The Sinclair QL Forum

It should stay triggered once you have raised the gate above the cathode voltage, and only reset once the current flow in the Anode to Cathode circuit has gone close to zero.

The resistance reading between gate and cathode seems very low, it's normally equivalent to a diode, and as it's a thyristor not a Triac - it is only unidirectional. I think it's only supposed to trigger if there is an overvoltage on the 5v output.

I ordered a couple. Cheap enough and easy to swap.

Using my crude methods the new thyristors test very similar to the removed part. So I'll fit a new (NOS) one and will then test voltages at various places on the secondary side. What I can also do - as various other boards are plugged into the main PSU board - is remove some/all of them and see if the problem goes away. It's all rather densely packed, hard to get at and there is a massive spaghetti of fragile connectors.

Fixed it!

Thanks to Pr0f for directing me away from the primary side of the PSU as there was nothing wrong with it! I measured D512 and it had no voltage. Unplugged the next most important board along (the Chassis board II) and the diode showed 5v. Tested again on the EPROM that is on the PSU board (Chassis board I), which is as far away from the PSU part as you can get and it too had 5v on it.

The problem was on Chassis board II and after some connection checking and chip re-seating it came to life again.

Then I spent hours tackling a mechanism error - which was my own fault as a few days previous I was impatient, got carried away and had used a little force to remove a cassette that was in place when I bought it and that ruined the alignment (it contained the 1996 Olympics start ceremony, a German car program and the Germany vs Czech Republic Euro '96 football game). Managed to fix that after a lot of faffing around and after watching 3 handy Youtube tutorials on the Panasonic G series VHS mechanism a few times each. The remote control was also faulty as its main electrolytic cap was dud and that is vital for generating short bursts of voltage higher than the 2x AA batteries can supply.

So far I have test played a video via the S-Video connection and it works. No idea about the other tons of obsolete features (the remove control folds out and has 144 buttons) but I am hopeful now that it's in good health.

Here is a not very good picture of the beast:

I have to say - that's one impressive looking VCR !

And Kudos for getting it all working again - I think mechanically they are as complex as electronically!