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Old 6th Dec 2022, 2:36 pm   #1
thermionic
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Default Defiant 17” TR1756T.

This set was in the auction at Royal Wootton Bassett this Sunday, and was described as ‘looking ugly’! The reason I bought it was because it looked a bit different, and as a change to my usual habit of just putting it on top of another TV that patiently awaits my attention, I put it straight on the bench!

It’s a 1955 model fitted with a Plessey chassis as was the norm for Co-oP sets of the time. Plessey we’re heavily into using Mazda Valves and tubes, and this set is no exception.....except for a couple of Mullard ECL80’s used as frame and sound outputs.

The CRT installed in my set is a CRM172, which I suspect is a replacement as the chassis mounted Visconol has seen cables in the past but is now disconnected. The CRM171 (which was supposed to be fitted) did not have a ‘dag coating so required this capacitor. I wonder if the original tube failed within the guarantee period?

Inspection reveals little, if any work carried out to the chassis. It all looks nice and dry & dusty - just as we like ‘em. The fuse fairy has been at work, however. Notice how both fuses have been ‘repaired’! I wonder what caused these to fail in the first place?


I wonder if there is any life in this CRM172..... let’s see!


Cheers.

SimonT.
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Old 6th Dec 2022, 9:37 pm   #2
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Default Re: Defiant 17” TR1756T.

Excellent receivers.
The split heater chains employ 0.1amp and 0.2amp heater valves. The two chains converge to supply the tuner valves, the two ECL80 triode-pentodes and the CRT heater.
Similar chassis with Emitron valves and CRT was fitted in some Marconi TV sets.

DFWB.
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Old 7th Dec 2022, 9:08 am   #3
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Default Re: Defiant 17” TR1756T.

Good old Plessey! As David says very good receivers. Plenty of waxies to change but please don't go mad and replace them all at the same time. Power supply, line output stage, frame output stage then I.F. sound and tuner etc. Just a suggestion to save mistakes and confusion LOPTS are reliable. Good luck. John.
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Old 7th Dec 2022, 4:07 pm   #4
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Default Re: Defiant 17” TR1756T.

Quote:
Originally Posted by Heatercathodeshort View Post
......Plenty of waxies to change but please don't go mad and replace them all at the same time........ Good luck. John.
No chance of that happening here, John, where’s the fun in that?! 😉.

Had a little time last night, and managed to get signs of life. The frame collapse image is much brighter than it actually was, - before you tell me off! 😬. I did get caught out on two things. First, whilst I could see the heaters glowing, there was no line stage activity....Hmm. I ‘scoped the grid of the line output valve where I found a healthy waveform. I soon discovered why,- the 200mA heater chain was dead, soon traced to R109 (375 Ohm) being open circuit on the dropper.

There was also no HT at the 500mA HT fuse, caused by R110 (30 Ohm) being open circuit. Checking the boost capacitor (C93, 0.5uF) I found this to be extremely leaky, and was replaced before further ado.

The other thing that caught me out ( apart from one of the heater chains) was the position of the brightness control which I assumed to be part of the dual concentric pot on the front of the set. Oh no! It’s tucked around the back, that being the Contrast control on the front!

Next, it’s happy hunting in the frame stage.

Stay tuned!

Cheers. SimonT.
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Old 7th Dec 2022, 8:49 pm   #5
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Default Re: Defiant 17” TR1756T.

Nice to follow the resurrection of an old telly on these dark winter nights. I'm staying tuned!
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Old 8th Dec 2022, 9:53 am   #6
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Default Re: Defiant 17” TR1756T.

Some more time, and some more progress.

After getting some sort of display on the CRT, (albeit frame collapse) I was keen to get a full raster to evaluate the condition of the Mazda CRT. After establishing that the main HT Rail voltage was where it should be, I checked that I had a reasonable voltage on the two anodes of the frame oscillator & output valve (ECL80 - V11) proving that, at least, the primary windings of both the frame output and blocking transformers were intact.

The Avo was then touched onto pin 2 of the triode section where a negative voltage was read, indicating that it was oscillating. A quick prod with the scope showed a healthy sawtooth here at around the right frequency but next to nothing on the grid of the pentode (output) section. Now, let’s be honest, we know that in order to get a full, linear scan the capacitors in the frame stage need to be 100%, don’t we?

In this chassis there are 4 main culprits,- C’s 63,64,65 & 66. Note that the cathodes in this circuit are returned directly to chassis, grid bias being provided as part of the frame hold arrangement, so no cathode bypass electrolytic to change.

My first suspect was C65. Yes, It was leaky but replacing this did little, if anything to increase the scan. My second suspect, C63, again very leaky produced a near full scan on its replacement. Replacement of the remaining 2 capacitors ensured I was able to set up a reasonably linear frame scan.

