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Old 12th Jul 2017, 10:29 am   #41
Focus Diode
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Default Re: Thorn 960 "Ignition" like interference

The planned replacement of the frame output transformer, assuming the replacement of Z2 doesn't solve the problem, looks to be more difficult than I envisaged due to the 1400 one being PCB mounted. Presumably they're electrically identical however so I'll find a way of mounting it on the 960 chassis frame via an adapter plate and possibly a tagstrip or two.

I'll study both 950 and 1400 circuits to determine where the tags should go.

Cheers
Brian
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Old 12th Jul 2017, 9:36 pm   #42
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Default Re: Thorn 960 "Ignition" like interference

Not unexpectedly replacing Z2 made no difference. Unfortunately adapting the 1400 PCB mounted frame output transformer isn't an option due to shortage of space . The only options are a similar sized replacement or a rewind.

The two original stick rectifiers in the modified doubler were also changed with the set on the bench. I'd retained the original stick rectifiers and added an EHT diode in the middle, replacing the original wire link. Now three BY8140s(?) are in the tray.

Some voltages I'm getting:
Z2: 200V and 221V (each end)
PCL85 Pin 1 67V
. Pin 2 -38V
. pin 4 -36V
. pin 6 200V
. pin 7 211
. pin 8 21V
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Old 12th Jul 2017, 10:33 pm   #43
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Default Re: Thorn 960 "Ignition" like interference

Hi Brian, would it not be possible to temporarily wire in the 1400 transformer on long leads as a test? That way you will at least know if the transformer is definitely faulty before going to the expense of a rewind.

I've never worked on a 960 series but certainly on the 1500 the PCL805 valve base often gave problems due to heat damage on high mileage sets.

John.
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Old 13th Jul 2017, 10:45 am   #44
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Default Re: Thorn 960 "Ignition" like interference

Hi John,
That's definitely worth a try. I'm reluctant to condemn the transformer to be honest as there's certainly no signs of overheating but of course that doesn't rule out problems deep within the windings. The only way to prove its innocence or guilt is to wire in the 1400 transformer as you say.

I've tried wobbling the PCL805 in its holder to no avail. David did find the cause of the intermittent fly back lines which turned out to be a poor connection to the earthy end of one of the linearity pre sets.

Unusually, mains neutral doesn't go directly to chassis in this version with its unique heater chain feature.

Cheers and many thanks again.
Brian
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Old 19th Jul 2017, 11:23 am   #45
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Default Re: Thorn 960 "Ignition" like interference

I've been very Kindly offered a replacement frame output transformer from a Thorn 981 model. Thanks Chris from the VRAT forum.

Physically the size looks to be identical but on checking the two circuit diagrams they are slight differences such as the resistance between the windings. The pin 6 anode voltages also differ, 190V on the 950, 165V on the 980. The 980 also employs a 30PL14 in place of the usual PCL85/805.

I don't envisage any serious problems but think it would be prudent to fit a feed resistor from HT to the primary of the transformer to keep the voltages down in the interest of preserving the replacement transformer. Z2 will also be fitted of course.

The transformer is due to arrive in the post at anytime. I'll keep you all posted.
Cheers
Brian
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Old 19th Jul 2017, 11:12 pm   #46
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Default Re: Thorn 960 "Ignition" like interference

Hi Brian, the winding resistances of the 980/1, 1400 and 1500 transformers are identical, I suspect they are essentially the same transformer with the 1400 and 1500 type modified for PCB mounting. If the 1400 type worked ok then I doubt you need to make any changes to the circuit.

John.
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Old 25th Jul 2017, 10:37 pm   #47
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Default Re: Thorn 960 "Ignition" like interference

Hi all,
Many thanks John. When the 981 transformer arrived it looked identical to the one in the set. I was gratified to find the original transformer complying with the resistances of later models despite the circuit suggesting otherwise.

Chris had kindly marked which wires go where but curiously the reverse to how fitted in the 960.

Seeing the transformers appear identical I decided to connect as the original transformer was in the interest in the existing lead lengths etc.

I got this interesting result! It later transpired I might have connected the secondary to the scan coils incorrectly. Unfortunately the original fault condition persisted, the line break up occurring at the bottom being upside down.

Unfortunately the innocent original suffered some damage on removal. It turns out the delicate soldering to the Jellypot LOPT is equally applicable to the frame output transformer. Thankfully it was easy to sort out.

When refitted the original fault condition persisted. Out of interest I removed the earth connection via the scan coils from the secondary. This resulted in shading to the left hand side as evident in the photo.

Boost HT to the frame transformer is a steady 209V approx. Doesn't vary with the creeping height. The height control can be set really excessively, in fact the photo wasn't taken when set to maximum as the frame wouldn't lock!

