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Old 28th Jul 2020, 5:28 pm   #1
SeasideSteve
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Join Date: Oct 2017
Location: Saltdean, Brighton, East Sussex, UK.
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Default Ever Ready Sky Queen. PSU modifications

I have been working on this radio following the advice from previous threads such as these:

https://www.vintage-radio.net/forum/....php?p=1042753

https://www.vintage-radio.net/forum/...=118410&page=2

I have replaced all the out of tolerance resistors and the mustard type caps.

Someone had added a high wattage resistor of 2.7 ohms between the LT+ brown wire from the battery connector and the switch where the LT+ connects into the circuit. There is another additional component not shown on the circuit diagram which is a 1000 uF electrolytic cap connected between LT+ and the chassis (LT-).

Am I right in assuming the functions of these were probably a dropper resistor when someone was using a power supply higher than the 1.5 v required for LT and the electrolytic was to provide provide smoothing for this AC derived power supply?

I have removed the 2.7 ohm resistor as I will be using a 1.5 V battery for the LT supply. I am wondering whether I need to remove the 1000 uF capacitor as well.
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Old 28th Jul 2020, 10:18 pm   #2
dave_n_t
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Default Re: Ever Ready Sky Queen. PSU modifications

Steve,

My guess is that either the original supply was a tad too high (by about 0.34V), possibly because it was loaded to 125mA instead of the 250mA more prevalent in earlier times; or, the owner was trying to give the valves a longer life by slightly under-running them (design voltage 1.4V, but they do still run at a somewhat reduced voltage).

If your chosen supply is rock solid at 1.5V there's no need to keep the 2.7ohm resistor, unless you want to explore long-life techniques.

As to the 1000uf capacitor, if fitted after the 2.7ohm resistor it would have slightly smoothed the turn-on inrush and decoupled the heater supply a bit more. Leaving it in place won't harm (if your power supply is happy); but it shouldn't be necessary.

hth.
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Old 29th Jul 2020, 7:23 pm   #3
SeasideSteve
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Default Re: Ever Ready Sky Queen. PSU modifications

Dave,

Thanks very much for your advice.

Steve
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Old 30th Jul 2020, 1:26 pm   #4
kalee20
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Default Re: Ever Ready Sky Queen. PSU modifications

The valve filament nonimal voltage is actually 1.4V. Higher disproportionately reduces life.

I've been using a Vidor CN420a (Dx96 series valves), the LT supply is battery-derived but regulated, at 1.38V. So just slightly below nominal.

It's clocked up well over 1,000 hours with no valve problems over the last 10 years.
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Old 30th Jul 2020, 1:49 pm   #5
Mike. Watterson
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Default Re: Ever Ready Sky Queen. PSU modifications

1.3V is the Philips recommendation for mains PSU on them.
1.38V is at the upper limit.

The end point of the Zinc cells is 0.9V, though usually the limit is the DK91, DK40, 1L6, DK92 or DK96 stops oscillating around 1.1V. A 0.9V to 1.4V variable LT supply is one way to test how good the DKnn is in circuit.

2 x 1N5408 in series to shunt an LT PSU with a series resistor is better than a regulator as it will also protect against inadvertent HT faults.
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Old 1st Aug 2020, 1:54 pm   #6
SeasideSteve
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Default Re: Ever Ready Sky Queen. PSU modifications

Thanks to Mike and Kalee for your posts.

As I will be using a an alkaline D cell for the LT which can be up to about 1.6 V, I reckon I should replace the 2.7 ohm resistor I removed with a 1.5 ohm resistor. This will drop 0.1875 V assuming 125 mA current is drawn.
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