HMV 901 restoration.

[Jac]

That's a pity - the EHT transformer.
Unfortunately it seems to happen often.
Luckily the CRT is in good condition, and the transformer can be rewound.

The Ebay EHT generator does not have a stable output and cannot generate enough current. The insulation is a bit flimsy as well.
Nice for experiments where you want to check the CRT in absence of EHT before going further though.

Jac

[FERNSEH]

Hello Jac,
Yesterday morning the picture was nice and bright but I noticed later on during the day the picture was becoming rather dim. The giveaway that the EHT was failing was the increased width and height. Then about eleven PM I noticed a burning smell, sure enough it was the EHT transformer.
That'll mean all my first generation EMI sets will have replacement EHT transformers.

DFWB.

[Jac]

Hello David,

I didn't know that it was that common for the EHT transformer to fail.
Ed Dinning or Mike Barker should set up a production line for them.

But I agree, a different way of generating the EHT might be an alternative.
You still have to take care of the CRT heater supply, which is not too difficult.
I don't think that it will be more reliable that an iron core transformer though.
Perhaps the best solution still is a rewound transformer.
If the set is kept in a dry place, the transformer could outlive the next 80 years.

A bit off-topic, but are the transformers in the 904/905/706/707 more reliable?
I suppose it helps that the EHT is half the value, but the winding wire will be just as thin.

Jac

[FERNSEH]

Originally Posted by Jac

A bit off-topic, but are the transformers in the 904/905/706/707 more reliable?
I suppose it helps that the EHT is half the value, but the winding wire will be just as thin.

Good evening Jac,
I've a suspicion that the EHT transformers in my HMV 904 and Marconi 707 have failed also. It's a long time since I fired up those two sets but I remember there was an almighty bang from the Marconi, and smoke!
The HMV 902 TV-radiogram I acquired in 1998 had an oil filled EHT transformer installed. In due course even that mil spec looking thing failed and had to be replaced.
Seems that I'm not having much luck with my HV transformers.

After the Marconi 702 is repaired the HMV 902 will be the next set to receive attention. The EMI K3 gram deck is to be replaced with a much better one.
Begs the question where the topic should be placed, TV or vintage audio?

DFWB.

[beery]

Hi Jac,

Originally Posted by Jac

A bit off-topic, but are the transformers in the 904/905/706/707 more reliable?
I suppose it helps that the EHT is half the value, but the winding wire will be just as thin.

The answer is not really. You might think that the lower EHT would make them more reliable, but I'm sure it has more to do with insulation used and the pitch coating. The EMI line output transformers also suffer from shorted turns...

The postwar Pye D16T has a relatively high EHT (6KV) produced from a massive transformer that powers everything and is very reliable, which I am sure is due to the fact that it is wax impregnated. I've found waxed transformers to be the most reliable (it is not so good for capacitors though!).

Cheers
Andy

[Jac]

Thanks David and Andy.

Are the winding details known for the TV-section-transformer of the 904 etc?
(Like we know the details of the EHT transformer of the 900 etc., courtesy of John Wakely.)
Would be very nice to have!

Jac

[brianc]

Mike Barker (Murphymad) has the details of all (most) of the EMI pre-war TVs including the mechanical details so he can also produce replicas for missing or incorrect Xfmrs. He made an exact replacement mains Xfmr for my Marconi 706 which had had a wrong Xfmr fitted during its life. He also made a Pye 815 jobbie for me too.

[Jac]

Thanks Brian.

I gather he has no problem in sharing the details of the 904 etc transformers, so I'll contact him about it.

Jac

[peter_scott]

I'm sorry to hear of your EHT transformer David. If it's any consolation when I bought mine it had nasty little ignition type "interference" on its screen that quickly turned into smoke signals. Fortunately I worked for a company with a transformer shop at the time.

Peter

[FERNSEH]

Hi Peter,
It's just as well there is hardly any modern stuff coming in for repair, now it's possible to do all that outstanding work on my old sets. But as you can read in this forum things are not going as well as I would like.

DFWB.