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Old 13th Sep 2018, 8:51 pm   #1
Focus Diode
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Default Thorn 960 Update (HMV 2633)

Regulars will be familiar with the saga of this set that's fitted with the original version of the Thorn 960 chassis. This is generally a 950 carved up into portable form with an unusually applied 4.33uf dropper capacitor and its twin heater chain and two stage warm up feature. Generally one set of valves powers the others. It takes about three minutes for it to fully warm up from cold.

The last fault I had was creeping verticals that was eventually traced to the Z1 thermistor that connects the height control from the boost rail to chassis. Thanks to the contributor on the VRAT forum for suggesting that. This was after replacing/checking all other possible causes including substituting the frame output transformer.

Results really are excellent. There's occasional ragged verticals on 405-lines I've yet to find the cause of. It's always suffered from this intermittent problem.

Here are some recent pictures.

Brian.
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Old 13th Sep 2018, 9:06 pm   #2
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Default Re: Thorn 960 Update (HMV 2633)

Nice job Brian, what a cracking looking little set, I'd love one of these just because of the way they look and their quirky heater chain design!

CRT doesn't seem to be that bad either.

We'll done.

Cheers
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Old 14th Sep 2018, 9:18 am   #3
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Default Re: Thorn 960 Update (HMV 2633)

Hi Brian,
I've just had a look at the circuit, very interesting.

Ragged verticals can be caused by faulty flywheel sync diodes, W401 and W402. The more unreliable early type of diodes are fitted in a three legged package looking a bit like a TO92 ahead of its time. A couple of 1N4148 diodes will do. Indeed later thorn sets used a couple of these diodes on a little plastic lead former to help them fit neatly in the 3 holes in the PCB.

Cheers
Andy
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Old 14th Sep 2018, 9:33 am   #4
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Default Re: Thorn 960 Update (HMV 2633)

That is looking good Brian, such nice looking sets.
Some friends of mine had the Ultra version of the 960 MK1 which they bought for the 1966 world cup. It was a really nice set which they used around the house. It was most intriguing with its strange warm up, plain raster appeared in normal warm up time then after about 20 minutes the sound and picture would eventually break through. Apparently nobody could clear the fault and they just lived with it. Once it did warm up it produced great pictures on both 405 line channels BBC1 and Westward TV all around their house just using its telescopic rod aerial which was impressive for this remote part of Cornwall,
I have been after a 960 mk1 set for some time and would love the challenge of getting one of these sets working properly.
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Old 14th Sep 2018, 11:16 am   #5
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Default Re: Thorn 960 Update (HMV 2633)

Ragged verticals can be caused by "Brushing".Poor crt earthing spring etc.
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Old 14th Sep 2018, 9:46 pm   #6
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Default Re: Thorn 960 Update (HMV 2633)

Replacing the valves in the delayed part of the heater chain would cure the very slow warm up. [I repaired many] This would produce a picture within 40 seconds of the raster appearing.
I don't think they liked the DC heater supply and went low resistance. Odd thing was the 'faulty' valves would work fine in a conventional heater chain.

They were modified by fitting a conventional mains dropper and a much deeper card back. It was quite a saga carrying out the mod! Regards, John.
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Old 16th Sep 2018, 8:35 am   #7
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Default Re: Thorn 960 Update (HMV 2633)

Thanks for your comments. I haven't tried replacing the flywheel sync discriminator diodes yet so I'll get round to doing that. Only 405-lines is affected.

Two of these sets have turned up recently but are later versions with the dropper resistor.

I would love to hear of another original version turning up. If one does don't forget to replace the 4.33uf dropper capacitor before thinking of powering up. 4uf high voltage motor capacitors are readily available quite cheaply and a 0.33uf external capacitor can be connected in parallel.

Very nasty things can happen as myself and Dazzlevision found out if the original is not replaced.

https://www.vintage-radio.net/forum/...d.php?t=103917

Brian

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Old 25th Sep 2018, 9:03 am   #8
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Default Re: Thorn 960 Update (HMV 2633)

Purchased another VHF/UHF modulator from eBay retailing at a very reasonable £19.99. I was able to assess the set's performance on 625-lines yesterday.

