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Old 11th Sep 2019, 4:54 pm   #21
TonyDuell
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Default Re: Alternative EHT system for HMV 902.

As of course do laser printers (I assume modern ones do, certainly the 'classic' ones I've worked on do). Laser printers and photocopiers have many similarities...

I seem to remember the voltage is a few kV and probably not enough for the EHT on a TV CRT.
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Old 11th Sep 2019, 6:37 pm   #22
FERNSEH
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Default Re: Alternative EHT system for HMV 902.

Good evening everyone,
Many thanks for all your ideas, we've now have many solutions to the problem of producing EHT for our old TVs. All will be given consideration.
The power unit is now removed from the HMV 902. The power pack you see in the attachment was made for the set by the late Gerry Welles in 1988. The original unit was completely rusted away because it had been buried in the garden of the house where the set was found.
The Majestic transformer is mounted on an adaptor plate because the transformer Gerry had fitted was Parmeko drop through oil filled type.
So later today it will be removed from the power chassis and refitted in the 901.

DFWB.
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Old 11th Sep 2019, 8:01 pm   #23
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Default Re: Alternative EHT system for HMV 902.

Quote:
Originally Posted by FERNSEH View Post
The Majestic transformer is mounted on an adaptor plate because the transformer Gerry had fitted was Parmeko drop through oil filled type.
Mmm, we like an adaptor plate
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Old 11th Sep 2019, 8:52 pm   #24
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Default Re: Alternative EHT system for HMV 902.

"Mmm, we like an adaptor plate2

Hi Andy,
nothing to do with vintage TV but these guys are great exponents of adaptor plates and brackets.

https://www.youtube.com/watch?v=SUeytYlxW7c

DFWB.
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Old 11th Sep 2019, 9:12 pm   #25
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Default Re: Alternative EHT system for HMV 902.

They've taken on an even bigger project.... just so they can make bigger brackets!

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Old 11th Sep 2019, 10:27 pm   #26
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Default Re: Alternative EHT system for HMV 902.

I just looked and the transformer type I posted are still available. The advantage of the split bobbin is that you can take off the primary and alter that to any mains voltage you like +/-heater winding and you will never have any trouble with the sealed secondary, its a real gem. They are an ideal donor for vintage TV repairs, that is if there is the space. just remember to open the air gap.I just used a file and lacquered the filed edges.

https://www.ebay.com/itm/3500v-Core-...0AAOSwGvhUGaYe

As I recall too they ran a very low flux density, nowhere near saturation, so probably you could get the peak voltage up higher too , or use a doubler.
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Old 12th Sep 2019, 8:47 am   #27
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Default Re: Alternative EHT system for HMV 902.

A nice transformer Hugo.

Indeed we need a higher output voltage (if the U16 rectifier tube is to remain in circuit): c. 4000 VAC.
Well, the primary would need to be changed anyway.

David, just out of curiosity, how high is the resistance of the EHT winding of your Majestic Transformer Co transformer?

Jac

Sorry, forgot to include the U16 characteristics graph.
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Old 12th Sep 2019, 12:18 pm   #28
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Default Re: Alternative EHT system for HMV 902.

"David, just out of curiosity, how high is the resistance of the EHT winding of your Majestic Transformer Co transformer?"
Hello Jac,
The resistance of the EHT winding is 7.23Kohms and the primary is 25.7ohms. Primary is wound for 230volt mains.
The laminations stack dimensions are: 115 X 95 X 45mm.

DFWB.
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Old 12th Sep 2019, 1:08 pm   #29
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Default Re: Alternative EHT system for HMV 902.

Thank you David.

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Old 12th Sep 2019, 11:12 pm   #30
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Default Re: Alternative EHT system for HMV 902.

It's oot! All twenty-seven inches of it. Now I can start work on the power unit.

DFWB.
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Old 16th Sep 2019, 10:26 am   #31
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Default Re: Alternative EHT system for HMV 902.

The complete chassis from a 5" telly (50p from a car boot) could go in a box to provide EHT.
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Old 16th Sep 2019, 12:09 pm   #32
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Default Re: Alternative EHT system for HMV 902.

Quote:
Originally Posted by llama View Post
The complete chassis from a 5" telly (50p from a car boot) could go in a box to provide EHT.
Graham
Hi Graham,
Using the 5" TV chassis wouldn't be a permanent solution but can be considered a safe source of EHT during servicing sessions.
Will be doing the transformer swop in the HMV 901 later today.

DFWB.
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Old 16th Sep 2019, 3:51 pm   #33
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Default Re: Alternative EHT system for HMV 902.

Old transformer oozes goo. EHT went down to 2KV and then the fuses blew.

DFWB.
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Old 16th Sep 2019, 5:05 pm   #34
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Default Re: Alternative EHT system for HMV 902.

The attachment shows how the Majestic transformer is fitted in the Marconi 702. No adaptor plate was needed.

DFWB.
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Old 16th Sep 2019, 5:11 pm   #35
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Default Re: Alternative EHT system for HMV 902.

A truly beautiful transformer David!
It looks very well made.

The 3.5 kV output may avoid a lot of problems of the 4 kV original transformer. A good choice, I think.
For the relatively thin CRT phosphor-layer, the difference in light-output will be small, while benefiting from a somewhat more generous deflection because of the somewhat lower EHT.

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Old 16th Sep 2019, 9:24 pm   #36
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Default Re: Alternative EHT system for HMV 902.

Hello Jac,
Because of the light load a 3.5KV transformer should produce a rectified output of just under 5KV DC.
I'm considering asking the Majestic Transformer Company for a quote for two more EHT transformers.
The new transformer required spacer washers because the screws that secure the smoothing choke protrude through the chassis.

DFWB.
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Old 16th Sep 2019, 9:36 pm   #37
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Default Re: Alternative EHT system for HMV 902.

It fits very well.

Peter
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Old 17th Sep 2019, 9:55 am   #38
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Default Re: Alternative EHT system for HMV 902.

Hi Peter,
Extra holes had to be drilled into the transformer end plates so that it can be fitted on the existing fixing points. It's much better practise to modify replacement parts rather than attempt to do any alterations to an eighty year old chassis.

DFWB.
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Old 17th Sep 2019, 4:28 pm   #39
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Default Re: Alternative EHT system for HMV 902.

Transformer is now fully wired into the circuit but before the power unit is reinstalled into the base of the cabinet a few tests must be carried out to confirm all is well with this heavy chassis. The attachment shows the EHT bleeder chain, one resistor has been found to have gone high value, that component will be replaced.
Got to get this thing right because I don't wish to be lifting the CRT out from the cabinet and refitting it too often.

DFWB.
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Old 17th Sep 2019, 5:56 pm   #40
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Default Re: Alternative EHT system for HMV 902.

Showing the sound receiver in the HMV 901. Valve line-up is: X41, VMP4G, MHD4 and N41. IF is 1.5Mc/s.

DFWB.
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