Hello, and thanks for trying to help me!
I'm searching for an help replacing/rebuilding the high-frequency transformer component of the PSU of a HeNe laser tube inside a laserdisc player (a glorious but broken Philips LV-720 from the 1978).
After quite a research and study (and painful translation from german
I'm able to first describe you a good part of the PSU operation, with the real schematics! (see attached)
The oscillator at 20KHz is driving a sort of darlington configuration to drive a step-up ferrite-core transformer from a mere 12.3V regulated supply (labeled 12.3A in the schematic). Reading the specs of the laser tube, the PSU should be able to feed 1150V at 5mA.
The voltage tripler labeled with 1002 is here only to create the spike necessary to light the laser tube on (something like 8Kv). After that the tube starts sucking up enough current (5mA) to completely disable the tripler effect.
The circuit on the right should be able to pickup the feedback tension to 'regulate' the PSU at 5mA, on the resistor 3067(+3066): 5V should be there. The transistor 6131 and the 6.2V zener 6022 closes the loop (limiting the amplitude of the square wave instead of his duty cycle actually?).
Now, after some test and a serious attempt to fry my multimeter (I don't have any tool for measuring HT) I can see that the PSU is only able to produce a mere 0.2mA. The 22V square wave at the collector of the driving transistor 6129 is there. Around 5V AC are across the primary windings.
I've tried to replace the electrolytic cap 2021, hoping for a dried one, with no luck.
Luckily, after a test with the room completely darkened (I was trying to spot flashes inside the laser tube), I've discovered a nice blue shiny arc coming from the inside the 5003 transformer! Found the culprit!
Ok, calm down. What now? How hack I'm supposed to replace that transformer? I will bet that the He-Ne technology is now only an memory from old nans. No way to buy a new one I think. You can find attached some photo of it (it doesn't seems a 40 years old transformer to me
And here the idea to 'rebuild' it from scratch. TME has the U-U ferrite of exactly this size, so most probably I could try to rebuild a new one with the plastic core as well. But... with what turn ratio? And with AWG wire gauge?
The primary is only 15+1 turns, so easy to count it! (I cannot explain the role of transistor 6128 driving this single coil turn BTW).
But the secondary (those that causes arcing) I cannot tell. The windings are not opened nor shorted: I can measure something like hundreds of ohm in the secondary. I don't have any inductance meter, nor capacitance meter...
Obviously I had tried to take the ferrite apart to reach the secondary coil and... simply count it.
But the damn glue seems invincible. Before breaking the ferrite I had tried to 'boil' the transformer in water, trying to soften the glue, but with no effect.
Ok, before crying on a pile of ferrite dust, I had tried to measure some voltage with... a tone generator app and an audio amplifier.
Here the readings using a 220K 1W load resistor (that actually become quite hot and fuming after few seconds), and using the 4-5 pins as primary (so 15 turns only):
5Khz sine wave: 4.5V~ -> 530V~
7KHz sine wave: 3.6V~ -> 510V~
So? 1:140 or 1:110?
Mmm my multimeter can be not so good at AC reading at these frequencies. In addition, is it possible that the transformer change its response at different frequencies? (it is designed to run at 20kHz but my ghetto tone generator made with a Lumia smartphone starts to cut at 10Khz 'by design'
No visible arcing during test, but who can say it?. The resistor ensured that I had sourced at least 1mA during the test.
Perhaps my only chance is to destroy this little thing only to count the exact turns, and then hoping to be able to rebuild a working replica.
I have harvested producers of SMPS transformers, but it seems that I'll be luck to find brand new parts up to 300/400V only.
Searching for complete PSU for laser tubes gives me only 'out-of-budget' results.
Now, what chances I have to fix this vintage (and heavy) brick in your opinion?
It will be funny to discover that the laser tube is kaput as well. And this time the 'rebuild' option will be unavailable this time
Well, thank you all, and be thankful for laser diodes!
Luciano