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CTV Thorn 3500 help needed...

 
slidertogrid
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Hi All It's a long time since I posted. Recently I have had a lot more time for my hobby. I need a little help with a 3500 that is giving me grief. I have watched Chris's youtube recordings which has brought back many memories.

I repaired a fair few of these sets back when I was in the trade but I admit to being a little rusty as it must be 40 years since I saw these on a weekly basis. This set has been a bit of a saga. I have had it working despite the power panel looking a bit rough with obvious dried up capacitors and a bodged up smoother on the 30V line.

The set worked with wavy verticals and various other problems, low sound. intermittent colour (later no colour). 

Then yesterday I decided to make a start sorting out the various faults. First I decided to run it up before replacing the grotty electrolytic capacitors in the power panel.

There was a rustle of EHT a very short hiss of sound from the speaker and then the set went dead!
Great! I had a check around with the meter and found the 30V Zener dead short. I replaced that and now the 30V supply is there but no 60V supply. There is HT on the collector of the chopper.
I checked VT602 W607 and VT603 and found VT602 to be open circuit presumably killed by the Zener going short. While I had the panel apart I also replaced the 400 UF C609 as it was very grotty and checked at virtually no capacitance. I also removed the bodged in capacitors which were connected in place of C607 1000uf which was disconnected. I hadn't got a 1000uf of suitable voltage rating so I fitted a 2200 uf which I assume will do?
Still the same. Tube alight, 30V present, 240v video supply present, 350 V on collector of VT604.
I need to check I have drive from the line osc panel next. But at the moment I don't have my 'scope at home. 

Am I on the right track? 

I should mention that the set is a "bitsa" I think it had been used as a panel donor as every panel has faults and a few were unplugged when I got the set. It came with a second stripped out set that had every panel missing apart from the line board. 

I would like to get it going again though as the tube was picking up nicely! 

A picture of the set when it was working before the colour failed and the PSU went dead! 

Any suggestions would be appreciated

thorn 3500

.

Rich.

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Topic starter Posted : 11/03/2022 12:59 pm
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crustytv
(@crustytv)
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Hi Rich, long time no see. 😉  a very warm welcome back to VRAT.

Need some more info really to be able to help. What are the voltages on base and emitter of VT604 (chopper). Have you checked W612? Also check voltages VT605 (chopper drive), VT606 (mono stable) and VT608 (Feedback amp). These will give us an idea what is going on. Suspect it related to drive transistor, mark space/feedback amp.

Reason I ask, a common fault I came up against was low collector volts on VT605, non-existent collector volts on VT608. Which meant the time-constant of the mono-stable was out, which in turn meant VT604 cannot be turning on and off correctly.

Have you seen these guides, they should prove very useful

https://www.radios-tv.co.uk/vintage-electronics-blog-forum/brc-30003500-diagnostics/

I've also done a fair few repair blogs on the 3k PSU, some of which might provide inspiration.

https://www.radios-tv.co.uk/thorn-3000-psu-repair-blog/

Edit:

In my new workshop storeroom, I've a dedicated workstation for 3K PSU repair. Though I've not done one in recent times as I'm in the middle of two 4K sets.

20220311 123634
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Posted : 11/03/2022 1:24 pm
slidertogrid
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@crustytv Hi Chris Many thanks for the reply I will do the checks you suggest and get back to you.

I have found that the R607 the 100R end of the dropper is getting hot. I have 45 volts on one tag and 4 volts on the other. I had a look at your repair blogs the other evening and found them interesting, I will have another look.

I got side tracked looking at the work you are doing to the 4000 sets. Making the panel was incredible you are a brave man!  I saw very few 4000 sets, two or three at the most. I can't remember the faults but if I repaired them they must have been simple! By the time they were in the rental sell off warehouses I knew to avoid them as the reputation of the tubes and thick film modules was well known by then!  

I wish I had a jig like yours! It does make things difficult trying to balance the PSU on a book on top of the chassis making voltage checks whilst trying not to short out the mains transformer or main smoother. 

I should get some more time in the workshop later today or tomorrow morning . Thanks for your offer of help it is much appreciated !

