Sansui QC-04 Not detecting 30kHz Signal

[user 19847]

Recently found a new in box QC-04 Demodulator. Works except the CD-4 LED does not light regardless of any position of the Carrier trim pot. Tried different low capacitance cables, and verified the wires are connected to the LED. Unit appears to decode CD-4 records properly when compared to other Demodulators I have. Anyone have similar problem that got it fixed?

 

[kfbkfb]

I guess I could ask an obvious question - perhaps the light has gone out (I believe the QC-04 is ~45 years old) - maybe could check and see if there's power to the CD-4 light when there should be (since demodulating is working).


Kirk Bayne

 

[user 19847]

Very possible it is the LED. Measured voltage on the transistor that controls it, but very difficult to measure downstream of this transistor to see if the LED controlling transistor is getting the signal it needs to turn the LED on due to the compactness of the circuit board. What is interesting that turning the carrier trim pot has no effect on the sound as it is suppose to hear the least amount of distortion when playing the 30kHz track. You know anyone with the service manual for the QC-04?

 

[proufo]

Very possible it is the LED. Measured voltage on the transistor that controls it, but very difficult to measure downstream of this transistor to see if the LED controlling transistor is getting the signal it needs to turn the LED on due to the compactness of the circuit board. What is interesting that turning the carrier trim pot has no effect on the sound as it is suppose to hear the least amount of distortion when playing the 30kHz track. You know anyone with the service manual for the QC-04?

Very possible it is not a LED but an incandescent bulb.

 

[user 19847]

The schematic for the QC-04 has the CD-4 indicator drawn as a LED, not an incandescent bulb

 

[barfle]

I’ve run across equiment that made me wonder how it ever got built, with wires running hither and yon making access to various part almost impossible, so I’m sympathetic to the difficulties in tracking down signals.

It seems you have a schematic - can you post it?

 

[user 19847]

Rather than post the schematic, I'll tell you where I found it at no cost to sign up to access this schematic and hundreds of other manuals and schematics:

https://www.hifiengine.com/manual_library/sansui/qc-04.shtml

 

[par4ken]

A copy here as well.

http://www.quadraphonic.info/manuals/
Thanks to Old Quad Guy