MX5050 Capstan hall effect sensor

[Humner]

I've been fixing up my MX5050. I noticed a new issue recently where the capstan would not start on its own in some positions, I would need to spin it by hand it get it going and would run fine after it starts

I've found the issue, but I need some further assistance to fix it.

One of the 3 hall effect/detector coil sensors in the capstan is always triggered, it appears its sensitivity is higher than the other 2. I have confirmed this by removing the hall effect sensor board from the motor and can measure the trigger distance.

When not triggered, the collector of Q7 sits at 1.35v for all sensors, however when triggered the voltage at the collector of Q7 varies - the 2 normal sensors drop to around 0.25v and the out of spec sensor drops to 0.45v

I have tested each component, and everything is to spec. Including testing the transistors and their gain with an atlas DCA55

I'm hoping to get some assistance in working out where I can adjust the sensitivity of this particular coil or at least where to focus my attention.

I have attached the schematic + highlighted the coil.

 

[Humner]

During further testing, the issue has resolved itself.

I'm suspecting the ferrous metal I was using to test the triggering may have had some effect on the magnetization of the out of spec coil?

It seems to be the only explanation.

Its now showing the same voltages as the others.