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Old 24th Jan 2023, 9:02 pm   #1
slidertogrid
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Default Pye CT72 Fire!

This was quite frightening ! Not because the set caught fire but because it happened after the set was switched off!
I watched a couple of episodes of Dr Who on the set and then switched off. About 10 minutes later while luckily I was still in the room just as I thought i could smell 'something' the smoke alarm in the workshop went off...
Peering into the back of the set I was horrified to see flames! I spun it around, wrenched the back off and blew the flames out. At that point it stank and smoked. Before there had been no visible smoke when the back was on and there had been no signs of picture disturbance when the set was on.
Panic over, it was only a small burn up but it could have quickly got worse! The set spent the night in the carport!
After a few days against the workshop radiator today I put it on the bench and had a look.
One of the mounting brackets on the tripler had burned up, this in turn had set fire to the rubber grommet and burning plastic had dripped onto the Boost capacitor.
I would have expected some arcing to have taken place if it was the EHT spraying that had caused the ignition but there wasn't any noise, Nothing!
Also I was watching the set it's not as if it was running in the workshop unattended .
This could have been nasty! I have a scrap timebase chassis that came with the set so a replacement tripler is no problem but I think once it is repaired I will buy a second smoke alarm which will be mounted directly above the set!
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Old 24th Jan 2023, 9:15 pm   #2
Roger Ramjet
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Default Re: Pye CT72 Fire!

That's scary... I have little knowledge of vintage TV internals but I do recall stories of sets catching fire overnight if left plugged in despite being switched off.

I did try to fettle an old TV once and what I understood to be a thermistor [recall CZ19] ? used to get very hot.

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Old 24th Jan 2023, 10:27 pm   #3
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Default Re: Pye CT72 Fire!

I imagine if the set had been operating for much longer, the EHT would have arced over more violently.
A very nasty event all the same and just goes to show how important it is to monitor any vintage set, even shortly after use.
Although it is probably rare for a fire to start after it is switched off of course, it is clearly a possibility.
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Old 24th Jan 2023, 10:57 pm   #4
Philips210
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Default Re: Pye CT72 Fire!

That was fortunate you were present when it started to burn. It could have been much worse. I remember the tripler on some of these sets often developed a hot spot, possibly due to a failing eht rectifier diode within. Maybe that's what's happened here. The black gondola focus unit could also be a fire risk too. The replacement grey version was supposed to be safe.

I repaired quite a few of these Pye Hybrid colour sets back in the early 1980s and these early 691/693 chassis versions were better than the later 697 with the toasty PCB version power/line panel. The line output transformer was generally OK and I never saw a burn-up if it failed. The red solder mask panels seemed better quality than the later green panels. The 691/693 power/LTB unit sometimes had problems with the mains transformer going up in smoke if the LT bridge rectifier went s/c. Later or replacement transformers were supplied with a built in thermal fuse. There were two versions of the metal timebase in the single standard sets. The first was entirely hand wired. The later version has two PCBs with hand wiring in other parts of the assembly. Both versions were preferable to the later 697.
I have many good memories of these sets despite their weaknesses. They were always easy and cheap to repair and I learnt a great deal in the process. The transistorised decoder based on the Mullard design was good when set up correctly. Shame about the CDA panel though.

Hope that you manage to fix your set.

Regards,
Symon
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Old 25th Jan 2023, 1:00 am   #5
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Default Re: Pye CT72 Fire!

I was up in the attic in the days before smoke alarms in homes and the rather expensive monitor on the computer in the radio shack caught fire. I smelled it, came down, saw actual yellow flame and put it straight out the front door.

Contacting the manufacturer produced no interest. The casing and PCBs were all supposed to be fire retardant. Photos went to ALL the safety agencies whose approvals marks appeared on the thing. Oddly enough, there was interest from them. They were very worried about the impact on their credibility. Having been involved in getting products through approvals, I had the advantage of good contacts with most of them.

None of this applies to vintage gear, but it served as a remainder that even new gear with all the required markings could be dangerous.

David
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Old 25th Jan 2023, 8:05 am   #6
toshiba tony
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Default Re: Pye CT72 Fire!

What chassis was this please? The 693
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Old 25th Jan 2023, 9:46 am   #7
dazzlevision
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Default Re: Pye CT72 Fire!

Quote:
Originally Posted by toshiba tony View Post
What chassis was this please? The 693
Pye CT72 = 691 chassis. I have one myself - a lovely looking set.
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Old 25th Jan 2023, 10:16 am   #8
Lloyd 1985
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Default Re: Pye CT72 Fire!

Thatís a bit scary! Good you were there to put it out!

I need a new smoke alarm, especially since I had coronavirus, I keep smelling smoke when there is none now, so if there actually was some Iíd probably ignore it!

Regards,
Lloyd
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Old 25th Jan 2023, 11:47 am   #9
toshiba tony
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Default Re: Pye CT72 Fire!

My error, was there a 693? I have dementia, remember the 697 with the pcb line timebase but I don't remember three variants, my fault, sorry.
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Old 25th Jan 2023, 11:52 am   #10
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Default Re: Pye CT72 Fire!

Quote:
Originally Posted by Lloyd 1985 View Post
I need a new smoke alarm...
Don't wait, order yourself one today and it could be up on the ceiling and doing its job for you tomorrow. They're not exactly expensive (compared with sorting out the effects of even a very minor house fire)
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Old 25th Jan 2023, 12:08 pm   #11
Philips210
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Default Re: Pye CT72 Fire!

Quote:
Originally Posted by toshiba tony View Post
My error, was there a 693? I have dementia, remember the 697 with the pcb line timebase but I don't remember three variants, my fault, sorry.
Tony, I think the 693 was basically a 691 but with a varicap tuner.

