SX-780 High Voltage on STK-0050 Inputs

[BadBoyBubby]

Schematic: hfe_pioneer_sx-780_ku_kc_schematic_en.jpg
Service Manual with Board Layout: http://akdatabase.com/AKview/albums/userpics/10007/Pioneer SX780 Service Manual.pdf

As a foreword, I'm not very advanced at troubleshooting electronics, but I gave it my go and would appreciate if anyone can give me some tips or point me in the right direction. I've mostly been checking transistors by measuring the forward and reverse voltage across all the PN junctions and making sure it seems reasonable. My initial assumption was that a transistor had shorted and that's why I was seeing rail voltage on my inputs.

I recently ran into an issue where I was under the impression I blew one of my power packs. Couldn't get power relay to click. So I went with the STK-0050-II discrete component replacement mentioned on the forum elsewhere. I soldered everything up, checked it out, and installed with a DBT. Powered on and the bulb was bright. Powered down, unsoldered the new amplifiers, and soldered in 2 x 1kohm resistors between pins 1-3 and 0-8 on both channels. I have the following voltages between these pins and chassis ground which leads me to believe there is something wrong in front of the right power amplifier that is pinning the input voltage to the rail.

Right
1: -37.2 V
2: -37.1 V
3: 0.00 V

8: 0.00 V
9: 37.4 V
0: 36.4 V

Left
1: -1.5 V
2: -37.1 V
3: 0.00 V

8: 0.00 V
9: 37.3 V
0: 1.5 V

Additionally, with the resistors wired in the protection relay will pull in.

I checked the forward and reverse voltage for the Q12, Q14, and Q16.

Q12 (NPN):
BC: 785 mV
BE: 810 mV
EC: 778 mV (Q14 BC measurement )
EB/BC/CB: OL
Q14 (NPN):
BC: 778 mV
BE: 804 mV
EB/CB/EC/CE: OL
Q16 (PNP):
CB: 752 mV
EB: 777 mV
BE/BC/CE/EC: OL

So everything seems okay with those transistors from what I can tell. (After being really confused why I had forward voltage across the emitter collector of Q12 for a minute lol)

So I've moved upstream to measure Q8 and Q10.

Q8 (dual PNP transistor (Not really sure what this is called)):
Pretty sure pin out is B1, C1, Shared Emitter, C2, B2 but maybe I'm wrong
C1B1: 769 mV
EB1: 803 mV
C2B1: 772 mV
EB2: 807 mV
All other directions OL

Q10 (dual NPN transistor (Not really sure what this is called)):
Pretty sure pin out is E1, C1, Shared Base, C2, E2 but maybe I'm wrong
BE1: 642 mV
BC1: Shorted
BE2: 762 mV
BC2: 756 mV
All other directions OL (besides C1B)

Finally measuring Q6
BC: 792 mV
BE: 811 mV
CE: 1085 mV? Measuring up through D2 or something?
All other directions OL


Am I approaching this the wrong way? I'm thinking now maybe I should be checking voltages at all of these points and looking for resistor issues? Sometimes the schematic and layout drawing confuse me because they don't totally match what I'm seeing on my PCB. I'm guessing it's due to revision changes though. One thing that does seem seem weird is that R252 has a wire ran next to it connecting the nodes. Same with R261. Seems factory, but I don't understand the purpose since it seems there would never be a voltage across these resistors. These wires are the blue ones in the photo I attached.

Also, am I even accomplishing anything by measuring reverse breakdown of the PN junctions and across the transistor emitter-collector and vice-versa?

Also, I measured the 0.22-ohm resistors and they seemed fine. After measuring my lead resistance and then them they're around 0.3-ohm which seems reasonable given the quality of my meters.

 

[rcs16]

One thing that does seem seem weird is that R252 has a wire ran next to it connecting the nodes. Same with R261.

Are they not the ones used to fine tune the DC offset adjustment range?

BC1: Shorted?? That's not correct, time for a change

 

[BadBoyBubby]

So I was looking into this some more and I realized I was looking at the wrong resistors and I saw that blue wire jumping over R252 and R251 (typo in my original said R261). What I was actually seeing was a wire jumping R238 and R239 which is extra adjustment (shown on the schematic) for the biasing circuit like you mentioned. R251 and R252 are not jumpered which matches the schematic and makes much more sense.

