[Takapuna]

I don’t like admitting defeat or giving up but a few weeks (and one Wisdom tooth extraction!) later I’m pretty well at the bottom of my bucket of ideas for the fault on this VCR.
Yet another quick summary:
The VCR works reliably other than generating noise across the tape end sensor and, to a lesser extent, the start sensor. The noise on both sensors is synchronised to the commutation noise on the reel motor and becomes of sufficient amplitude to generate a tape-end signal causing the controller IC to command stop and then rewind. I’m convinced that a stop signal is being generated by the controller rather than the motor running out of torque or stalling etc.

So far, I’ve tried the following:
1). Changed the reel motor.
2). Replaced the M54647L reel motor driver IC with one taken from another VCR.
3). Hooked-up a 15V supply to replace the “unreg 17V” rail from which the reel motor IC is powered.
4). Removed the series pass transistor Q601 and sourced its 5V output from a bench supply. This is the 5V supply for the controller IC and also the rail which supplies the end sensor pull-up resistors etc.
5). Re-checked grounding paths for the deck signals, noting that the earthing screw on the so-called relay board to which the end sensor is connected is essential for correct operation. This is in spite of the manual suggesting that a secondary ground connection is carried through from CN601 pin 1.
6). Synchronised application of a bench 12V supply connected directly to the reel motor to selection of FF using the front panel controls. Low amplitude spikes can still be seen across the end sensor but they are bi-directional. They appear to be reasonably regularly spaced but not so regular as to allow the scope to trigger and freeze them. There is a clue here somewhere but I'm not sure what it's telling me.

I have not replaced the STK 5490 regulator chip and probably have nothing to lose by doing so. It’s just that under fault conditions the only motor being powered is the reel motor and its power is the “unreg 17V” directly from the PSU. Similarly, 5V for the controller IC and sensor conditioning is from a series pass transistor which I’ve sourced independently already.

I bought this VCR in a non-working condition and it soon came back to life after replacing the 1uF start-up capacitor on the PSU board and replacing a couple of other electrolytics. I don’t know if these symptoms were present previously or if my repair effort including removal cleaning re-lubricating and re-belting of the deck mechanism has provoked them.
I’ll put feelers out for a replacement M3 (main) board from a scrap machine including a request in the Wanted column. At the few car-boot sales I’ve attended I’ve only seen one VCR and it goes without saying that it wasn’t a JVC HR-D 750!

Again, any thoughts or comments very welcome.
Thanks,
Phil
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