Pioneer sx780 troubleshooting, high dc offset

KSA992's bonded together thermally with heatsink tubing & thermal paste, and these trannies being hFe matched, are generally used to replace the 2SA types of dual trannies.

I dont see why the KSC1845 cant be used in the same way to replace the 2SC dual trannie. However someone with a bit more experience with these exact types of dual trannie would be best to confirm this.

These dual trannies do tend to break down over time, causing unstable DC offset & noise.

If this mod is sounding a bit technical - you may be best to get a tech to do it for you
 
KSA992's bonded together thermally with heatsink tubing & thermal paste, and these trannies being hFe matched, are generally used to replace the 2SA types of dual trannies.

I dont see why the KSC1845 cant be used in the same way to replace the 2SC dual trannie. However someone with a bit more experience with these exact types of dual trannie would be best to confirm this.

These dual trannies do tend to break down over time, causing unstable DC offset & noise.

If this mod is sounding a bit technical - you may be best to get a tech to do it for you
I am interested.

If I can get the pinouts right and see how exactly to best bond them together thermally I am confident I could pull this off.

Any pics of this? Google image search came up short, but maybe I used the wrong words.
 
Put a dab of heatsink grease on the flat sides of the TO-92 cases, hold them flat-to-flat, slip a piece of heatshrink tubing over them and shrink it down. A small ziptie would do as well. Insert the two common leads into the appropriate hole in the PCB, and the other individual leads into their proper holes and solder it in. You may have to bend the leads around each other, and even slip a bit of insulating tubing on the leads to protect from shorting. You may have to slightly enlarge the hole that gets the twin leads with a pin vise drill, by hand. You can also buy thermal conductive adhesive, or double sided sticky thermal tape to put between them. Couldn't find any pictures though I have seen them in the past.
 
OK - this is where you start - download the service manual from hifiengine.com and look at page 25 of that.

You have one 2SA979 and one 2SC2291 on each channel of the input stage of your poweramp stage, it appears.

Personally, before these are shotgunned, I would use a tiny bit of freezespray on each component (once the unit is warmed up) to confirm if they are indeed problematic. If you replace one - I would replace them all.

if you end up replacing these - use 2 x KSA992 to replace each old device - personally I would match their hFe with a component tester (I have seen a great deal of variation in batches), bond them together as merlynski says with a small piece of heatshrink tubing & thermal paste - solder/tie the EMITTERS together and ensure that the remaining 5 legs are not touching. Ensure that the new device pinout will lineup nicely with the PCB holes. The joined emitters will go into the central hole where the old component was, and will need enlarging with a small drill bit.

Do the same thing to replace the 2SC2291 - only use 2 x KSC1845 for this - however you instead need to join the BASES of the two new trannies together - otherwise follow the above steps.

The above is subject to you finding out if these new trannie recommendations are indeed the best as I am not 100% on that. The best way is for you to search the datasheets of the 2SA979 and 2SC2291 and check their voltage ratings and tolerances against the new devices.

Capture.PNG
 
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There is another way to check if the dc balance is good, without risking a good STK (or other components). With the STK uninstalled add a 1kΩ resistor to the pcb between STK pins 1 and 3; 0 and 8. This will close the dc feedback loop. If the output of the channel's power amp is 0vdc the components are good, and the STK was at fault. You will not get sound from that channel because the STK (current gain amp) is not in place, but it will tell you if there are other bad components in that channel. See attached:

Pioneer SX-780 red dc-test.jpg

If the dc offset is good the protection relay will pull in, unless there is a different failure.
 
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We'll have to wait for the OP to get back to us on your questions. I am at least as curious as you are :biggrin:
This project is now front and center.

I will start with stk voltage readings. I am not entirely sure how to troubleshoot a problem like this and any feedback would be great.

1551544776602-1293364448.jpg
 
Great!
Are you on a DBT at this time?
Have you removed the bad channel STK and added the resistors?
What are the DC voltages on the PCB for pins 1, 2, 3, 8, 9 and 0?
 
No problems there. No indication of a short in the power supply. 100watt bulb never even illuminated, 60watt bulb stayed dim.
 
Pins 6 and 7 do not have any function in this unit. 1. 2. 3. 8, 9, and 0 are the only used pins.
 
DC Voltages...

1.... -1.25
2....-39.89
3.....0.011
8.....0.0305
9.....39.9
10....1.323
 
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Another thing, this problem is intermittant. Sometimes I turn it on and there is no problem. Other times, one channel has very high dc voltage and the power meter for that channel is at 5 watts.
 
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Offset reading. Left channel at 1.9 VOLTS and the power meter shows 5watts and the amp is at idle.
 
Left side has a problem, let's figure out if it is the STK or something else.
Power off, replace the left STK with the resistors (post 25), power up and measure again.
Does the protection relay click?
Protection relay does indeed click after the usual 2-3 seconds. After the click, the power meter jumps up.
 
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