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Old 19th May 2017, 8:09 pm   #1
Focus Diode
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Default Thorn 960 "Ignition" like interference

This set has been featured on these pages before. Basically its a 16" Portable version of the original Thorn 950 chassis with the line, frame and sound output stages mounted seperately with a unique 2 stage warm up sequence not seen on any other set.

Anyway, the set's been through a lot in my hands eventually requiring a replacement line output transformer when the original red jellypot failed.

It works well on adapting a 1400 LOPT and modified 1580 EHT tray.

Anyway, to the present.... I know I should run my sets more often than I should but this evening the set developed an unusual fault condition.

The blank raster appeared as it should, followed by the signals after another 45 seconds. However I noticed ignition like crackle causing white blotches onthe screen with accompaniying crackle on sound regardless of thee setting of the volume control. Switching to 625 gave continued interference with the white blotches becoming black of course.

The "interference" results in the picture jumping up and down in sympathy though the line output stage appears to be OK.

To rule out any external interference I ran up another 405-line set (Regentone Pessey model), which functioned perfectly, not even picking up the interference from the 960.

So the fault must originate on the Thorn portable therefore.

Any suggestions where to look? Could be the CRT aquadag spring requiring a clean but any other suggestions?

Cheers,
Brian
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Old 19th May 2017, 8:39 pm   #2
linescan87
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Default Re: Thorn 960 "Ignition" like interference

Sounds like eht leakage around the final anode cap. Remove the cap and give the glass and cap a good clean with isopropal alcohol. Gin or some other spirit will do.

If that does not clear it, run the set with the back off in the dark and look for corona discharge from the lopt area.

Cheers,
John Joe.
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Old 19th May 2017, 10:11 pm   #3
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Default Re: Thorn 960 "Ignition" like interference

Thanks John Joe. I'll certainly check these points. When I last used the set it worked perfectly.

Interesting that the fault only showed up when the signals came through. The first stage of the warm up gave the expected clean unmodulated raster. Only when the signals faded in did the problem become apparent

I have the set running at the moment. Less crackle and dots yet the frame is still a bit juddery. The line hold and width are rock sold.

Last edited by Focus Diode; 19th May 2017 at 10:24 pm.
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Old 21st May 2017, 11:18 am   #4
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Default Re: Thorn 960 "Ignition" like interference

Well the cause was embarrassingly easy. The tripler had virtually detached itself from the LOPT! Cramping the PP3 like stud to fit more securely has completely solved the problem.

The tripler is mounted upside down in this model. Being a portable it's bound to work loose after movement. I'll fit a restraining strap to prevent this happening again.
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Old 21st May 2017, 1:45 pm   #5
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Default Re: Thorn 960 "Ignition" like interference

I found the original tray securing strap. I'd omitted to re-fit it when I replaced the LOPT. Very fiddly job getting it back on with the narrow space available. No excuse though. I must've been tired.

The LOPT screening can has of course been removed when taking the photos.

Photos all on 405-lines.

Cheers,
Brian

EDIT- Correction to the above post. Tripler should read Doubler of course.
The set originally had a half wave rectifier three stick tray but needed a doubler to work with the 1400 LOPT. I modified a 2-stick Thorn 1580 tray to a 3-stick doubler to fit in the small space avaliable.
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Last edited by Focus Diode; 21st May 2017 at 1:58 pm. Reason: Added correction.
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Old 21st May 2017, 1:49 pm   #6
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Default Re: Thorn 960 "Ignition" like interference

That's a great picture. Glad it was an easy fix!

Cheers,
John Joe.
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Old 21st May 2017, 1:52 pm   #7
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Default Re: Thorn 960 "Ignition" like interference

The tag strips at the top of the chassis with the wire wounds on used to fail spectacular back in the day, tracking and arcing, sometimes went bang.

Lawrence.
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Old 21st May 2017, 2:07 pm   #8
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Default Re: Thorn 960 "Ignition" like interference

Thanks to you both. The set certainly works better on 405-lines with the 1400 LOPT, the intensity of the line whistle is considerably reduced too.

You'll probably have noticed the replacement dropper capacitor. This was fitted inside the original can with an additional 0.33uF wired externally.

Horrible things can happen with the original capacitor as I found out, so it's essential this is replaced before first testing the set if any more examples turn up. 4uF "motor" capacitors can be obtained quite cheaply.

Some work had been carried out with the tagstrips before I got the set.

Cheers,
Brian
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Old 21st May 2017, 2:11 pm   #9
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Default Re: Thorn 960 "Ignition" like interference

Hi Brian,

Nice easy fix thankfully and doesn't that set produce a nice picture !
I could to to find a some doubler/triplers to get a couple of sets running here.

Marc.
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Old 21st May 2017, 2:37 pm   #10
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Default Re: Thorn 960 "Ignition" like interference

Hi Marc,

The ABC caption came from one of the very few surviving 405-line VT episodes of "Public Eye", electronically converted to 625 and displayed on 405! The picture quality is stunning even when viewed on a flat screen TV.

Sadly I don't have any EHT trays left, only components to repair an open one.

The original half wave rectifier tray failed giving the usual ballooned image with smoke pouring from the back. I repaired it using EHT diodes, then soon after the LOPT itself packed up!

With the 1580 tray I kept the two existing stick rectifiers and added a third one plus the capacitors needed to make it into a doubler.

A request in "Parts wanted" could result in a tray or two hopefully. I got a 1500 RS sealed type this way which turned out to be unsuitable for the 960 physically, so as one good turn deserved another I passed it on to someone else who wanted one.

