04-02-2011, 05:15 PM
Here are pics of the former 37-620 and the LED tuning indicator schematic. The schematic is my design and is for Philco's primarily. Most other manufacturers do not have the same B+ wiring as the Philco's so it will not be always easy to adapt to these non-Philcos.
On the LED circuit I found that 5mm 4K brilliance LED's work best. If you try the 10cm or the super brights, they won't work well at all. The 4k's are plenty bright enough. The theory is very simple: if you were to put an LED across a B+ line resistor when the load circuit is at resonance, the LED will go dark since current is minimal. What's done here is to create another voltage close to the B+ using the zener; when the load is tuned to resonance, the LED will see enough voltage difference from B+ minus the zener voltage relative to the B+ and it will light.
Other value zeners work as well and none of the components are critical; they may need to be customized for whatever LED's one has handy. Feel free to spread the schematic around.
On the LED circuit I found that 5mm 4K brilliance LED's work best. If you try the 10cm or the super brights, they won't work well at all. The 4k's are plenty bright enough. The theory is very simple: if you were to put an LED across a B+ line resistor when the load circuit is at resonance, the LED will go dark since current is minimal. What's done here is to create another voltage close to the B+ using the zener; when the load is tuned to resonance, the LED will see enough voltage difference from B+ minus the zener voltage relative to the B+ and it will light.
Other value zeners work as well and none of the components are critical; they may need to be customized for whatever LED's one has handy. Feel free to spread the schematic around.
Pete AI2V
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