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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 30th Nov 2021, 8:39 pm   #1
Sundog-
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Default CRT Heater to cathode leakage

Where the CRT heater is fed from a transformer or a series heater chain, H to K leakage can seriously impair the display. Low capacitance transformers were sold to remedy such problems.
Another way is to create a winding using well insulated cable on the line output transformer, as later TVs tended to do.
Deciding the number of turns is a bit hit and miss but start expecting about 1 turn per volt. Keep away from the EHT overwind.
The original heater connections should have been referenced to a potential with 1 or 2 resistors and possibly a capacitor. This was to prevent static build-up of the heaters and discharge through the H-K insulation. This will need to be removed and referenced instead to the cathode (if colour the cathode of the gun with a problem)

The horizontal deflection circuit is unlikely to complain about the small extra load, but you might have to re-adjust slightly for optimum.

I used this method in the 70s when a large batch of TVs had the heater reference circuit missed during manufacture. Most of the CRTs failed H-K. I can't remember which colour gun it was.
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Old 1st Dec 2021, 5:51 am   #2
FrankB
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Default Re: CRT Heater to cathode leakage

H-K shorts in Zenith & Sony TV sets were a common problem when I was repairing them full time. We used an isolation transformer to solve the problem.
There were a couple of articles on how to solve the problem in Electronic Servicing Magazine IIRC. The article told how to build your own isolation transformer quite cheaply.
The commercially made ones were at $25.00 -$29.00 Dealer Net.
Also a really top end CRT tester could also blow out the short.
Sadly, the shop I worked at, the owner was too cheap to buy a really good CRT tester for many years (And one tech blew it up in just a few weeks), so I was often able to charge up an electrolytic and blow out the short with it.
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Old 1st Dec 2021, 7:32 am   #3
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Default Re: CRT Heater to cathode leakage

I still have a Radiospares heater isolation transformer somewhere in my workshop.
I have used 5 or 6 turns of insulated 14 x 0076 wound around the ferrite on the LOPTX for PYE 11U and some early Bush models to great success.
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Old 2nd Dec 2021, 1:29 pm   #4
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Default Re: CRT Heater to cathode leakage

I have used the LOPT additional winding trick many times in the past. It always worked very well and saved the customer the cost of a new colour CRT. I seem to remember it occurred mainly in receivers of the late 80s early 90s period.

Two 2V isolation transformers from an earlier period for the notorious Mazda triodes! Note the percentage data.

In the UK I never encountered the USA plug in type 'brightener' probably because most of our chassis were AC/DC. The isolation transformers were well made and cheap, probably another advantage. In 1966 a Radiospares isolation/boost transformer cost 15/- [75p] They were produced with many secondary voltages to cope with the crazy range of CRT heater voltages that were common back then. John
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Old 8th Dec 2021, 5:36 am   #5
FrankB
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Default Re: CRT Heater to cathode leakage

I forgot to mention that you can charge up a higher value capacitor, 100 mfd +, with about 400+V and "zap" out the short.
Basically what the "Remove Short" function of a CRT tester does.
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Old 8th Dec 2021, 10:37 pm   #6
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Default Re: CRT Heater to cathode leakage

Hi.
On a older black and white set with a valve video amp often adding an isolating transformer will degrade the HF response somewhat. The idea if physically possible is to add a cathode follower after the video amp, the low output impedance restores the HF content.
Although a few turns round the lopt core can drive the heaters on older sets you can get a little shading on screen. I doubt a TV22 or Pye V4 would have enough puff to drive a CRT heater without complaining a little bit, I could be wrong though.
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Old 10th Dec 2021, 9:51 am   #7
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Default Re: CRT Heater to cathode leakage

Remarkably the capacity between the secondary winding and core of the Radiospares transformer is only 66pf. the larger one is 68pf. It is 25pf between windings and RS recommended it was fixed to the cabinet rail rather than the chassis. This very low capacity would have had an insignificant effect on definition.
As you mention Trevor the older mono receivers would have had great difficulty in providing .3a from the LOPT! It was very successful with the later transistor colour tvs.
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Old 10th Dec 2021, 9:30 pm   #8
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Default Re: CRT Heater to cathode leakage

Quote:
Originally Posted by Heatercathodeshort View Post
....
As you mention Trevor the older mono receivers would have had great difficulty in providing .3a from the LOPT! It was very successful with the later transistor colour tvs.
John.
Good points. I should have qualified that my experience was with 90 degree deflection colour TVs. I would hope that 110 degree sets would have even more power to spare.
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