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Old 17th May 2018, 12:15 am   #1
1100 man
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Default Ultra 6823 (Thorn 1500 chassis)

As I get older, I find nostalgia plays a bigger part in choosing the sets I want to play with. I seem to be on a mission to re-discover sets that I first encountered when I was a kid in the early '80's.
The Thorn 1500 was something that came my way fairly often. I was given several, repaired others for use with ZX81's at school, and often found the remnants of them dumped in ditches and farm gateways near our house.

TV's then were free. People, knowing I had an interest, just gave them to me, much to the annoyance of my mother. Even now, I can't accept that they are worth the price that Ebay would have us believe! So the sets that I am now re-acquiring have to be attractively priced- i.e. free, or very near it!

I had been on the lookout for a 1500 which met these criteria for a while, when Neil29 offered one which I was lucky enough to secure.

Thanks to the generosity of Neil and Colourstar, it found it's way from Wales via Sollihull, to the NVCF where I collected it on Sunday.

Back at base, on the workbench with the back off, I gave it a quick check over. I was surprised to see a green pcb:- I remembered them as red but maybe that was the 1400?
There were a couple of tired looking caps with crusty bottoms and the spring off resistor feeding the line stage had sprung off! As I like to do, I hooked the tube up to the tube tester just to see what to expect picture wise. Considering the B&K tends to read low, the emission showed well over into the green, so that was encouraging.

So I plugged it in and switched on. Valve heaters lit and eventually the frame timebase could be heard purring away. Nothing much else though, no line whistle. I'd forgotten this was 625 and that I'm not 20 any more! I really must remember that I can't hear 625 line whistle these days!!
Looking round the front, showed a bright raster. Connecting a signal showed a very bright picture. The brightness control didn't do anything.

Amazingly, I still have the very tatty photocopy of the circuit that I last used in about 1985! Referring to this showed that the wiper of the brightness control (hence the cathode of the CRT) was at 2V, hence the very bright screen. Further investigation revealed that instead of the 470K control, someone had fitted a 3k9 instead! The same had happened to the volume control as well.
Fitting a temporary rotary control of the correct value cured the fault.

As we are now back in the 1980's, my test card of choice is the channel 4 one. This has always been my favourite and I still maintain that it's the best one for setting picture geometry and linearity. I much prefer it to test card 'F'. I have a VHS tape recorded in the mid '80's on my 3V16, which I have used many, many times for this purpose.

After a few tweaks and a new width pot which was open circuit, the picture was excellent. The tube is fantastic and displays a sharp contrasty picture- even the focus at the edges is good!! Thankfully, it's a Mullard tube.

Unsurprisingly, as it uses full AC coupling to the cathode, the black level performance is awful. The background brightness is all over the place! This will definitely be a candidate for a black level clamp mod.

Also, it still has all it's Lockfit transistors present! I know people seem to condemn these on sight but I must confess, I've always found them to be very reliable;- I really can't remember ever having a failure!

So all in all, this is going to be a cracking set, especially once the black level clamp has been sorted. Cheers neil

All the best
Nick
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Old 17th May 2018, 11:28 pm   #2
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Here are a couple of screen shots which, due to my poor camera, don't do the picture justice! Don't forget that the Channel 4 test card was recorded on a 3V16 in the mid '80's.
Very conveniently, the flywheel sync handles the signal from the VCR fine without modification.
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Last edited by 1100 man; 17th May 2018 at 11:35 pm. Reason: extra text added
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Old 18th May 2018, 12:06 am   #3
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Great chassis the 1500, a workhorse of the rental market. The fact that it has a green coloured PCB is evidence of it being late schedule set, most production was red. With a good CRT and the occasional repair they had long lives, the problem with the very high mileage sets was cooking and eventual conductivity of the PCB around the PCL805. I never saw one fitted but Thorn produced a transistorised field stage add on pcb to keep them going (here was also a board produced fitted with a TBA120 to finally put an end to intercarrier buzz on the 1400).
In my experience the only lockfits to give trouble in these were VT7 and VT8, both BF197s the BF196 fitted in the AGC controlled IF stages were much more reliable. At one time BF197s became scarce for some reason and BF199s were supplied as a replacement, a definite drop in performance could be noticed.

Yes the black level did wander around a bit but very few people complained, as an apprentice at a Bush/Murphy dealer believe it or not more complained about the screen going almost totally black during the gaps in the adverts on the legendary A640 sets, some viewed it as a fault not a feature. Just goes to show how fickle the general public could be.

John.
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Old 18th May 2018, 12:21 am   #4
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Default Re: Ultra 6823 (Thorn 1500 chassis)

I'm fairly sure it was me who replaced the front controls with new ones as the old ones had only half of the two prongs?? on them. and I was sure that they were the same value as the originals but on occasion my mind does wander and I might have thought they were the same for some reason. still no harm done and its good to see it working well.