The pictures posted below, once again, appear brighter than they actually are, as increasing the brightness control to give a normal viewing picture makes the raster balloon and fade out. Checking the EHT ( which should be 12kV) I have 9.5kV with no raster, 8 kV just before it balloons then dropping to around 5kV. The LOPT overwind remains cool to the touch, so it doesn’t seem to be saturation, hopefully it will just be a tired U25 EHT rectifier.

Off now to search through the TV valve boxes..........

SimonT.
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Old 8th Dec 2022, 11:06 am   #7
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Default Re: Defiant 17” TR1756T.

Remarkable! I like your sensible no no nonsense approach. Like Ekco you can always get a Plessey working. Regards, John.
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Old 8th Dec 2022, 1:12 pm   #8
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Default Re: Defiant 17” TR1756T.

The frame timebase circuit resembles the same in the Bush TV22.
Output pentode receives its grid bias from the DC component in the negative going sawtooth waveform. The linearity control adjusts the amount of bias supplied to the pentode.
Frame sync is simple and effective. C61 and R67 acts as a frame sync pulse differentiator. GD2 remains in conduction and is only cut-off on the negative edge of the differentiated frame sync pulse. At least that's my take on how the sync circuit works.
Except for the frame sync circuit the Regentone 143T is similar, same valves types used. 17". Regentone 173T has Mullard valves.

DFWB.
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Old 11th Dec 2022, 5:43 pm   #9
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Default Re: Defiant 17” TR1756T.

Well, after tracking down a NOS U25 valve in the chaos of my valve boxes, I was hoping this would cure my ballooning picture.

Guess what? No, it didn’t! In fact it made no difference at all. To save my sanity, I removed the U25 again and fitted a series pair of BY8410, 10kV diodes to provide the EHT.

There was a very marginal improvement, but nothing to get excited about. As previously mentioned, the LOPT windings were not getting hot, which would normally suggest dampness causing inefficiency and saturation, but I decided to pass a DC current through the overwind for 24 hours to see if this made any difference in any case. A current of 100mA produced a warm to the touch overwind, so I left it alone and started to de wax some of the removed capacitors ready for re stuffing.

Whilst I was working in the area of the Lopty, I discovered that both the line linearity and width variable inductors were jammed solid. These live beneath the transformer and it could be seen that the wax impregnation of the lopty had migrated downwards over the years and had settled into both of these inductors. I resigned myself, therefore that this would also need to be attended to before I could check to see if my ‘ drying out’ exercise had bore fruit.


All great fun!

Cheers. SimonT.
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Old 12th Dec 2022, 5:22 pm   #10
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Default Re: Defiant 17” TR1756T.

That certainly looks like LOPT dampness. Definitely worth giving it a good dry out. Another waxie on the base to change but I doubt if it will be the problem. I agree that the overwind usually becomes quite warm when impregnated with dampness but you never know with these things. I presume it starts off OK then fades after 20 mins or so. John.
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Old 13th Dec 2022, 7:51 am   #11
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Default Re: Defiant 17” TR1756T.

Hi John.

No, it remains the same from switch on. It doesn’t get any better or worse.

It happens almost at reasonable viewing brightness, just advancing the brightness control around this sweet spot and the raster balloons and EHT plummets!

I monitored the HT during this happening and it only varies a few Volts. The cathode voltage of the 20L4 line output valve is approximately 2 Volts lower than the spec, and again shows no more than 0.5 Volt variation.

This evening I’ll re assemble the Lopty and see if there is any difference. 🤞


Cheers.

SimonT.
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Old 13th Dec 2022, 8:56 am   #12
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Default Re: Defiant 17” TR1756T.

I hope you get it back together OK. Were there any gapping papers between the cores? It may be a CASLAM core made by Plessey. I'm not 100% sure of the core construction.
The overwind looks OK from your pictures but you can never tell. They were a very reliable transformer. Why did you take it apart? Recheck the 20P4 lop valve, the screen feed resistor to pin 4 and any components connected to pin 8 if it is not earthed of course! John.

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Old 14th Dec 2022, 8:01 pm   #13
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Default Re: Defiant 17” TR1756T.

A busy night last night......


After disassembly of the Line Output Transformer I put the windings in my ‘slow cooker, ( I’ll explain later........) to soften and remove the wax ‘tyre’ that had accumulated on its base, some of this wax causing the width and line linearity controls to jam up. ( this was why I took it apart, John).

I also hoped that this heating, in addition to me passing a DC current through the overwind, would solve my sagging EHT and ballooning picture problem. After careful reassembly and re instatement onto the chassis, it was time to see if I had improved the situation or made it worse!

Well there’s good & bad news. The Lopty still worked, but the fault was exactly as before! The only positive that came out of this exercise was that both the width & line lin controls now worked.

Time for a think..... I had jumped to the conclusion that the lopty was not capable of supplying enough current to the final anode, caused by either a soft U25 rectifier valve or a transformer in trouble. Both appeared innocent. I checked the voltages on the screen and cathode of the (20P4) line output valve...... pretty close to the manual. I then swapped both the 20P4 and the U301 boost rectifier valves with known good valves.....just in case......no change!

