This set needs attention to the power supply. The ST&C metal rectifier and the HT smoothing capacitor C20A an C20B need replacing.
Replacing the capacitor presents no problem but where I have to be careful is replacing the metal rectifier. The circuit designer will have factored in the forward voltage drop across the rectifier which would have been about 20 volts. The replacement will be a 1N4007 and a series resistor. The filament current is 50mA + the current through the compensating resistors across the filaments of V2 and V4.
The HT current will be 10mA. Assuming the HT current through the rectifier will be 70mA a series resistor in the region of 285 ohms will be required, but the reservoir capacitor current will have to be taken into consideration so will be some adjustment of the series resistor will be required.
Bias for the output valve is provided by the 7.5volt filament supply which is higher than the recommended 4.5V for the DL94, 1P11 and 3V4.
Till Eulenspiegel.
The cabinet of the Coronation Twin.
Nice radios.... Every time i see one, I'm mindful of "All Creatures Great And Small" (original series) and Mrs Hall (Mary Hignet) in the kitchen with Coronation Twin, trying to drown out Siegfried's (Robert Hardy) Wagner, bellowing forth from his radiogram. A rather unequal contest I think!
I wouldn't mind one of these - sort of counterpoint to my Bush MB60.
Won’t the bias voltage on V4 be the 7.5v minus the voltage on its filament so the bias will be much less than 7.5v.
Or have I got that completely wrong?
Frank
Hi Frank, this matter was discussed on the vintage-radio forum.
My take on how the output valve is biased is that there is 7.5V on pin 7 and 4.5V on pin 1. So it follows there is a voltage gradient across the directly heated filament. The mean bias voltage will be 6V.
Of course I could be wrong.
To sum up one end of the filament is higher than the other. The HT current is bypassed through R18 and R19. Without these components the filament current will be 60mA.
Till Eulenspiegel.
Some information about the type of metal rectifier that was employed in the Ultra Coronation Twin T786 and other AC/DC mains-battery radios.
The data applies to the DRM2B which might have similar characteristics as the DRM2R used in the Ultra T786.
These rectifiers have a high forward resistance. Checkout the input voltage versus current. Silicon diode much better.
Till Eulenspiegel.
That looks like some significant loss there! but I guess it was just accepted as normal in those days?
That's right, designers had to make allowances for the short comings of many components. The Ultra now has a silicon diode to supply the HT. Also added for safety an internal mains fuse. Total current drawn from the mains is only 65mA.
Careful - the silicon diode will give you much higher HT. It would be wise to check it, especially the filament voltage. 10% over-voltage on the fils will shorten their life considerably.
Sorry! Just-re-read your earlier post! Something about Grandma and eggs comes to mind....
The voltage supplied to the filament series chain is under 7.5volts. These valves have 1.4volt filaments and this voltage has been the norm since the "all-dry" type valve was introduced just before WWII.
From the World Radio History website the Australian Radio World magazine which has articles about radio receivers using the then new 1.4volt valves:
Till Eulenspiegel.
The Vidor Lady Anne CN430 had a variable resistor to set the filament current to 23.7ma, always thought that was a control begging to be adjusted by the customer and ruin the valves.
Frank
From the February 1959 Radio Constructor magazine "In Your Workshop". A description how the series filament chain in battery radio receivers need bypass resistors to compensate for the HT currents. Without the resistors the valve filaments in the lower part of the chain will be over-run.
Till Eulenspiegel.
The Ultra Coronation Twin is now fully reassembled and working very well indeed. Reception of BBC radio 4 on Long Waves is surprisingly good considering the station is difficult to receive in the North-East of England. Some sets can receive the station but others are hopeless.
The chassis showing the the new components used to replace the metal rectifier. Mains fuse fitted for added safety.
Provision is made to store the mains lead with without having to remove the back.
Till Eulenspiegel.
I've got a really manky one of these, part of a job lot from my Uncle many years ago (he charged me 40 quid for them though!!), most notably the tuning dial glass has been replaced on mine with a clear piece, without the brown bit with the numbers on it, and the scale underneath is rusty, in fact, most of the set is rusty! The output TX was O/C too, and so I gave up on it. Maybe I should give it another go? The cabinet isn't bad, and it even has the original mains lead and connector coiled up in the back. Maybe something to play with on my next holiday that's probably going to get cancelled!
Regards,
Lloyd
