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Old 4th Aug 2022, 9:07 am   #1
19Seventy7
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Default Thorn 3000 - flywheel test point

Hi all

I’ve been asked to work on a couple of old sets at work to see if I can get them working, a Thorn 1500 and 3000. The Thorn 1500 has a dead IF stage, looking like VT7 is faulty, but I digress

I’ve managed to get the 3000 up and running, it took very little work, someone has been here before in the 80s and the PSU was overhauled - thankfully!

I’m now faced with a very fast line oscillator. I’ve read that to adjust the coil you need to short the “flywheel test point” to chassis, easy enough, if only I knew where the test point was! So here comes the daft question - where is it?!

I’ve had a look on a trader sheet but I can see no mention of the actual point nor can I see anything on the circuit diagrams provided.

Thanks
‘77
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Old 4th Aug 2022, 9:17 am   #2
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Default Re: Thorn 3000 - flywheel test point

Have a look and see if you can find a copy of the original Thorn manual, I have one at home, it should go into more detail in that than the trader sheet.

Regards
Lloyd
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Old 4th Aug 2022, 12:04 pm   #3
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Default Re: Thorn 3000 - flywheel test point

Bottom right hand corner just inside the edge of the chassis. You will find a black test point with a couple of wires soldered to it. Short it to chassis and set the line osc coil for a floating picture.

John.
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Old 4th Aug 2022, 1:12 pm   #4
19Seventy7
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Default Re: Thorn 3000 - flywheel test point

Ah brilliant thank you! Would never ever have guessed that, what an odd arrangement…

Will get to doing that and will let you know how it goes!

Thanks
‘77
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Old 4th Aug 2022, 3:35 pm   #5
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Default Re: Thorn 3000 - flywheel test point

Here's a quick & dirty scan from the manual...
Attached Files
File Type: pdf BRC3K 04-Aug-2022 16-29-24.pdf (1.88 MB, 91 views)
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Old 5th Aug 2022, 5:47 pm   #6
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Default Re: Thorn 3000 - flywheel test point

Oh thank you, I think that'll come in very handy actually!

As promised too, I said I'd let you all know how it goes so here's the before and afters!

Much more work needs doing, brightness and contrast seem to have little effect and the picture sometimes fades to a pure white raster, but I'd say we've got a good starting point. Picture was provided through VHS too which I doubt would work too well, so I'm going to provide it with a better signal tomorrow.

Thanks all
'77
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Old 5th Aug 2022, 6:29 pm   #7
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Default Re: Thorn 3000 - flywheel test point

That looks pretty good! Tubes nice and bright by the look of it, which is always a bonus!

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Lloyd
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Old 5th Aug 2022, 7:09 pm   #8
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Default Re: Thorn 3000 - flywheel test point

The tube is painfully bright! It's fitted with a Thorn New-Life tube which is handy. Though actually that's one of the faults I have to clear, it's so bright and neither the brightness or contrast control seems to have an effect! It seems C901 or C902 can be leaky or the beam limiter/preset brightness could be incorrectly adjusted, which wouldn't be surprising as I found a few things to be one ended for no reason, for some odd reason

Thanks
'77

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Old 6th Aug 2022, 6:28 am   #9
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Default Re: Thorn 3000 - flywheel test point

Quote:
Originally Posted by 19Seventy7 View Post
Hi all

. The Thorn 1500 has a dead IF stage, looking like VT7 is faulty, but I digress.
Its a long time ago, but VT7 rings a bell

As for the uncontrollable brilliance etc, there's a set up procedure involving the set white switch and at least two presets, I think one is in the Frame Board, cant remember now once done they can produce a half decent picture, not sure if a Thorn New Life CRT is much of a positive They had a poor reputation back in the day.
As for the loss of Video, check the Decoder PCB, if it is fitted with the Thick Film Video out put module, they were notorious for intermittent faults, such as blank white raster, smearing etc etc.
Ken G6HZG.
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Old 6th Aug 2022, 8:46 am   #10
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Default Re: Thorn 3000 - flywheel test point

what exactly was the reason for shorting out the sync? was it find the best position for the line hold? i used to merely guess the optimum position by having equal overlap of the picture in the "screen area", I was usually fairly accurate, never did a return call for line hold problems, ta anyone.
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Old 6th Aug 2022, 9:02 am   #11
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Default Re: Thorn 3000 - flywheel test point

