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Friday, 29 July 2016

CCT811 Video modulator massive failure.

Remember my warnings and concern about the CCT811 video modulator?

This is the offending model in question. Apparently it's also being sold under various guises, one model quoted to me is the RF9000. If it looks like this, I'd be seriously concerned....




I wrote about it here. Please take a moment to read this, if you haven't seen it before.

Well, settling down for an afternoon of vintage TV fun, I switched the video rack on....

Crack, crack, crack, bang! Uh-oh...

"It's bound to be the modulator" I thought.


I was right. It had opened the fuse I'd retro-fitted. (If you have one of these awful modulators, time to stop using it!) Damned glad I fitted one... lord knows what would have happened if it had tried to short out the mains without the protection of a fuse... Fire would have been a REAL possibility.

I opened it up and expected to find a mass of blackened bits, but no.  Now I've giving up on this I thought... but it's so useful!

It looks as though the insulation had failed on the transformer, and destroyed the semi-conductors in the drive-side of the supply.

I thought I'd remove the ghastly existing "switched-mode" blocking oscillator, and replace it with something ...

1) Electrically safe.
2) Reliable.

OK. First things first. To find out if it still functions.

I removed the transformer, and connected the workshop power supply between the end of the rectifier diode, and ground....






It seems to want about 6.5 VDC to operate, and has even remembered the settings it had before the power supply failed... good.

It's drawing about 165mA at that voltage, so a supply is not too challenging!






I decided to have a look in the drawer of redundant wall-warts to see if I could find anything suitable, and I spy one of my favourites, an old Nokia phone charger!

Now I always pick these up from boot-sales for a few pence, and are very useful. This one is rated at 3.7 Volts, at 350mA...

"But Andy, you said the modulator needs 6.5 volts to work" ... and so it does, but this particular Nokia charger is unregulated. Off-load it makes about 10 Volts.... Will it make our 6.5 volts at 165mA?

I solder the white +VE lead to the anode of D107, and the black lead to the far right hole (viewed from the rear of the modulator) left behind when I removed the transformer...










... and switch on....

Eureka! It works.










So it's now electrically safe, and I can sit back, and watch some "proper" TV....








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