Friday, 10 April 2015

Vidor CN429 "My Lady Margaret" restoration.

This is a little battery portable, manufactured in the mid 1950's, and this particular set dates from 1955, and has been in the family for years.

 Most of these sets appear to be red and white, whilst this one is the more unusual green & cream. I think I've also seen a blue and white one. They're very common.

This was last used in the early 80's, when the 90V HT battery, and 1.5V LT battery became unobtainable. It's a 4 valve set, using very delicate low voltage, low current filaments. The 1.5 volts originally specified for the filament supply is a little hard on these valves, and 1.4 volts would ensure a little more longevity for these fragile tubes.

Valve line up is as follows:-
DK96 (Freq. Changer)
DF96 (IF Amplifier)
DAF96 (Detector, AF Amplifier, AGC)
DL96 (Audio Output)

All 4 valves were replaced on this set, and I originally set about creating a switched mode supply to supply the 1.5v and 90v required, but the filament supply is so susceptible to noise (there's no large DHT style cathode to provide thermal inertia with these battery valves) I've shelved this idea for the present time.

5 wax capacitors were replaced as a matter of course, and the 2uF 200v Electrolytic (nicely dated!) refused to show acceptable leakage, so was replaced with a 2.2uF polyester.

I created a 90v HT battery by connecting 10 9v PP3 batteries in series, so that gives me 550mAH, and the set draws a little over 8mA from the HT supply, so it should have some acceptable life.

For the LT supply, I've used a 1.5V D cell, in series with 1.5 ohms, just to lower the filament supply to 1.25 volts. This should increase the life expectancy of the valves, whilst it will lower the performance slightly.

The guilty party....

And, of course... a video of the set in action!

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