Rich 

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Topic starter Posted : 11/03/2022 2:35 pm
crustytv
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Posted by: @slidertogrid

I have found that the R607 the 100R end of the dropper is getting hot.

That suggests there are excessive volts at the junction of R607/C607 and makes me think VT605 is, indeed, worthy of further investigation and the others I mentioned, VT606, VT608 including W610.

I'm just an amateur though, the chap who you really need is John @jayceebee, he's one of our resident Thorn guru's he pretty much taught me what I know. I'm sure John will pop in, but he's a busy lad still out in the field, only now it's with 'puters'.

Posted by: @slidertogrid

I wish I had a jig like yours! It does make things difficult trying to balance the PSU on a book on top of the chassis making voltage checks whilst trying not to short out the mains transformer or main smoother. 

Absolutely, I have full admiration for the engineers who had to do what you're doing, not easy, and I'd be always fearful of  it slamming down on my hand whilst taking readings. At least on my rig I can, as I termed it, 'Spatch-cock' the PSU module, and have the line board connected, easily take readings with a meter and scope as well.

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Posted : 11/03/2022 3:15 pm
Jayceebee
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Hi Rich, R607 running hot is indicative VT605 the chopper driver conducting heavily. The causes could be many, no feedback from the line osc (very rare), monostable not running or a s/c VT605. VT603,5,6 need to be checked but before you dive in too deep have a look at R620. This 2.7k resistor goes low in value and can be responsible for some odd symptoms, low 60v rail and the complete loss of the rail after running for a while. Eventually it falls so low in value it causes VT605,6 to conduct leaving a dead set with the symptoms as you describe.

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Posted : 11/03/2022 9:08 pm
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slidertogrid
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3500 capacitor bodge

@jayceebee Many thanks for the advice that is very helpful I will have a look tomorrow and report my findings. I am annoyed with myself that I didn't tidy up the bodg

thorn 3500

ed 1000uf and replace the dried up 400uf while the PSU was still working. I do wonder if the bodged capacitors that were wired in place of the 1000uf shorted to the 30V Zener, they were very close.

3500 capacitor bodge

 

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Topic starter Posted : 11/03/2022 11:55 pm
slidertogrid
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Many many thanks for your help Chris and Jayceebee. I have a working Power Panel!

I Checked each Transistor in turn as suggested followed by the diodes. I found R620 had risen to 3.8K so I replaced it. A couple of the chopper transformer soldered joints looked a bit ropey so I re soldered them while I was at it.

Plugging in and testing resulted in a 60V line! So either it was the resistor or drys on the transformer ? Thanks so much for the help and encouragement ! I just have the low sound, one ended focus control, bent verticals and no colour to deal with now!

I may be back for more help!

3500 picture
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Topic starter Posted : 13/03/2022 5:55 pm
slidertogrid
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Many thanks to all who helped with this repair it has been a bit long winded, not helped by the set throwing up 'new' faults along the way. My mate Mick came round yesterday and got the 'scope on it, he found two faulty transistors one that had become disconnected. It was hard to see as it looks as if one of the legs had corroded away where it went into the print. I followed the diagram as he went along muttering about "burst" and chroma amps . Anyway after a lot of tea and two diodes and three transistors we have colour!
Today I decided to evict the nasty 'welded up' RGB panel (see picture!) and repair the much better one that had no blue. That was simple the output Transistor was the problem despite my checker saying it was OK !
I then

3500 empty II

set up the frame panel. I am very pleased with the results. Considering I started with an empty cabinet and chassis and an incomplete varicap set that had been butchered and robbed of its panels the parts have combined to be quite a nice set. There are still a few little jobs like find some better tuner knobs and a speaker as the one from the Baird varicap set is too large to fit the Ultra cabinet. That may present a problem as the speaker is 80 Ohms. so at the moment the one that won't fit is residing in the bottom of the cabinet.
This one has been some fun and has done my memory no end of good!