The flywheel sync tended to 'pull in' sync when the frequency was a little off. Changing channels or just breaking the signal would result in line hold loss. [The notorious PYE V14]
Removing the sync allows the oscillator to be set at the correct frequency without the influence of the flywheel action.
It was certainly not uncommon. The early PYE FV2C from 1951 had a toggle switch on the back to remove the sync when correctly setting the oscillator.

I had a customer with one of these receivers when I was first in the trade. It was a 16" metal tube model, Mullard MW41-1. To think it was 15 years old then, past it's sell by date in 1964. It had a PYE box converter on it for the ITA. It worked really well! John.
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Old 6th Aug 2022, 9:12 am   #12
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Default Re: Thorn 3000 - flywheel test point

sorry I've butted in on this post, was this the reason for the Thorn 2000 being able to disable the sync by pushing the line hold in, sorry, I have dementia now, low on logic.
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Old 6th Aug 2022, 9:16 am   #13
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Default Re: Thorn 3000 - flywheel test point

That's correct, you had to keep the 625 pushed in when you set the 405 control too.

Flywheel sync did not guarantee picture would be centred in the scan. Picture centre
magnets could be used on mono sets, but colour sets may have a horizontal shift.
The line oscillator in the 3000 was the same as used in the 1590 portable.
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Old 6th Aug 2022, 9:23 am   #14
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Default Re: Thorn 3000 - flywheel test point

Yes of course the Ferguson push and turn method. Used on most of their flywheel sync models from the 305T to the final one the 505T. J.
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Old 7th Aug 2022, 9:36 pm   #15
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Default Re: Thorn 3000 - flywheel test point

Quote:
Originally Posted by its ur aerial View Post
Quote:
Originally Posted by 19Seventy7 View Post
Hi all

. The Thorn 1500 has a dead IF stage, looking like VT7 is faulty, but I digress.
Its a long time ago, but VT7 rings a bell

As for the uncontrollable brilliance etc, there's a set up procedure involving the set white switch and at least two presets, I think one is in the Frame Board, cant remember now once done they can produce a half decent picture, not sure if a Thorn New Life CRT is much of a positive They had a poor reputation back in the day.
As for the loss of Video, check the Decoder PCB, if it is fitted with the Thick Film Video out put module, they were notorious for intermittent faults, such as blank white raster, smearing etc etc.
Ken G6HZG.
I have to admit I cheated a little and googled the fault, and found that there was a fairly extensive thread on the same fault, I did all the same checks on "my" set and found VT7 to be well out of spec.

I think you're dead on with the thick film module too, it will just fade into a pure white raster and the video is pretty smeary, but I originally put that down to being from VHS, when it did the same from DVD I realised it was the TV at fault, I've got some 12k and 56k resistors on order to replace that with. Being it'll be a somewhat daily runner I think it'd be good practice anyway just for the sake of reliability.

Thanks for the heads-up on that one, as I don't think I'd have suspected the module to cause that

Thanks
'77
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Old 8th Aug 2022, 9:54 am   #16
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Default Re: Thorn 3000 - flywheel test point

Details below on how to convert the thick film unit back to discrete resistors.

https://www.radios-tv.co.uk/vintage-...lmvideo-panel/

John.
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Old 11th Aug 2022, 8:38 pm   #17
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Default Re: Thorn 3000 - flywheel test point

Oh sorry I completely missed your post somehow!

That'll definitely be of use, if only I was of use and remembered to order the resistors themselves!

Thanks for linking it
'77
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