SX 780 and intermittent operation

Test the amp as described in post #28, if that shows good, the STK is the problem. If the amp shows bad then fix it it, note what you replaced to fix it. Put the old STK, or a new one, back in. If it works the amp was the problem, if it still doesn't work the amp and the STK were/are both bad. If the old STK is bad it may damage the repaired channel in the amp. These are the risks you take (and part of the challenge) owning and repairing vintage audio gear. You could try moving the good (other channel) STK over, after verifying the amp. This does risk the good STK, however. Look at the link in post #2 about custom replacements for the STK with modern replaceable discrete components. If you can solder and turn a screwdriver they are a very good upgrade as well.
 
The meter negative probe really needs to be in the COM jack for proper readings. Theoretically, it will work in the 10A jack but will read resistance through the internal shunt and parallel circuit in the meter, potentially skewing the results.

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If the readings are 1.423, 1.105, and 1.186, they are probably O.K.
 
Looks to me like the left pack is toast. If you have (or can get) a couple of 1kΩ resistors, you can solder them in where the STK was and it will help the troubleshooting process. On the PCB, after the pack is removed, connect one from pin 0 to pin 8, one from pin 1 to pin 3. See pictures:
View attachment 1443439 View attachment 1443452
This closes the DC feedback loop to see if the rest of the power amp section is ok or not.


I couldn’t locate any 1k ohm resisters but the hobby store said I should be ok if I used (2) 560 ohm 1/2 watt resistors. Is this ok to test as such?

And I have them soldered in - fairly ugly but good connection

If this is correct - what next ?
 

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The meter negative probe really needs to be in the COM jack for proper readings. Theoretically, it will work in the 10A jack but will read resistance through the internal shunt and parallel circuit in the meter, potentially skewing the results.

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If the readings are 1.423, 1.105, and 1.186, they are probably O.K.


Here are the reading with negative in COM port :

1-0 1385 (1 direction)
1-2 1085 (1 direction)
1-3 shows open (1 direction - swapping leads around shows voltage but then goes open)

0-8
shows open (1 direction - swapping leads around shows voltage but then goes open)

0-9 1167 (1 direction)
 
I couldn’t locate any 1k ohm resisters but the hobby store said I should be ok if I used (2) 560 ohm 1/2 watt resistors. Is this ok to test as such?

And I have them soldered in - fairly ugly but good connection

If this is correct - what next ?
I suspect they intended for you to put them in series so you had 1120Ω. In parallel it is 280Ω, which may be too low, try just one 560Ω on each connection, 0-8 & 1-3. Be careful there are no solder bridges to 2 and 9, then bring it up on DBT and measure Vdc on 1, 2, 3, 8, 9, and 0.
 
I suspect they intended for you to put them in series so you had 1120Ω. In parallel it is 280Ω, which may be too low, try just one 560Ω on each connection, 0-8 & 1-3. Be careful there are no solder bridges to 2 and 9, then bring it up on DBT and measure Vdc on 1, 2, 3, 8, 9, and 0.

Ok - have only one on each side. Not sure how or what I’m doing to bring it up on DBT? Just turn receiver on with no speaker connections and measure at the locations you mentioned ? Sorry still new territory for me
 
No problem, everybody was new at this sometime.
DBT = Dim Bulb Tester, which is an incandescent light bulb in series with the hot wire between the power outlet and the amp. It acts as a ballast to protect the amp circuits from overload while troubleshooting. 60 to 100 watts is a good bulb for an SX-780. If you don't have one you can build one, there are multiple threads on AK about it. If you have not been using one you have lucked out. When you turn the amp on using the DBT the bulb will flash bright as the power supply caps charge up, then dim down if there is no overload. If there is an overload it will stay bright. If no overload it is ok to power up without it.
If you want to continue without it you can. Power up with no speakers, volume at minimum, Aux input selected with no input and measure those pins.
 
