Troubleshooting my O-scope LV supply

Discussion in 'General Electronics Chat' started by dareposte, May 20, 2012.

  1. dareposte

    Thread Starter New Member

    May 20, 2012
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    It's got issues, and I'm not sure I understand how this circuit is really supposed to work. It looks to me like all the transistors are biased backwards!!

    I would much appreciate it if you could have a peek and give any advice or direction, I'm beating my head against the wall on this one.

    Circuit fragment is attached, with measured voltages marked in blue.

    Thanks in advance,

    Dare

    http://derekyates.net/5103N_schematic.pdf

    Or if that's not enough for some reason, the whole PS circuit is here on page 55 of the manual.
    http://derekyates.net/Tek 5103N_v6.pdf

    So far I have tried the following:
    1) Replace Q850 with new 2N3904 (after I did I realized the old one was OK)
    2) Replace Q845 with new 2N222A (also no change, old transistor measures fine)
    3) Lift and check R858 and R859 for drift, both are at 6.03K-6.035k on my meter
    4) Checked VR850, CR841, and each of the capacitors to make sure they function normally
    5) Measure each voltage shown in blue, to try to formulate a plan for what's going on there. I'm still stumped though!
     
    Last edited: May 20, 2012
  2. absf

    Senior Member

    Dec 29, 2010
    1,493
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    So what was the actual problem? +30V not steady or too high? R858 100 ohm preset not functioning? Power transistor too hot?

    Did you try to clean the preset with contact cleaner?

    Allen
     
  3. dareposte

    Thread Starter New Member

    May 20, 2012
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    The problem is that the +30v regulated rail is at +38V and stays there. The accepted tolerance is +29.925 / +30.975 volts per the manual. The circuit is also not matching up with the values shown in the manual at various other points which makes me think there's probably an issue with it, though I'm not super familiar with this type of regulator circuit so maybe it just needs a big load and it will come down.

    The -30V supply is hovering at -33V after I disconnected this board, but it was adjusted to -30V fine with the trimmer pot when it was all hooked up. The +30V supply hasn't dipped below 38V for anything I've tried yet.

    The interface board and HV supply are both disconnected, so there's no load on the supply other than it's own circuits which use the 30v supply to generate 5v and a couple reference voltages. 5V rail is at 5.4v, proportionally too high since it references the 30v rail at 38v.

    R858 100 ohm trimmer functions fine, from 10 ohms to 104 ohms, but adjusting it won't pull the Q850's base to 0V as is shown on the schematic as a set-point. It moves the Q850's base voltage over the range of 1.8v to 2.5v depending on which way it's trimmed, and of course the emitter stays about 0.6v below the base. There was no voltage change up or down on the collector at any trim setting, which is why I tried swapping it out for a different transistor.

    Any ideas?
     
    Last edited: May 20, 2012
  4. absf

    Senior Member

    Dec 29, 2010
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    What is the voltage drop across R842 2.2 ohm? From the schematics, it should be 0.2V so the current to the load should be roughly 400mA. Q845 is an over-current protection that will shut down Q840 when there is a S/C on the +30V rail.

    Q850 is the main actor that controls how much Q840 should conduct. I think all the passive components around Q850 should be checked especially C852. From your voltage indication, voltage across VR850 is 86.6V, so it could be one of the problems too.

    Cant understand why you cannot get 0V on the base of Q850. What are the voltages on both sides of the 100 ohm preset?
     
  5. dareposte

    Thread Starter New Member

    May 20, 2012
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    All right, I understand the circuit better now now and it does seem to be working as it should in this case.

    The drop across R842 is -1.5 volts (yes negative) from the voltages I measured, indicating about 680 mA flowing *into* the circuit. Which explains why Q840 is biased backwards and not conducting any forward current.

    I didn't explicitly check CR851 but it appears to be functioning as I would expect it to based on the voltages I measured. It looks like the goal of CR851 is to keep the base from going more than 0.6v negative, and since the base is positive it's doing it's job fine. C852 checked OK.

    I lifted and checked R852, R853, R850, R857, R858, R859 and they all check as expected with one leg up. The voltages on either side of the 100-ohm trimmer R858 are about 1.8 and 2.5 volts respectively, meaning I'm getting about 7mA through that leg when the reference circuit indicates I should be getting < 5mA. The 7mA is consistent with my 38V rail vs 30V rail though.

    At this point it seems that something else is pulling the +30V rail high other than this circuit, and is back-flooding this circuit with a current through R842 (2.2 ohm). Could CR842 be having some bizarre failure mode that's pulling the rail up? I'll lift a leg and check it tonight with the mA meter but it seemed to be OK when I was checking diodes with the meter last night, having the right forward drop and reverse resistance.
     
    Last edited: May 21, 2012
  6. absf

    Senior Member

    Dec 29, 2010
    1,493
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    Have you checked CR841, the diode across CE of Q840? If it is leaking, it may disturb the voltage on +30V rail....Just a thought:)

    Allen
     
  7. dareposte

    Thread Starter New Member

    May 20, 2012
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    Yeah I suspected it already, but when I lifted a leg and checked it the forward drop was 0.7 and reverse was infinite on my meter.