Cheers
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Old 18th May 2018, 9:03 am   #5
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Default Re: Ultra 6823 (Thorn 1500 chassis)

That's a nice example of a good 1500 you have there. Just a word of caution if this is an early version of the 1500 there was a modification to the width chain with a change in value of the width control and its feed resistor. The modification was to reduce the current through the width control. This was the same issue that we were familiar with on the 1400 chassis.
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Old 19th May 2018, 10:12 am   #6
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Simon's right but this does appear to be a late model with the green PCB so presumably the modification will already have been done.

The control was originally a 1M pre-set with the feed resistor being 330K. They were changed to 2M2 and 680K respectively.

Pre-sets themselves are worth a mention. They seem to be hard to obtain nowadays unless they're the plastic bodied black and white types. They never seem to last very long in older valved equipment unless I've been unlucky.

A word about the ETP-1; it was rightly criticised for lacking a circle. The E-W castellations were also too wide when compared to TC F and the Philips PM5544.

Nevertheless the image on 1100 Man's set is well done!
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Old 19th May 2018, 11:48 am   #7
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Default Re: Ultra 6823 (Thorn 1500 chassis)

The 1500 is an ideal candidate for a proper black level clamp. Following the rules of using the tech of the times when the set was designed we can use an emitter follower stage as the video amplifier to CRT cathode buffer.
I believe that even the schools version of the 1500 didn't have this refinement.

DFWB.
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Old 19th May 2018, 11:44 pm   #8
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Hi Folks,
Many thanks for the comments!

Jayceebee.
Yet another interesting comment suggesting that customers didn't like black level clamping! That has cropped up several times from people in the trade at the time. Maybe, then, it was more of an active decision by manufacturers not to clamp the black level as they found customers didn't like it?

When was the 1500 released and when did it cease production? Thorn were experienced with all transistor TV's, and so on the face of it, continuing to use valves in the output stages was a retrograde step.
However, they must have realised that it made for a simpler design and greater reliability. After all, I don't think transistor LOP stages were ever really more reliable than valve ones- right to the end of CRT TV's, I was still changing many line output transistors!

Neil29
I can easily forgive the wrong value brightness & volume controls as you have given me a set with such a fantastic tube!! I didn't expect it to be so sharp!

Hybrid tellies
My circuit shows a 1meg width control and a 330K. My set had a 2m2 and a 680K. Not that the 2m2 had any track left on it!! Now replaced with a black and white preset type which is all I had to hand.

Focus Diode
I have probably spent more time looking at the Channel 4 test card (ETP-1 if you insist!) over the years than anything else on a CRT!
I was at school in the early '80's, just when the ZX81 and then the Spectrum were all the rage. Parents would donate old scrap TV's to the school and I appropriated the job of fixing them. I commandeered the side bench in the only classroom in the science block which had a TV aerial and spent all my lunch hours and after school there fixing TV's.

It's hard to believe now that they would allow a 14 year old to play with 'live chassis' valve TV's totally unsupervised but nobody seemed to mind- they were just glad to have working TV's for the computers!
At the time, 1981/82, Channel 4 were getting started and that test card was frequently available at lunch times. After school was annoying as they replaced a perfectly good test card with 'Countdown'
So that's why I have such fondness for it and have used it ever since! I've always found it better for adjusting frame linearity as the grid covers the whole screen whereas the circle on TC-F only covers the middle bit!

Fernseh
Don't worry, after my education on DC restoration, black level clamping is a must! I'll have to order some suitable transistors for the emitter follower though- BU508D's aren't really the thing

This is already turning into a rewarding project!

All the best
Nick
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Old 20th May 2018, 7:53 am   #9
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Quote:
Originally Posted by 1100 man View Post
Yet another interesting comment suggesting that customers didn't like black level clamping! That has cropped up several times from people in the trade at the time. Maybe, then, it was more of an active decision by manufacturers not to clamp the black level as they found customers didn't like it?

When was the 1500 released and when did it cease production? Thorn were experienced with all transistor TV's, and so on the face of it, continuing to use valves in the output stages was a retrograde step.
The black level clamp did cause some problems but I didnít have any sets returned because of it and when CTV came along those were clamped. The 1500 chassis like most of Thorns output was made for rental and keeping costs down was the order of the day. Black level clamping was not a necessity.

I think the 1500 chassis was introduced for the start of the 3 stations transmitting on UHF 625 in November 1969 or thereabouts. We didnít sell many but I think they were still in production in the middle 70ís, not many other large screen BW sets available then. They had poorer quality CRTís in some of the late sets, same with the valves, not a criticism of Thorn, had to get what was available.

I think they kept using the design because it was proven and low volume any major BW redesign would take resources from other R&D so I can understand them running this chassis as long as possible.

I didnít see many 1500 sets but like all Thorn large screen BW sets they were easy to fix.
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Old 20th May 2018, 10:04 pm   #10
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Default Re: Ultra 6823 (Thorn 1500 chassis)

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Originally Posted by Nuvistor View Post
They had poorer quality CRT’s in some of the late sets, same with the valves, not a criticism of Thorn, had to get what was available.

I think they kept using the design because it was proven and low volume any major BW redesign would take resources from other R&D so I can understand them running this chassis as long as possible.

