QX-949

Discussion in 'Pioneer Audio' started by KeithD, Feb 11, 2018.

  1. KeithD

    KeithD Active Member

    Messages:
    421
    Location:
    La Crosse, WI
    Recently picked up a QX-949 with known issue: the unit drops in and out of protection, though it is mostly in protection. I don't seen anything obvious from visual inspection of the boards (from what I can see without disassembly). Will do a de-oxit of all pots and switches, but after that figured I will start with the power supply board and the two amp boards. Is there a recap list posted somewhere for this unit?

    Will also replace all the suspect transistors, but first have to look at what is on those boards. The service manual lists several transistors for each position. For example Q1 on the equalizer board is listed as 2SC1327-T or U and then lists in parenthesis (2SC1312-G or H, 2SC1344-E or F). I assume the transistor used depended on the board revision?
     

     

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  2. markthefixer

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

    Messages:
    20,984
    Location:
    Bensenville,Illinois
    Transistor used is whatever the factory had on hand, from what their supplier had on hand.

    Get a PILE of ksc1845's and ksa992's, and go on a MISSION to get all the CSSA (and CSZA) caps. There are a LOT of them.
    If you cannot fit film caps into some spots that the CSSA (and CSZA) caps come out of, use Nichicon UKL sub 1uf electrolytic caps.They're small and the unit is ultra cramped.

    Pay SPECIAL attention to deoxiting the 2 channel power boosting switch / plug on the rear panel..

    Sorry, I haven't done a QX-949 cap / part spreadsheet. (yet). (I checked)
    If you want to make up the raw parts list in an excel spreadsheet, I'd be happy to annotate replacement parts to an emailed copy, the slowest part is getting the initial parts in. See the recent SX-636 thread for an example ("standard") parts list spreadsheet format.
     
    Last edited: Feb 11, 2018
  3. KeithD

    KeithD Active Member

    Messages:
    421
    Location:
    La Crosse, WI
    Thanks Mark! Forgot about the power boost switch, will deoxit that tonight.

    Looking at some of the boards, I am seeing quite a few 2SC1312s along with 2SK30A and 2SA763P. There are also a few 2SA720s and I did see a couple of 2SA726s on the amp boards. I know the 1312s, 720s, and 726s should be replaced, what about the 2SK30A and 763P?

    Will start making a parts list for all e-caps and suspect transistors, which will take me a while. I've got the service manual, but will confirm with what's actually on the board. I've been waiting for Mouser to have 1845s back in stock before putting in another order, so this is a good time.

    It's going to be an effort to get the power amp boards out so that I can actually see what's on those boards. Looks like QSilver got access without having to disconnect all the wires http://audiokarma.org/forums/index.php?threads/qx-949-adventures.772234/#post-10572675 I'll see if that works for me.
     
  4. markthefixer

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

    Messages:
    20,984
    Location:
    Bensenville,Illinois
    You're in deep enough with all the other transistors that NOT doing the 2sa763's would be a mistake - even though they might not be a troublemaker, I don't expect them to stack up as well as a new transistor with 30+ years of manufacturing improvements. Especially in these preamp and decoding circuits.

    The Jfets like 2sk30 don't have modern equivalents anymore (some come close)especially if they capitalized on the 50v ratings it had - I leave the Jfets unless they are blown, and then I sweat the replacement.

    With the list, to simplify your end don't discriminate on transistor types, just do every board's e-caps, and silicon (transistors AND diodes) - that's a huge boost for me, I can add the few other components as I go through them. I'm thinking of adding categories of parts - like recap and repair - as separate columns in the spreadsheet to make it quick and easy to differentiate and also to quick and easy to grab parts lists.
     
  5. KeithD

    KeithD Active Member

    Messages:
    421
    Location:
    La Crosse, WI
    Thought I'd do a little bit of troubleshooting tonight. At this point when I turn the unit on it does not come out of protection. Measured some voltages with volume down, selector set to AUX, no speakers connected:

    Front power amp board:
    Pin 5: 1.8mV (should be zero)
    Pin 23: -37.4V (should be zero!)
    Pins 1-2:19.3mV (should be 20mV)
    Pins 25-26: 0.9mV, but can't adjust with trimmer (should be 20mV)

    This led me to check pin 17 (one of the channel inputs to the front board): -11.9V
    Pin 17 of the front board appears to be connected to pin 10 on the control amp board, so checked that: -12.5V
    Pin 10 on the control amp traces back to the input pin 13, so read that: 1.9mV

    If I've read the schematics right, this indicates the problem may be on the control amp board with the circuit between pins 13 and 10. There are three transistors on that circuit: 2SK30A, 2SA725, and 2SC1312, the latter two are known to go bad. Am I on the right track here?

    Rear power amp board readings are fine and in accord with the service manual recommendations.
     
  6. markthefixer

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

    Messages:
    20,984
    Location:
    Bensenville,Illinois
    Without breaking out the service manual to research the contrary possition, I would say you are on the right track.
     

     

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  7. KeithD

    KeithD Active Member

    Messages:
    421
    Location:
    La Crosse, WI
    Thanks Mark. Started making the parts spreadsheet and will share when I have the lists done (there are a LOT of 2SC1312s in this thing!). Per your advice I'll be listing all e-caps, transistors, and diodes. However, if possible, I'd like to see if I can't at least get the unit running by tracing back the specific problem. I'm hoping its either the 725 or 1312 on that circuit causing the protection problem.
     
