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Thorn 981 chassis. Picture ripple.

 
sideband
(@sideband)
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All this activity in the other thread prompted me to have another look at my HMV (981) chassis. It's all working quite well except it has always been troubled by a slight ripple in the picture. At first switch-on, all seems well. Leave it for around 15 minutes and a ripple starts to be evident (most noticeable on a testcard). It seems to get to a maximum after around 20 minutes or so and sometimes even seems to correct itself. Forget things like smoothing (tried), HT rectifier (replaced), heater rectifier (replaced) and all valves substituted. Also it's running on normal mains, not a variac or isolating transformer.

I've run out of ideas at the moment and wondered if there were any Thorn experts who may have any suggestions.  I was starting to think leakage across the PCB somewhere but the only area that shows any overheating (but only slight) is around the video amp. 

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Topic starter Posted : 17/08/2018 10:26 pm
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Jayceebee
(@jayceebee)
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Hi SB, not really come across any ripple faults as such but if it looks like it's in the line osc then there are several resistors in the anodes of V7 which tend to go high, also C36 which might be worth replacing.

In my time with DER these sets were used as loans and to be honest when they went faulty they were scrapped, they were treated very poorly by the field staff with most showing signs of sliding around the back of a van face down.

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Posted : 17/08/2018 10:54 pm
sideband
(@sideband)
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I can imagine them being used as loans. It always seemed unusual that Thorn would manufacture a 405 only set as late as 1967 when dual-standard sets were the norm.

Maybe they always had a slight hum problem but being portable and generally a loan set, no-one bothered.

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Topic starter Posted : 18/08/2018 4:03 pm
PYE625
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Hi, my DER 981 set is free from any hum so it must be a fault  ? 

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Posted : 18/08/2018 7:34 pm
sideband
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Changing the PY801 for another new one seems to have effected a cure. When I got the set it was fitted with a PY83 but that was slow to warm up. I just fitted the correct PY801 (a new Pinnacle) which cured the slow warm-up. Tonight I tried another new PY801 (a Mazda) and even after an hour there was no hum apparent.

I have to say I'm impressed with the picture.....I watched the end of Star Wars tonight...the fact that it was B/W didn't detract from the film at all.

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Topic starter Posted : 18/08/2018 10:44 pm
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PYE625
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Ah, you see it was only happy with a Mazda PY801 valve. ?

Must have been heater to cathode leakage with the Pinnacle valve perhaps.

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Posted : 18/08/2018 10:56 pm
sideband
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Hunting through my stash of valves (some are sorted, other remain in a box) I found two more new PY801's and tried them. One is another Pinnacle the other is a Mullard and both appear to be OK....neither show any sort of hum after half an hour or so. The original new one I fitted however does show some hum so as you say must have a slight heater-cathode leak. I think it might just be slightly out of the three-months warranty though.....! I'll probably keep it as a 'test only' valve with a note on the box.

Come to think of it, the original new Pinnacle I fitted was in a plain white box with PY801 stamped at one end only. All the other Pinnacle valves I have are in branded boxes so I wonder if it was a 'reboxed reject'?

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Topic starter Posted : 19/08/2018 7:43 am
sideband
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Posted by: PYE625

Ah, you see it was only happy with a Mazda PY801 valve. ?

All the other valves are original Mazda except for the line output which is a new Mullard PL81A....seems happy enough with that!

 

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Topic starter Posted : 19/08/2018 8:07 am