Hi all,

The other day, I had my THOMSON Telecontrol TS-3652 (from around the late 70's/80's) operating, and I heard a crackling sound coming from it. That was followed by a loud pop and a lot of smoke.

Upon inspection, I found that the 275v 0.47μF (?) metallized paper capacitor on a circuit mounted on the transformer had exploded. A Positive coefficient thermistor (posistor) appears to have been damaged by it too. The middle leg of the thermistor (which is connected to the capacitor) has been discoloured by heat.

Would this be from the cap, or just from normal operation?
Strangely the 250v 1A fuse didn't blow. I also noticed that upon inspection of the main PCB, that the legs of the flyback transformer had a browny red gunk buildup around them (not shorting them though). It looks like some sort of bubbly rust. Could this be a contributing factor?

What could've caused all this? My guess is that the cap was just to old, or maybe there was a mains power surge (should've blown the fuse?), but I'm suspicous about the flyback. Any thoughts?

I can also provide photos of the cap, thermistor and flyback.

Thanks,

Chris

A power surge usually consists of a sharp increase in voltage, not current. The fuse will not defend against this as it will only blow once its current rating has been exceeded.

If there was a surge it could be the reason for the expired condenser. They'll blow once their working voltage rating has been exceeded by more than around 10%. What actually caused it to blow could remain a mystery for all time though.

As for the transformer, the only thing I can think of without looking at a photo is that the brown gunk you speak of is the varnish used to hold the laminations and coils together to prevent vibration and breakdown of insulation. We'd need to see a photo of this to confirm.

Need to see pictures to be precise but from your description it sounds like a interference suppression capacitor wired across the incoming 240v AC supply.

With a 275 VAC rating these capacitors are marginal in specification for Australian conditions, they were designed for Europe's 220v AC supply.

If it is a clear (or translucent) epoxy coated capacitor these are notorious for failing in the way you describe even if they have not been exposed to over-voltage conditions. I doubt the TV has any fault that caused this, it was something that was going to happen no matter what due to a poor quality part.

The fuse not failing is not unusual. The capacitor becomes resistive and starts to heat up. The leakage current through the capacitor is enough to generate a lot of localized heat but is still well below the fuse current rating.

The PTC thermistor is probably for the degaussing coils. The discolouring may be OK but will need to see a photo of it.

Send some pictures to Brad and I'll be able to give you some more accurate advice.

Thanks for all help! I'll try and send the photos to Brad tonight. If they don't get sent, it's because we are packing to fly to New Zealand tomorrow morning.

Cheers,

Chris

I've just sent an email with the photos. Just incase he posts the photos before I regain internet access, I'll briefly describe them.

1. The blown 275V 0.47μF metallized paper capacitor.
2. The C52250 Positive coefficiet thermstor with the discoloured leg.
3. Possible replacements for the C52250 PCT?
4. The flyback legs (soldered under PCB).
5. My THOMSON Telecontrol TS-3652.

Would an MKP (metallized polypropylene) capacitor do? I have a nice yellow one, which appears to be rated correctly. It had .47k275v~X2 on it, though it also has 250V~ under the name

I have two more MKPs. One is an OKAYA 0.47μF 1414. 125V, 1283 250V and the other is a 47n 275V one.

Cheers,

Chris.

P.S. Anyone know of any good antique stores in Auckland, NZ? Preferrably with old televisions...

First off I need to make a correction to something I said earlier.

After checking some parts catalogues, it seems all mains rated capacitors are normally rated at 275v AC so my comments about the rating being marginal is not true!

Any mains service (X rated) MKP capacitor will be a suitable replacement. The yellow one you have that is 0.47μF 275v X2 rated is ideal.

Okay, the brown gunk on the underside of the PCB isn't varnish as first thought. It is flux from the solder and it appears the component above has been replaced before.

The blown condenser looks like a simple case of component failure and replacing it may fix that problem although in many cases it can be that another failed component (perhaps one going short-circuit) has caused the condenser to explode.

I doubt the 275V was paper, or it should not have been. Polypropylene, or Polyester, I would believe. Proper mains type caps should fail open. Was what was there definately a cap?

I am wondering if there may have been a MOV (Metal oxide varistor) there? These are the surge protectors and are also liable to have 275V on them.

These have been known to explode, with a big surge.

Marc

It's definately a metalized paper capacitor. Do you think that there would be an issue with the PCT, since it connects to the MP cap?

Chris

(P.s. Currently in Auckland. Amazing how the antique stores around here seem to have lots of old spare CRTs from televisons, radars and oscilloscopes. Also tonnes of cool valves too!)

I agree, the blown cap is metalized paper capacitor. They are a well known problem. "Google" Rifa Capacitors and the first few things that show up are blown up X class capacitors.
While they are not supposed to fail short circuit they often do and produce lots of smoke in the process!

Chris, once you replace this capacitor your Thomson will probably be ok again. I doubt it could cause any harm to the TV apart from blowing the primary fuse as it is in the circuit between the mains and the transformer (connected between Active and Neutral).

The PTC thermistor should also be ok. The capacitor would not harm it electrically. I would not worry too much about the discoloured leg at this stage. The case isn't melted looking so it probably works ok still. It's job is to allow the degaussing coils to only operate for a few seconds when the TV is turned on. If your picture quality is good (even colour across the screen) then the coils are probably working still.

The flux around the line output (flyback) transformer won't cause any harm but looks messy. Unless you are in a humid or salty area you can leave it alone. Otherwise you can gently pick it off with a toothpick or similar.