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Decoder Memory Jog

 
crustytv
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Watching Star Trek on my A823 this evening and I noticed a very subtle one inch shaded disturbance has appeared on the right-hand side of the picture, accompanied by what appears to be tiny lines/streaks on the right-hand side of spocks head and body. Really difficult to spot on normal scenes, you only really notice it on darker scenes. The colours are all correct.

Without breaking out my Tricorder, and as It's been a long while since I've partaken in D-D-G (Decoder Diagnostic Gymnastics) I'm trying to remember what the cause of this is.  Searching my memory banks, I seem to recall I had this self same issue on my PYE CT203. On that I spotted it on the Test-card. I believe  this is related to the 7.8kHz ident, it needs adjusting for phase, but I'm not certain my memory is correct, what say ye all?

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Topic starter Posted : 05/01/2021 12:03 am
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Marconi_MPT4
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Posted by: @crustytv

..disturbance has appeared on the right-hand side of the picture, accompanied by what appears to be tiny lines/streaks on the right-hand side of spocks head and body.

Spock being attacked by the Klingons perhaps, but seriously though, are the lines/streaks on every other line? If so that might indicate a possible stability problem or setting issue around the phase detector / phase lock loop leading to a miss-triggering of the PAL switch.

Last time I worked on this chassis was in the late 1970's when we had a pantechnicon deliver hundreds of Rediffusion badged sets for refurbishment, all with low emission Sylvania CRT's!

Rich

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Posted : 05/01/2021 2:07 am
Nuvistor
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Worth setting the decoder up as per manual, may just be slight drift. These decoders could be a bit touchy with the setup.

 

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Posted : 05/01/2021 10:59 am
Jayceebee
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Looking at the decoder circuit the first thing I would check would be a slight adjustment to 3T5. Looking at the alignment instructions it mentions to start with that the core should be flush with the top of the former, I don't if it's me but I can't find any mention of a final set up procedure? It's a long long time ago but there is something at the back of my mind where the glue on some could degrade to the point where the individual windings could be able to move along the former 😱. An awful lot of variable inductors in this design.

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Posted : 05/01/2021 11:28 pm
crustytv
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Hi Rich, Frank & John,

Thanks for taking the time to read and respond, much appreciated.

I suppose I should put some background to this. The set was performing perfectly fine over Christmas, providing hours of entertainment. One afternoon I noticed some glitches, these were indicative of decoder trouble. As I had not done anything to this set other than refurbish the PSU I decided to take a look.

The first thing noticed was 8C11, this takes the line pulse which in turn presents a clamp pulse to the decoder. One of those horrible Plessey duo colour caps and seriously looking, baked to be precise and the worse for wear, I'm surprised it had not gone boom! So it was replaced which wasn't too difficult as a simple push made one leg break off.
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Upon removing the decoder I then spotted 3C21, it showed electrolyte discharge, a 100uf 25V. Upon testing, it had gone low in value and was also acting as a resistor with very high ESR. This was replaced.
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I tested the adjacent Callins 3C16 also 100uF 25V, although this was OK with regard to ESR it was on its way to doubling in value, so that was also replaced too.
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At this point I felt these three faulty components were enough for now and put the set back into operation. Happily rewarded with stability returned to the decoder and the picture, or so I thought until I spotted this ident trouble. The Picture below shows the set working after I had replaced the above mentioned components, at this juncture the disturbance was not evident to me, but you can just make it out on the far right side.
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Taking all the above into account, I have been pondering over the past couple of days the possibility that maybe the decoder has in the past been tweaked to compensate for a degraded clamp pulse via 8C11. This clamp pulse is presented to the cathode of 3D3 and subsequently through to the collector of 3VT7/3T5. Now that 8C11 has been replaced with a nice spanking new cap, perhaps the gate & burst switching needs readjusting (3T5) back or the gate pulse (3T1). So I had been wondering about adjusting 3T1, 3T5 or both. 3T5 is flush so perhaps 3T1, I'll have to ponder a little further or is that procrastinate 😉 

 
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Topic starter Posted : 06/01/2021 12:15 am
Nuvistor
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@crustytv

Has noted they are touchy on set up so it may cure the fault.

Those capacitors were well past their sell by date, appreciate you like to fix faults has they arise. With fixing things for a living all my life after fixing the fault I tended to look for parts that were not up to spec and could degrade the output or reliability. Saved call backs and gave some confidence for the owner, I always discussed it with the owner of the equipment so they had the ultimate decision.

 

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Posted : 06/01/2021 9:49 am
Marconi_MPT4
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@crustytv

A board with Callins, Plessey and Tants. Had capacitor UXB (8C11) developed a dielectric leakage?  I wonder if 8D4 and 8D5 are still working properly as this would have an indirect effect the the gate pulse timing.

It might be worth checking the timing of the burst gate signal on diode 3D3 to coincide during the colour burst and that it is not advanced or delayed.

Does the line effect show up on any particular colour displayed using a static colour bar test pattern?

Rich

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Posted : 06/01/2021 5:19 pm