Thanks Keith! That's why I called BS on the tech, how can you not just remember which channel caused the short?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.
Now I'm even more eager to bring her back to life as it is in pristine cosmetic condition.For whatever it's worth, the rebuilt one sounded incredible, and looked amazing as well. These are definitely worth rescuing.
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.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.
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.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
Have not removed them. Also found a bad Sanyo blue cap on channel 1.YOU CANT POWER UP WITH THE STV DIODES REMOVED FROM THE HEAT SINKS!!!!
Replaced all Sanyo caps, waiting on the transistors, channel one both SA726 were shorted/bad. Making progress. Tedious work.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
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.Replaced all Sanyo caps, waiting on the transistors, channel one both SA726 were shorted/bad. Making progress. Tedious work.
Indeed. Feels very therapeutic as well. Putting some good music on and going at it with a steady hand it's a calming experience.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.