Let's try this again.
I'm updating this post to document what I discovered to solve the auto-stop EOT issue. It summarizes my troubleshooting with advice from AK member perryinva who was very helpful in helping me understand how the auto-stop circuit worked.
My previous post had nothing to do with the solution, just coincidence.
I decided to look at the auto shut-off signal output of the Auto Shut-Off PCB to the Logic PCB. It is the output of the photo transistor which picks up the light through the aperture on the spoked disc that connects to the counter which rotates with the take-up reel. When the take-up reel is spinning it produces a square wave, but when it stops (like at EOT), it is a steady signal.
I put a probe on CN-13 Pin 11 on the Logic PCB.
Here is what I found:
1. This is when the tape is playing:
2. This is when it comes to the end of tape and the deck goes to Stop and the head retracts (like it should):
3. Finally, this is when I caught it when it did not stop at End of Tape and head & capstan were still engaged (noise on the signal):
I'll verify again, but it looks like the signal sourced from the Shut-Off PCB goes bad.
Next step, I'll check the wiring/connectors between the boards, then start scoping the Shut-Off PCB for a bad component.
I put a scope probe on the collector of Q449 on the Shut-Off PCB and it looks just like the photos above.
OK, with the transport in Stop, I manually rotated the Auto-stop pully. When I keep it rotating it looks like a square wave. When I stop sometimes it remains high, but occasionally I can get the noise as shown. Looks like it can drive it low, but sometimes has trouble driving it high? It looks like the transistor since it has trouble driving it high sometimes?
Q449 is a 2SC945. I have a KSC945CYTA (center collector) on hand. The pin configuration looks the same and I'll try to swap. I hope I can get to it without taking the transport out again.
Scope on Q449 collector while manually rotating the Auto-Stop pully in transport Stop:
I think Q449 is intermittent BAD. I'll try to reflow the solder first to make sure it's not a cold solder joint.
Latest update: I replaced the transistor (what a pain to get to from the bottom) and so far it appears to work fine. Here are the new waveforms.
Conn CN-13 pin 11, during Play then EOT Stop:
Same connector pin when in Stop.
What is interesting is that there is still noise present, but the signal is above 6V and it seems to work OK.
My thanks to perryinva for taking time to answer my questions on how the auto stop was supposed to work and providing suggestions.
I hope this post can help someone that has a similar issue with a Nakamichi tape deck.