  8. KeithD

    KeithD Active Member

    Messages:
    421
    Location:
    La Crosse, WI
    Decided to do some more troubleshooting. Pin 10 on control amp board (AWG-023-0) was putting out about 12VDC to the power amp. This is one of the four channels (which I later discovered is the Front Right channel). Replaced Q6 on control amp board: 2SA725 with KSA992. Also replaced Q10: 2SC1312 with KSC1845. The third transistor on that circuit is Q2: 2SK30A, but I don't know the replacement for that. Changing the transistors did not change the voltage output on that circuit. The output of that circuit on the control amp (pin 10) is connected to pin 17A on the power amp A board (front). The output of that circuit (pin 23A), which is connected to protection board pin 14 was putting out -38VDC. Thought I'd check to see if I brought pin 17A down to zero, would pin 23A come down. So, I grounded pin 17A, but pin 23A was still outputting -38V leading me to believe the problem is really on the the front R channel circuit of the power amp board. Decided to disconnect the wire from pin 23A on the power amp A board. After doing this, the unit comes out of protection and all three remaining channels are fully operating. I'll need to take a look at that circuit. There are two 2SA726 transistors, a 2SC1451, and one each of 2SC869, 2SB528, 2SD358, 2SA679, and 2SC1079.

    So, my plan is replace the two 2SA726 with KSA992, and replace 2SC1451 with KSC3503. Don't know if any of the others are problematic? I probably don't have enough of the right caps on hand to change the e-caps in that circuit, so if changing transistors does not work, I will have to wait until I've got the re-cap list done and checked by MTF to place an order.

    I continue to work on the re-cap and transistor/diode replacement list. I've got the power supply, power amp, protection, equalizer, and sub channel lists.... but several more boards to go...

    Edit: corrected typo ("resistors" should have been "transistors")
     
    Last edited: Feb 19, 2018
  9. markthefixer

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

    Messages:
    20,984
    Location:
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    PIn 10 of the AWG-023 control amp board is capacitor coupled - NO DC should be getting through to the next part of the circuit.
    C58 2.2uf CSSA 2r2p 25 2.2uf 25vdc
    It IS a DREADED CSSA Sanyo sky-blue capsistor - a guaranteed to short out capacitor.

    I looked to see if the power amp was capable of back feeding voltage to that point, and the input of the power amp is capacitor coupled with a 0.22uf Tantalum capacitor - far less of a shorting candidate - just not the clearest audio - passer around. So That power backfeed is extremely UN likely.

    Do not change the 2sk30 jfet transistors. Leave them be.

    Yes the power amp has an internal problem.
     
  10. KeithD

    KeithD Active Member

    Messages:
    421
    Location:
    La Crosse, WI
    Thanks Mark, so you recommend replacing AWG-023 C58? I do have several 2.2/50V nichicon caps on hand:

    UKL1H2R2KDDANA
    UKL1H2R2MDDANA
    UKL1J2R2MDDANA

    Not sure which is best for this application or if it matters. If that does not solve the problem, where to look next? Thanks again for the help!
     
  11. markthefixer

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

    Messages:
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    Location:
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    They are all equally useful in this replacement awg-023 C58 but that will not cure the amp problem.

    what's next = I agree with
    That will be fine, no CSSA caps in the amp, some CSZA caps that will get film caps (and some really fine sound) when we get around to replacement parts lists.
    I don't expect the CSZA or CEA caps to have caused this bad of a fault, thus my suspects are the named transistors.
     

     

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  12. KeithD

    KeithD Active Member

    Messages:
    421
    Location:
    La Crosse, WI
    Success! Replaced the 2SC1451 with KSC3503. Voltage on pin 23A came down to 35V. Replaced the two 2SA726 on that circuit with KSA992 and voltage came right down to a few milivolts. Reconnected the wire from pin 23A to the protection circuit, and all is working! Thanks for your expert help, Mark! Will now focus my attention on finishing the parts list.

    By the way, I was able to get at the back of the power amp A board without disconnecting any wires. I unscrewed all of the brackets holding the board and the large heat sink in place (about 14 screws) and I was able to swing the large heat sink up and out of the way without disconnecting the output transistors from the heat sink or the power amp board. Still a little cramped, but I was able to see what I was doing and get the soldering iron in there without much difficulty.
     
  13. markthefixer

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

    Messages:
    20,984
    Location:
    Bensenville,Illinois
    Congratulations!!! Now remember I am swamped so it won't be an instant response of the replacement parts numbers. The knee and leg are getting better so I should not be online as much as I have gotten used to recently.
     
  14. KeithD

    KeithD Active Member

    Messages:
    421
    Location:
    La Crosse, WI
    No problem at all Mark. I really appreciate your help on this. I have the list of all e-caps, transistors and diodes complete as per the service manual. Will need to double check some of the boards to see if what is in the service manual matches what I actually have. Found a couple of discrepancies already. Not in any rush at all. Really glad we located the problem! I can fill in the known replacements for some transistors (2SC1451, etc.), others will need your expert advice. I can also try my hand at e-cap replacement suggestions if it would be helpful to you?

    Thanks again, and glad you are on the mend!
     

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