Sorry folks, for the lack of news. Bear with me, impossible workload at the moment.
That said, I thought I had another nightmare on my hands and will just share it with you briefly ...
Since the mini-meet at Piero's a few weeks back, my prototype controller has been with Colin. All was going well, until I got this PM from Colin (I don't think he'll mind me quoting it here) -
I have been using the speed controller for a couple of weeks with no problem, but on Sunday I switched it on and after a second or two the led went out and there is no output. The only thing I can think of as different was that the Lenco was left in the on position from the previous session - I had up till then only turned the controller on when the Lenco switch was in the off position. This time I did not notice it was left on.
When using the controller at Piero's his Lenco was always left on and actually switched on/off by the controller, so I don't think it should have been a problem .....
OH no! Another dead controller!! Immediately I thought that we had another case of exploding chip amps and was almost ready to abandon the project.
Thankfully, that isn't what happened. Colin returned the controller to me and in fact all that had happened, was that the fuses on the supply transformer secondary had blown. I fitted new fuses and it is now working perfectly again.
So, why did the fuses blow? Well, I think it's due to the large inrush(surge) current which will be drawn when the unit is powered up with a turntable connected and switched on. That's not to say that you shouldn't do that - it's bound to happen occasionally, even if unintentionally. So, the fuses I have used are not adequately rated for the job.
Fuses come in various "flavours" - quick blow, semi-delay, anti-surge, etc, etc and I will need to either change to a fuse with a longer delay, or else increase the current rating a little. This is a pretty trivial detail, I will look into it.
(The reason that Colin saw a one second delay, before the LED went out, is because the amplifiers are held in mute for about a second after power-up, by the way.)
Meanwhile, there is ONE final detail that I need to resolve, before I can start taking PCB orders and I will need some input from all of you. No time to write about it now, though. I'll get back to you ....