HeathKit Euw-15 Power Supply Voltage Regulator Failure

Discussion in 'General Electronics Chat' started by harkkam, Jan 6, 2011.

  1. harkkam

    Thread Starter New Member

    Jan 6, 2011
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    Hey guys I just bought a used Heathkit EUW-15 Power supply from ebay and I havent done made any changes in it. It has all the original caps and fuses that it came with along with the tubes.

    Here is the schematic and specs. Attached below as well

    https://docs.google.com/viewer?a=v&...DItNzkwZS00MDM0LTljZGUtZjk1M2YwZDUwY2Jm&hl=en

    Now it has two selections a voltage divider and voltage regulator.

    The first time I powered her on, I had it in the voltage regulator position and I used my Ohm/DC meter to measure the voltages it was putting out and it was working as I expected it to.

    As I turned the regulator knob the voltage went from 150V to about 380V

    Then I turned her off.

    I turned her on again and the voltage regulator knob has stopped working, to my surprise it is putting out a constant 520V which is much higher than what it is rated for. The capacitors inside are only 350V so I shut it off to avoid damaging them.

    No matter which direction I turned the voltage regulator knob, it was a constant 520V.

    However whats interesting is that the voltage divider selection works just perfectly, it puts out about 480V at its highest setting which it is supposed to do based on the specs sheet.

    I get different fractions of voltgages at every knob turn of the voltage divider potentiometer. As it should be.

    So First I tested the potentiometer to see that it was not open, and had the proper resistance values and it is working just fine.

    The schematic gives me voltages to measure and I am going to do that next however I am wondering if I should change the caps and fuses anyway?

    And I am curious as to what could be causing the voltage regulator to be putting out 520V when its not designed to do so.

    More interestingly the voltage regulator worked perfectly the first time as I turend the knob the voltage went from 150V to about 350V, and now has stopped working putting out constant 520V which leads me to believe it could be a bad capacitor. Now the turning the knob makes no difference which is very strange it keeps the voltage at a constant 520V.

    I tested the voltage regulator output with no load but it is designed to work without any load present and still put out the same rated voltage.




    Here is what it looks like:

    http://cgi.ebay.com/ws/eBayISAPI.dl...witem=&sspagename=STRK:MEWNX:IT#ht_500wt_1145

    It has pictures of the insides as well

    Attached as well, sorry guys im new dont know protocols yet
     
    Last edited: Jan 6, 2011
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The first link in your post does not work.
    That way we can not be of any help.

    Bertus
     
  3. harkkam

    Thread Starter New Member

    Jan 6, 2011
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    sorry fixed hopefully
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    We do not like the sharing sites over here.
    Why not upload the file here?

    Bertus
     
  5. harkkam

    Thread Starter New Member

    Jan 6, 2011
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    Oh didnt know you could do that, but now its attached. Thank you
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    [​IMG]

    Look at these caps first.

    Turn off and unplug the unit. Let it sit unpowered for MORE than hour, or discharge the caps yourself if you know how and don't want to wait.

    Use, or borrow an ohmmeter, and see if any of the caps show a steady low ohms reading that doesn't seem to increase. (the meter will charge the capacitor and its resistance will increase as seen on the meter. If the caps are ok.)

    The best course of action for the long term, if you plan on using this power supply, would be replacement of ALL the caps and large carbon resistors. This would bring its operation much closer to the original spec.
     
  7. #12

    Expert

    Nov 30, 2010
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    If the wiper on the control pot burned open, the feedback loop will be open and the 6BH6 wil not lower the grid voltage of the 6L6.

    That's a first idea, loss of feedback loop. Then there are a bunch of switches that take more than 5 minutes to figure out. Check back later.
     
  8. #12

    Expert

    Nov 30, 2010
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    The 6BH6 and the 0B2 are a series string used to control the grid of the 6L6. Both of them must be good or the grid floats and the output goes high.
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Yea, hate to tell you but the OB-2 tubes didn't have a very long life, especially as time caught up with the sealing/glass and the gas leaked out but they're still out there but many have created a zener circuit to replace them and there's probably a plug in solution.

    Here's a source for originals, take it from there:
    http://electrontubestore.com/index.php?main_page=product_info&cPath=78&products_id=229

    http://cgi.ebay.com/1x-Shuguang-WY-...626?pt=LH_DefaultDomain_0&hash=item5640dda3a2http://electrontubestore.com/index.php?main_page=product_info&cPath=78&products_id=229
     
    Last edited: Jan 6, 2011
  10. harkkam

    Thread Starter New Member

    Jan 6, 2011
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    Well, Well, Well.

    I used the ohmmeter like you suggested and all the caps on the voltage divider circuit seem to be fine they show an increase in resistance.

    However the caps on the voltage regulator circuit dont show an increase of resistance, the ohmmeter just shows a 1 even at the highest setting, meaning its open and bad like you said.

    C6 just shows a 1 even at the 2000k setting on my ohmmeter

    As does C8

    but C1 and C2 their resistance goes up.

    I also realized that it uses FP style capacitors for C3 which cost 17-20 per capacitor. Thats nuts. Can i replace the FP style capacitor for other kinds of caps?