3500 empty
incomplete Baird Varicap
3500 Colour!
3500 bodged panel

 

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Topic starter Posted : 20/03/2022 7:36 pm
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Jayceebee
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Well done, it's coming along very well indeed. Back in the day I used to buy these from the ex rental outlets dotted around Leeds/Bradford. With a handful of electrolytics, sometimes a regun CRT from Teletronic in Seaham and the known trouble spots attended to like R620. I could sell them on and rarely see them again, happy days.

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Posted : 20/03/2022 9:23 pm
Sundog
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My goodness, seeing those photos and the component references has brought back a flood of memories from the mid 70s. A shop I worked for sold a lot of 3000s and 3500s. A lot came back for repairs but once we got an understanding of the circuitry they were straightforward to repair.

I remember the CRT was mounted upside down, why was that?

Also that the geometry of the 3000 was awful with a pincushion bow at the bottom.

Nice colours though 🙂

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Posted : 31/03/2022 8:46 pm
Sundog
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I forgot to commend slidertogrid, great work!

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Posted : 31/03/2022 8:47 pm
slidertogrid
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@sundog Thanks I needed a bit of guidance to sort the set out it was a state! As you say it soon starts to look familiar again. It must be early 1980's since I worked on these sets. I think that they mounted the tube upside down as the 3000 didn't have raster correction and they thought that the distortion at the bottom wouldn't be noticed! When the 3500 came along the die was set... 

This set started off as two incomplete scrap sets found by a skip at a house clearance place. The chassis, line panel and cabinet came from a mechanical tuner Ultra, the tube, back cover, tripler and most of the panels from a incomplete varicap set with a damaged cabinet. The tuner electrical bit and decoder from a box of 3000 spares which I had in the loft and the tuner mechanical part from a scrap 1591 portable.  A bit of a 'Frankenthorn' but better it lives on like this than go to landfill !

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Topic starter Posted : 31/03/2022 9:38 pm
slidertogrid
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This is the sets as found.

Skip finds
Skip finds
Skip find 3500 s

 

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Topic starter Posted : 31/03/2022 9:47 pm
Jayceebee
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Posted by: @sundog .

I remember the CRT was mounted upside down, why was that?

Also that the geometry of the 3000 was awful with a pincushion bow at the bottom.

The CRT was run in Blue Gun Down mode because the designers decided not to include pincushion correction on the 3000 and the later 8000/8500. Like this the distortion because of the triad arrangement of the CRT gun together with rounder edges of 19", 25" and A55-14X 22" CRTs was less apparent at the bottom of the screen than at the top. I think some survey was done and joe public were apparently less like to complain about distortion at the top rather than the bottom of the screen. Actually the Ultra cabinet styling used came first with the A55-14X and later the square A56-120X, this later version was fitted with the 3500 chassis which did include a transductor for correction.

The biggest mistake with blue gun down was with customers who used LPG portable units like the SuperSer for heating. The huge amounts of water vapour generated by these would condense on the cold glass of the CRT and collect around the final anode with lots of fireworks, the resultant tracking often cracking the glass resulting in a gassed CRT with it's beautiful purple glow from the neck. Why this mode was continued into the 3500 and 8800 I will never understand.

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Posted : 31/03/2022 10:29 pm
Jayceebee
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…final anode connector of course which was now situated at the bottom of the CRT. A number of Japanese sets also used blue gun down I think but their CRTs were manufactured with the final anode connection still at the top like the 17” one used in the 8000

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Posted : 31/03/2022 10:54 pm
Sundog
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Posted by: @jayceebee

..A number of Japanese sets also used blue gun down I think but their CRTs were manufactured with the final anode connection still at the top ...

That's interesting. I wonder if it was just a matter of fitting the gun and shadow mask the other way?

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Posted : 01/04/2022 8:57 pm
Nuvistor
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One thing the Japanese sets had was a decent final anode connector for the CRT compared to many UK sets. I always presumed it was due to the design being used in countries with high humidity and they used the same type for all sets.

I don’t remember any problems with those connectors compared to the UK connectors.

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Posted : 01/04/2022 9:10 pm
Sundog
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@nuvistor Yes the Japanese type were a lot more reliable but could be a bugger to get off sometimes.

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Posted : 02/04/2022 8:13 pm