Mains transformers in Eddystone receivers

[Wendymott]

HI Peeps. As you all may know, my interest in Valve Eddystone receivers and modifications to them have thrown up some modern day problems.
My main concern now is that both the 888 and 750 have 230V primary mains transformers... Yes they have taps, but only 110 and 230V. The mains supply at my QTH is 240V. This means that the heater voltage and HT are approx 4% high. The heater voltages measured are 6.6V and the HT pre choke is 270V.
Is the consensus to leave it and say c'est la vie... As both receivers are used sporadically and not often, my thoughts are "leave it", the purists among you I guess would be having the mains transformers re wound or Auto transformers fitted externally.

Another problem that has come to light is the HT. I have replaced the VR150 regulator with a 3W zener diode, as both the 1st local oscillator and BFO are now FET ised, the only use for 150V is the second osc. However removing the VR150 has made the HT shoot up to 300V pre choke.
Both smoothing caps are rated at 450V. The series res to the Zener was increased to 7.5K 7W and the whole circuit consumes 20 m/A.

Obviously the VR150 was also keeping the HT in check by virtue of grabbing a whole lot of current down it.

As a note I post the modified Zener, this is rated at 3W and gets a bit warm. Bolt mounted ones seem not to be available at 150V, so I Made my own. Using a M5 cheese head 10mm bolt. It was drilled in the lathe at 3mm then opened out to 3.2mm. The diode body is 3.3mm... as I have no 3.3mm drill I used a needle file to make the diode a nice interference fit, with a little heat sink paste on the diode body. It now runs very cool attached to the outer chassis.

Your comments would be most welcome

[mole42uk]

A nice piece of work...

Good old valves work with a large range of voltages, the data sheets are (where) a bit of damage limitation exercise. Nice job but in my honest opinion, not worth doing.

[G6Tanuki]

I'd consider that your voltage-rating paranoia's rather getting in the way of your enjoyment of the radios!

+/- 10% is not to be worried about; these receivers were built using 20%-tolerance components, after all.

I consider '6.3v' valves to be OK for anything varying between 5.7 and 7V; remember that the '6.3v' was designed for 6-volt cars and fed from lead-acid batteries which could give significant voltage variations between 'well-drained after a really cold-start' and 'blasting down the interstate in the summer'.

Sidenote: in 1982 I was involved in a 14-day NATO exercise which involved plenty of running 'blue-side' radios non-stop in ~interesting~ situations. We had plenty of Eddy 730/4, 6200, EC958/7 and Racal RA117 and RA217/MA323 installs alongside the usual Clansman and TRIFFID stuff and a couple of racks of Watkins-Johnson gear the Americans thoughtfully provided. All powered from a range of local mains, radio-trucks and heavy generators. We took power from wherever we could. This usually meant something wildly between 190 and 260V - so we set all the radios which had voltage-plugs to 220V and got on with the real stuff. Nothing actually failed - though us Brits took satisfaction from the wails of the German types whose Siemens teleprinters were fricked by wild voltage/frequency fluctuations.

Them: You stupid Brits! You plug your kettles in to make your tea and you break our HF RTTY link!

Us: if NATO's defence can be defeated by a mere kettle...

[Ed_Dinning]

Hi Wendy, is there room between the core and windings of the transformer to get some turns of insulated wire? If so you will need 30 or 40 turns connected in anti phase with the pri winding to reduce the voltage.

It would also work, but not nearly so well if wound over the outside of the lams.

Ed

[bikerhifinut]

This is similar to the situation I found myself in when i had some transformers custom made for a project.
They came with a 230V nominal primary and so with our local mains running at between 245 and 253v depending on time of day i was a tad concerned about lobbing the thick end of 7V up an EL34.
In the end I decided not to worry as it was still (just) inside the tolerance allowed for the heaters.
Mind you, next time i get a transformer custom wound commercially I'll specify the primary to be wound with either a nominal 240V rating or with a tap for 230 and 240 which i reckon will get me nearer. i guess if you are that worried you could put a well rated series resistor in the heater circuit. Perhaps not the ideal way of doing it but it does work.
A.

[Wendymott]

Thanks peeps. Paranoia ?? haa haa... Me ?? Thanks guys all noted and put to bed. Ed, If on a dark cold night with nothing to do I may just do what you suggest... great idea, but I tell you guys something, the FET ising of the local osc and BFO is really worth doing. Could be a synthesized receiver.

[M0FYA Andy]

I'm surprised no-one has suggested using an external 'bucking' transformer in series with the mains input.
This is, of course, functionally identical to Ed's solution.
A 240v to 10v transformer with the 10v secondary wired in series antiphase with the mains input to the Rx.
Andy

[Wendymott]

Thanks for your input Andy. I may invest in an auto transformer as there are two Eddys and I don't want to be switching between them. Its bad enough switching the 5RV... yes, "get an antenna switch", will do soon.

[carnivalpete]

I'd forgotten about voltage standardisation. But the European move from 220v or 240v to 230V is fairly recent (at least by Eddystone standards). By incorporating 230v trf's in the 1960's, were Eddys ahead of the game?
If all new stuff, including my household lamps, is now being designed for 230v it's no wonder that my lamps don't seem to last as long as I think they should on my 240v domestic supply.
If you want to read more about voltage standards you might like to take a look at:
http://www.nottingham.ac.uk/etc/guid...timisation.php
As an after thought, maybe you could simply incorporate a 230v voltage regulator in your Eddy.
By the way, ''should get out more'' seems like a good idea to me. GL, pete

[Wendymott]

Hi Pete..... and others...... problem solved...... I had a couple of 200 V/A transformers giving nominally 12V AC Out for soldering irons...... One has a 240, 230, 220, tap sequence... So I inputted at 240 and outputted ar 230V Voila....As only one receiver will be used at any one time, the transformer will be well on top of the requirement....