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QX-949 Power supply issue

Discussion in 'Pioneer Audio' started by KeithD, May 6, 2018.

  1. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    Pin 7 - ground: 0.09 ohm
    Pin 9 - ground: 0.09 ohm

    Will remove q4 and put in new KSC2073TU...
     

     

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  2. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    Q4 replaced, ECB readings unchanged: 11.4, 34.5, 11.6
     
  3. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,103
    Location:
    uk.. the middle bit
    check c15 to rule it out .. you could lift one end to see if it makes any difference .
    am thinking you are best checking every component for being right way round and in tolerance .
    do you have a spare 14v zener for d7 ?
     
  4. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    Unfortunately, Pete, I think you are right. The only things I have not replaced or checked on that circuit are the small capacitors: C2, 3, 8, 9, 15. So, at this point, I need to check those caps.

    Already replaced D2, 3, 6, 7 (post #22).
     
  5. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    C15 is 1.27nF, should be 1nF. Leaving it disconnected and powered on gives 29.5V on pin 8, and 13V on pin 6 with pin 6 connected to tuner, unit comes out of protection. Seems like that small cap is the problem?
     
  6. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,103
    Location:
    uk.. the middle bit
    must be leaky .. try a new one .
     

     

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  7. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    Yes, don't have those in stock. Mouser has many options. Any suggestions of which one or will any work that are 1nF/50V?
     
  8. Unjoy

    Unjoy New Member

    Messages:
    49
    Location:
    Apeldoorn, Netherlands
    I think you nailed it Keith, congratulations!
    Pioneer lists C15 as a ceramic type. Anything will do here I would say, something like 810-FA28X7R1H10200 from Mouser.
     
  9. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    Thanks for the recommendation. I will probably order a bunch of these and 10nF ones as well. I strongly suspect a bad capacitor on the power supply board B is causing variation in the power coming from that board, which in turn (I think) is causing low level noise on the rear channels it feeds on the power amp board.
     
  10. Unjoy

    Unjoy New Member

    Messages:
    49
    Location:
    Apeldoorn, Netherlands
    That could very well be the case. You have seen how the 13V supply affects the 30V supply, so a defective C15 can definitely cause audible rumbling noise.
    Film or ceramic capacitors usually don't break, but always good to have some common values at hand. In that case, order a couple with a higher max voltage, just in case you need them.
    Let us know if changing C15 also gets rid of the noise.
     
  11. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    Parts arrived and I replaced C15. This did not solve the issue, but it did change the voltage on pin 6 when disconnected from tuner. When tuner is disconnected Pin 6 is 16V and pin 8 is 29.4V and unit comes out of protection. When pin 6 is connected to the tuner, pin 6 is 11.3V and pin 8 is 26.2V, unit does not come out of protection.

    I'm scratching my head at this also. Only things connected to pin 6 are C15 and Q4. Replaced Q4 twice and checked grounding.
     

     

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  12. Unjoy

    Unjoy New Member

    Messages:
    49
    Location:
    Apeldoorn, Netherlands
    Do you have some resistors lying around? Can you try changing R7 (36k) to 33k?
     
  13. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    Yes, I do have a 33k high wattage resistor I could try.

    I think I will also check the remaining ceramic caps, since C15 was out of spec. I bought enough of those to do replacements as necessary.

    As I mentioned in post 18, I'm seeing fluctuation in the B2 voltage from power supply B board. I'm thinking that may be a bad ceramic cap there.
     
  14. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    Replaced C8 and 9. With tuner power disconnected, pin 8 is unchanged, but pin 6 is now 13.5V as it should be. However, connecting tuner to pin 6 has same results as before: pulls down pin 6 to about 11V and pin 8 to about 26V. C8 and 9 were out of spec by 25 - 35%

    To be complete I also replaced C2 and 3. These tested good after I pulled them, but replaced the anyway.

    Will try powering the tuner from a separate power supply tomorrow. Need to borrow one from work.
     
