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Thorn 8000A troubles.

 
Tazman1966
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Hoping that there's a few people around as it seems to have gone a little quiet in this colour tv section of late ☹️

Anyway...one of the sets that gets used on rotation in the kitchen/diner in our house is this Baird 8770 which is fitted with the Thorn 8000A which uses a upgraded signals/chroma/video panel designated PC856. This was one of the latest versions and had all the improved performance mods. The set is also fitted with a varicap tuner and a six button selector bank.

The problem is that there is an intermittent fault which can only be described as a picture disturbance somewhat akin to the aerial plug being wiggled around. It does not always do it and is highly intermittent in nature. The fault can be cleared temporarily by very quickly switching on and off. 

I had some time today to sling the set on the bench and see what could be done. The circuit in the signals path is quite different to the original panel but applying heat and freezer to the IF amplifier transistors (all Lockfit types) and their nearby components could not tempt the fault to show itself, surprise, surprise! Moving across to IC1 a TCA270S which contains the functions of IF AGC, vision detector and various other associated duties and a blast with the hairdryer caused to fault to happen but not for very long. Alternate sprays of freezer and hairdryer could not provoke this to happen again. 

What I did notice though, its how hot this IC runs. It is almost too hot to touch. Surely for a small signals type of IC, it shouldn't be too hot should it? Any thoughts on this?

Meanwhile, here's a few shots of the set on the bench.

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All the best for now,

Tas

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Topic starter Posted : 01/06/2019 6:47 pm
crustytv
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Posted by: Tazman1966
 
What I did notice though, its how hot this IC runs. It is almost too hot to touch. Surely for a small signals type of IC, it shouldn't be too hot should it? Any thoughts on this?
That doesn't sound right Tas, I believe the supply voltage and current for the TCA270S is 12V @ 47mA so that's only about half a watt, I wouldn't expect it to be too hot to touch.
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Posted : 01/06/2019 7:36 pm
Nuvistor
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Yes that’s the thought I had, just checked the specification.

https://www.digchip.com/datasheets/parts/datasheet/1019/TCA270S-pdf.php

Perhaps worth checking the components around the chip before condemning the chip.

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Posted : 01/06/2019 7:38 pm
Tazman1966
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Thanks Chris, thanks Frank.

I'll have a further check on it in a little while and see how the land lies.

 

Tas

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Topic starter Posted : 01/06/2019 7:44 pm
Tazman1966
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Back in from the workshop and I've checked the components on the periphery of the IC and all is okay. The voltages on the pins of the IC are all roughly correct as per the circuit. Obviously the 25V rail is also spot on - well, it's actually 25.2V!  I did change a couple of the decouplers as in the words of the late great Les Lawry-Johns "beware the blue tants"! He was also referring to this type of board!

A further development now though is that I cannot for love nor money provoke the fault! Should I wrap it up and hope for the best? Anyone else got an 8000A/8004/8800 with this designation of signals/chroma/video board that could check the operating temperature of the IC. The PCB is slightly darkened in the vicinity of the IC so I reckon it may always have been like it...

Tas

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Topic starter Posted : 01/06/2019 10:19 pm
PYE625
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I might be tempted simply to replace the IC as they may be had for a couple of quid from ebay and it would rule it out.

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Posted : 01/06/2019 11:16 pm
crustytv
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Hi Tas,

my 8800 signals panel is not the PC856 but the previous incarnation the PC819. Like yours it uses the multi-signals processor TCA270s. There's no darkening of the surrounding PCB on mine and I don't recall it being too hot to touch when I was mapping voltages during diagnosing a fault on my 8800. I remember this because I removed it and installed another to eliminate it from suspicion and it was not hot.

I can't imagine Thorn would have a cool running PC819 signals processor, then move to a hot running one for the modded PC856 version, still you never know. I can't get to my 8800 it present but here's a photo I took at the time of the signals PCB, as you can see, no toasting around IC1.

signals

 

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Posted : 02/06/2019 7:56 am
Nuvistor
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As Andrew said it is worth changing, it is in a socket as well so it’s an easy test.

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Posted : 02/06/2019 9:56 am
Tazman1966
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Thanks chaps!

I'm going to pop down to Cricklewood Electronics after work tomorrow and pick one up and also a couple of other bits and bobs. They're only just down the road from where I work in Edgware and their stock is really good indeed. Not many shops like this left nowadays.

TTFN, 

Tas

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Topic starter Posted : 02/06/2019 11:36 am
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PYE625
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Ah !  Cricklewood.... been using them since the early 90's for mail order bits and bobs.

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Posted : 02/06/2019 12:00 pm
Jayceebee
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A thumbs up here also for Cricklewood Electronics. Managed to source NEC transistors for the reel drive bridge and also belts for my 8924.

Just for completeness the 8770 uses the 8004 chassis, it should be fitted with the PSU from the 8800 with the overvoltage circuit.

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Posted : 02/06/2019 9:52 pm
Tazman1966
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Sure enough, I did pop to Cricklewood Electronics and got the required IC plus a few more bits and bobs.

I put the new IC in and switched on and as far as can be seen, the picture "disturbance" has gone. The IC does still run very warm but not too hot to touch now. All this was done a few days ago but it's only now that I had the opportunity to post on here.

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I really do like this little set and I know it would be even better with a first class CRT like my 17" Ferguson 3727 (Thorn 8004). I'm still after an HMV version of the 8000 with the gorgeous bevelled edged real wood cabinet. One day maybe...

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Topic starter Posted : 07/06/2019 8:41 pm
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PYE625
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I dare say the thick film resistor just below the IC generates a fair proportion of localised heat, judging by the discolouration of the pcb and that of the yellow wire above it.

A cracking little set and one to be proud of. ? 

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Posted : 07/06/2019 9:03 pm
Tazman1966
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You're right. That gets hot too!

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Topic starter Posted : 07/06/2019 10:15 pm
Nuvistor
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Nice bright picture, glad the chip sorted it out and was easy to obtain.

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Posted : 07/06/2019 10:36 pm
Jayceebee
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Looks very good ? . That later PC856 five IC decoder was a big improvement over the earlier three IC version and performance more uniform, although with a good signal the three IC type performed reasonably well it was not so good in marginal areas. Does anyone remember the white inverters hung on the back of sets with it's edgewise volume control in Rediffusion cabled areas used to feed aerial sets? These did not not sit well with the early 8000/8500 decoder.

I have to say that from your pictures the raster correction doesn't look too bad at all even though there's no circuitry fitted.

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Posted : 08/06/2019 6:39 pm
malcscott
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One of my first jobs when i started work for Rediffusion in 1975 was fitting inverters to aerial sets. Went round customers homes fitting improved audio t/x to them in cases of low volume, Malc.

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Posted : 09/06/2019 12:13 am
Tazman1966
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Thanks for the comments, chaps! We had relatives over a couple of weekends ago and this set was trundled outside into the garden and had Glastonbury on it and it performed impeccably even in the strong sunlight. No lumbering of sets for me though for the next couple of months though following a groin hernia operation on Monday just gone.

All the best,

Tas

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Topic starter Posted : 12/07/2019 11:53 am
Nuvistor
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Speedy recovery Tas, feet up for now.

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Posted : 12/07/2019 12:08 pm
Tazman1966
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Thanks Frank 🙂

 

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Topic starter Posted : 12/07/2019 12:22 pm