Pioneer QX-949A - WTF!?!

M Jarve

Audio Geek and NGE Freak
A friend just picked up a Pioneer QX-949A receiver. Plugs it in and turns it on, and no sound, but lights up and what not.

So he brings it over to me, seeing as I have some knowledge of electronics, and can repair most things.

I plug it in, turn it on, and wait for the protection relay to kick out. It doesn't. I check the relay, and it tests fine. I bypass the relay, and get DC at the speaker jacks.

Okay, one or more bad output transistor(s). I take the grate off the power amps, and LO and BEHOLD! none of the output transistors match or are in pairs. Each output transistor is different.

From what I can tell, no one has been in there ever, or at least for some long time, as the dust on the screws internally was undisturbed.

So, if someone has one of these units (or the service manual for one) I would greatly appreciate knowing at least what the output transistors are.

Thank you!
 
M Jarve said:
A friend just picked up a Pioneer QX-949A receiver. Plugs it in and turns it on, and no sound, but lights up and what not.

So he brings it over to me, seeing as I have some knowledge of electronics, and can repair most things.

I plug it in, turn it on, and wait for the protection relay to kick out. It doesn't. I check the relay, and it tests fine. I bypass the relay, and get DC at the speaker jacks.

Okay, one or more bad output transistor(s). I take the grate off the power amps, and LO and BEHOLD! none of the output transistors match or are in pairs. Each output transistor is different.

From what I can tell, no one has been in there ever, or at least for some long time, as the dust on the screws internally was undisturbed.

So, if someone has one of these units (or the service manual for one) I would greatly appreciate knowing at least what the output transistors are.

Thank you!

Mike -- My 949A is in the shop getting "tuned up". When she returns, I'll take some interior photos so you can see what things should look like. Sounds like someone really messed that unit you're working on up!! :thumbsdn:
 
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Thank you. It looks like they just threw in anything that would fit in there!
 
checked the transistors on a QX-949 indeed they are all different! I guess the reason is this: As a four channel receiver (4x 40 watts) it was switchable to 2 channel operation (2x70 watts ??) using only the 2 front channel amps. When switching to 2 channel the front amps got a higher supply voltage from power supply. Thats why the front output transistors have a higher rating.
Front transistors: 2SA679/2SC1079 (120 Volt 12A)
Rear channel trans. 2SD370/2SB530 (110Volt 6A) All Toshiba Transistors
A good substitute for these old TO-3 transistors are MJ 15024/ MJ 15025 or MJ15003/4 in this case.Be sure to buy only ON- semi or old Motorolas, as there are a lot faked MJ transistors on the market. (Google: counterfeit transistor)

...by the way, what is the difference between QX-949 and QX-949A ??


Helmut
 
The difference between the 949 and the 949a model was the type of decoders used in the 4ch modes. The 'A' model had a new decoder that had better seperation between channels than the standard model. The A model was a little more $$ new, and is harder to find in most cases. Other than the decoders, both models are the same.
 
ruesselschorf said:
checked the transistors on a QX-949 indeed they are all different! I guess the reason is this: As a four channel receiver (4x 40 watts) it was switchable to 2 channel operation (2x70 watts ??) using only the 2 front channel amps. When switching to 2 channel the front amps got a higher supply voltage from power supply. Thats why the front output transistors have a higher rating.
Front transistors: 2SA679/2SC1079 (120 Volt 12A)
Rear channel trans. 2SD370/2SB530 (110Volt 6A) All Toshiba Transistors
A good substitute for these old TO-3 transistors are MJ 15024/ MJ 15025 or MJ15003/4 in this case.Be sure to buy only ON- semi or old Motorolas, as there are a lot faked MJ transistors on the market. (Google: counterfeit transistor)

Helmut

Thank you.

In my case, each transistor itself is different, with 4 Sankens (L->R, front facing away, 2SA747, 2SC1116, 2SC1403, 2SA745) on the forward heatsink, and 4 Toshibas ( L-R, front facing away, 2SB530, 2SD370, 2SC1079, 2SA679) on the rearward heatsink.

All totally mismatched. I can only assume that the Sanken parts were x-referenced with their Toshiba counterparts. I'm not even sure of what goes where. I think I need to see a picture of the poweramp modules from a properly configured one, or have a picture drawn. That way when the new parts come, I can at least put them in the correct places!
 
Hey Mike,
I've got a 949 under about 4 other receivers. I'll pull it out in the AM and look at it, if you don't have an answer by then. I'm pitching the cover that's on it now, and have a replacement for it, so....looks like it's time to take that ratty cover off. :) I think I have an original SM too, so maybe I can find what you need.
 
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Hello M Jarve,

the heatsink with the Toshiba trans. is the original.
I got 2 QX949 for parts here in Germany so shipping is expensive.
Find out the bad transistors and let me know

@jpdylon, thank you for your update on Pioneer QX-949A

Helmut
 
I found the SM, but have no scanner. Here's some pics. If this doesn't work, I can still drag the unit out and look. This is power amp assembly AWH-027-0
This is the 949, not the "A" unit. Son of a gun...the outputs are all different! :yes:
 
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