, it only held the charge for about 30-40 seconds, only tapping my meter on it periodically to measure the voltage. (I;m supposing this is the nature of oil caps... They have very high leakage from what I've read, since they use a paper dielectric). However, I see almost no ripple on the output, only about 10-15mV at full voltage, and >5mV at nearly 30V. But honestly though, would I have any real benefit replacing the oil caps? Why are they there alongside electrolytics? I mean I get the low capacitance now, but why oil specifically?

    I'm considering the overhaul because it used to be able to go down to 0V, which I like, but now lately it won't go below 30V, which when it started acting up before, it wouldn't go past 50V.
     
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