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No problem, everybody was new at this sometime.
DBT = Dim Bulb Tester, which is an incandescent light bulb in series with the hot wire between the power outlet and the amp. I acts as a ballast to protect the amp circuits from overload while troubleshooting. 60 to 100 watts is a good bulb for an SX-780. If you don't have one you can build one, there are multiple threads on AK about it. If you have not been using one you have lucked out. When you turn the amp on using the DBT the bulb will flash bright as the power supply caps charge up, then dim down if there is no overload. If there is an overload it will stay bright. If no overload it is ok to power up without it.
If you want to continue without it you can. Power up with no speakers, volume at minimum, Aux input selected with no input and measure those pins.

Awesome! I'll start reading up on that and report back!
 
Ok I have the DBT built and ran a test- all seems fine. Bulb lights up dim, appears to get a slight bit brighter but then dims again and seems to hold a steady dim rate. I can hear the click of the protection circuit turn on on the amp and no other adverse reactions noted. No smoking and I did not get shocked so we must be doing something right!

I Have the volume at a minimum and the auxiliary function is selected. What is my next step? Do I measure voltage at the pin locations where the Darlington pack used to be?
 

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Do I measure voltage at the pin locations where the Darlington pack used to be?
Excellent, yes, now that it is stable with the DBT, power up without DBT and check the voltages on 1, 2, 3, 8, 9, & 0. Also check the other STK, same pin numbers. Be very careful not to short between pins with your meter probe. Sometimes it is better to use a little grab hook, connect and disconnect with the power off. 1 and 0 will not be the same where the pack is removed and where the other one is still installed.
 
Excellent, yes, now that it is stable with the DBT, power up without DBT and check the voltages on 1, 2, 3, 8, 9, & 0. Also check the other STK, same pin numbers. Be very careful not to short between pins with your meter probe. Sometimes it is better to use a little grab hook, connect and disconnect with the power off. 1 and 0 will not be the same where the pack is removed and where the other one is still installed.

Left STK:

1: 3.49
2: 1 (open)
3: 0.01
8: 0.01
9: 1 (open)
0: 3.65

Right STK

1: 11.30 (varied a little up and down into 10’s)
2: 1 (open)
3: 0.00
8: 0.00
9: 1 (open)
0: 15.21
 
Left STK:

1: 3.44
2: 1 (open)
3: 0.01
8: 0.01
9: 1 (open)
0: 3.60

Right STK

1: 12.30
2: 1 (open)
3: 0.00
8: 0.00
9: 1 (open)
0: 14.30
 
I don't think you are understanding your meter correctly. The black lead should always be in the COMmon port. The red lead should always be in the V Ω mA port, unless you are using the 10 Amp range, then and only then the red should be in the grey 10A port (see the grey 10A position for the switch).
The "Ω Ohms" is the label for the meter's ohms range to the above-right of the label, not for the port..
You may have a problem with your meter. ?? If you ever tried to read a voltage with the meter on one of the Adc current scale positions you may have popped the internal fuse.
The black lead should be clipped to the chassis ground for the tests we are doing now.
The STK pins 2 and 9 should be the power supply rails, and read about +40Vdc and -40Vdc.If you are not reading about this voltage with the meter switch in the 200 Vdc position you do have a meter problem.
 
On second thought it might be a power supply problem, though I would expect 2 to be the same on both, and 9 to be the same on both. Try reading the Vdc of a 9v battery if you have one
 
On second thought it might be a power supply problem, though I would expect 2 to be the same on both, and 9 to be the same on both. Try reading the Vdc of a 9v battery if you have one

I tested a 9v battery - Read 9.5

Went back and tested - your right I had things way off on meter-

Left side

1: 3.4
2: 41.7
3: 0
8: 0
9: 41.7
0: 3.6

Right side

1: 10.8
2: 41.7
3: 0
8: 0
9: 41.7
0: 14.0
 
My phone is giving me problems on trying to edit - but on #2 pins above, both of those readings are

-41.7

And pins #9 are both

41.7
 
Pin 2 and 9 are good for both STKs.
Some of the readings I expected to be negative volts, on both STK pins 1 for instance. Always watch for polarity when measuring DC volts. Please remeasure 1, 3, 8, and 0 on both STKs using the 20 Vdc scale on your meter.
 
Ok - new readings are Set on 20 Vdc:

Left pins
1: -3.56
3: 0.01
8: 0.01
0: 3.63 (positive)

Right pins
1: 12.37 (positive)
3: 0
8: 0
0: 15.13 (positive)
 
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