Alternative EHT system for HMV 902.

[FERNSEH]

The EHT transformer has failed (again) in my HMV 901. That's bad enough but to gain access to the power unit the CRT has to come out and as we know the less we handle these things the better.
So, here's the plan. In the HMV 902 there is a very high quality EHT transformer that was made by the Majestic Transformer Company in Poole Dorset. Take that transformer out and fit in the 901. While the 901 is in a dismantled state change any hi-viz capacitors with restuffed EMI ones.
For the 902 I'm considering the voltage multiplier circuit shown in the attachment. The circuit was designed to provide 5KV from a 350V - 0 -350V transformer. However, the mains transformer in the 902 has a 280 - 280 volt secondary so to achieve 5KV a quadrupler circuit will have to be employed.
It all depends on if the circuit can provide good regulation. The CRT takes only 100uA but the current drawn by the bleeder chain has to be considered.
The 0.1uF capacitors in the multiplier network need to be good for 2KV.

DFWB.

[Jac]

Yes, the total current is over 1 mA.
Perhaps you can tap the approx 1 kV from the multiplier along the way, and use much higher load resistors to free the top end multiplier section of the total originally high 5 kV current?

Was the failed transformer rewound some time?

Jac

[Jac]

Please find attached an extract out of WW May 1948 about EHT supplies.

Jac

[FERNSEH]

Hello Jac,
The transformer in the 901 was rewound sometime in the nineties. I guess it's safe to say the set is now on it's third EHT transformer, or fourth.
That's something to consider, tap off the multipilier the focus and accelerator anode voltages in the same manner as was done in modern EHT triper modules.
Cornell-Dubilier 0.1uF 2000V capacitors are available from Mouser UK.
https://www.mouser.co.uk/ProductDeta...4Om9gNYk98CC4=

DFWB.

[Radio Wrangler]

The Cockroft-Walton multiplier, particularly for high multiplication ratios, needs quite large capacitors at 50Hz.

Things become much easier at higher frequencies, and the TV industry has created plenty of high voltage fast diodes.

I'd be tempted to make a little sealed box and put a transistor inverter in it, ferrite transformer and a cockroft-walton multiplier. Forwards converter not flyback for a smoother output, and a good output filter so you get smooth DC out and no patterning. High freq means plenty of voltage per turn, so the number of turns is reasonable.

Perhaps build it all into a restuffed EHT capacitor?

With the high frequency source, you need a lot less capacitance so there is a lot less danger.

Such a design could be duplicated by others with EHT transformer woes.

As a kid, I made oscilloscopes and I used a real EHT mains transformer. I didn't realise the danger. Somehow I survived!

David

[FERNSEH]

Hi David,
As Jac mentions the EHT current requirement in the first generation EMI TVs is >1mA so it's not certain of the multiplier can deliver the power. Only thing to do is construct the EHT unit and see if it is up to the job in the 902.
Back in the late forties Westinghouse made and marketed the "Westeht" unit, (I think I got the name right) for use as an economical replacement of TV EHT transformers.

DFWB.

[Jac]

Originally Posted by Jac

Please find attached an extract out of WW May 1948 about EHT supplies.

Hi David.

P.S. Please note that the internal impedance get rapidly very, very high with adding stages. The load resistors (even at very high value) would not be helping. There is a point where more stages actually decrease the output voltages.
I do not expect the multiplier to keep much voltage...
The Westeht unit is also in the WW article.

A rewind might not be the worst thing to do, or is it terribly expensive? It may not be easy to find a suitable replacement transformer.

[FERNSEH]

Hi Jac,
I could try the multiplier in the 902 and if it isn't any good I'll have the transformer rewound. Two sets of three 0.1mfd capacitors in series operating on 50hz cannot offer good regulation.

DFWB.

[Argus25]

Going down an alternative route one problem that always remains at these voltages is the insulation. It is generally the reason why most EHT supplies, regardless of design fail.

In my Argus TV this is one reason I went for a modified neon sign transformer, because of they way they had potted the EHT winding, but that particular one required a primary re-wind and it was only a 2kV system.

David's suggestion is very good, go transistorized. You can easily rectify the heater voltage to get DC and run a standard transistor TV Lopty, just to get EHT. The lopty of a modern design has all the insulation requirements looked after and the EHT rectifier in there too.

All you need to drive it at around 15kHz is a 555 IC, an output transistor, a damper diode and a tuning capacitor, which you can adjust to get the flyback peak the right level.

Most of these lopty transformers have multiple taps and with that, the power supply voltage and the tuning cap can all be manipulated as variables to get the right EHT and you could fit it in one box. A small lopty from a 9" monitor would be fine or in fact just about any type could be made to work, there are many on ebay found searching flyback transformer:

https://www.ebay.co.uk/itm/NEW-2-X-F...MAAOSwZZlbmWMN

[Chris55000]

Hi!

Another idea to try, and possibly worth thinking about is RF Oscillator E.H.T., it's very much safer and more reliable than 50Hz supplies, and if you want to make your own to try, winding details of the E.H.T. Oscillator Coils L1-L5 can be found in early 1950s Practical Television!

The trimmer shown across the primary is for tuning and a padder trimmer of about 500p from an old radio chassis can be tried. Note that R.F. e.h.t. doesn't need a LOP rated valve with a top-cap anode, by the way, an ordinary audio beam tetrode is fine!

Adding modern-type circuits & components partly takes the "Gilt off the Gingerbread" a bit, whereas R.F. E.H.T. is still in keeping with the general era of the set.

