I went through this 780 with a restoration kit I got off of eBay. I also installed the stk-0050 replacement power packs. Everything lowers up and sounds amazing. It drives my large ESS speakers very well. Amazing deep bass. It however will randomly go into protection mode. It doesn’t matter if I’m playing it at exclusively loud volume or if it’s barely turned up. I was playing it for the first time after the restoration fairly hard for anbout 30 minutes and it went into protection mode. Next day I turned it on and it was working perfectly. I ran it very hard for an hour and it didn’t skip a beat. Then turned it off and later that evening I was using it with my tv for sound at a normal watching volume and it kicked out again. I turned it off and back on a few times and nothing. I turned it off for a few minutes and it was working again. I left it on with the volume all the way down and a few minutes later I heard a click… turned the volume back up and no sounds. Again after being off for a bit the unit worked again. Any thoughts on where to start looking? All caps have been replaced. Several of the transistors were replaced with the restoration kit as well. Thanks for any help I receive!
Pioneer SX-780 goes into protection mode after being on for a while
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dr*audio
Fish fingers and custard!
I don't like restoration kits. Buy your own caps and semiconductors from Digikey or Mouser and then you know what you are getting. Are they Chinese caps, or Nichicon / Panasonic / Elna? Did you replace the caps in the protection circuit? Have you measured the DC offset on the output? You have to measure it before the speaker relay if the protection is on.
3 things could cause the protection to trip:
1. Bad caps in the protection circuit.
2. DC offset on the output.
3. Bad transistor in the protection circuit.
3 things could cause the protection to trip:
1. Bad caps in the protection circuit.
2. DC offset on the output.
3. Bad transistor in the protection circuit.
this is what you really want:
https://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/
https://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/
I set the dc offset per the service manual instructions. It’s extremely close to 0. I set the bias on the replacement power packs to 6.8 mv. All caps were replaced in the entire unit. They were all Nichicon or Elna capacitors. Both brands were used in the kit I purchased.
dr.audio already mentioned my next thought: bad transistor in protect circuit.
it is q26 which is a 2sc1384 . replace it with a ksc2690a if you have any or a ttc004b.
i can not find my list from the 780 i did and with search down, i can not find the thread i was thinking of but this one has some good info:
https://audiokarma.org/forums/index.php?threads/sx-780-refurb-bom-to-vet.841234/
q19,20,25 all need reflowed at the very least
someone more familiar with the unit may come along and offer more advice.
You're doing good, Toad. The only question I have is on offset and bias adjustment. Did you use the manual, or did you use the instructions that came with the modules? And which modules did you get? Name names....... Pictures would be better.......
Like earlier stated I set dc offset per the service manual. I set bias to 6.8mv. Here is a like to the modules I purchased. No instructions are with. I know there is a thread about them but there are pages and pages of nonsense on that thread and I couldn’t find the install info. Odd think is it happens so random and even when the volume is all the way down. I replaced the 3 voltage regulator transistors already.
Re-look at the DC Balance (Offset) on both channels closely. See if there is any fluctuations in the readings. They should be stable and within only a few mV’s of fluctuations. No inputs, no volume, no speakers. Monitor for more than a few minutes. If one is glitchy it could be the clue you need.
Do you trust those speakers? They could be causing the issue.
Check the regulated supplies for the proper B+- voltage. They should be very even with only a .75v of a volt difference between the two. Monitor those voltages as well.
The B+- voltages are the voltages that supply the front stages of the amps. They should be even, or very close.
If all that above is good, focus on the protection circuit. The relay driver replacement was a good suggestion.
This issue might be an original problem?
If you have time, post the protection board pin voltages. Some pins might be in AC. Consult the schematic for the supply voltages and input pins from the amps.
Do you trust those speakers? They could be causing the issue.
Check the regulated supplies for the proper B+- voltage. They should be very even with only a .75v of a volt difference between the two. Monitor those voltages as well.
