The PHILCO Phorum

In case anyone missed it in an earlier post, here's the schematic of the shadow meter to LED conversion mod. Please comment on success or failure but NOT if you used a 10mm LED or one brighter than 4K mcd- there is usually not enough current to drive those. Also, don't get fancy and expect much light through both a band mask and a dial together. None of the values are critical, not even the zener diode. The 8.2 volt zener works fine but so do other zeners within a volt or two.

This circuit has worked well in several manufactures' superhet radios, not just Philco's. If you attempt non-Philco's, you must separate out the B+ lines to the tuned RF amp and IF tubes only, not any oscillator, detector, mixer, or audio tubes. This circuit will not work in a radio with only a two section tuning cap.

Thanks for the neat info. As I look at your schematic it occurred to me that a similar circuit could be devised using a zener diode to fire a conventional 6v lamp. I.E use the shadowmeter lamp without the meter.

TA- I think you would find that an incandescent bulb would require much more current than the circuit would allow. That's the reason that I chose the 5mm LED's; even the 10mm LED's drew too much current to be usable.

A lamp or larger LED could be used if someone wants to do a design including an active circuit with a FET or transistor; that wouldn't be very difficult but would require many more parts and a low voltage supply.

Yes, that's right. I often forget the low voltage and current requirements that modern components have. 99.99% of the stuff I work on is 1930-38, a whole different set of operating parameters.

Some have asked for a quick explanation of how this circuit works: the zener sets a fixed voltage (e1) below the B+. As the receiver is tuned to a station, the voltage at the load end of the 8K (e2) rises and approaches the full B+. This difference between these two voltages (e2-e1) lights the LED.

If an LED were simply placed across the 8K, the LED would become dark when tuned to a station, just the opposite of what's desired.