Hi all.
Over the weekend a mate of mine Tony (a member of this forum) came round and badgered me into getting my finger out and repairing an old Philips 1446U TV from 1954 that I’d had lying around for the best part of 17 years. I say “repair” rather than “restore” because there was so little (technically) to do to get it working. The set, an astonishingly plain but somehow quite endearing monstrosity, hadn’t been powered up once since I got the old thing many many years ago.
In the best tradition of things around here we decided not to plug the set straight in but to use a toaster as some kind of protection (what protection?) against short circuits. On powering the set up and waiting a good few minutes a faint line whistle was heard and some slight scratching noises from the loudspeaker. After some time had passed (not really very long actually) we decided to throw caution to the wind and plug the old girl into the full force of the mains. Faint line whistle and no picture at all. Hmm.
“Okay”, we thought, “let’s put a signal into the aerial socket to see if we get any picture or sound now”. So, on stringing an aerial lead from the bench to my big cabinet containing the Aurora, a loud yelp and a sudden spasm up my arm after touching the metal cabinet told me that something was not quite right. It turned out that somebody had once changed the mains lead but they’d connected the live (red) wire to the chassis – this combined with a very leaky capacitor between the aerial socket and the chassis had led to there being a nasty charge on the aerial socket outer, and my cabinet, being earthed, made quite a nice path to deck through my body.
After rewiring the mains lead and replacing the leaky cap there was still no picture. On checking the EHT, it appeared a little bit low but not so low that a picture shouldn’t have been displayed. We then started prodding around the tube base to see what was there. Around 700V on the A1 and pretty normal readings on the control grid and cathode (I can’t remember what they were, but Tony, a much cleverer and more resourceful chap than me, said they were fine). We even shorted the cathode to the control grid and got nothing at all on the screen.
Okay, perhaps the ion-trap was set incorrectly – It looked about right but we loosened it off and spun it around without any effect whatsoever – Not a glimmer on the screen. Surely the tube couldn’t be so duff that there was not even the slightest glimmer of a picture could it? After all it must have come out of somebody’s house – Would they have still been using it if the tube had gotten so bad that there was not even the faintest hint of a picture?
But then we thought – "Hmm. Perhaps there isn’t enough heater voltage for some reason". So we checked – and – there was only 4.5V instead of 6.3V. On disconnecting the heater connection to the tube and powering it via a separate power supply, a reasonably bright but very squashed picture was shown. However, a look at the current being drawn (450mA) revealed that the heater had an internal short somewhere.
Luckily I had kept a slightly tattier (but otherwise identical) spare set. After gingerly powering that set up too (not even through a toaster this time) there was a reasonably bright but, again, very squashed, picture. The chassis of the two sets were removed and the tubes duly swapped over. Putting both sets back together and powering up the original now revealed a fairly bright but still squashed picture.
Since we’d been working on the set upside down I decided to make sure the smoother capacitor was okay after its rather rude awakening. On touching it, it was pretty warm so we decided to re-form it. I’d been meaning to build a re-former for some time and I just happened to have all the parts (all 3 or 4 of them) to hand – a small 1:1 isolating transformer, a handful of medium to high value resistors and a load of croc clips. Everything in fact except a suitable diode. On rummaging through my collection of diodes, I picked out what I thought looked like a suitably beefy monster – I forget the number now – but we decided to look up the spec to make sure it was okay. Looking through my diode book, it only gave equivalents and checking the Interweb just revealed plenty of promises to data but very little actual data. All I got was “click here for datasheets” simply to be taken to another site with more “click here for datasheets” and so on. “Not to worry” I thought, “It looks pretty hefty so I’ll give it a shot anyway”. The diode was duly connected up to the transformer with an 8W 18K resistor, a multimeter measuring the voltage, and the capacitor to be re-formed. We then left it cooking nicely on the side and went back to the set.
We temporarily tacked a much smaller more modern cap in place of the old smoother and carried on with the repair. To cut a long story short, after deciding to not just do a blanket-change of all the black-wax-pitch Philips capacitors, every one we took out to check was duff – either incredibly leaky, low value or reading high value. In the end we decided that a blanket-change was what was actually required after all. And a blanket-change was what the set got. We’d been powering the set up after each few capacitors that we changed and the picture was getting better and better after each batch replaced in the line and frame sections. In the I.F. section though there was virtually no difference made to the picture or sound but at least it would be more reliable we decided.
After a couple of hours the voltage across the capacitor on our impromptu reformer had only risen to 30V (suspiciously exact in hindsight). The capacitor was also a bit warm but not alarmingly so. “What the hell”, we thought, “let’s give it something to think about” and bridged the 18K resistor with a big fat 10K. “Hmm. That’s odd” we thought after another while – the voltage is still stuck at 30V and the capacitor and resistors are getting warm. It wasn’t until we’d almost exploded the capacitor and burnt the house down with yet further smouldering resistors (I think this was a case of “It seemed like a good idea at the time”) that we decided enough was enough. A slightly more persistent search on the Interweb revealed that what we’d supposed was a big hefty power-diode was in fact a 30V zener diode! We then ripped the diode out and put in what we should have put in to start with – a plain old 1N4009. After a couple of hours with the new diode and the original high-value resistor, gradually working our way down to lower values, the capacitor had re-formed and was holding its charge nicely, none the worse for its terrifying ordeal.
The newly re-formed cap was then installed, the set boxed up, and, after a few tweaks here-and-there, the set was displaying a lovely picture. Not wonderfully bright, but still very watchable.
So, a couple of morals to this tale I think – First don’t assume the chassis you are working on is not live – even if the plug is wired correctly, and next – don’t assume that the aerial-isolator is actually isolated! As for the diode – I don’t think there’s any accounting for stupidity on this scale.
Thanks all.
Kind regards.
From Mike.