Pioneer QX-949 in protect mode / no audio

[CapLeaker]

Calling all badcappers to help me rescuing and restore this oldie! I have an old mid 70's Pioneer QX-949 quad receiver that I am doing as more of a kind of long term go over everything repair / restore. It is in a decent shape too and even everything still lights up. So I really want to get it back working 100% as it supposed to.

The problem right now is that the receiver amp has absolutely no audio. None on the headphone jack either. I turn the amp on and there is no click from the protection relay. The tuner does work, as the needles do something while turning the dial.
So off I go directly to the output of the PA and check for DC voltages. Didn't have to look very long and found on one PA -39V on one of its output pins (pin 22).
I went and took the two TO3 transistors out and checked them in diode mode. Nothing wrong. Checked the driver and nothing wrong there either. So what am I missing? Why is there -39V when there shouldn't be any?

[R_J]

Re: Pioneer QX-949 in protect mode / no audio

I would check the emitter resistors (.5Ω5W) then check the other resistors like R44. R48. R40 & R42 It should'nt be too hard to repair, compair resistances with a working channel sometimes its can point to the faulty area.
If the diode array D6 (STV4H) is mounted to the heatsink, make sure the leads are not broken.

[Andrew F. Ali]

Re: Pioneer QX-949 in protect mode / no audio

The big feed thru cap from the outputs to the speaker terminals could be shorting. Do you have a Signal Tracer for tracing AF signals? Feed a signal to one of the inputs and trace to see where the signal is lost.

[R_J]

Re: Pioneer QX-949 in protect mode / no audio

To have -39 volts at the output either Q16 is turned on, causing the the negative supply to show up at the output or Q14 is turned competely off so that the positve supply is not allowed to ballance out the circuit, +39 added to -39 = 0

[petehall347]

Re: Pioneer QX-949 in protect mode / no audio

start with power supply voltages ..
am helping with one of these on audiokarma right now ..

[R_J]

Re: Pioneer QX-949 in protect mode / no audio

With the fault only in one of the power amb circuits, that would eliminate any power supply issues. Likely with the 2 output transistors removed from that circuit the rest of the amp should function

[petehall347]

Re: Pioneer QX-949 in protect mode / no audio

not the case with the one on audiokarma .. 30v low on supply . its looking like a tuner issue as 13.5v supply has some sort of a short going on and its affecting 30v supply ...
with any amp repair i check supply first . if something wrong there you will chase your tail looking for other problems that might not even exist .

[CapLeaker]

Re: Pioneer QX-949 in protect mode / no audio

I would check the emitter resistors (.5Ω5W) then check the other resistors like R44. R48. R40 & R42 It should'nt be too hard to repair, compair resistances with a working channel sometimes its can point to the faulty area.
If the diode array D6 (STV4H) is mounted to the heatsink, make sure the leads are not broken.

Checked these and they compared to the others and good.

To have -39 volts at the output either Q16 is turned on, causing the the negative supply to show up at the output or Q14 is turned competely off so that the positve supply is not allowed to ballance out the circuit, +39 added to -39 = 0

That is what I thought too. So I took Q14 and Q16 out and compared them to the other PA. I also put them through my transistor tester fine. No difference. Same with Q12 and Q10 (measured in circuit). I compared the 2 PA's component by component. There isn't any difference to speak of, besides one component that one PA has, but not the other. The "good" PA has on pin 19 a 103Z 250V ceramic disc cap that goes to chassis GND. The "bad" board doesn't have that.

With the fault only in one of the power amb circuits, that would eliminate any power supply issues. Likely with the 2 output transistors removed from that circuit the rest of the amp should function

My thinking too. I am going to install the PA's again and check what happens. If not, I guess I am going to swap the TO-3 transistors from one side to the other and see if the problem "moves".

[CapLeaker]

Re: Pioneer QX-949 in protect mode / no audio

The big feed thru cap from the outputs to the speaker terminals could be shorting. Do you have a Signal Tracer for tracing AF signals? Feed a signal to one of the inputs and trace to see where the signal is lost.