First signs revealed low gain which I thought was rather odd as I had it tuned to ch64 (Pontop BBC2). Perhaps I should explain:

On first getting the set working they were clearly problems within the UHF tuner with flickering and occasional loss of signals.

The manufacturers used to always say no attempts should be made at servicing them.

Fifty years on with the service department long closed, not another tuner to hand and with nothing to lose.....

One of the tuner vanes appeared dry jointed. A quick touch with the iron should sort that out.... What I didn't bargain for is for the whole thing to become detached. Disaster!

With care I was able to solder it back on, but the alignment against the other two would have been slightly out which can seriously affect performance.

It worked when re- fitted to the set though the lower UHF channels have low gain with near normal performance at the top of the band. The intermittency had thankfully been cured. It would have to do. This is a Group C/D area after all!

Back to the present- it would have helped had I hadn't connected to modulator to the VHF aerial socket!

The off screen image shows the excellent results achieved. I can confirm the occasional ragged verticals appears to be a 405-line only fault. What probably isn't one I noticed, more like a feature (!) is some "breathing", similar and probably is related to EHT regulation. It settles after a few minutes with the set performing like a monitor.

The instruction book advises a few more minutes may be needed for the circuits to settle down.

The picture quality on the 900+ series never fails to impress me on both systems.

Cheers
Brian
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Old 25th Sep 2018, 9:31 am   #9
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Default Re: Thorn 960 Update (HMV 2633)

The 900/950 certainly gives a splendid high contrast picture due partly to the use of the Mullard PFL200 sync separator and video output valve. The gm is 21Ma/v for the video output section.

With the acquisition of Mazda in the early 60's Thorn naturally decided to employ Mazda valves, now manufactured by their own company.

With the arrival of the 1400 in 1967 they decided to employ one of their own family in the video output stage, namely the 6F28 with a gain of 12.5 Ma/v. Still good but almost half that of the PFL200.

Maybe it was not such a odd decision. Like all highly strung brats the PFL200 suffered from annoying oddities.

When operating on 625 the valve would run into grid current causing the picture to rise in brightness over a 15-20 min period. Eventually it would be unviewable with the brightness at minimum. This was due to the different bias requirements required for the two standards. 405 was unaffected.

Another head scratcher was it's tendency to create a 'zizzy' buzz on the intercarrier FM sound on 625. The sync section would fail at odd times.

The 6F28 was a completely reliable valve and I never replaced a single example. Has anybody else?

Noisy UHF tuner operation on 900/1400/1500 is normally caused by the earthing clips on the tuner rotor not doing their job properly. Thorn supplied a modified clip that was silver plated both sides. I must have replaced a bucket full in my time!

Looks good Brian and nice to see the old chassis working again. That was my 'White Heat' period. Regards, John.
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Old 25th Sep 2018, 10:57 pm   #10
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Default Re: Thorn 960 Update (HMV 2633)

No, I don't think I've ever had to replace a 6F28 either.

In my earlier days I often re-valved sets just through age which I now know to be completely unnecessary.

On first getting the 960 there was no line output stage activity when full mains was first applied. I replaced the Mullard PL504 and PY801 which had the older style logo thus probably '60s replacements.

This got the line time base working so I disposed of the old valves.

Only when I later changed the boost capacitor which is wired in this version I noticed it was dry jointed. The action in replacing the valves disturbed it to make contact.

I then felt terribly guilty as the valves I disposed were probably ok, but by then it was too late to retrieve them from the bin.

Back to the 6F28. Works remarkably well as an emergency substitute for the PL802 luminance output valve in the Pye hybrid series if Pin 6 to the valvebase is cut. I could honestly not see any difference in performance when I tried it. Obviously life would be short due to the higher heater voltage.

I had a later blue printed Mazda EF184 that was also marked 6F28 rather than 6F30. It functioned perfectly well in both positions in a 1400. Never tried a 6F30 original in the video output position though.

