power transformer questions

Discussion in 'Pioneer Audio' started by gonebytim, Aug 13, 2017 at 1:08 AM.

  1. gonebytim

    gonebytim Active Member

    Messages:
    138
    Hi,

    I'm by no means an electronics expert, but I have managed to do a fair bit of fixing work on my own equipment in the past, however , i'm not experienced with power transformers.

    I just stumbled across a Pioneer A-5 amplifier which is completely dead (wont power on).

    I put my multimeter on the secondary side of the power transformer and measured the voltage between the positive red leads and the black ground lead. It is showing absolutely no voltage. Am I testing it correctly? and does this confirm that the transformer is dead?

    also - I noticed that there is continuity between the positive and negative leads of the transformer (tested on the secondary side) - is this normal, or also indicating a dead transformer?
     
  2. mbz

    mbz Well-Known Member

    Messages:
    664
    Location:
    Melbourne, Victoria
    The transformer will be putting out AC voltage. Better to measure between the 2 red wires, pins 9 and 10.

    Suggest check the primary by power off/unpluging the unit then measure the resistance between the grey and white wires (pins 3 & 7
    on the fuse assembly). There looks to be a thermal fuse in the transformer. Expect a few ohms if all ok. An open circuit reading would
    be a bad sign. Should also check for continuity on the secondary by measuring the resistance between the 2 red wire, pins 9-10. Again expect
    near short. Open circuit is bad.
     
  3. gonebytim

    gonebytim Active Member

    Messages:
    138
    Hi, thanks for the advice!
    I performed these checks.

    The resistance checks seems ok, a few ohms and nearly short.

    But I'm not measuring any voltage between the two red wires of the secondary
     
  4. redk9258

    redk9258 Well-Known Member

    Messages:
    974
    Location:
    Illinois, close to St. Louis.
    Is there voltage at the input of the transformer?
     
  5. mbz

    mbz Well-Known Member

    Messages:
    664
    Location:
    Melbourne, Victoria
    Assume you've checked the two fuses FU1, FU2 for continuity (amp powered off/unplugged).
    Check primary by measuring AC voltage between pins 3 and 7.
    You may have a problem with the power switch, measure AC between pins 7 & 5, expect ok.
    Measure Ac between pins 7 & 12, expect ok. Measure AC between pins 7 & 1, maybe not ok indicating faulty switch.
     
  6. gonebytim

    gonebytim Active Member

    Messages:
    138
    Thanks for the tips guys, turns out that it was bad soldering on the switch. Resoldered it and it can power on now.

    Powered it on with a dim bulb tester first and the buld is glowing full brightness without any power light coming on on the front of the unit
     
  7. mbz

    mbz Well-Known Member

    Messages:
    664
    Location:
    Melbourne, Victoria
    What lamp wattage are you using for DBT.
     
  8. gonebytim

    gonebytim Active Member

    Messages:
    138
    To be honest - I don't recall what wattage I put in it about a year ago, but I've used it on many amps and the wattage seems to be a nice little sweet spot where the bulb dims down to a faint little glow when an amp is working fine and burns full brightness when ever I've had a dead short.

    I've never come across an amp so far that had a short for me to troubleshoot - but I've seen shorts on the bulb tester when I've caused them myself during repair and troubleshooting.

    I've just checked all four of the big transistors on the heatsink. I found that one of them had its emitter disconnected due to a broken solder joint. Fixing this didn't solve the issue and while I was at it - I desoldered all four transistors and tested them (the regular six way diode test between legs) and they appear to be fine (no shorts at least). Then I resoldered them all back properly onto the board.

    I've also tested the 4 rectifier diodes and they appear to be fine. I've also had a good look over the board with my eye looking for anything suspicious but can't see anything.
     
  9. mbz

    mbz Well-Known Member

    Messages:
    664
    Location:
    Melbourne, Victoria
    Do you get a relay click at power ON, maybe insuffient voltage due to DBT. Maybe excessive dc on line.
    I will search for test points.
     
  10. mbz

    mbz Well-Known Member

    Messages:
    664
    Location:
    Melbourne, Victoria
    If no relay click, measure voltaged at TP1 and ground. Then repeat for TP2, TP3 and TP4 and ground in each case.
    Expect less than +/-1Vdc in each case. TP's are in the power amp stage used for biasing.
     
  11. gonebytim

    gonebytim Active Member

    Messages:
    138
    I just checked for a click, and I'll be honest, it's hard to tell since the power button itself makes a loud click - but I don't think the relay is clicking
     
  12. mbz

    mbz Well-Known Member

    Messages:
    664
    Location:
    Melbourne, Victoria
    The click should be 3-8 seconds after power ON, allowing time for transients to settle before connecting the speakers.
    You probably have a dc offset issue caused by a transistor short. First step is to identify faulty channel using measurements
    above. If you are not comfortable with measurements on live units then let me know.

    Need to step out for a bit...
     
  13. gonebytim

    gonebytim Active Member

    Messages:
    138
    TP1 = -3.29v
    TP2 = -3.49v
    TP3 = -3.57v
    TP4 = -3.45v
     
  14. mbz

    mbz Well-Known Member

    Messages:
    664
    Location:
    Melbourne, Victoria
    The voltages are a little close and maybe inconclusive. However if we run with it, it
    looks like the right channel is at fault |TP3| > |TP2|, |TP4| > |TP1|. Also TP3 is closer
    to the source than TP4. however this would point to Q4 (diode tested OK).

    Also more likely to be component associated with negative power rail/half of amp.
    As a guess, you could diode test Q424 and Q422, in circuit should be ok.

    If you can follow the schematic, the task is to track down the source of this negative
    voltage. Check Q424, Q422 first and I'll try and work out a test procedure.
     
  15. mbz

    mbz Well-Known Member

    Messages:
    664
    Location:
    Melbourne, Victoria
    Right channel suspect faulty.
    Suggest in circuit diode test of Q422 and Q424.

    The fault finding technique is to find the source of the -3.x Vdc by starting from some
    reference (the speaker line) and moving in the direction of increasing voltage. Where
    possible measure voltages at resistors, diodes,,, rather that at transistor legs, one slip...

    Assuming Q422, Q424 passed diode test.

    With black meter probe connected to the speaker line anywhere between D412 and D414 junction
    and R462 and R464 junction, eg, at R462 and R464 junction, measure the voltage at,
    - Q4emitter (mV scale)

    a negative voltage indicates suspect faulty.

    -Q4b
    -Q422base
    -Q2b
    -Q420b
    -D414, R432 junction
    -D412, R430 junction
     
  16. merlynski

    merlynski Curmudgeon Electronicist Subscriber

    Messages:
    59
    Location:
    High Plains, Eastern Colorado
    Did you get the dim bulb tester to dim like normal yet? Until you determine why it is still bright and fix that, that should be your priority.
    Approx -3.5 volts on the TPs is way too negative, they should be close to 0 volts, looks to me like your +39 volt supply is loaded down or failed.
     
    Last edited: Aug 15, 2017 at 5:52 PM

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