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Easter Sunday Test; Thorn 3000 Chroma Module Fault

 
crustytv
(@crustytv)
Vrat Founder Admin
Posts: 10843

As I mentioned in another thread, bored, bored and even more bored.

So had a look in the PCB stock room and pulled out a board from the service section. This board still has its original trade service tag, it states "No Colour".

chr2
chr1

Now after putting it in the PCB mount and spending just a few mins looking and prodding, I believe I've sussed this without needing to place it in a test rig, I wonder why it was never repaired and put back in service. It was sneaky, not obvious at all, but gentle prodding is a good diagnostic habit. It would be unfair to leave as it was presented to me because as I say, not obvious. So for you to try to remotely diagnose, I've left a clue. See if you can spot it.

chr3

No prizes and I can guess now who's likely to be the first to spot it. ? 

p.s.

I'll action the repair a little later, do you think the customer will complain about the 45-year wait ? 

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Topic starter Posted : 12/04/2020 6:15 pm
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PYE625
(@pye625)
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Posts: 4980

I know nothing of the 3000, but T301 is clearly broken.

It may be possible to glue the former back in place, if a replacement is unavailable.

Mind you, enlarging the photo, it looks as if a wire may be broken from the winding too.

W313 at the bottom right looks to have been knocked out of position, but hard to see if it is damaged.

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Posted : 12/04/2020 6:56 pm
Lloyd
(@lloyd)
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Posts: 1539

Also looks like L306 is a bit squished?

Also just spotted C358 is missing, unless it's supposed to be?

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Posted : 12/04/2020 6:59 pm
PYE625
(@pye625)
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Yeah, L306 looks a bit pushed over. Either that or it moved during the soldering process.

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Posted : 12/04/2020 7:02 pm
abctelevision
(@abctelevision)
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Posts: 179

Could the broken L306 and T301 have happened in the 40 odd years whilst waiting to be repaired?

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Posted : 12/04/2020 7:05 pm
Jayceebee
(@jayceebee)
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Posts: 1631

The damage to T301 and L306 are probably collateral from the back of the field engineers van. A broken T301 would not cause no colour but a lack of R-Y, L306 which is in the delay line driver stage I suspect if damaged and o/c would cause severe Hanover bars. 

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Posted : 12/04/2020 9:55 pm
crustytv
(@crustytv)
Vrat Founder Admin
Posts: 10843

Indeed, T301, L306 and W313 were all o/c so well spotted, the later edition chroma boards did away with C358. T301 on initial inspection look OK but fell apart when gently prodded, L306 was lifted out of trace on one side and W313 is cracked on the right. I must admit I wondered whether the ticket stating "No colour" might have meant no correct colour. I thought with all the damage in the R-Y demodulator coupled with the delay line driver, would account for pretty "messed-up" or no colour.

As John and Dave pointed out much of this is likely storage damage, though not under my care as all boards here are stored (see below) carefully in storage slots, or hung from the ceiling in the workshop PCB store. Anyway, John knows this chassis like the back of his hand, so I'll go with his analysis. Perhaps VT309, VT310, the only way to find out what's what is to stick it in a set. Shame you cannot stick the PCB on a bench supply and scope all the waveforms

store
st1
st3

Workshop PCB Store

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Topic starter Posted : 13/04/2020 9:40 am