    Is there a possibility it might be leaking and still measure OK in that test?
     
  8. absf

    Senior Member

    Dec 29, 2010
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    I think not likely.

    You said the +30V rail is measuring +38V, but C842 is 10uF 35V. Could this cap been punctured by over-voltage? Try replace it with a 50V one....

    Allen
     
    Last edited: May 21, 2012
  9. absf

    Senior Member

    Dec 29, 2010
    1,493
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    Does the interface board and HV board used the +30V supply? If yes, you should add a 180 ohm 5W resistance across +30V and GND to increase the load and see if the voltage goes down a bit.

    Allen
     
  10. dareposte

    Thread Starter New Member

    May 20, 2012
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    I checked C842, and it appears to be fine. I don't have an ESR meter, but the regular capacitor tester shows a normal value for it, and it doesn't look like its leaking or otherwise having troubles. It holds 30VDC across it's terminal with no current draw, which is the best I can test it with what I've got.

    I think the 180 ohm / 5W resistor idea is a great idea, but I'll have to order one so it will be a couple days before I can test it.
     
  11. absf

    Senior Member

    Dec 29, 2010
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    If they are not too expensive, might as well order 150, 120 & 100 ohms so you can connect them in series or parallel for other values for testing.

    Allen
     
  12. dareposte

    Thread Starter New Member

    May 20, 2012
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    Good idea, I've got some on order from Mouser. I will feel like an idiot if I hook them up and it drops to a nice steady 30.0V, but I am doubtful that will happen.

    I went back through and checked the voltage drops across each resistor and they appear more normal than I anticipated.

    Right now I'm wondering if there's not some way that Q840 has failed and I just haven't identified it yet. If it failed open and CR841 was partially rectifying the voltage peaks just enough to drop it from 42 to 38v on my DC meter.... Hmm.

    I got my HP scope working today so I'll have a peak and see what the waveforms look like while I'm waiting for my power resistors to come in.
     
  13. absf

    Senior Member

    Dec 29, 2010
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    Dont count the chicken before they are hatched. May be 600mA is enough to keep the load steady but I am just not sure.....

    BTW, what is part number of Q840? TO-3 or TO220 casing? Dont know if one of the darlington transistors can get leaking while the other one is good.:rolleyes:

    Well, what else can you think of?:(

    Allen
     
  14. dareposte

    Thread Starter New Member

    May 20, 2012
    16
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    [​IMG]

    Settings 0.5 V/ div, 10x probe & 10ms/ div.

    It looks to me like 1.2V ripple peak to peak at about 100Hz. The 30V and 5V rails are completely quiet, even unloaded.

    But what does this mean?
     
  15. dareposte

    Thread Starter New Member

    May 20, 2012
    16
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    Q840 is a d40c5 GE Darlington. It looks like a TO220 case but the datasheet says its a TO202 case.
     
  16. dareposte

    Thread Starter New Member

    May 20, 2012
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    I'm wondering if I could get away with replacing the Q840 with a modern LM317T voltage regulator IC, and just bypass most of that other mess going on. Keep the protection diodes and smoothing capacitors but the rest could just "go away".

    Hmm.
     
  17. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Clarification please. That blue thing next to the fuse, labeled 42.3
    That is not a standard value for a resistor. Is it a resistor?

    Oops. My bad. I found my mistake.
     
    Last edited: May 22, 2012
  18. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Problem with LM317T.
    40 volt limit or the chip blows up.

    I'm not too "proper" to redesign the 30 volt power supply, I just needed to point out the voltage limit on the chip.
    I believe a 60 volt version is available.

    CR841 will not "rectify" in the manner you suggested, however, if it is shorted, that would be a way to explain the whole problem. Lift a leg and check.

    ESR is not a likely candidate for this problem. The capacitor would have to look like it's open before this kind of fault would cause this much error. (Don't worry about not having an ESR meter.)

    Referring to post#4, I can see why you can't get zero volts on the base of Q850. As long as the +30 and -30 aren't balanced, your adjustment range will not center. The problem is that this adjustment should cause the balancing to happen, and it isn't. Q850 is responsible for pulling the down base of Q840 and it isn't. This makes Q850 a prime suspect. Either it's open or it isn't being fed properly.

    LM317HV
     
    Last edited: May 22, 2012
  19. absf

    Senior Member

    Dec 29, 2010
    1,493
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    I totally agreed with #12. I think the fault should be around Q850 and dont give up too early!

    Where does the +200V come from? Was there any ripple on it? I'd suggest putting a 10uF 50V cap on the collector of Q850 and GND and see if situation improves.;) BTW what part no is Q850?

    Allen
     
  20. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Diagnostic procedure: Connect a 10k resistor from the collector of Q850 to ground...NOT to negative 30 volts, Not to emitter, to ground. If the +30 line colapses to about 15 volts, you have found the part that is not dumping the "go" signal and it is Q850.

    Edit: Q850 can be a TO-92 (small) as it is only dumping about 2ma.
     
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