I didn’t see many 1500 sets but like all Thorn large screen BW sets they were easy to fix.
Hi Frank,
I somehow suspect that the total production volume of 1500's would be pretty large. I also suspect they were very reliable and so were not regular visitors to service departments. Would be interesting to know what the numbers were though.
As it would seem that mine is a late production due to the green pcb, I was surprised that it had a Mullard tube. I was half expecting it to have a Unitra one !
The PL504 was branded 'Lloyds', which is not one I have come across before. It was very lazy giving a narrow picture until it had warmed up for 5 mins!

All the best
Nick

Last edited by 1100 man; 20th May 2018 at 10:07 pm. Reason: more text added
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Old 20th May 2018, 11:54 pm   #11
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Perhaps I phrased it poorly, yes there were lots of 1500 sets made but the point I was trying to make was that when the early to middle 70's came along the push was for colour sets from the public and not a great demand for large screen BW sets. It would not have been a good use of R&D to design a new chassis for a shrinking market.

It was the Unitra CRT's I was think of, rough looking phosphor and poor edge focus.
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Old 22nd May 2018, 2:18 am   #12
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Default Re: Ultra 6823 (Thorn 1500 chassis)

My 1500 was manufactured in 1977, according to the date codes inside.
I was quite surprised as I didn't think they were being made that late.

Michael.
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Old 22nd May 2018, 7:48 am   #13
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Probably one of the few large screen BW sets being produced at that time.
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Old 22nd May 2018, 8:43 am   #14
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Default Re: Ultra 6823 (Thorn 1500 chassis)

I started to sell the Marconi 20" 4800 in late 1969 just before the change to 625 UHF.
The early models had a few stock faults, tuner gang earthing strips and those nasty circular black electrolytics.

The last models I sold must have been around 1977 with the green printed panel. They were great receivers and regardless of fault could be repaired in less than 15 mins. You could even change the tube in less than 5 mins!

With a good tube the picture is excellent and well exceeds what the customer expected.

Customers hated the blacked out screen, correct of course with black level clamp. It gave the appearance of a breakdown and it took a long time to convince customers this was correct particularly with the fantastic RBM A640.

I will always have happy memories of the Thorn 1500, in fact of all the Thorn television chassis. [ OK except the 1600 and 4000] Only 2 difficult chassis in over 30 years production of millions of units. John.
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Old 22nd May 2018, 8:46 am   #15
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Default Re: Ultra 6823 (Thorn 1500 chassis)

I remember a rental (Baird) model in 1977. Usual 3 control 4 push button layout yet the latter were actually light action switches, but the same size as conventional tuner buttons.

There was a door at the back with 4 pre set tuning controls like those seen on some colour models (usually had six of course).

Never seen another example of this varicap tuner version.
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Old 22nd May 2018, 9:22 am   #16
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Thanks for the interesting comments. 1969 to 1977 was a long production run. There can't have been many other chassis that endured for that long. No wonder by the end the technology was a bit dated! They were probably wise to leave it as it was with it's valves as I'm sure it would have been a reliable chassis.
Is there any way to date a particular chassis?

All the best
Nick
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Old 22nd May 2018, 10:13 am   #17
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Quote:
Originally Posted by 1100 man View Post
Thanks for the interesting comments. 1969 to 1977 was a long production run. Is there any way to date a particular chassis? Nick
Many electrolytic will have a date code, such as 09 71 (week 09, in 1971). The cabinet may also be stamped internally with its date of manufacture. The later yellow coloured axial Plessey/TCC paper dielectric capacitors and their successors (in grey plastic) will also have a date code, as per the above.

I don't think Thorn applied a date code to the chassis, just a Schedule letter, to indicate production modifications.
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Old 22nd May 2018, 11:41 am   #18
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Default Re: Ultra 6823 (Thorn 1500 chassis)

The 1500 was launched just as I was entering the trade as a 16 year-old apprentice. I was impressed by the neat design, both inside and out. One thing that troubled me a little was the slightly 'soft' video compared to contemporary sets that used valve video output stages. My colleagues would say things like 'it hasn't got that sparkle that you get with valves' etc, but it may just have been prejudice against solid-state.
Other than that, they were great sets and we had lots of them out on rent. The handful of stock faults were easily dealt with.
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Old 22nd May 2018, 11:58 am   #19
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Default Re: Ultra 6823 (Thorn 1500 chassis)

As to the comments above concerning lack of black level clamping. My father who bought a Bush TV166u in 1968 and who loved a good war film, often complained he could not make out what was going on on dark scenes. Consequently we always had the brightness set slightly higher than it should have been. He prefered the picture on my grand dads Ferguson 3810. From this I soon learned so set a picture up to what the customer wanted instead of what I thought was correct.

Alan.
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Old 22nd May 2018, 1:58 pm   #20
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Default Re: Ultra 6823 (Thorn 1500 chassis)

Quote:
Originally Posted by 1100 man View Post
It's hard to believe now that they would allow a 14 year old to play with 'live chassis' valve TV's totally unsupervised but nobody seemed to mind
Yes, looking back I'm surprised I was allowed to play with TVs with (potentially) live chassis and 15Kv EHT when I was 14.
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