Oscilloscope repair.

Discussion in 'Technical Repair' started by redrooster01, Jan 18, 2015.

  1. redrooster01

    Thread Starter Member

    Jul 15, 2014
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    I have a Huang Chang: Protek 6502 20mhz oscilloscope that has developed arcing across D1012 in the power supply next to the fly-back transformer. I found 3, ISS83 diodes had failed in the circuit so I ordered 3 Ultrafast UF4004 diodes by post because I couldn't get the ISS83s? The UF4004s are twice as fast and have the same capacitance according to the datasheets. The thing I would like to know is what could have caused these diodes to fail? I don't want to replace these diodes only to have them fail again? My knowledge of circuit theory isn't up to scratch by a long way? So I thought the best place to ask this is at the AAC circuit gurus hang-out? I bought the scope through the Trading Post online,when it arrived it had a broken plastic voltage adjuster for 230v,120v,100v etc... I put it back together using heatshrink so that its on the 230volts it needs here in Western Australia.(until I get a new one) All the voltages appear to read OK except for the 1900volts that I haven't measured yet? I'm waiting for a high voltage measuring kit to arrive in the post so I can do that. Ive uploaded the service manual for you, its a 3 in one schematic that includes 40mhz and 60mhz models.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    I think that is the orginal designation problem for the parts.
    The data info about 1SS83:
    Peak reverse voltage : 300V
    Reverse voltage : 250V

    1. If you counting from +75V to -1900V that the voltage has 1975V, the hz22(1SS83) zener diode only about 22V and the VR1021 has 100K, when it at the middle position then it could be only 50K, you can calculate the current, the voltage crossed on the reverse diode is very high.

    2. If you counting from -1650V to -1900V that the voltage has 250V, the R1023 only 4.7Ω as a shorted circuit and the D1024(1SS83) has 300V peak reverse voltage, C1023(1000P) will be as shorted circuit when the power up, when it has any current leakage occurs then it will be more seriously, I hope the reverse diode at least has 250V*1.5=375V or some more.

    So the voltage is too high for the reverse diode when it was using in a long term application.
     
  3. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    This scope is the 20mhz model with 1900v rail and no HZ22 zener? So you think it will be OK after I replace the 3 ISS83 diodes with the UF4004s? Thanks for the reply Scott. Sorry about the delay,I didn't realize I had a reply because the email alert didn't work.
     
    Last edited: Jan 21, 2015
  4. ScottWang

    Moderator

    Aug 23, 2012
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    If the VR1021(100K) turn to the R1021 then the voltage cross on the D1021(1SS83) will pretty high.
    To me that I will using UF4007 or some other diode has higher rated voltage, as the rectifier diodes that we can't identify their difference from outward appearance, and the price almost the same, so I bought the 1N4007 to use for all the other diodes as 1N4001~4006.
     
  5. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    I don't quite understand what you are saying? Are you saying that I can use the UF4004 going to R1021 and I don't have to use UF4004 diodes on the others and that IN4001-4006 will be OK?
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    I just said what I did for the other rectifiers as 1N4001~1N4007, but they are not for this case, and other people may not do that.
    In your case that I just tried to tell you the D1023, D1024 can be replaced by UF4004, but using UF4007 to replace the D1021 is better, in another method as what I did then I will using UF4007 to replace all of diodes including D1021, D1022, D1023, D1024, for the money and stock then I will do that.
     
  7. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    Oh OK, sorry about that Scott, there's a little bit of language difference. Is this what you mean? Or just use UF4007s on them all?
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    Don't say sorry, it's ok.
    I say what will I do, you may also do that to using UF4007s to replace all of them, I just provide an idea, what to do it's up to you.
     
  9. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    I forgot to upload the schematic in my last post? Ive only ordered UF4004 diodes,so I hope that's going to be OK? I wanted to point out that the D1020 Zener isn't installed on this 20MHZ model.Just in case it makes any difference?
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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    For a 1975V cross voltage comparing to 22V that it just a little bit, so it didn't affected anything.
    I can't see the cross voltage of D1022, but it connected to 1900V, so maybe change to UF4007 is better although it didn't damaged at this times.

    What you labeled is the normal way, they can be work and those are better than the original spec of parts, what I suggested of the spec was for repairing, the specs for our high voltage products are more than that.
     
  11. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    Yes I saw that the UF4xxx series is twice as fast as the ISS83.Thanks again Scott you've been a big help to me.
     
  12. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    Well after receiving the wrong diodes in the post twice in a row!Then being ran over by a car while riding a bike on the footpath.
    First scope.png
    Can you believe it? I finally got the UF4004 diodes. I installed them,then powered up,it started arcing on D1012 and D1022 and all the ISS83 equivalents failed except for D1022? I ordered some UF4007s and installed them but I used a dim bulb tester this time with a 40 watt bulb installed and sure enough it lit up? So its shorting out /dragging current somewhere? So I disconnected 2SD880(TR1005)Thinking that must be where its coming from But the bulb lit again at power on, with a 75 watt bulb in the DBT? All the components seem to read OK on my DMM? Ive built a high voltage tester kit to try and figure out whats going on with this thing? Should I remove just the diodes and power it on to measure the high voltage rail, or is there a chance of blowing up anything else? Ive uploaded a schematic showing the diodes that failed,the 10mega ohm resistor(R1024) read shorted when I measured it in circuit but when I removed the shorted diodes it read OK?

    Edited by Scott Wang : You attached too many circuits, so I deleted the rest, maybe you should check the switch of your mouse, mine also have that kind of problem.
     
    Last edited by a moderator: Mar 10, 2015
  13. ScottWang

    Moderator

    Aug 23, 2012
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    D1023 and D1024 crossed on R1024, when D1023 and D1024 shorted then R1024 will also shorted.

    If also need to check the C1023,C1025, if your meter have ESR function then you can measure two caps.

    Do other circuits have any shorted, you need to check all the circuit around the diodes.
     
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