And...yet another sx-780 restoration thread

[grantly]

Hi all,

I went to an estate sale yesterday on a whim, and I was one of the first few people in the house. There was an incredible Barzilay cabinet with KLH 6s in custom cabinets. Way too much $$$ for me, so I decided to go to the basement. Right at the front was a cosmetically pristine sx-780 for low low $. I grabbed it, and took it home a little later (pics attached). Seriously, it looks like it just came out of the original packaging.

I probably should have fired it up on a DBT first, but I went ahead and plugged it in. I tested dc offset, and it was around 20mv on left channel, 13mv on the right. I adjusted DC offset according to the service manual, and I got it to about ~5mv on each channel. All good, right?

Well, I hooked it up, and while the right channel sounds excellent, the left channel is heavily distorted. It cuts in and out and bass especially breaks up. The meters seem to work fine (although left channel seemed to fluctuate a little with the extra distortion?). The distortion existed on multiple inputs - FM, aux, and phono with a TT hooked up. I probably should have tried it in mono, but I didn’t. I’ll also note that I haven’t touched anything with DeOxit either yet.

My first guess is the left STK Power pack. It doesn’t look like the internals of this have ever been touched - it’s super clean inside and out.

With all that said, I’m new to working on amps (other than using a good ol’ spray of DeOxit). I’ve built and rebuilt plenty of speaker crossovers, but 2 way and 3 way crossover schematics are relatively simple.

I thought maybe I should spring for the STK replacement modules that are available on the auction site (and buy the materials listed on the BOM in the details on the purchase page), but that repair would be a new level for me. I’m willing to do it, though. Frankly, I’m ready for the challenge.

Any feedback or direction would be appreciated. I plan on reading other sx-780 threads as well.

I’ll also note that this repair will take time - like a loooong time. I’ll update and post as I get to next steps. I’m a teacher and a coach, and my summer break is coming to a close very soon. I also have a one year old daughter, so a lot of the free time I once had is no longer free (but still good!).

My goal is to ideally get the problem diagnosed and repaired by the end of July 2020. You can quote me on that.

 

[merlynski]

If you can solder and run a DMM (apparently so) you can fix that SX-780. You already have had the cover off and tweaked dc offset, you are ready to go.

 

[grantly]

If you can solder and run a DMM (apparently so) you can fix that SX-780. You already have had the cover off and tweaked dc offset, you are ready to go.

Thanks! I appreciate the encouragement. I have a fine soldering station and I can use a
DMM, so I’m set there. I’ll be asking questions if I’m stumped, but any pointers are appreciated.

 

[merlynski]

OK, cover the tip of your (red) DMM probe with heatshrink or tape except for the very point to avoid shorting between component leads when you slip making measurements, we have all done that and let the magic smoke and harsh language pop out.
My First (OCD) thing to check is the power supply DC voltages. Pins 15 and 16 of the main PCB are the +/- 40vdc supplies that feed the power amp STKs, or you can probe the STK pins 9 (+40vdc) and 2 (-40vdc). Then check the + side of C314 for +51.7vdc and the - side of C315 for -53.5vdc. These are the power supplies that drive the regulators and everything else on the board. The numbers for the voltages come from the schematic, are unregulated and may be +/- a few volts. One of the most common failures on the SX-780 is PS regulators Q19, Q20, and Q25 having bad solder connections from years of heating up and cooling down. It is a good idea to just reflow their solder at the beginning of the project. Those are theTO-220 transistors with the aluminum heatsinks toward the left side.
In the red rectangle in the picture. (I can't remember who to credit for the picture.)
Sometimes I get long winded, please forgive me.
If those are good check STK pins 0 and 1 for +/- about 1.4vdc (with no input, Aux selected, volume at minimum, tone controls and balance centered.) Since the DC offset was adjustable on both channels I expect the previous measurement to be good.

 

[grantly]

This is super helpful. I’ll plan on taking these measurements in the next day or so.

 

[grantly]

probe the STK pins 9 (+40vdc) and 2 (-40vdc). Then check the + side of C314 for +51.7vdc and the - side of C315 for -53.5vdc.

Ok - I checked voltages on the STK pins 9 and 2 and they measured correctly. + c314 and - c315 were the same vdc as listed above give or take 0.1 or 0.2 vdc.

However, STK pin 1 was correct at 1.4, but STK pin 0 was only at 1.0 vdc, not 1.4.

 

[grantly]

Solder on Q19, Q20, and Q25 has been reflowed.

 

[merlynski]

Ok - I checked voltages on the STK pins 9 and 2 and they measured correctly. + c314 and - c315 were the same vdc as listed above give or take 0.1 or 0.2 vdc.

However, STK pin 1 was correct at 1.4, but STK pin 0 was only at 1.0 vdc, not 1.4.

That is a problem indication, pin 1 and 0 must be balanced, pin 0 positive, pin 1 negative, but both very close to the same voltage. Was pin 1 -1.4vdc? And pin 0 +1.0 vdc? polarity matters. Test each of the transistors, Q5, Q7, Q9, Q11, Q13 and Q15. Note Q7 (PNP) and Q9 (NPN) are 5 legged transistors, with 2 transistors in one package. Q7 has their emitters in common on the center leg, Q9 has their bases in common on the center leg.Test D1. If those are all ok we will measure the resistors.

 

[grantly]

Got it. Thanks again for your help.

I’ll double check the polarity either tonight or tomorrow at some point. I’ll check the transistors as well then.

 

[grantly]

Ok - I double checked pin 1 and 0 of the left STK. Pin 1 is -1.39 vdc and Pin 0 is 1.04 vdc.

Test each of the transistors, Q5, Q7, Q9, Q11, Q13 and Q15. Note Q7 (PNP) and Q9 (NPN) are 5 legged transistors, with 2 transistors in one package. Q7 has their emitters in common on the center leg, Q9 has their bases in common on the center leg.

Also, I have to admit my ignorance here...when you say to test these, am I simply checking resistance without power to the unit? I can figure out whether they're PNP or NPN, but do I need to remove the transistors from the board to test them?

 

[merlynski]

Preliminary testing of transistors is frequently done in place, more detailed testing requires removing them. Testing does not always identify transistors that are failing while under load and in circuit, sometimes you just have to replace them. Fortunately most transistors are inexpensive.
Here is a link to Echowars post of how to test transistors: https://www.audiokarma.org/forums/i...lar-junction-transistor-testing-basics.43186/
That link comes from another link that has many other excellent topic links in it: https://www.audiokarma.org/forums/i...-repair-restoration-reference-threads.426532/
If you do need to buy parts buy them from reputable suppliers, sellers on the big auction site frequently sell fake, fraudulent or second-rate parts, especially caps and semiconductors.

 

[merlynski]

Check the DC voltage at pin 9 of the main PCB, that is the output of the whole power amp stage (left channel), but before the protection relay. It should be near 0vdc and move +/- a bit with the dc offset adjustment (VR5).
There is a way to tell if the Voltage amp and Bias circuitry before the STK is working by removing the STK and putting two ≈1kΩ resistors in its place. It is non-destructive and does not risk parts damage. Let me know if you want to try it.

 

[grantly]

Here is a link to Echowars post of how to test transistors: https://www.audiokarma.org/forums/i...lar-junction-transistor-testing-basics.43186/

Thank you for the link. I’ll get through Echowars post and I’ll read through the other link you sent. This is all extremely helpful.

 

[merlynski]

You are welcome. Echowars has not been on AK for a couple of years but his post are very useful. Two others here that are top tier (Pioneer) are markthefixer and mattsd, amongst many others