In July I picked up a very dusty SM-80. It appeared complete with the exception of the phono section EMI cover and mounting hardware.
It looks like someone was trying to troubleshoot that section, gave up and didn't bother to reassemble the parts.
After changing every capacitor in the rig, cleaning all the switches and pots, testing and replacing worn tubes, I got to the point where I could diagnose the remaining issues.
First to be fixed was a spotty volume control.
This was tough, it needed to be a dual 500KΩ with loudness tap. The best I could source was an Alps 250KΩ with loudness, it worked.
Along the way I made some very interesting observations:
This Receiver is not an FM stereo receiver but an AM stereo receiver!
The FM is mono only, but could be FM stereo using an optional MPX adaptor for which there is a port provided.
The crazy AM stereo concept never took off, but this is how Sansui approached it:
There are two separate AM tuners with their own separate tuning knob and strength meters.
Left and right would have been on two separate stations therefore would need to be tuned independently.
I thought the meters were VU meters, by following the schematic and wires, they led to the independent AM tuners.
I got AM channel 1 to work (the same variable capacitor the FM circuit is on) but AM channel 2 is still out. FM also works.
The meter I thought was an FM channel centering meter, instead is a balance meter tied between the 32Ω taps of the output transformer.
If someone could explain the purpose of monitoring the balance between channels, I love to know about it.
The sound... In a word, Outstanding! Couldn't be more pleased.
My ears are telling me about 25 watts, very clean, very quite between notes, neutrally voiced as tube amps go, that may be on the count of the coupling caps I chose.
Or It may be the result of the 25E5 sweep tubes Sansui employed for output tubes, they have a unique sound signature I've not heard before.
I forgot to take some pictures. When I get back to addressing the phono section, I circle back to this thread.
It looks like someone was trying to troubleshoot that section, gave up and didn't bother to reassemble the parts.
After changing every capacitor in the rig, cleaning all the switches and pots, testing and replacing worn tubes, I got to the point where I could diagnose the remaining issues.
First to be fixed was a spotty volume control.
This was tough, it needed to be a dual 500KΩ with loudness tap. The best I could source was an Alps 250KΩ with loudness, it worked.
Along the way I made some very interesting observations:
This Receiver is not an FM stereo receiver but an AM stereo receiver!
The FM is mono only, but could be FM stereo using an optional MPX adaptor for which there is a port provided.
The crazy AM stereo concept never took off, but this is how Sansui approached it:
There are two separate AM tuners with their own separate tuning knob and strength meters.
Left and right would have been on two separate stations therefore would need to be tuned independently.
I thought the meters were VU meters, by following the schematic and wires, they led to the independent AM tuners.
I got AM channel 1 to work (the same variable capacitor the FM circuit is on) but AM channel 2 is still out. FM also works.
The meter I thought was an FM channel centering meter, instead is a balance meter tied between the 32Ω taps of the output transformer.
If someone could explain the purpose of monitoring the balance between channels, I love to know about it.
The sound... In a word, Outstanding! Couldn't be more pleased.
My ears are telling me about 25 watts, very clean, very quite between notes, neutrally voiced as tube amps go, that may be on the count of the coupling caps I chose.
Or It may be the result of the 25E5 sweep tubes Sansui employed for output tubes, they have a unique sound signature I've not heard before.
I forgot to take some pictures. When I get back to addressing the phono section, I circle back to this thread.