    Like a can capacitor that has the same voltage and MFD
     
    Last edited: Jan 6, 2011
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I'm assuming you mean a decrease in resistance right after you hook up to them? Some meters won't respond quickly enough to measure a cap like we used to be able to do in our old analog days.

    Bingo, (I hope) - replace those first and I'd advise you to pick up an extra OB-2 tube when you've got the funds to do so. I don't recall that exact tube in operation but I know the basis behind them, some glowed a bit orange, others more a greenish-blue depending on the Argon content as very few are still available until Russia starts making them again - and they will in time.

    After you get the caps replaced measure the voltages again, I think the OB-2 was a bit over 100V regulation. There's one of these or something similar in my old DuMont scope that failed but since it was barely usable at anything over audio range it was retired to my garage ages ago along with a couple of old Teks that had far outlived their time. Often thought about selling them on eBay but to properly crate and ship the monsters would probably far outweigh the trouble of doing so thus I'll probably just donate them to the local Bell Labs museum we have here.
     
  12. harkkam

    Thread Starter New Member

    Jan 6, 2011
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    Well I changed all of the resistors and most of the capacitors however C4 and C5 are actually a multisection capacitor and I am waiting for the replacement to come.

    I have gotten new tubes as well and tried them but still no good.

    So I took some voltages and I started where the two diodes D5 and D6 feed into C4 and C5.

    Right there I am getting crazy voltages, In the schematic it is telling me that after C5 the voltages should read 560V between that point and the DC- point. Mr kermit was nice enough to draw a red arrow on the schematic to were I am talking about

    However I am getting 670V here. I also tested another spot pin 1,5 of OB2 and I am getting 107 V which is what the schematic says it should be.

    However I could be taking the wrong measurements because at the bottom right (full schematic in first post) it says voltages taken with 11 M ohm resistor.

    But for dc voltage that made no sense so I put the negative lead in DC - and the positive lead on the point to be measured.



    Can a bad C4, C5 multisection cap cause a 100V increase?

    Thanks guys
     
    Last edited: Jan 11, 2011
  13. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The combined drift of circuit component value with extreme age means it could do lots of things; none of which are good for the unit.

    Lots of components need to be replaced, if checked and found out of tolerance, or like the electrolytics, have a limited lifetime to begin with.

    Even the tubes may have changed in output specs due to age. Restoring it might be worthwhile, but relying on it to be daily use test equipment will soon cause it fail completely due simply to its age and delicate condition.

    The more you replace with new components, the more reliable and predictable the circuits functioning will be.

    Troubleshooting something that old, is tricky. Something gets replaced and the new operating voltages and currents could cause other weak or out of range components to fail next. It is up to you to make a call on whether it is financially worth pursuing this far enough to get a 'reliable' power supply from it.
     
  14. retched

    AAC Fanatic!

    Dec 5, 2009
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    Be sure to note the polarity of the caps you are replacing.

    And does there appear to be any other attempts at repairing this PS?
     
  15. harkkam

    Thread Starter New Member

    Jan 6, 2011
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    No, no other attempts have been made to repair the PSU.

    Ive invested some money into the unit and I am going to replace the caps and see what happens, but I am reaching the end of where I want to keep putting more money into it.

    I am going to replace C4 and C5 and see what happens.
     
  16. harkkam

    Thread Starter New Member

    Jan 6, 2011
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    I did some more testing because I dont want to give up on her just yet.

    I didnt look carefully before but the guy who sold it to me put a 6t6l6b tube, instead of the 6l6GC.

    Would that make a difference?

    Also I threw in a larger resistor before pin 4 of the 6l6gc socket and brought the voltage down to 560V.

    However on the measurement on pin 7 and 8 of the 6l6gc socket instead of getting 300v I am getting 450V.

    Ive changed the OB2 and 6BH6 and every resistor and capacitor.

    All that is left is the pot and the 6l6GC tube which is really in my case 6t6l6b tube
     
  17. #12

    Expert

    Nov 30, 2010
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    Did I say "pot" in post 7?

    Can you measure the voltage on pin 1 of the BH6?

    Does the pot affect the voltage?
     
  18. harkkam

    Thread Starter New Member

    Jan 6, 2011
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    When the pot is turned down the minimum setting pin 1 reads 145 volts when it is turned to max it reads 61 volts.

    I measured the voltage by putting the negative lead of my volt ohmmeter into the DC- terminal and used the red pin to probe.
     
  19. #12

    Expert

    Nov 30, 2010
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    When the voltage on the wiper is higher than the voltage at the 0b2 tube (labeled 105 V), the BH6 is supposed to dump the grid of the 6L6 towards 105 volts, and the cathode of the 6L6 should follow. If you have 500 volts at the cathode of the 6L6, it should show up in the voltage divider R7,8,9 and not be loaded by the BH6 tube grid.

    Apparently you aren't getting 50% of 500 volts at the voltage adjust pot. If you don't have that voltage telling the BH6 to dump the grid of the 6L6, there is your problem, the feedback loop. Find out why the 500 volts is not showing up properly in the R7,8,9 divider.
     
  20. harkkam

    Thread Starter New Member

    Jan 6, 2011
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    Ill be honest I wouldnt know how to do that, Im not very familiar with tube circuitry.

    I was hoping that replacing all the parts would fix it.
     
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