The Pye Hybrid Colour sets started with the dual standard version, typically models CT70/1. There was no chassis designation, it was just referred to as the dual standard chassis
The early single standard version was the 691 chassis, the later version was the 693. There were two versions of the metal Power/LTB unit as mentioned earlier. I'm not sure when the production change occured for the introduction of the twin PCB metal power/LTB unit, it was probably part of the updates to the new 693 chassis.
The final version was the 697 chassis

Regards,
Symon

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Old 25th Jan 2023, 12:24 pm   #12
greg_simons
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Default Re: Pye CT72 Fire!

Interesting failure, don't recall any particular dramatics with the 691/3/7 chassis apart from the crispy cda board, good job you were on hand to deal with it promptly, the Bush ctv25 on the othet hand, saw a couple of those that caused severe damage.
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Old 25th Jan 2023, 12:50 pm   #13
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Default Re: Pye CT72 Fire!

So am I understanding this correctly: The stored EHT left after switchoff has caused something to breakdown in the tripler which has then set fire to the grommet?

I assume there is no bleeder and any remaining EHT is normally expected to just leak slowly away?
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Old 25th Jan 2023, 12:54 pm   #14
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Default Re: Pye CT72 Fire!

I wonder if any arcing was accelerated by the use of a metal self-tapper. I remember
our Skantic had the tripler attached using nylon screws and nuts.

Beware of later grommets, as the plasticizer can be tranferred to some plastics which it
is in contact with.
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Old 25th Jan 2023, 1:01 pm   #15
Lucien Nunes
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Default Re: Pye CT72 Fire!

I don't think there's enough stored EHT energy to ignite anything, it must already have reached a critical temperature while the set was on, but not enough material was burning at that point to notice. It's possible that switching the set off, therefore reducing convection currents past certain hot components, changed the airflow speed and direction around the smouldering material in such a way as to accelerate the combustion. I think Gerry Wells made this point, possibly while discussing GEC convergence panels, as a possible cause of things getting worse after switch-off.
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Old 25th Jan 2023, 1:32 pm   #16
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Default Re: Pye CT72 Fire!

I immediately thought you'd installed the later 'toasty' PCB line panel which was known for this where the mains came in. I'd like to think it would have gone out of its own accord, but you never know.
I'm sure you remember a Tomorrow's World episode which demonstrated a tube filled with an extinguishing gas and a pressure switch to isolate the TV in the evenmt of a fire. This was after the much publicised fire in a TV shop due to the Burning Bush Mk 1 CTV25. Never caught on, surprisingly.
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Old 25th Jan 2023, 2:15 pm   #17
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Default Re: Pye CT72 Fire!

It certainly makes you think. I guess the back got damaged round its fixings when you wrenched it off, but what else could you have done.

The only thing I can think of that I've had happen like this was with a food mixer that caught fire on the kitchen table a few minutes after it had been switched off and disconnected. I had to grab it and put it outside PDQ! It was caused by congealed flour that had built up over the years, being sucked in by the motor, and sparks from the brushes had ignited it and it must have sat there smouldering after the mixer had been used. Check those old food mixers, folks!
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Old 25th Jan 2023, 3:29 pm   #18
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Default Re: Pye CT72 Fire!

I worked for a Pye dealer after leaving school. They had hundreds of Hybrid Pye sets out on rental the mechanical tuner sets were getting a bit long in the tooth by then (1977) but we kept them going as decontrolled rentals. Around 1980 they were sold off as they came in ex-rental but the varicap sets were rebuilt at any cost. As Symon has said the black focus assembly sometimes overheated I saw a few where the plastic had really melted I think the VDR went low resistance and glowed red hot. Later ones didn't suffer and the grey plastic was supposed to not ignite. I never heard of one of 'our' sets catching fire, but a local (Deeping) TV shop had a nasty fire in the workshop when a hybrid Pye they had left running on test over the lunch hour went up!

I think my set must have been smouldering away when I switched off and then ignited once off. it was strange that I saw no smoke until I blew the fire out...
Luckily the damage to the back is minimal the back was fitted with some of the GEC type screws with the built in washer which are a little undersize for the slots in the Pye back so they let go without too much of a struggle. I was going to fit some PYE screws with the brown washers as original but now I am thinking I may keep the quick release ones!
I am going to fit the replacement tripler shortly but for now I have carried out the official CES approved modification....
Rich
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Old 25th Jan 2023, 4:23 pm   #19
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Default Re: Pye CT72 Fire!

That's worrying!

I'm kinda glad that the vast majority of my gear here has steel or aluminium cases which would contain any 'internal' conflagrations.
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Old 25th Jan 2023, 7:52 pm   #20
slidertogrid
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Default Re: Pye CT72 Fire!

I replaced the tripler and ran the set up, the EHT was spot on the width correct and it looked as if the fault was cured, but there was a just audible hiss from the timebase chassis and sure enough a faint smell of ozone. I took the side off the chassis turned off the bench lamp and had a look.

It took a second to spot the problem as it was on the underside of a disc capacitor but there it was tracking away! Out came the timebase chassis again and the capacitor replaced. No more hissing!

I decided to investigate the burnt tripler to find out if there was a pinhole under the melted bracket that had tracked and caused the bracket to track and combust. I chopped the burnt plastic away and then got it on the bench grinder, having ground away the case a fair bit I could see a tiny black dot in the casing so I think the case may well be punctured, the dot being where it had tracked.
I cannot see how the arcing or tracking capacitor could have ignited or had any affect on the tripler so it looks as if I have had two faults? Both tracking and in the same area! The arc can just be seen in the second picture.
I will give the set a run and keep an eye on it!
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