So......if the EHT was sagging under load, then what if it was being asked for more current that it was designed to supply? What would cause this? I checked the tube base voltages. Again, all close to those specified with the exception of the cathode. This was given as 110 Volts, but mine was 90 Volts. Again, not too much of a difference but worthy of investigation.

Well.....the culprit was soon found! C98, (see below) a 0.1uF waxy was very leaky. It measured only 40 KOhms on my Avo, and with this replaced, this fault was cured! It just goes to show that sometimes you need to think a little differently than the norm. I still learn something new from all this stuff.

There may be some out there shouting at their screens that if I had replaced all the wax capacitors in the beginning, I would not have had to go through all this saga. Maybe not, but I’ve had a puzzle to solve, fun doing it and also learnt some more.😉

There are still issues to remedy. There is an occasional line twitch & mild top tearing that I guess will point me in the direction of the sync separator, and I’ve yet to test the audio...

So, still more fun in store!

Cheers. SimonT.
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Old 2nd Jan 2023, 9:21 pm   #14
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Default Re: Defiant 17” TR1756T.

Over the Christmas period I had a chance to do some more on the Defiant.

After achieving a good picture, it was time to see if the sound channel required much work. I had already clipped out C88 during my initial pre power up checks. I didn’t want the audio output stage drawing excessive current with the possibility of damaging the audio output transformer and ECL80 valve during my power supply and timebase works.

As it happened just 3, interstage coupling capacitors required replacement to get good clear sound. The 2 associated electrolytic capacitors tested fine, and were left alone.

The underside of the chassis shows the ‘yellow perils’ tacked in for testing purposes, and I’ll now start to replace these with the re stuffed original capacitors.


SimonT.
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Old 8th Jan 2023, 7:24 pm   #15
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Default Re: Defiant 17” TR1756T.

Today, I’ve completed the re stuffing of the old wax capacitors. The pictures below show how it looked originally under chassis, and with the re stuffed capacitors. Hopefully it doesn’t look too different!


I have two issues left to resolve. 1, the volume control (which is part of the dual concentric control) is intermittent and hasn’t responded to switch cleaner, and 2, the line oscillator tends to drift after the set has been on for 15 - 20 minutes. I think this is probably due to the 10C2 valve so I’ll hunt for a replacement.

Interestingly this chassis only has a complement of 5 electrolytic capacitors. 3 of these are sections in the main reservoir & smoothing can, so there are only 2 more. These electrolytics have all proven serviceable and all were manufactured by BEC ( British Electrolytic Capacitors).

Also interesting is the fact that this set does not utilise a HT choke, which most contemporary sets did. Instead, the reservoir and smoothing block consists of 200uF +250uF +250uF sections which are higher than usually found in sets of this type, along with a 30 Ohm & 60 Ohm wirewound resistors.
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Old 9th Jan 2023, 12:52 am   #16
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Default Re: Defiant 17” TR1756T.

Quote:
Originally Posted by thermionic View Post
Today, I’ve completed the re stuffing of the old wax capacitors. The pictures below show how it looked originally under chassis, and with the re stuffed capacitors. Hopefully it doesn’t look too different!
Hi Simon,

Well, if you hadn't said anything. I would never have known! I'm very impressed with that!!

Cheers
Nick
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Old 25th Feb 2023, 7:58 pm   #17
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Default Re: Defiant 17” TR1756T.

[/QUOTE]

Hi Simon,

Well, if you hadn't said anything. I would never have known! I'm very impressed with that!!

Cheers
Nick[/QUOTE]

Thanks for your kind words, Nick... all encouragement helps!

Anyway, it’s time I updated this thread and put it to bed.

My drifting line oscillator problem was indeed rectified on the fitting of a NOS 10C2 valve......phew!

The concentric volume & contrast control was another matter, however. Whilst the contrast control worked smoothly after cleaning, the resistance track for the volume was in very poor condition. A lot of the carbon track had simply come away from the substrate which I was surprised to see.

A 1Meg logarithmic track was required, so after a lot of searching through my junk ( treasure chest) I found an ‘AB Metals’ switched pot where I was able to transplant the resistance track into my concentric control without too much trouble.


The set performs very well and is stable throughout a sessions viewing. I feel very lucky to have a very decent CRM172 and am counting my blessings. This good luck helps make amends for some other sets I have with Mazda airbags!

Cheers all for your interest & comments.

SimonT..
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Old 25th Feb 2023, 8:55 pm   #18
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Default Re: Defiant 17” TR1756T.

A truly marvellous job. The under chassis looks just as it did originally. A proper restoration!

Rich
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Old 26th Feb 2023, 6:37 pm   #19
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Default Re: Defiant 17” TR1756T.

Excellent results. That's how I remember those sets. Very popular model in North-East England.

DFWB.
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