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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 25th May 2024, 1:21 pm   #1
Astroradio
Diode
 
Join Date: May 2024
Location: Chester, Cheshire, UK.
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Default Philco A2 Radio

I had been restoring an Philco A2 radio from 1939, which looks as though it had been previously modified with various ad hoc solutions over the years, so I have been returning it to its schematic, with the exception of the elusive PenDD61, which I have replaced with a Mazda AC2/penDD (with dropper resistor for the 4v heater). I have it working after replacing some components, but I'm unsure what the grid voltages should be reading - I'm getting about -6.5v at the top cap off of the penDD. Is this in the right ball-park?

Last edited by Cobaltblue; 25th May 2024 at 1:27 pm. Reason: Rule B8
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Old 25th May 2024, 3:06 pm   #2
Silicon
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Default Re: Philco A2 Radio

It is probably OK.

The schematic uses 'grid bias' resistors between the HT windings centre-tap and chassis to create a negative voltage which is fed to all four valves.

There is no cathode resistor. I am looking at the Trader service sheet.

I like to check the cathode current of the output valve to confirm that the valve is operating OK.
Since the cathode is wired directly to the ground (chassis), this is not easy to achieve.

If you measured the actual resistance of the 'grid bias' resistors and then measure the voltage across them you could calculate the HT current taken by the complete radio circuit.
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Old 25th May 2024, 4:05 pm   #3
Astroradio
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Default Re: Philco A2 Radio

Many thanks for your reply. I have measured across R18 to chassis as -2.81v and an actual resistance of 62.3 ohms. Also measured R19+R18 to ground as 5.63v and 124.2 ohms. Both calcs give about 45mA. The voltage to the penDD grid is now slightly less negative than I measured earlier, now at -5.54v, so it has wandered.
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Old 25th May 2024, 6:40 pm   #4
Silicon
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Default Re: Philco A2 Radio

Have you replaced all the paper insulated capacitors?

Leakage in C16 will alter the cathode current in the output valve and may damage it.

Leakage in C22 will also affect the cathode current.
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