The B+- voltages are the voltages that supply the front stages of the amps. They should be even, or very close.
If all that above is good, focus on the protection circuit. The relay driver replacement was a good suggestion.
This issue might be an original problem?
If you have time, post the protection board pin voltages. Some pins might be in AC. Consult the schematic for the supply voltages and input pins from the amps.
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So I’m working on this for a friend. Unit has been sitting for years. He knew one channel was dead but didn’t mention this issue but when I told him what it was doing it sparked his memory and he said that it done that for a very long time. So at least I know it’s not a result of the caps and transistors I replaced. I will check voltages tonight. One thing I noticed is you hear a pop from the speakers when it goes into protection mode. Thanks for the advice.
The 780 is a wonderful receiver. It sounds fantastic for the value it is (was?).
A unique circuit in the day, that uses the TA7136 op amps for the tone control. These can become quirky and cause issues like you now have with the protection.
The only way to zero in on it, is to watch the Balance for spikes or replace the op amps and cross your fingers.
There are many options for the Op replacement other than the original TA chip. Search the AK threads using the TA7136 as a base for the inquires.
I'm advising on a hunch and a little experience, so the time and costs are yours.
A unique circuit in the day, that uses the TA7136 op amps for the tone control. These can become quirky and cause issues like you now have with the protection.
The only way to zero in on it, is to watch the Balance for spikes or replace the op amps and cross your fingers.
There are many options for the Op replacement other than the original TA chip. Search the AK threads using the TA7136 as a base for the inquires.
I'm advising on a hunch and a little experience, so the time and costs are yours.
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dr*audio
Fish fingers and custard!
Bingo! The pop is a large DC offset or excursion. Both speakers or just one? If it's one channel I'd be looking at the small transistors in the output stage. Get some freeze spray and spray a small amount on each transistor. The one that causes the problem is bad. If the pop is coming from BOTH speakers it has to be a power supply or ground problem. If the ground at pin 5 has poor continuity or there is a problem with the +/- 39V supplies it would cause a pop in both channels.
I’d love to search this site for what you said but apparently the search feature hasn’t been working for a while now. At least on any device I’ve tried I get an error message. So there is no searching for anything and that really sucks. Leaves me with no way to find useful information.
At pin one on the op amps I am getting 20.84 V. The service manual says I should be getting 10.4. That seems like a very significant difference.
Also this resistor is hooked to a different pin on each one. Is that correct? I can’t find this in the service manual drawings. All voltages are correct except pin one that is 20 v instead of 10v.
Attachments
Pinout
Check out the Datasheet attachment.
Link to a post that should get you started.
Just use Google and add "Audiokarma" after your search query. I think the AK search feature is a google search function anyway.
Doc had a good suggestion to freeze spray the small signal transistors when it acts up. That might be worth a try.
1. Supply Voltage : Pins 4 and 7 : should have +-20 or so. Pin 7, +20v, Pin 4, -20v's.
2. Pin 1 should have 10.4 volts.
3. Pins 2, 3, 6 should be zero volts.
4. Pin 5 should be lower than pin 7 and check both chip pin 5 for a comparison.
Check out the Datasheet attachment.
Link to a post that should get you started.
Just use Google and add "Audiokarma" after your search query. I think the AK search feature is a google search function anyway.
Doc had a good suggestion to freeze spray the small signal transistors when it acts up. That might be worth a try.
1. Supply Voltage : Pins 4 and 7 : should have +-20 or so. Pin 7, +20v, Pin 4, -20v's.
2. Pin 1 should have 10.4 volts.
3. Pins 2, 3, 6 should be zero volts.
4. Pin 5 should be lower than pin 7 and check both chip pin 5 for a comparison.
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Use Google to search the site. It works better than the site search function anyway. The syntax would be:
"site: audiokarma.com (search terms)"
Don't use the quotation marks or parentheses when entering the search.
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