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Old 30th Apr 2019, 6:36 am   #1
Mikey405
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Default Spark-gap Shenanigans

Hi all.

I suspect this will be a very short thread and nobody will have much of an idea what I'm whingeing about.

However, that aside, I was wondering whether I was the only person with problems with spark gaps in old CTVs. I've had a number of sets where the focus SG either cracks over at the slightest provocation or sits there sniggering at me producing corona and poisonous ozone smells whilst intermittently blurring the picture.

This isn't confined to those red plastic things, but the ceramic whatsits with the slit in the top. Of course filing the electrodes actually makes matters worse because of the sharp edges; I've even resorted to removing them altogether in the past.

Thanks all.

Kind regards.

From Mike.
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Old 30th Apr 2019, 8:02 am   #2
peter_scott
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Default Re: Spark-gap Shenanigans

Hi Mike,

I wonder if a dehumidifier would help?

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Old 30th Apr 2019, 8:03 am   #3
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Default Re: Spark-gap Shenanigans

I've followed your posts with interest over the years with great interest re old tv's and spark gaps have always been a pain for me. I think most people will admit to snipping them. I only went to LCD\Plasma last year and till then stuck to CRT. Spark gaps always were a problem, and whilst manufactureres must have fitted them for a reason I personally haven't had any problems with removing them. Certain sets seemed more prone to them flashing over. Bush 823 and some of the continental sets certainly did.
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Old 30th Apr 2019, 8:26 am   #4
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Default Re: Spark-gap Shenanigans

Never had too many issues except when...

1) The set smoked 40+ a day....
2) Room was heated by a gas or parafin heater. This created a high humidity.

In the above cases i'd more likley see a breakdown elsewhere such as final anode cap / overwinding.
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Old 30th Apr 2019, 10:07 am   #5
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Default Re: Spark-gap Shenanigans

I have had a bit of experience second-hand with spark gaps. The ideal spark gap passes zero current until voltage reaches a well-defined figure, then it abruptly goes conductive (and passes current until voltage has fallen to a lower level).

As Mike observed, filing edges is a really bad thing to do because it creates sharp points. The best spark gap has rounded edges, ideally two spheres. Breakdown is then about as sharp as it can be, at a reasonably well defined voltage (depending on air pressure). Anything else and you are likely to get corona discharge, which bleeds charge away, ionising air, at lower levels. And insulation between the electrodes needs to be air - anything else, of length comparable with spark gap distance, is likely to collect dust, moisture, etc which again acts to steadily bleed away charge, or create a lower voltage trigger effect to breakdown.

Only suggestions are to clean the area with IPA and a toothbrush!
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Old 30th Apr 2019, 10:28 am   #6
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Default Re: Spark-gap Shenanigans

Over the years I have seen focus spark gaps continually arcing due to high humidity and dampness in customers houses, faulty triplers, LOPT's and focus chain resistors especially that 4.7M resister between the focus VDR and chassis going high.
A few years back my own Decca Bradford lost focus. It was a hot humid summers day and I found that a small flying insect had found its way into the back of the set and melted itself across the focus spark gap totally melting the red plastic surround. Fortunately I had a few spare Decca tube bases lying around so a replacement spark gap was soon found. After fitting it and with a few adjustments and checks normal service was resumed.
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Old 30th Apr 2019, 3:23 pm   #7
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Default Re: Spark-gap Shenanigans

I had the “ring trap” fail on BW tubes due to mechanical failure, the wire ring breaking out of its plastic holder. Simple fix, they just pushed on over the tube pins, kept spares on the van.

Only failures on CTV’s was the focus trap on the base, due to contamination, smoking etc, again bit more work to replace but straightforward and kept on the van.
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Old 30th Apr 2019, 7:07 pm   #8
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Default Re: Spark-gap Shenanigans

Hi.

I recall problems with the 20AX tube base sockets as used by the RRI T20. It was made by Pressac and I think it was similar to the type in the Philips G11 and Decca 100 though I don't remember it being an issue with these chassis.
The problem was associated with the built in focus spark gap, it seemed prone to tracking. There was I think an improved version issued.

I also had a few faulty red coloured spark gaps as used in many delta gun sets. I suspect a lot of the trouble was due to high humidity where the set resided.

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Old 30th Apr 2019, 11:30 pm   #9
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Default Re: Spark-gap Shenanigans

Yes I have had problems with spark gaps as well, and have generally replaced them with new ones from the likes of Farnell and RS.
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Old 1st May 2019, 2:59 pm   #10
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Default Re: Spark-gap Shenanigans

Hi
Yes, the Pressac ones always gave problems - I think the other makes uses a different type. Stripping them down revealed green corrsosion and often the plastic would have melted by the time we were called in!
Those skeleton red ones could catch fire if left too long. A duff tripler or O/C lower ladder esistor would result in a continuous arc. Korting ones used a ceramic enclosed unit which still could give trouble - you could see it arcing in the dark.
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Old 2nd May 2019, 7:23 am   #11
Mikey405
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Default Re: Spark-gap Shenanigans

Hi all.

Thank you for the interesting responses. It seems to be mainly Rank sets with those red plastic polo mint spark gaps that are the worst. As for Bobby's comment, I had no idea you could still buy them. I shall do some investigating. Well done that man.

Thanks all.

Kind regards.

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Old 4th May 2019, 11:29 am   #12
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Default Re: Spark-gap Shenanigans

Farnell (for example) sell them, listed under "Circuit protection - Gas discharge tubes".

there is one rated at 5.5 KV and the next one up is 7.44 KV; I am presuming this sort of thing is suitable...
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Old 4th May 2019, 2:49 pm   #13
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Default Re: Spark-gap Shenanigans

7.44kV should be suitable as replacement for a G3 spark gap in a colour set, but in other cases a lower protection voltage would be needed.
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Old 7th May 2019, 7:05 am   #14
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Default Re: Spark-gap Shenanigans

Thanks Bobby and Maarten. I'll check those out.

Kind regards.

From Mike.
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Old 7th May 2019, 11:46 am   #15
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Default Re: Spark-gap Shenanigans

Lower than 5.5kV I meant, though I'm not sure on te best voltage. Maybe there's something in the databooks.
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