And BC1 on Q10 being shorted makes sense because I realize that the Base and the Collector on this transistor are connected on the schematic.

I ended up taking several voltage measurements and this is what I'm seeing. The Left channel (top on schematic) is functioning normally so I'm using that as my baseline. Most of my voltages are a little high vs what is called out on the schematic, so that is something I'll try and deal with eventually Also, my voltages on the DC biasing circuit are way off, but I adjusted to 0V on pins 6 and 9 to ground like is called out in the service manual.

I've pulled the power amplifiers while I'm troubleshooting to take them out of the equation. I soldered in some 1kOhm resistors instead. It doesn't appear like I'm getting any input into this stage of my amplifier (Q5/Q6 base), yet I'm still seeing +/-36.5V on my 0 and 1 pins for the power amplifier on the Right Channel. I'm starting to think the issue is with Q12 and Q14 because that is where things start to go a little wonky. I'm going to pull them off the board and verify nothing is wrong with them. But if there is anything else jumping out at anyone let me know.

 

[BadBoyBubby]

I'm an idiot. I had 220Ohm resistors on the right channel, and 10kOhm resistors on the left.... Now that I have 220Ohm resistors on both, everything matches. My relay clicks in too.

I did have to replace the Q7 and Q8 transistors (2SA979) with two HN4A06J as Q8 was shorted (not sure if I blew it troubleshooting or if it was like that originally). So I'm going to unwire the resistors and wire the power amplifier back in. Hopefully the relay clicks in still after that, otherwise the power amplifier is the problem.

 

[BadBoyBubby]

I solved my issue. In case anyone ever sees this in the future, this is what happened. Full story:

1. Found speaker wires had fallen out of speaker on right channel and landed shorted. Stereo was no longer working, relay would not click.
2. Did some troubleshooting, determined STK pack on the right channel was bad. Decided to do discrete component STK replacement instead of trying to find a Darlington.
3. Build replacement following instructions closely. Installed replacement in stereo.
4. Turned on stereo on DBT, and hoped for the best. Bulb was bright and relay did not click in. Started troubleshooting and found high voltage on inputs to new power packs.
5. Soldered in resistors (but did not pay close enough attention and grabbed two different kinds).
6. Spent a ton of time troubleshooting by checking transistors and voltages, basically working my way through the schematic. The whole time most things seemed fine other than I had high voltages on my right channel inputs.
7. Accidentally shorted something while measuring voltage and blew up Q8.
8. Replace Q7 and Q8 with new transistors.
9. Eventually discovered that I had 220ohm resistors on the Left channel, and 10kOhm resistors on the right. Solved the issue why my right channel was high. Once I had 220ohm resistors everywhere I could verify that the amplifier stage before my powerpack was okay.
10. Installed powerpack again, started checking every component to see if I had used a wrong component, shorted something, blew something, etc.
11. Couldn't figure it out. I checked the instructions which specifically said to turn the potentiometers fully CCW and verified that was the case. As a sanity check, I turned them fully CW for a moment, and the DBT went dim and the relay clicked in.

The whole time I've been troubleshooting this it was because I blindly followed the instructions that came with the PCB for my power packs. For whatever reason my potentiometer or how I installed it (not sure how I could install it wrong because the footprint only fits one way) was not accurate and my bias in the powerpack was WAY too high.

So glad I finally got this figured out. Hopefully if someone else is having the same problem they'll find this and save themselves the headache I created for myself.

 

[rcs16]

On the A0 PCB it was designed for a 3362R pot which is reverse of a 3362P. Later versions use the 3362P.

 

[BadBoyBubby]

That's helpful to know the background of the change. I figured it was some sort of component change between what I purchased and what was called out.

I should have looked into it a bit closer far earlier than I did, but I wrote it off as something being wrong upstream from the powerpacks in my stereo and was too focused on that. This is the first time I dug into anything like this so lots of lessons were learned. It was definitely extremely satisfying to power it up and have everything working well again