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QX-747A No Sound...is the service tech full of ***t or am I paranoid?

Discussion in 'Pioneer Audio' started by Sharky B, Nov 28, 2018.

  1. Sharky B

    Sharky B New Member

    Messages:
    22
    Hi all,

    I have a Pioneer QX-747A that has no sound (everything else seems to work, lights, tuner and so on) but the relay does not engage/click, so I dropped it off to a local tech to have it checked out (paid him in advance to look at it) and he would tell me what's wrong with it. Initially he said it would take a few days to get to it, but to my surprise he called the next day and told me that there is a short somewhere and I would have to replace all capacitors and it would be a $300-400 job. When I pressed further he said that he was able to get one channel working and had disconnected the other ones, when I asked him which one he said he didn't remember and that was that. Now call me paranoid, but if he was able to get sound out of one channel wouldn't that isolate the short? Any thoughts?

    Should I take it to another tech for a second opinion or let it go at this point?
     

     

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  2. mbz

    mbz Super Member

    Messages:
    2,256
    Location:
    Melbourne, Victoria
    The QX-747A is quite old. The tech opens it up and sees many caps near/at the point of failure. When he eventually fixes the "short"
    does he return it only for one of those caps to fail the following week. What would be your reaction in that case.

    The amp probably has excessive dc voltage on the speaker line, so it's not activating the relay, ie no click. The tech identified the
    faulty channel then isolated it, so the fault does not appear so the relay engages.

    The cause of the short is typically a failed transistor. It might be a failed resistor, diode cap or...

    Your options are for him to only fix the protect issue. You then have a working amp with suspect caps. I wouldn't
    give any warranty that covers the caps in this case. $300-400 does not sound unreasonable, depends on how many caps
    and amp layout.
     
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  3. Sharky B

    Sharky B New Member

    Messages:
    22
    I hear you, but he didn't even give me that option. I agree $300-400 is not unreasonable. I would've been happy with a temporary fix and take a chance on other things failing down the road, it's still better than a bricked receiver.
     
    mbz likes this.
  4. mbz

    mbz Super Member

    Messages:
    2,256
    Location:
    Melbourne, Victoria
    It's your dollars, your choice. If you've lost confidence in the tech then it's hard to stay with him. Will the next tech be any better?
    Suggest that you seek out a tech vetted by AK, maybe give your location and members can put names forward.
     
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  5. Sharky B

    Sharky B New Member

    Messages:
    22
    Might have to do just that. Thanks for the suggestion and the reply mbz. Appreciate it.
     
  6. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,662
    Location:
    Milwaukee, WI
    The problem with the QX-747 is that it’s jammed with the sky-blue Sanyo caps and full of bad transistors. There are SO MANY parts to replace. Realistically most are probably fine, but it’s hard to tell what’s causing the issue. If you do have it redone, though, there are plenty of transistors that are more trouble than the caps. Tons of C458s and 2SC1451s in there.
     
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  7. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

    Messages:
    21,110
    Location:
    Bensenville,Illinois
    Seconded
    don't forget:
    2sa725, 2sa-726>>> ksa992
    2sc1312, 2sc1313 >>> ksc1845
    2sc1451 >>ksc3503
     
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  8. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    645
    Location:
    La Crosse, WI
    I would not spend that kind of money on a QX-747. They sell for less than that working. One sold on CL here recently for $150 and had been serviced (I know the tech that worked on it).

    In addition, if you want real quad sound you need to get a Sansui. Pioneer did not have good quad decoding, some might say any at all. So, unless you are using a 4 channel source for quad, you’re not getting much for the money you are spending with a Pioneer quad receiver.
     
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  9. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
    669
    Location:
    philippines
    i successfully rebuilt a qx949 its quite a labor of love. the amount of caps and transistors excluding the main power caps is a DOUBLE HANDFULL. it is the most difficult job ive ever done as not only is it cramped with little to none wiring slack the layout of the board components is also jammed packed, i have a professional grade grade solder extractor which speeds things up ENORMOUSLY. the 747 is quite similar and has some boards in common with 949. given the amount of man hour's involved NO business could make money going COMPLETELY through that thing and then GUARANTEE the job on top of that for less than 1000$ and that is a conservative estimate. there are 4 adjustments on the sub channel board that are not called out in the service manual and as far as i can deduce were pioneer trade secrets. i rolled the dice and recapped my subchannel and was elated when it played a cd4 test record. its singing in my office as i type this. I WOULDNT RELISH DOING ANOTHER UNLESS IT WAS FOR A GOOD FRIEND. i did this set out of sentiment as i bought one new overseas,from what ive read keith d is correct about the superiority of the sansui units.whos got any 4 channel records? maybe someone is pressing them i dont know but even in '74 4 channel records were not available in all titles
     
    Last edited: Nov 29, 2018
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  10. Sharky B

    Sharky B New Member

    Messages:
    22
    Thanks for the feedback guysI The unit I have is in excellent cosmetic condition and it's such a shame to see such a beauty no longer produce sound. Perhaps I can put it on a pedestal and use it as a decorative "objet d'art"? :D
     
  11. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
    669
    Location:
    philippines
    you could of course have a go your self and try to fix whats wrong with it at the moment. it beats the hell out using it as a door stop.my comments were based on a total rebuild.nothing ventured,nothing gained.the amp boards themselves arent that bad and it sounds like that might be your problem they are kind of bxxxh to get at but not impossible.
     

