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Nakamichi Dragon CT - Center Search does not stop

DCP

DCP

AK Subscriber
Subscriber
Hi all,

I'm working on a Nakamichi Dragon CT turntable where center search does not stop. It neither makes a center adjustment nor finds absolute center. The problem initially seemed to be a non-functioning LED emitter in the arm assembly. Replaced the LED with an IR LED and was able to make adjustments to get proper voltages at the test points. On an actual offset record, I see the sensor signal varies between 1.18 and 1.12 volts at the test points and it looks like a smooth sine wave on a scope. But there is no response from the logic that should stop the platter and attempt an adjustment or determine center has been achieved. There is a strobe signal present from the platter strobe. It seems to be getting what it needs.

If anyone has experience with this center search logic, I would appreciate any guidance on where to look next.
 
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Although I received no replies, I'm posting what I found in hopes it might help someone in the future. There is another LED used for the motor speed control. The signal from that LED is also used for the platter position reference. That LED is located beneath the platter and shines through slots in the platter to a receiver. On this machine the tip of the LED was slightly proud of the little housing it is in. Somewhere over the years, removing and replacing the platter, the tip of the LED lens was chipped. The receiver was getting enough signal for speed control, but the signal was too low to activate a downstream inverter chip (IC108). Without info from IC108 the CPU (IC105) would wait forever and not complete the search process. I replaced the LED and the signal improved enough to trigger IC108 to activate. After that, the center search cycle operates normally.

FYI - I'm not that smart. I used a paid version of ChatGPT to guide me through what to check and where. I uploaded a scan of the service manual, and it was able to "read" the schematics and analyze the signal paths. That is where we found that IC108 was not passing pulses to the CPU. After measuring the signal coming into IC108, ChatGPT "looked up" the specs for IC108 by its part number and suggested that the signal level may be below threshold. It was up to me to find out why.
 
Thanks for the follow up, these are really uncommon so anytime you can document a fix you're helping out folks in the future.
 
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