Friday, 12 February 2016

Video rack video modulator repair and warning! Model CCT811

Remember the video rack I built a while ago? It's details are here.

Well, it's developed a fault. The UHF modulator has been giving very poor pictures after a few minutes of operation. I decided to take it out, and repair it.


I purchased it ages ago from eBay, and it came fitted with an unfused euro plug, which I cut off, and fitted a standard UK Plug, and fused it at 1 amp. I'm glad I did... read on ...

It's a useful thing. Is stable (when working properly), is adjustable throughout European VHF allocations, as well as UHF, and has switchable FM sound between 5.5 MHz and 6 MHz sound sub-carriers....
Disassembling the unit reveals a few horrors....

Check the mains input to the diminutive switched-mode supply ..... No fuse is evident!

What you can't see by this picture is the bottom of the case, the black bit. It's metal. There's no earth, which would be fine if the thing met Double Insulated (Class II) standards, which I doubt it does. You can read about classes here.

There's also zero filtering on the mains input, so any noise from the switched mode makes it's merry way back down the mains lead and out onto our mains, spoiling our radio reception (and, ironically, our TV reception too!) if it radiates (which it will).

Grim. Glad I fitted a fused plug ....

Examination of the power supply shows it to be nothing more than a simple blocking oscillator.

There's a simple zener on the output attempting to provide a little regulation. You can see where the board is a little discoloured, as it's been running warm. There are two capacitors in the primary, the mains smoothing capacitor (4.7uF 400V) proved to be very low in capacity. I fitted a 10uF , as I had one to hand. I also changed the smaller cap (10uF 50v) whilst I was there.

 A quick check shows the unit to be working again.... but what to do about the safety issues?

Mounting it back in the rack, I've fitted an in line filter and fuse (100mA). I'm not so concerned about earthing the case, as the modulator is inaccessible when the rack is assembled, that doesn't mean you should be though!

Pity really ... "for a ha'p'orth of tar" an otherwise good unit is spoiled.... and possibly electrically unsafe and a fire risk. It carries a CE mark, which I've no doubt it doesn't deserve.

 This gives you some idea of the size of that supply transformer, that's a 1p piece!
The guilty parties. Caps. As usual!

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