Pioneer SX-1250/5590 EQ board issue

Having a problem with an SX-5590 that has a problem on the equalizer board. The receiver had already been serviced and recapped by a previous tech.

I couldn't duplicate the issue at first but then noticed a bulged cap (470uf, C12). When I pulled the cap it read 63uf so, I replaced it. After I replaced it I monitored the output of a 1khz sine wave on my scope, through the phono inputs (I use a little RIAA circuit in-line to match the signal before going to the input). What I noticed was at about 9:00 on the volume, the wave would get distorted and then it would trip into protection. Lowering the volume took it out of protection but I didn't like that distortion. So I pulled the board and replaced all the transistor pairs in the right channel where I saw the bulged cap, and when I tested, R44 and R45 (150r) smoked. Nothing was installed backwards, and I used the correct subs (KSA992, KSC1845) or so I presume based on research and other threads that give viable substitutes. Dim bulb does not protect the resistors from smoking. So I pulled the board again and went through rebuilding it; new caps (again), all transistors replaced including both 2SA850/628A with KSA1013, and replaced the burned resistors. Put back together, tested on the bulb, no smoke. Ran a signal through the phono stage for about 15 minutes and everything was fine but then R44 smoked again. R45 was fine. I checked D5 and it was shorted/open. Replaced it with a 1N5252B, replaced the resistor again, and the same thing happened on power up.

I've never had an EQ board smoke before. Any insights on where to look? Obviously I'd have to disconnect power to the EQ board to test the power supply otherwise it will just keep frying that resistor in the meantime.

IMG_5714.jpeg
IMG_5713.jpeg
 
I mean all the pinouts on the modern replacements are ECB and i triple checked that they’re in correctly… but then again I used the PCB screen print and maybe something is wrong there. I’ll check again. Just seems like if it were in backwards the problem would have been immediate.
 
The dim bulb tester is of no use, it’s meant for a quick go no go test to determine if their is gross fault only, once the lamp dims take it off immediately or your supply voltages will be to low and wrong in some cases stressing components into failures.
I am on my phone so would have to go to my computer to look at the schematic.
I do not think the silkscreen is wrong but it’s always a good idea to verify that it is correct.
You did not say what the signal levels were, so you might be clipping the eq. it has a regulated +/- 49v iirc, that is massive and should never have a steady state signal be run that high.
What is the voltage rating on c12?
 
Original rating was 6.3v which I upped to 10v simply because that’s what I had on hand.

Also I don’t have a better copy of the schematic; I’ve been relying on the individual board schematics and then following the interconnects which is a real pain in the rear.
 
Okay i see what cap that is, the voltage rating is fine. I have a better big overall schematic, need to get in the computer to upload it for you, someone posted it on audiokarma, which is a site where would get more more support, refer to the pioneer mfr section
 
For signal levels= I wasn’t measuring. When it was going into protection I was running a signal generator through it and I thought about that also, that it may have been too high and clipped. But after it was rebuilt, I was just playing some computer audio through my RIAA circuit into the phono input at fairly low volume.
 
Ok so then logically, if it smoked after being on for a bit, we’ve ruled out a backwards component. It blew up that one resistor and the diode, which I replaced and tested again with no signal input or load, and it smoked again. So- I have a cascading failure on that channel now and will have to pull it and check every component I replaced.
 
If you smoked R44, you have an excessive amount of current on the +49V supply so yes you need to check every component to determine what's at fault. No easy way.
I can't upload the file its too big, sorry, try AudioKarma web site
 
Yeah, I know. I disconnected the phono input from the board because I had played around with the jacks for a different issue and thought I possibly caused a short or something to touch, but it's not that. So I'm basically smoking R44, shorting Q12 and diode D5 - which is a WZ-240 and I read that I can replace this with a 1N5252B 24v zener. But I will pull the board and check again.
 
I tested with the bench supply.
Something didn't look right; I was getting negative voltages where there should have been positive.

I was looking over the board layout and I realized I put PNP transistors where there were supposed to be NPN 😗 I reached into the wrong drawer and grabbed 2SA1013 instead of 2SC2383....

So far we're good. I'm still not sure how that was able to work for 15 minutes or so without instantly baking the resistor but.
 
The schematics are in the service manual available on the net.
Easy fix by the sounds of it. Looks like you did not verify voltages against the schematic after changing the components, it would have identified your mistake. Its possible to pass a signal if the supply voltages are messed up until something fails as you witnessed.