     

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  12. Sharky B

    Sharky B New Member

    Messages:
    22
    The tech said that there is a short in one of the output channels and was able to get it going with one though he forgot which one, so there is hope, I would love to get my hands dirty and give it a go myself, but this monster looks intimidating. I would like to replicate what he did, he said that he had to disconnect the output channels, how he did that I have no clue. Assumption: would disconnecting all output channels allow the relay to engage?
     
  13. Sharky B

    Sharky B New Member

    Messages:
    22
    You're right. I'm looking at the service manual and they are everywhere! Damn!
     
  14. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,662
    Location:
    Milwaukee, WI
    I rebuilt a QX-747 a few months ago. I put in 72 capacitors and 75 transistors, and that was only like half the electrolytics in there, and not all the transistors. I just did all the tantalum and sky-blue caps and a few others, and left the rest of the electrolytics. I think I hit the majority of the problem transistors though. There are SO MANY boards in there and it's just ridiculous. I actually have two more QX-747s to rebuild. They're pretty fun to do, though--it's fairly easy to get to the boards and the parts are all cheap and available from Mouser.
     
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  15. Sharky B

    Sharky B New Member

    Messages:
    22
    I just counted eight 2sa726 on the output board. Noob question, when you rebuilt the output board what was your procedure for taking it out? I'm thinking of starting there as that's where the short is happening according to the tech.

    [​IMG]

    [​IMG]
     
    Last edited: Nov 29, 2018
  16. rcs16

    rcs16 Super Member

    Messages:
    4,057
    Location:
    Baldwin, Ontario, Canada
    I have never done a QX-747, but I have a QX-9900, similar issues
    Big problem with the economics of servicing these units. It is mostly time.
    Usually the wires are so tight you can not get to the back side to access the solder points.
    Start by un-bundling the wires, removing tie wraps to loosen up the tension on the wires.
    Some or all connections have to be removed to get access. Un-solder, do not unwrap, the square wire wrap posts to get access.
     
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  17. Sharky B

    Sharky B New Member

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    Good tips. Thank you!
     
  18. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,662
    Location:
    Milwaukee, WI
    I can't remember offhand, unfortunately. I'm sure I'll get the next one opened up fairly soon though. I don't recall the output board being super-hard. I have found that Pioneer gear almost always has enough slack in the wires if you play with them, and I usually just get the board out as much as I can and go to town.
     
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  19. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    645
    Location:
    La Crosse, WI
    This is very similar to the QX-949 setup, and I have had two of them. You are going to have to unscrew the heatsinks and pop those small plastic pins attaching each board to the metal frame. You need enough slack to then move and swing the heatsink out of the way to get at the back of the amp boards.

    In my opinion, this is one of Pioneer's worst designs for dissipating heat from critical components: they've mounted the power amp board right on top of the heatsink for the output transistors! Heat is the nemesis of electronics components - especially electrolytic capacitors. This is one reason I personally would not spend time on a QX-747. They can be purchased working for $200. As you mention you've got 8 2SA726 transistors (two per channel). Those should go. It's entirely possible they are causing your problem.

    I'm not sure how much you trust the "tech" you took it to, but I can't believe he "forgot" which channel had a "short." I mean, really? All you need is for one of the channels to be putting out bad voltage for the unit to not come out of protection. Test the output voltage on each channel. Follow the schematic for which pins these are. If one is way out of line with zero (no inputs, volume at zero), then focus on that channel. Odds are high you've got a bad 2SA726. Eventually, they should all be replaced as odds are high they will go bad. There is no pre-out/power-in on this receiver, so the only way to isolate the power amp board is to ground the inputs or disconnect them from the pre-amp by removing the wires, but I'd just carefully ground them to start. If the receiver does not come out of protection, your problem is on the power amp board or after in the signal path (protection comes next.) If it does, then you need to look further upstream.
     
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  20. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    645
    Location:
    La Crosse, WI
    Yes, because then there should be 0V on all channels going to the protection board, which is what it wants to see (or at least a balanced low voltage) to close the relay and send signal to the speakers. However, if you have an issue on the protection board, then the relay would remain open.
     
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