  15. Unjoy

    Unjoy New Member

    Messages:
    49
    Location:
    Apeldoorn, Netherlands
    Man, this is frustrating! Only a handful of components, and almost all replaced. What else can be wrong?
    In your first posts you mentioned the voltages fluctuating quite a bit. You don't see that anymore now?
    The only thing connecting the 30V and 13V supply is R9. As an experiment, can you lift the leg of R9 that connects it to pin 8 and instead connect it to the 'C' of Q4? Then first measure pin 6, disconnected from the tuner. Voltage should be 13.5V. If ok, what happens when you connect the tuner?
     
  16. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
    La Crosse, WI
    Here's what I did yesterday. Connected tuner power wire to a 13.5VDC regulated power supply. Everything worked fine. Then connected tuner to pin 6 with one leg of C15 out. Again, everything worked fine, pin 8 and 6 voltages were normal. Voltage from pin 8 (B3) is very steady. However, -B1, +B1, -B2, +B2 voltages vary. Range is 0.15 to 0.2V. These voltages are coming from the B power supply board. All that is on there are diodes and ceramic caps. Wondering if one of the caps is failing? However, this should not affect the problems on the main PS board, unless I am missing something.

    I will be away for a day or two, but I will try what you suggested. Also, need to take a close look at the back of the PS board to make sure I don't have any stray solder causing a short. Lots of stuff got replaced and I need to double check all the joints.
     

     

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  17. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    648
    Location:
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    OK, think I found the power supply shorting issue. There was some stray solder on the back of the board that I believe was shorting pin 5 to ground. I've cleaned that up and now all is working for voltages on pins 6 and 8 with tuner connected and C15 in. :)

    Now, on to the next issue (which is why I started this re-cap in the first place), static on all four channels, but most on rear left and right. As I mentioned in previous post, I'm seeing variability in both B1 and B2 of around 0.2V, which is about the level of the static on the output pins from the power amp boards (pin 5 on each board). My guess is there has to be a bad cap or more on Power Supply board B that is causing this ripple in the voltage. I can pull a leg one at a time on that board and see if any of them test out of spec. Any other thoughts on the cause?
     
  18. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,103
    Location:
    uk.. the middle bit
    both channels suggests power supply issue as a first call .
     
  19. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
    669
    Location:
    philippines
    the thing is loaded to the gill's with 2sc1312's and some 2sa725's and 2sa1000's and some 2sc1451's thrown in, all on the crap transistor list's. ive done a dozen pioneers now,and while the accepted wisdom is to locate the issues before attempting a recap or restore (AND FOR GOOD REASON),this set may be a possible exception if it as you say just static you just might be better off going through the thing and replace all the junk and test it AFTER EACH BOARD. the reason being is that its REALLY TIGHT getting at the boards in most cases in the first place with little or no slack available to access the boards. on the equalizer i guarantee that you wont be able to lift it without removing at least one wire wrap and even then its a real headache.you will be forced to move and manipulate the wires way to much as it is and thus run the risk of broken wire's. i broke three and normally dont break any. i dont think its wise to add more troubleshooting to locate offending transistors which by the way are more than one and sprinkled like confetti throughout the unit. double and triple check each wire wrap on every board after you are done. aside from the static evaluate now so you have some reference to its ccondition before you begin. by the way,i also did the subchannel board and surprisingly it still functions on a cd4 test record.whether it could be improved by alignment is an unknown as its alignment procedure is lost to antiquity. i do also think you would be well served using a solder extractor as there are a helluva lot of parts to change.just my two cents worth and just trying to be of assistance.
     
    Last edited: Jun 5, 2018
  20. Unjoy

    Unjoy New Member

    Messages:
    49
    Location:
    Apeldoorn, Netherlands
    I agree with John, there are lots of parts that tend to be unreliable. When restoring my QX-949a I removed the boards completely to make working on them easier. Instead of unwrapping the wires from the pins, I desoldered te wire wrap pins from the boards. Getting the boards out completely makes it easier to work on. I then replaced all electrolytic caps, trimmers and suspicious transistors. A lot of work, but certainly worth the effort. It's a beautiful piece.
     

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