DiegoV
New Member
Newb here. I’ve had an SX-780 since 1985 that I purchased from the original owner. I had it serviced twice to replace STKs. I believe the right channel was replaced both times because the left channel STK looks original. Regardless, any replacements were done in 1990 or before.
It has not been in use for around 15 years, so I attempted to hook it up. I cleaned the pots and switches with Deoxit. Before connecting speakers, the unit powered up and the relay clicked on. I turned off the unit, connected speakers, and powered up. The relay clicked on, but as soon as I began to turn the volume up, the 4A fuse blew.
After reading through a bunch of threads on AK, I decided to try and diagnose the problem since there aren't any vintage audio folks in my area.
I found that the right STK was shorted between pins 2-3 and 8-9. I ordered supposed new-old-stock STKs from a seller on eBay. They seemed the most legit I could find quickly, although who knows if they are genuine. They look exactly like what I believe to be original in the left channel.
I installed the “new” STK in the right channel and replaced the 4A fuse. The unit powered up and the relay clicked on. I got sound from both channels, although the left channel was a bit staticky.
Great, right?
Next, I attempted to set the DC balance, and this is where the problem comes in. I connected the COM lead of the DMM to the chassis for ground, then connected the V lead to Pin 9. A small puff of white smoke came from near the left STK, but I couldn't pinpoint from where exactly. I should probably have stopped there, but decided to go ahead and try the right channel. I connected the V lead to Pin 6 and was able to adjust very near 0V.
Now, there is output from the right channel but nothing from the left. The right channel power meter is working but no movement on the left.
I decided to check voltages on the STKs and here is what I found.
STK-0050 voltages
Input AM radio station
Volume set to 0, no speaker connections
Right: Newly replaced, claimed new old stock.
1 -1.222 V
2 -40.95 V
3 -16.5 mV
8 +17.1 mv
9 +40.93 V
0 +1.298 V
Left: Probably original. If not, replaced no later than 1990.
1 -27.10 V
2 -40.92 V
3 0.0 V
8 0.0 v
9 +40.96 V
0 -24.65 V
For what it's worth, when reading pins 3 and 8 on the left, there was no fluctuation when reading them. Meter just pegged to 0.
Based on what I’ve seen in other threads, looks like there is a problem on pins 1 and 0 on the left STK, but I don’t know what the numbers mean. Is it the STK or something else? I have another of the NOS STKs but wondered if there is something else I should do before installing it. I’m not too skilled at this type of thing, so go easy on me. I do have the service manual but most of it is way over my head. Any ideas would be greatly appreciated.
It has not been in use for around 15 years, so I attempted to hook it up. I cleaned the pots and switches with Deoxit. Before connecting speakers, the unit powered up and the relay clicked on. I turned off the unit, connected speakers, and powered up. The relay clicked on, but as soon as I began to turn the volume up, the 4A fuse blew.
After reading through a bunch of threads on AK, I decided to try and diagnose the problem since there aren't any vintage audio folks in my area.
I found that the right STK was shorted between pins 2-3 and 8-9. I ordered supposed new-old-stock STKs from a seller on eBay. They seemed the most legit I could find quickly, although who knows if they are genuine. They look exactly like what I believe to be original in the left channel.
I installed the “new” STK in the right channel and replaced the 4A fuse. The unit powered up and the relay clicked on. I got sound from both channels, although the left channel was a bit staticky.
Great, right?
Next, I attempted to set the DC balance, and this is where the problem comes in. I connected the COM lead of the DMM to the chassis for ground, then connected the V lead to Pin 9. A small puff of white smoke came from near the left STK, but I couldn't pinpoint from where exactly. I should probably have stopped there, but decided to go ahead and try the right channel. I connected the V lead to Pin 6 and was able to adjust very near 0V.
Now, there is output from the right channel but nothing from the left. The right channel power meter is working but no movement on the left.
I decided to check voltages on the STKs and here is what I found.
STK-0050 voltages
Input AM radio station
Volume set to 0, no speaker connections
Right: Newly replaced, claimed new old stock.
1 -1.222 V
2 -40.95 V
3 -16.5 mV
8 +17.1 mv
9 +40.93 V
0 +1.298 V
Left: Probably original. If not, replaced no later than 1990.
1 -27.10 V
2 -40.92 V
3 0.0 V
8 0.0 v
9 +40.96 V
0 -24.65 V
For what it's worth, when reading pins 3 and 8 on the left, there was no fluctuation when reading them. Meter just pegged to 0.
Based on what I’ve seen in other threads, looks like there is a problem on pins 1 and 0 on the left STK, but I don’t know what the numbers mean. Is it the STK or something else? I have another of the NOS STKs but wondered if there is something else I should do before installing it. I’m not too skilled at this type of thing, so go easy on me. I do have the service manual but most of it is way over my head. Any ideas would be greatly appreciated.
