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Pioneer QX-949A in protection, one channel high VR 3/4 voltage.

Nashou66

Nashou66

AK Subscriber
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Hi I have this nice shape QX-949A where it is in Protection.

When I first got it to look over The relay would click after a long while and I was able to get a few DC measurements off the speaker Terminals but then its click out again going into protection.

I have the Service manual and need some guidance , first in naming the channels.

I measured the output voltages on the 3 output terminals of the power transistors and all but one measured in a similar voltage readings its the amp ,if looking from the front face, at the top right corner. the one in question measure over 39 volts at all pins IIRC as I did this latte last night.

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The service manuals lines indicating the parts is faded and you can not make out what description goes to
what part.

So On that amp the readings across pins 1 and 2 was .5 volts way above the 20mV's it needs to be adjusted to.
Adjusting VR3 got me to 240Mv's but it was jumping around as I tried sometimes hitting 3 volts dc.

So something is amiss on that amp section possible. Where do I begin diagnosing this?

I did a diode test of the output 's and all seem to measure ok , this was done in circuit for now. Removing them right now seems a task I'll wait for till they actually need to be removed.

Athanasios
 
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The amp board with the C1451s should get all new transistors. Those C1451s are trash and the other transistors on there are pretty bad too. There should be some blue Sanyo caps on there too—those are bad as well. You can replace the other caps on it but it’s not urgent.
 
EastPoint said:
The amp board with the C1451s should get all new transistors. Those C1451s are trash and the other transistors on there are pretty bad too. There should be some blue Sanyo caps on there too—those are bad as well. You can replace the other caps on it but it’s not urgent.
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What are replacements for those transistors? I just ordered a bunch if you have seen my thread on What transistors to stock. Hopefully I have replacements on hand for those.
 
Just checked my Last digikey order online and I have both . One is 160v the other 300v , does it make a difference?

Nashou
 
Use 3503. Good luck, I've had a 949 in my garage for months. The amp boards are atrocious to work on.
 
ivandezande said:
Use 3503. Good luck, I've had a 949 in my garage for months. The amp boards are atrocious to work on.
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Thanks, I think I'll check the voltages on page 98 of the service manual first when I get time, then go from there.

Like to find the bad parts first before shot gunning it. But advice from those who have gone before is always appreciated and welcome.

Looks like a fun project, but I do hate desoldering all those wire pins to get the boards out. ugh

Nashou
 
I have two versions of the 3503's one is 40 hFE and the other is 60 hFE . which should I use when I get there?

Nashou
 
Well Measured the voltages on the amp boards and pin 4 on the Suspect channel is 13.xx VDC , should be 0 , this pin goes to pin 16 on the Protection board. Therefore tripping the relay open.
 
Well the one 1451 was bad as suspected but also the 2SD357 on the one heat sink .What should I use for that?

Removing that amp assembly was not that bad actually if you go methodically and slow.

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ok for the 2SD357 i'll use a KSC2690AYS .

Now the 2SC1451 had a round heat sink on it should I add a heat sink to its replacement?

Nashou
 
EastPoint said:
No, you don't need to put heat sinks on those.
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Thanks, but I created another issue and its a doozy !!! I never removed the heat sink connected components while working on the board.
Well, the bending back and forth of the board broke the wire leads of the two varistors !!! Crap those are unobtanium !!

I tried to resolver the leads back by eating away of the potting on the part and then I was able to solder it back.

But not sure if these went bad with the other parts.

here is a reading on my parts tester. I never used it for a varistor so not sure if its reading it correctly, but I think it is.

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I used a piece of clear shrink tube that fit perfectly over the STV-4H case and filled it with Clear JB weld to keep the solder connection nice and secure.
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