You will need to provide all-round screening - a LOP cage and lid from a scrapper TV will do for this!

R.F. E.H.T. is very uncritical with regard to component values & types, the circuit idea I've attached will give you a starting point!

Chris Williams

PS!

Ed Dinning or another Member might be able to make an E.H.T. Oscillator Coil for you if you ferret out the winding plans and send them to him - R.F. E.H.T works at typically 500kHz - 2MHz and can be made with air-core coils!

PPS!

The P.D.A. supply from a cannibalised valve scope can also be used, altho' you might have to remove one of the multiplier valves - many of these use a R.F. or "ringing-choke" system to provide it - try asking around under "Wanted".

[Chris55000]

PPPS!

I remember in the early 1950s "P.W." and "P.T.", several firms regularly advertised R.F. E.H.T. Oscillator Transformers & ready-built units (the "Nera" was one) to replace unreliable (and potentially incendiary!) E.H.T. Mains Transformers - early post-war T.V. got a very bad reputation with local Fire Services and Insurance Cos. for exactly this reason!

Try asking in "Wanted" - someone may have something!

Chris Williams

[beery]

Hi David,
As a temporary measure, using any EHT generator and fitting the majestic transformer to the 900 would seem a good idea...
I'm assuming thart the eventual aim is to re-wind the burnt out transformer and fit it to th 902 (where I guess access to the transformer does not involve removing the CRT).
What is the diagonal distance across the corners of the transformer bobbin (ie the maximum diameter)? I just wonder if it will fit on my winder.

Cheers
Andy

[Argus25]

RF Oscillators came with a single 6V6 and a 1B3 in a shielded box in many USA made post war TV's to power the 7JP4 CRT at around 6kV. They take the feedback from a clip on the glass envelope of the 1B3.

Steve McVoy of the ETF is likely to have quite a few of these on old TV chassis.

Many have been affected by moisture in the overwinds. Also they run at around 100kHz, the interference from them is V difficult to shield which was why they were always in a tight box. I'm sure Steve came up with a quick way to replace these units in many of his museum demo sets, drop him a line.

[FERNSEH]

Argus25 wrote: "RF Oscillators came with a single 6V6 and a 1B3 in a shielded box in many USA made post war TV's to power the 7JP4 CRT at around 6kV. They take the feedback from a clip on the glass envelope of the 1B3".

Here's an example: in the post-war Sentinel 7" TV the EHT RF oscillator is contained in a screening box. The 6Y6G oscillator valve is positioned outside the box.

DFWB.

[FERNSEH]

The attachments show the EHT transformer in the HMV 902. This is the second transformer to be fitted in this set during my ownership.

DFWB.

[Lloyd 1985]

Hi David,

That Motorola 7vt2 that I was working on uses an RF EHT generator, I think the circuit is available online. Another daft thought for you, what about the line stage from one of those cheapo 5” Chinese black and white TV’s? Plenty of them about!

Regards
Lloyd

[Argus25]

The transformer in the attached photo was a small neon tube transformer.

These have a closed around air gap which bypasses many of the magnetic flux lines (if you like to think of flux this way) way from the secondary coil, which has the effect of acting like a ballast for neon tubes, a high striking voltage and then acting a little like a series inductor. In effect it raises their internal impedance. You don't actually want that though for a TV CRT, but..

They are easily converted to an ordinary transformer by widening the air gap (like I did for the one in my my ArgusTV).

One of these sorts of transformers could easily be voltage doubled for your application.

The one I used, I rewound the primary, with a heater winding for a single 2x2 rectifier , you can see the polyamide tape. Not necessary if you use silicon rectifiers. The best semiconductor EHT rectifiers the planet are made by VMI (voltage multipliers INC)...

..but actually the "cheap as chips 10kV rated microwave oven diodes" all over ebay work just fine.

The thing is though in these neon tube trannys, the high voltage windings are professionally potted in clear resin, its not just a varnish dip... and they never(practically never) fail because the insulation and hermetic seal is outstanding, which is why I chose this type of thing for my Argus TV.

If you wanted to go down the silicon rectifier (rather than valve route) these are the sort of outstanding products made by VMI. They are used in military and spacecraft and rocket motor exciter applications:

https://www.ebay.com/itm/10PC-VMI-X1...0AAOSwX2xbqV5j

Going down the valve rectifier route for medium EHT voltages, its extremely difficult to beat a 2X2. The 2X2A also has superior mechanical construction and was rated for airborne use, just in case you decide to go Rock & Roll and throw your TV off the balcony into a swimming pool.

[peter_scott]

Originally Posted by FERNSEH

Hello Jac,
The transformer in the 901 was rewound sometime in the nineties. I guess it's safe to say the set is now on it's third EHT transformer, or fourth.

DFWB.

Thanks to John HKS we do have the EMI original transformer design but back in the late 1970s I didn't have access to that but got the transformer shop at my work to build me a replacement using the original core. The first attempt suffered from insulation problems but by using PTFE covered wire for the heater windings and using those windings to separate the mains and EHT windings from each other and the core I appear to have a transformer that still works as well as it did 40 years ago. The set has had quite extensive use.

Peter

[PJL]

A source for a PSU might be electrostatic air cleaners:

https://sqboer.en.made-in-china.com/...10W-CF02B.html

Anyone want to go for a sample?

[Heatercathodeshort]

Not sure what the potential is but I believe the old photocopiers had a high voltage souce to charge the toner drum? Possibly 2kv? J.