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Old 20th Oct 2020, 3:08 pm   #1
FERNSEH
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Default Ultra Coronation Twin. Model R786.

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.

DFWB.
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Old 20th Oct 2020, 3:19 pm   #2
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Default Re: Ultra Coronation Twin. Model R786.

Hi David.
Why not consider using a bridge rectifier a cap dropper and you then can dispence with the resistive dropper. This gives a degree of soft start, less hum (100hz ripple) with the only disadvantage of chassis floating above mains neutral.
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Old 20th Oct 2020, 3:36 pm   #3
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Default Re: Ultra Coronation Twin. Model R786.

Hi Trevor,
that's something to consider. What is certain the set will never be used on batteries and in fact the mains-battery switch S4 has been removed.
Smoothing capacitor C20 is a 50 + 50mfd component. It needs replacing.

DFWB.
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Old 20th Oct 2020, 3:45 pm   #4
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Default Re: Ultra Coronation Twin. Model R786.

Vg1 of the output valve is normally referenced to pin 1 when the filaments are series connected, the bias voltage would be developed across V1, V2 and V3 valve filaments not the full filament supply voltage.

Lawrence.
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Old 20th Oct 2020, 4:08 pm   #5
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Default Re: Ultra Coronation Twin. Model R786.

Directly heated valves will have a voltage gradient across the filament which has to be taken into consideration so with pin 7 at 7.5V and pin 1 at 4.5V the mean bias will be about 6V.

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Old 20th Oct 2020, 4:18 pm   #6
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Default Re: Ultra Coronation Twin. Model R786.

Vg1 is referenced to pin 1 for series filaments so in the valve specs for the different design operating conditions that's what it will be referenced to:

https://frank.pocnet.net/sheets/129/d/DL94.pdf

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Old 20th Oct 2020, 10:10 pm   #7
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Default Re: Ultra Coronation Twin. Model R786.

I've got a pye P27UBQ with a similar setup, rectifier very low, I cheated and subbed a rheostat in series with the diode and simply adjusted that to get the right readings, due care taken of course. I think a 4W resistor was all that was required, can't remember the value, I mounted it and diode on a short piece of tag strip.
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Old 21st Oct 2020, 10:00 am   #8
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Default Re: Ultra Coronation Twin. Model R786.

Nothing wrong with that. I'm sure some sets did use a variable resistance to make sure the filament voltage was correct.

DFWB.
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Old 21st Oct 2020, 11:27 am   #9
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Default Re: Ultra Coronation Twin. Model R786.

I would use a 7808 or LM317 to control the filament voltage.
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Old 21st Oct 2020, 11:58 am   #10
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Default Re: Ultra Coronation Twin. Model R786.

no..I didnt leave the rheostat in, just used to find the correct value of resistor!
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Old 21st Oct 2020, 2:18 pm   #11
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Default Re: Ultra Coronation Twin. Model R786.

Have you read my blog in success stories it covers all the work you are doing on this set i have done all this work on two of them both are working fine today
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Old 23rd Oct 2020, 11:46 am   #12
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Default Re: Ultra Coronation Twin. Model R786.

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.

DFWB.
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