Exploding Resevoir Capacitor and Terrified Budgie
Submitted by: Steve Webb
When I worked for Radio Rentals as a fresh faced youth we encountered all ages of TV from the very modern to the very old. One of my service calls was to an elderly lady who rented a fifteen year old Baird 420 17″ 405 line only TV. The picture looked very dull and tired and exhibited both low width and height. Voltage tests on the main HT with my trusty AVO 8 indeed revealed low HT. The old selenium rectifier had gone high forward resistance.
My solution was to fit a nice new silicon BY127. I can’t remember if I incorporated a surge limiter. Experience tells me I should have done with the events which were about to unfold. The replacement rectifier produced a superb picture and a delighted old lady. The next day however, I received a call from her which just said ‘TV exploded!’ The nice new HT rectifier I had fitted had proved too much for the elderly reservoir capacitor and it had sure enough exploded. She said the resulting bang had frightened her to death and I don’t think her nearby budgie was too impressed either.
Oh the inexperience of youth!
That reminds me of an ageing Murphy that my brother a I were given, way back in the early 70’s. Not being fully genned up with the habits of old capacitors, we plugged this thing in and switched on. All was well for about half an hour, when suddenly and without warning, there was an almighty bang from the telly, and all fell silent. My poor cat, who spent most of her time perched on my shoulders, leapt for the top of a wardrobe and buried herself behind a collection of boxes – followed a few seconds later by the unmistakeable aroma of cat ‘doo-doos’.
Now the capacitor… On investigating the bang, once the back was off the set, there was a definite flocked appearance to much of the internals, and right down by the on/off switch, two spiral discs on the ends of two rigid wires where once the mains filter cap had been. As the two of us looked toward each other, my brother just mouthed the words “Where the ****’s that gone?”. The mains filter cap had literally blown itself to smithereens, and it wasn’t a small one to begin with, about the diameter of a cotton reel and about half as long again – a true wax-bomb. As I recall, there were a few shreds of foil, but not a vast amount, but the most striking thing was the apparent flocking inside the cabinet – and a the pile of cat crap on top of the wardrobe!
I tried to keep explosions to a minimum in the customers house, it did not do much for their confidence and their impression of my competence however in the scheme of things I suppose you are bound to have a few mishaps along the way.
I remember a Thorn 8500 where the mains input filter capacitor had gone short circuit. I replaced the on/off switch which was also damaged and I also replaced the mains fuse in the plug. Unfortunately and foolishly whilst talking to the customer, I did not replace the faulty short circuit capacitor. I inserted the mains plug into the wall and bang! welding the nice new switch closed, doh. The best of it was I had replaced the capacitor and fuse for the same reason on many occasions and on many TVs so there was no excuse. Most times however the on/off survived.
Oh that did make me laugh Marion! Poor cat but I can imagine the sudden flight, the knocking down of certain items and the poor thing not appearing for several moments……!
Best of it was, the cat was called Smokey – She was the classic tortoiseshell, and a bit highly strung. Despite the ‘big bang’ though, it didn’t put her off snoozing on top of television sets!
Hello all,
Long time, no post, but I saw this and my memory was jogged.
I remember once having a proper battle with a Thorn 1500 B/W chassis. One of sections of the dropper resistor was open circuit. Line O/P valve was glowing, due to an open circuit grid bias resistor. Got all that done, but the tripler has caput, so in went a new of those as well. The resulting picture had poor line lock and terrible vertical linearity. Did a flywheel mod to take care of the line sync. The next thing was to change various caps, reistors and frame O/P valve to get the linearity back. After all of this, I was feeling pleased…… Then BOOM….. HISSS. The smoothing cap decided to let go!
The chassis was opened out in the service position at the time and the aluminium top had smashed off the neck of the tube on the way through to the wall, where it had left a neat circular dent, surrounded by oily paper and foil pieces. With all the noise, the manager ran in and asked me what had happened. I said “I think it’s f*****, Tom”. It really shook me as I had just finished adjusting the picture shift magnets at the time. Had I spent another 5 seconds adjusting the magnets that cap would have got me. As it was everything in the room, including me, was covered with capacitor guts. The smell you can’t forget.
After all of that, we had a cuppa and laughed about it. From that point on, we would say “I think it’s f*****, Tom” whenever something turned out to be U/S. I must admit I called that out from the back with customers in the front (unbeknownst to me), more than once…… Oops!
Happy days!
Love the site. Keep it going 🙂
All the best,
Tony
That reminds me of a picture I saw once in a book of a TV with a wrap-round cabinet. The set had been running on soak test for some fault or other. Unbeknown to anyone, at some previous time in its history it had obviously started blowing fuses, so someone had ‘cured’ the problem by up-rating the HT fuse!
The fault was obviously a leaky smoothing capacitor but, otherwise, the set had been working until the intermittent fault it was being soak tested for occurred. Suddenly there had been a terrific bang and whilst the contents of the can liberally distributed themselves around the interior of the set, the can itself had set off at high speed in the opposite direction!
The picture clearly showed the neat hole it had punched in the top of the cabinet! The can had finally embedded itself in the workshop ceiling tiles.
Early on in my career I learned a valuable lesson – to the discomfort of my colleague Ray.
Ken, one of our field engineers. marched into the workshop carrying an elderly radio which he smashed down on the bench in front of a startled Ray and proceeded to lay into him!
Ken had returned the set to an elderly customer after repair. He’d plugged it in, waited for it to warm up and sweet music appeared. He picked up the budgie cage and placed it in its normal position – on top of the radio – and prepared to leave.
There was a terrific bang and it was touch and go which might be the one to have the first heart attack – the elderly customer or the budgie! Certainly there were a few loose feathers flying around in the later’s cage! A quick check told Ken all he needed to know, which is why Ray was now on the receiving end of a very strongly worded lecture on the ripple current rating of electrolytic capacitors!
The rating of canned electrolytics is much higher than tubular types and this is particularly important when replacing the main smoothing capacitors. Ray had, of course, fitted a tubular electrolytic in this set.
The message was spelled out loud and clear – in fact I’m surprised it wasn’t written on the workshop wall in (Ray’s) blood!
NEVER, EVER, REPLACE THE MAIN SMOOTHING WITH A TUBULAR ELECTROLYTIC!
In fairness to Ray, the set was so old that it might have been fitted with one of those blocks in a cardboard box, so there may not have been a physical can to replace. In which case, if I’d repaired the set, I might have fallen into the same trap (but I was two years younger than Ray and, thus, not so experienced). That’s my story and I’m sticking to it!
Of course, these days, with the low ESR required for Switch Mode Power Supplies, things are different but, back then, if the smoothing consisted of more that one capacitor in the same can – even if they were the same value – one was always identified by a red paint dot on its solder tag as the ‘Outer Foil’. The significance of this only came to me much later on – that the improved cooling of the capacitor close to the outside of the can was important.
I remember a time working in an electronic measuring instrument factory, and some chassis had come for initial testing and set up.
A batch of 10 or more were powered up. A few seconds later there was a lot of noise and capacitor cans being blown off into the ceiling tiles above, the entire room filled with acrid smoke and spirals of capacitor foils! It turned out that the screen-printing for the smoothing capacitors had been printed backwards, resulting in them being assembled backwards! It appears that the draughtsman had made an error and over 2000 pcbs had been wrongly printed!