Much more detective work to do. I'd welcome any suggestions.

Cheers
Brian
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Old 26th Jul 2017, 7:15 am   #48
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Default Re: Thorn 960 "Ignition" like interference

Hello Brian,
Just another log on the fire. have you run the receiver with the lin sleeve removed? It may be arcing to the line coils. I had this on many occasions. John.
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Old 26th Jul 2017, 10:51 am   #49
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Default Re: Thorn 960 "Ignition" like interference

Hi John,

Many thanks. I'll try that. It's occurred to me the scan coils may be mounted upside down with the result I was shorting out the wrong side of the coils, ie the opposite to where the troublesome thermistor is so I'll try that too.

There was a peculiar effect when the frame transformer's chassis connection was lifted. It resulted in the left side of the picture being shaded.

Unlikely to happen of course but worth noting as one would assume the fault may be in the line output stage!

Cheers
Brian
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Old 27th Jul 2017, 12:35 pm   #50
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Default Re: Thorn 960 "Ignition" like interference

Unfortunately doing both had no affect. The only difference removing the linearity sleeve did was to result in unacceptable linearity yet the frame continued to judder as before.

The only things left are the small components. Two Suflex capacitors for example. I understand silver micas can be fitted in their place.
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Old 29th Jul 2017, 5:02 pm   #51
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Default Re: Thorn 960 "Ignition" like interference

Quote:
Originally Posted by jayceebee View Post
Hi Brian, the winding resistances of the 980/1, 1400 and 1500 transformers are identical, I suspect they are essentially the same transformer with the 1400 and 1500 type modified for PCB mounting. If the 1400 type worked ok then I doubt you need to make any changes to the circuit.
Hi John,
Just been looking at the 950mkII circuit which shows identical resistance readings to the 980/1400/1500. Seems the original 950 is slightly different but it turned out the one in the 960 has more or less the same readings as the 981 one. The 950 mkII also has a different Jellypot LOPT, the same as used in the 1400 series.

I'll be getting back to the set in due course.

Cheers
Brian
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Old 30th Jul 2017, 9:38 am   #52
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Default Re: Thorn 960 "Ignition" like interference

Had another occasion to work on the set last night. I established removing the EHT tray had no effect ruling that out. I also replaced the 180pF disc capacitor I had to add on fitting the 1400 LOPT. It ran extremely hot which didn't inspire confidence. Found a suitable 160pF type on the 1500 LOPT.

Perhaps you're wondering why I didn't do this earlier? Some 30 years ago I ruined a perfectly good Jellypot LOPT on attempting to replace the capacitor. I also wanted to ensure I had the correct type to hand which I recently found.

Thankfully the replacement was fitted without ruining the LOPT, important seeing I no longer have a spare 1400 tx. I'm also pleased to report the replacement runs quite cool.

There's still brief ragged verticals and picture breathing soon after the signals appear. Must look for the probe and check the EHT as it's definitely a bit lower than it was when working with the original Lopt.

To the original fault condition: System switch jamming made me quick to unplug the set. The switch was lubricated then on switching back on the fault condition appeared to have been cured! Nice solidly stable frame scan I'd almost forgotten about. I was able to establish the two linearity controls are touchy so will need replacing.

A further switch on saw the original fault back, but in a far less extreme form. In fact the vertices only varied slightly, quite watchable in fact unlike before. I ran the set for a good hour displaying Test Card "C" no apparent additional problems until the signals suddenly disappeared. Yes, you guessed it the VHF tuner has now developed a fault!!! Tapping the tuner brought the signals back....

Another thing I need to sort out is the dust seal between the CRT and Feinbridge guard sheet. It's beyond useless with dirt quickly accumulating between the CRT screen and guard. I wouldn't be surprised if this is the cause of the fault with static building up that can't be released.

The un banded CRT is crudely fixed via a plastic cradle with the face resting directly on to the guard. I'll remove everything, give it a thorough clean and add draft excluder material to help prevent a re-occurance.

This early photograph shows the arrangement.

Cheers and thanks again for your helpful suggestions and advice.
Brian
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Old 4th Aug 2017, 9:52 am   #53
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Default Re: Thorn 960 "Ignition" like interference

CRT was removed for cleaning and application of draught excluder to act as a dust seal. No provision had been made during manufacture it seems. It looks as if the set was originally designed to take a re-enforced CRT but none were available hence the crude lash up of a flat plastic implosion sheet as an afterthought. Perhaps later models were fitted with a re-enforced CRT I don't know.

A thorough clean of the CRT seems to have finally cured the occasional ragged vertices and ballooning but the occasional frame judder and top scan break up persists. I'm beginning to suspect the CRT itself for the problem.