Cheers,
Brian
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Old 21st May 2017, 6:37 pm   #11
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Default Re: Thorn 960 "Ignition" like interference

The set's giving me the runaround again! Intermittent ragged vertices developed which turned out to be the EF80 on the flywheel sync panel, tapping it made the fault come and go.

With that sorted slight but noticeable height variations developed. I had to replace the height control sometime ago as the original developed a dud spot.

Had to use a plastic bodied type which doesn't seem man enough for the job. Raising heat has probably distorted the shell too.

Sure enough it proved to be at fault when attempting adjustment. Will have to find an original type for longer term reliability.

Bah! These sets, good job we love them.....

Brian
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Old 25th Jun 2017, 7:45 pm   #12
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Default Re: Thorn 960 "Ignition" like interference

I'm beginning to wonder if this set qualifies as a "lemon", as encountered by Chas E Miller with a Rank set and in 1981 with a GEC single std hybrid colour set?!

It's beginning to look like it is. Since getting the set working I've had the original half wave rectifier tray fai, then the 4.33uf dropper capacitor (should have replaced it earlier), then the LOPT itself. There's also been audio and frame problems.

It's gone wrong again!

The original ignition like interference fault returned. A loose doubler can of course be ruled out. No arcing apparent when looking in a darkened room. Apart from the the spluttering on sound they were no "ticking" or other uncomfortable sounds emerging from within.

I ran the set for a while. A bit later very ragged vertices developed with the picture size varying. Suddenly the contrast and sync failed intermittently, this seems connected to the PFL200, IF and audio stages being dependent on the first heater chain. Clearly something is breaking down.

Reducing the brilliance stopped the ragged vertices suggesting either something in the doubler is breaking down or the disc capacitor on the LOPT tags.

The doubler is modified from a Thorn 1580 two stick arrangement (a similar unit was fitted in the 980 chassis). I retained the original sticks and fitted An EHT diode (BY1810?) in the centre. I'll try replacing the original sticks with diodes then go on from there.

Brian.

Last edited by Focus Diode; 25th Jun 2017 at 7:53 pm.
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Old 26th Jun 2017, 6:52 am   #13
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Default Re: Thorn 960 "Ignition" like interference

You have certainly been unlucky with this one Brian. I have never encountered a faulty heater dropper capacitor. Other than that daft heater arrangement that required the replacement of all four valves, or wait up to 30 mins for them to warm up, I had very few problems.

I did have a service call and on removing the back discovered the main smoothing block spattered everywhere inside the case. It must have gone off with a violent bang but the customer never noticed it! [Or they told me they didn't]

Well if you run out of patience I will loan you the IGNITION key to my Atkinson lorry and you can bring the irritating thing down to be crushed. Regards, John.
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Old 26th Jun 2017, 10:29 am   #14
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Default Re: Thorn 960 "Ignition" like interference

Hi John,

The only thing the set has so far not suffered from is delayed warm up. The only valves I've had to replace so far have been the PY800, PL504, PCL86 and EF80.

I ran it up again for closer examination with the Lopt screening can off. This time the fault symptoms were different! It revealed an impending break down in the frame stage as tapping it gave intermittent frame collapse and fly back lines which I've had before with the set.

Clearly the modern plastic bodied pre set controls aren't man enough for the job. In due course I'll look for a more conventional replacement type fed by a 1W feed resistor to reduce the dissipation and hopefully give longer service.

Funnily enough the apparent EHT tray problem etc. didn't show up.

I'll attempt re-restoration to see if that improves reliability!

Cheers
Brian

Last edited by Focus Diode; 26th Jun 2017 at 10:35 am.
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Old 26th Jun 2017, 10:47 am   #15
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Default Re: Thorn 960 "Ignition" like interference

I know you have fixed it now but I have had a similar fault due to a dry joint on the degaussing thermistor.

Peter
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Old 26th Jun 2017, 11:14 am   #16
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Default Re: Thorn 960 "Ignition" like interference

Thanks Peter. Every little helps. Faults like this are usually visible and audible (I mean not from the speaker). I thought it was interference until I ran up another set. The HMV's fault didn't cause interference to the other set!

Next weekend I'll dismantle the set for thorough investigation and once and for all (hopefully) sort out the frame problems. It's beginning to look as if the fault exists here. In this version the line, sound and frame stages are conventionally wired. Unless the chassis is removed from the cabinet it's impossible to see the associated components. The metal frame for the audio and frame stages can be lifted up for service, again not possible without chassis removal.

Cheers
Brian
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Old 26th Jun 2017, 11:45 am   #17
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Default Re: Thorn 960 "Ignition" like interference

Degaussing thermistor? These sets were certainly unusual!
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Old 26th Jun 2017, 12:56 pm   #18
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Default Re: Thorn 960 "Ignition" like interference

Certainly not a degaussing thermistor but these sets do employ them elsewhere of course, scan coils for example.
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Old 27th Jun 2017, 9:59 am   #19
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Default Re: Thorn 960 "Ignition" like interference

Sorry, I'm colour blind - and losing my marbles.

Peter
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Old 27th Jun 2017, 11:21 am   #20
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Default Re: Thorn 960 "Ignition" like interference

I've had all manner of interference symptoms caused by poor dagging on the CRT. Sometimes just ragged verticals, sometimes 'brush a discharge' appearance. Sometimes just random white 'splats'.
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Last edited by Andrew2; 27th Jun 2017 at 11:39 am.
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