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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 4th Jan 2021, 12:48 pm   #21
Heatercathodeshort
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Default Re: Dos and don'ts with Pre War TVs + Marconi 706 pickup!

A quick check to see if the tube has any emission is to pull out the HT rectifier U52/5U4G. Check the EHT for any obvious shorts then apply mains through a variac and gradually increase the input. Do this slowly giving the tube and EHT rect plenty of time to warm up.

The tube is a triode with electromagnetic focus so no risk of a screen burn. You should see a bright circle about 2" wide at the center of the screen.

It may appear similar to a picture of the moon with dark spots on it's surface. These are dead areas of emissive material from the cathode but will not stop the display of a reasonable picture. A large ion burn is normal with these Emiscope 3/2 CRTs.

The EHT is around 2500 but still very dangerous.

John.
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Old 4th Jan 2021, 12:58 pm   #22
HamishBoxer
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Default Re: Dos and don'ts with Pre War TVs + Marconi 706 pickup!

250 DC direct mains would have you, never mind 2500 Volts!
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Old 4th Jan 2021, 1:39 pm   #23
em536716
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Default Re: Dos and don'ts with Pre War TVs + Marconi 706 pickup!

Quote:
Originally Posted by Heatercathodeshort View Post
A quick check to see if the tube has any emission is to pull out the HT rectifier U52/5U4G. Check the EHT for any obvious shorts then apply mains through a variac and gradually increase the input. Do this slowly giving the tube and EHT rect plenty of time to warm up.

The tube is a triode with electromagnetic focus so no risk of a screen burn. You should see a bright circle about 2" wide at the center of the screen.

It may appear similar to a picture of the moon with dark spots on it's surface. These are dead areas of emissive material from the cathode but will not stop the display of a reasonable picture. A large ion burn is normal with these Emiscope 3/2 CRTs.

The EHT is around 2500 but still very dangerous.

John.
Thanks for the tip John. Thought about doing this. Will try this soon. Nearly home now, so will start a new thread on the set.

Regards,
Ed
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Old 4th Jan 2021, 2:12 pm   #24
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Default Re: Dos and don'ts with Pre War TVs + Marconi 706 pickup!

Yes, do take great care. These sets are potential death traps - more so than the mirror-lid models. The way things are set out below decks mean touching parts at EHT potential is easily accidentally possible. In these 5-inch sets, everything is crammed in too, and there are parts at EHT potential that could spark over to nearby parts.

Steve
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Old 4th Jan 2021, 4:25 pm   #25
Heatercathodeshort
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Default Re: Dos and don'ts with Pre War TVs + Marconi 706 pickup!

Yes! No EHT 'guard' that is present underneath the chassis on the later 1804 series. The EHT rectifier if I remember is a U17 and can rectify quite a high current.

When working on the chassis use wooden blocks to rest it firmly on the bench. A number of fatalities were caused by chassis toppling onto guys that were working on them. [particularly the PYE D/B 16T] Make sure the bleeder circuit is not O/C. J.
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