Pin 22 goes to the protection board and straight to the relay. No cap there.

[CapLeaker]

Re: Pioneer QX-949 in protect mode / no audio

More news:
Changed all TO-3 transistors from one PA over to the other PA. No change. The problem persists at the same PA. Did some more measurements:

Pin 22, 24, 25, 26 = -39V still.
Pin 27, 28, 29 (2SA747 TO-3) = -39V
On the 2SC1116 TO-3, pin 32 has +39V. Pins 30 and 31 have -39V.
Pin 17 has -18V and should have 0V
That STV4H diode D6 has also -38V on both sides.

[R_J]

Re: Pioneer QX-949 in protect mode / no audio

What about pins 15,16? Whats the voltage on C14 (+)? it should be +19 volts, Whats on the base of Q8, should be +1.8 volts The collector of Q8 should be +39, the emitter should be +1.2 but its -39 so Q8 is NOT biased on
If Q8 base is not +1.8 and is instead -39v then that could be caused by a shorted Q6
Also check on R8 (56K) you should have +30 on one side and +(I'm guessing 19) volts

[petehall347]

Re: Pioneer QX-949 in protect mode / no audio

17 goes to 10 control amp . whats pin 20 voltage ? yes it will be wrong .. as above ..30v supply to board .. ? 11 and 12 pins .then at resistors after there if ok

[CapLeaker]

Re: Pioneer QX-949 in protect mode / no audio

What about pins 15,16? Whats the voltage on C14 (+)? it should be +19 volts, Whats on the base of Q8, should be +1.8 volts The collector of Q8 should be +39, the emitter should be +1.2 but its -39 so Q8 is NOT biased on
If Q8 base is not +1.8 and is instead -39v then that could be caused by a shorted Q6
Also check on R8 (56K) you should have +30 on one side and +(I'm guessing 19) volts

This is the latest state off the PA with everything hooked up as it supposed to be:
pin 15, 16: +30V
pin 17: -18V
pin 20: 0V
pin 18, 19: -46V
pin 21: 0V

pin 14: +39V
pin 26, 22, 25, 24, 40: -39V

Q6 has -45V on B C E (all around). The base of Q8 has -45V. I checked Q6 and Q8 out of circuit and they are fine. Then I checked Q2 and Q4 out of circuit and low and behold Q2 2SA726 is bad and Q4 is good.

[R_J]

Re: Pioneer QX-949 in protect mode / no audio

Good to here, so its fixed? You should check the bias and off-set after the repair and adjust as required, page 16 (21) of service manual

[CapLeaker]

Re: Pioneer QX-949 in protect mode / no audio

Not yet. I wish I'd have the parts, but I don't. So I cross referenced the 2SA726 and the best I can do is a KSA1015. There are two to choose from that have different current gain. Either the KSA1015YTA (lower gain) or the KSA1015GRTA (higher gain). I am thinking I should order the higher gain ones and replace Q2 and Q4 as a pair. The good 2SA726 had an HFE of around 400 when I measured it. I am placing an order right now.

[petehall347]

Re: Pioneer QX-949 in protect mode / no audio

high gain matched pairs for those differentials . order a bagful and find a pair that closely match .

[CapLeaker]

Re: Pioneer QX-949 in protect mode / no audio

I've ordered a dozen of KSA1015GRTA and match a pair up, put it in and see what happens.

[petehall347]

Re: Pioneer QX-949 in protect mode / no audio

matching is not so critical as dc offset has a trimmer .. best to match anyway .

[kc8adu]

Re: Pioneer QX-949 in protect mode / no audio

while you are in it replace at least all the light blue sanyo caps.
working on a qx-747 now that had distortion and random relay tripping.
none of the blue sanyo caps passed my quick test.
1 was nearly shorted and shoving dc into one channel of the amp.
these same caps are trouble in the kenwood ts-120-140 hf radios too.

[petehall347]

Re: Pioneer QX-949 in protect mode / no audio

yep blue sanyo are also known as capisitors