At least it's now intermittent and the set can now be used unlike before.
Cheers
Brian

Edit- photos won't upload for some reason but here's a link to them via the Radio-TV site.
https://www.radios-tv.co.uk/forum/bl...rc-960/page-2/

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Old 14th Aug 2017, 3:45 pm   #54
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Default Re: Thorn 960 "Ignition" like interference

As an update I confess to being not much further forward with the set. Resorting to desperate measures I temporarily wired in an 0.0047uF 250V AC capacitor in place of C79 and also the 500pF Suflex capacitor. Absolutely no change!

There's nothing left to check in the frame timebase!

Tried new valves in the initial heater chain, no change. Admit defeat for now and reassemble set. The system switch jammed again. Switch off quick, sort out and switch on. A perfectly still raster appeared!

Now I can't imagine the system switch being the cause of the fault as, after all the frame standard's are the same. When the fault is present both systems are equally affected. Changing systems doesn't affect the fault in any way but somehow when it jams it seems to!

Wonder if the solenoid itself is the problem? A faulty smoother can cause the time bases to break up yet there's no hum bar.

What do others think?

The set gave a very good account of itself when used for over an hour today.
Definitely a set to use in the cold dark winter evenings as the heat emerging from the set would warm up a room in no time!

Cheers
Brian
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Old 15th Aug 2017, 11:16 am   #55
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Default Re: Thorn 960 "Ignition" like interference

I got it wrong when I mentioned the system switch has no effect on the frame timebase operation. It was pointed out to me S2M infact functions to compensate the height setting on 625-lines. Not relevant to the fault here but relevant to the 900 11" portable which gives much reduced height at 625 which can be set but results in excessive height on 405. Clear the feed 620K resistor has changed value in that set.

Back to this one it seems I've been barking up the wrong tree from the outset. Carrying out much maintenance in the frame timebase certainly has improved the overall frame sync and locking so it wasn't a complete waste of time.

Appears the fault is in the system switch solenoid or at the system switch AC point tracking over causing the disturbance on the screen. This area of the set is well hidden. Dismantling including removal of the VHF tuner will be needed to get at it.

This is for another day, but at last there's light at the end of the tunnel!

Brian
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Old 16th Aug 2017, 3:03 pm   #56
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Default Re: Thorn 960 "Ignition" like interference

Well, I'm now confident the cause all along has been the system switch, possibly the solenoid itself or the switching point on the slider. I had problems on another set due to the latter which allowed the system to be changed but not back on some occasions with much hum from the solenoid. Replacing the system switch slider gave a permanent cure.

The original fault had somehow been "cured" by system switch jamming. I removed the supply to the solenoid then temporarily re-connected it. They were some crackling noises from the speaker which immediately went on removing the supply to the system switch while the set was running.

Thanks for all your help and suggestions. The set will be used on 405 until time allows for further investigation.

Brian

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Old 23rd Aug 2017, 11:54 am   #57
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Default Re: Thorn 960 "Ignition" like interference

Switching the set on nearly a week later showed the fault had returned!!! The solenoid, switching can now be ruled out as the AC supply has been disconnected.

Could be the switching sliders? The last time the fault appeared to have been cured I left it a week or so before switching the set on again to find the fault had returned.

Could be significant?

Cheers
Brian
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Old 23rd Aug 2017, 1:45 pm   #58
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Default Re: Thorn 960 "Ignition" like interference

Sounds like part of the switch sliders is becoming conductive when its cold and possibly slightly damp.
Viewing in total darkness may show it.
I have found that some switch cleaners can soak into the insulation of switch wafers rendering them conductive especially if voltage is applied before they have totally evaporated.
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Old 23rd Aug 2017, 2:42 pm   #59
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Default Re: Thorn 960 "Ignition" like interference

Hi Sam,

The sliders are virtually concealed in situ which might explain why I'm unable to see any signs of arcing in a darkened room.

The fact system switch jamming appears to cure the fault could possibly be due to a dry joint on the PCB side, the jamming sealing a dodgy connection.

I'll look into it in due course.

Cheers and thanks for the feedback Sam.
Brian
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Old 24th Aug 2017, 9:08 am   #60
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Default Re: Thorn 960 "Ignition" like interference

Had a look at the print side of the main panel yesterday evening. There was indeed an obvious dry joint at the centre tag on one of the sliders. I also re-connected the supply to the system switch solenoid.

All was well on re-assembly and switching on, but I still feel the fault could re-occur at anytime!

Despite the CRT being past its best the pictures really are stunning.

Unfortunately but inevitable the fragile VHF tuning knob has succumbed to wear to the point it's no longer possible to change channels. The lesser stressed UHF knob is OK. I doubt very much anyone will have a spare but any advice to help repair would be welcome.


Cheers
Brian
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