Cheers,
Brian
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Old 26th Sep 2018, 9:37 am   #11
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Default Re: Thorn 960 Update (HMV 2633)

I never thought of that one Brian! The extra 10V across the heater chain would have made very little difference and the 6F28 would probably have outlived a new PL802. The solid state version lacked sparkle. John.
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Old 26th Sep 2018, 11:01 pm   #12
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Default Re: Thorn 960 Update (HMV 2633)

The solid state one certainly did.

We have to thank our very own Mike Phelan for the tip. I honestly thought, "Pull the other one" until I tried it myself. I remember an EF184 worked well too.

Would've been useful to have known that when the PL802 suddenly became hard to find and very expensive.

Cheers
Brian
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Old 27th Sep 2018, 12:20 am   #13
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Default Re: Thorn 960 Update (HMV 2633)

Hi!

My experiences with the PL802/T were never very gratifying either - the picture always seemed to have a "plasticy" feel to it - I suspect that was down to inevitable self-capacitances between "electrodes" which always would have been inferior to the true thermionic device, and then I think this device had an inherently poorer transient-response than the valve.

I didn't think about trying EF184s then in those days - I wished I had!

Chris Williams
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Old 27th Sep 2018, 9:10 am   #14
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Default Re: Thorn 960 Update (HMV 2633)

If only I'd known at 20 what I know now...

John.
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Old 27th Sep 2018, 12:52 pm   #15
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Default Re: Thorn 960 Update (HMV 2633)

The Mazda 6F28 displayed by the Valve Museum has the appearance of an EF184. I've seen a few late production Thorn AEI Mazda 6F28s which are relabeled EF184. Also PCL83 triode-pentodes sourced from Mullard marketed as Mazda 30PL1.
http://www.r-type.org/exhib/aai0123.htm

DFWB
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Old 27th Sep 2018, 10:09 pm   #16
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Default Re: Thorn 960 Update (HMV 2633)

I see the 6F28 was introduced in 1961. I don't know of any UK sets prior to the Thorn 1400 that employ this valve.

It's worth mentioning the 30FL14 which is applicable to the 900-960 and 1400s seeing this valve is used in both chassis.

Original Mazda types are marked 30FL14 and PCF 808 which suggests either replacement can be used.

Not apparently so I found. There's blue printed late Mazda's that are marked PCF808. I tried one which resulted in very bizarre symptoms, extremely low pitched line whistle with the line hold miles out.

The new valve could've been faulty of course but I've been cautious ever since. I was able to get a Selectronic valve branded replacement marked just 30FL14 which worked ok.

Brian
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Old 27th Sep 2018, 10:29 pm   #17
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Default Re: Thorn 960 Update (HMV 2633)

Hi Brian,
Rediffusion used the 6F28 in the TD20 system frequency unit which was fitted in fully dual standard Mark 8 models. Also the valve was employed in the 1962 Ekco dual standard model T398. The Murphy 625 line adaptor for the later Astra models also employed the 6F28. Formally a Mazda valve adherent Ultra had been taken over by Thorn in 1961 so except possibly in the 625 adaptor used for the convertible model 1984C I can't remember any other sets that used the 6F28.

DFWB

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Old 28th Sep 2018, 8:59 am   #18
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Default Re: Thorn 960 Update (HMV 2633)

After the Mazda valve factory in Rochester was closed by Thorn in the 1970s, Thorn Consumer Electronics supplied 30FL14/PCF808 marked valves that were clearly made by Mullard. They worked in the 900/950/960 and 1400 chassis sets, but the IF gain was noticeably lower than the genuine Mazda made version.
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Old 28th Sep 2018, 10:16 am   #19
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Default Re: Thorn 960 Update (HMV 2633)

The quoted gm of the PL802 is a staggering 40mA/V, so I'm surprised that a 6F28 gives even comparable results in a Pye 693(etc). Maybe it just goes to show how speculative a lot of these numbers are, or that they reflect only a small steep portion of any otherwise fairly flat curve. The RCA 12HG7 (used in the B&O hybrids here) is like a "super PL802" and the latter, along with its solid state counterpart, do not give such good results in these sets.
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