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

For whatever it's worth, the rebuilt one sounded incredible, and looked amazing as well. These are definitely worth rescuing.
 
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.
Thanks Keith! That's why I called BS on the tech, how can you not just remember which channel caused the short?
 
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.
Got it. I'll start with the output channels and by process of elimination go from there. Crossing fingers that after I disconnect the output board the relay will click.
 
i would not be so hard on the tech not everyone is out to screw you,it just seems that way!.do you know from just looking at a opened up set which is right or left front and rear? but as the amp boards are all the same it would be easy to find out which was creating a problem and sending an error signal to the protect board and NOT KNOW exactly which channel is which.when you say WHICH what do you mean by that? if you are going to attempt this be advised its one of the most difficult to repair and takes a LOT OF PATIENCE. rebulid the power supply FIRST. dont forget to deoxit the 2/4 ch plug on the rear if the 747 has one. as far as the WORST design as far as heat goes well maybe ive never lived in an aircon house i bought one as new in 74 and i used it constantly it seems for 18 yrs before it gave trouble it lived in the philippines three times. central california HOT AS HELL in summer and then in hawaii. 18 yrs is LONG TIME
 
Last edited:
i would not be so hard on the tech not everyone is out to screw you,it just seems that way!.do you know from just looking at a opened up set which is right or left front and rear? but as the amp boards are all the same it would be easy to find out which was creating a problem and sending an error signal to the protect board and NOT KNOW exactly which channel is which.when you say WHICH what do you mean by that? if you are going to attempt this be advised its one of the most difficult to repair and takes a LOT OF PATIENCE. rebulid the power supply FIRST. dont forget to deoxit the 2/4 ch plug on the rear if the 747 has one. as far as the WORST design as far as heat goes well maybe ive never lived in an aircon house i bought one as new in 74 and i used it constantly it seems for 18 yrs before it gave trouble it lived in the philippines three times. central california HOT AS HELL in summer and then in hawaii. 18 yrs is LONG TIME
I was taken aback by the fact that the tech was pretty broad on his prognosis, it's like going to the doctor and he tells you, you have a broken bone just can't remember which body part. Especially since he said he was able to get sound from one channel, anyways I'm about to give this a go on my own and see where it leads. Ordering some parts today. :)
 
Last edited:
Following some tips from KeithD (thanks KeithD) I was able to get the channel board out to a manageable position where I can visually inspect the back and front of it.

Any suggestions/shortcuts as to where I would measure here? Currently I only have a multimeter. Do I connect to 9 & 10 to check voltage for channel 1?





 
Last edited:
9 + 10 (CH1,3) No. 3 and 13 + 14 (CH1,3) No. 5 have voltage also when I run an audible continuity check on those I get no beep so that might be the culprit/short? 11+12 and 15+16 check out (no voltage, beeps).
 
Last edited:
thats almost as bad,if its like any other pioneer (i dont own a 747) the leads are quite fragile and you can break them off at the component quite easily and they arent made any more
 
thats almost as bad,if its like any other pioneer (i dont own a 747) the leads are quite fragile and you can break them off at the component quite easily and they arent made any more
Replaced all Sanyo caps, waiting on the transistors, channel one both SA726 were shorted/bad. Making progress. Tedious work.
 
Replaced all Sanyo caps, waiting on the transistors, channel one both SA726 were shorted/bad. Making progress. Tedious work.
Yes, tedious, but keep at it. You are making good progress. This is a labor of love, but will also be a good experience for other stereo equipment you may want to service.
 
Yes, tedious, but keep at it. You are making good progress. This is a labor of love, but will also be a good experience for other stereo equipment you may want to service.
Indeed. Feels very therapeutic as well. Putting some good music on and going at it with a steady hand it's a calming experience.
 
You'll need to look at the datasheet, which can be found here:

https://www.mouser.com/datasheet/2/308/KSA992-1301291.pdf

I encourage you to always check those so that you are clear on the pinouts. Datasheet indicates the 992 is ECB. For the 2SA726, look at the schematic as well as what is printed on the circuit board. They should match. Sometimes you can't find the old datasheets for transistors, as is the case for the 726. Once in a while they are listed in a service manual. I don't have the QX-747 manual. I checked and it's not in the QX-949 manual. But, I did find this in the SA-8100 manual:

Transistors.jpg

So, it appears the 726 is also ECB. Regardless, you'll need to correctly orient the KSA992 with the markings on the board (and double checking the board is right by checking the schematic). If you look at how the 2SA726 is oriented before you remove them, that will be a third check for the pinout orientation.

Pioneer uses this common system for the transistor symbol in schematics and printed on the circuit board:

npn-and-pnp-transistor-symbols-425x300.jpg
 
The trick to replacing transistors is to go slow and test it after every couple of them. If you lose sound in one or both channels, you put it in wrong.
 
Yup. You want to try to do the work in short sections and then reassemble and test the unit out on a DBT, to make sure you havent mucked anything up. We all do it, its best to do this work very methodically and carefully. Dont trust yourself, recheck very thing.
 
Thanks guys! Also for good measure I just ordered the Mega328 ESR transistor/diode/capacitor checker from Amazon should help identifying this stuff.
 
Just received the transistor replacement (KSA992 & KSC1845)
Old 2SA726 is PNP BCE
The KSA992 replacement is PNP ECB
I'll triple, quadruple check before I solder.
 
Back
Top Bottom