[TenTec] Adding Pilot Light to RX-320D

[Charles P. Steinmetz]

K8JHR wrote:

>I would like to add a pilot light to the front panel of my TenTec 
>RX-320D.   I have SOME home brewing skills... having built several 
>kits, including a few TT receiver kits, and having designed some 
>small project circuits on my own.  My experience suggests doing this 
>is not as easy as merely soldering a LED into the circuit, and 
>sticking it through a hole in the front panel.

You need a power supply, an LED, and a current limiting 
resistor.  Typically, you use a push-in plastic bezel to mount the 
LED to the panel.

The schematic shows that a nominal 15 V enters the radio, is filtered 
with an RF choke and a 100 uF capacitor, and is available as the 
internal 15 V supply.  I'd mount the current-limiting resistor on the 
PCB near C7 (one end to the +15 V PCB trace with a very short lead, 
one end floating).  A wire (22-24  ga. stranded, insulated hook-up 
wire) would run from the floating end of the resistor to the anode of 
the LED (the one with the longer lead and without the flat on the 
diode body).  Another wire (also 22-24 ga. stranded, insulated 
hook-up wire) would run from the LED cathode (shorter lead, flat on 
diode body) back to the PCB to ground (also preferably somewhere near 
C7).  If you want to get fancy, make the anode wire red and the 
cathode wire black.  Doing it this way (resistor on the PCB +15 V 
trace), if something goes wrong with the wiring you won't 
short-circuit the +15 V supply.

LEDs have forward drops between 1.5 and 3 V, depending on technology 
and color.  Assume ~2 V, so the resistor will have ~13 V across 
it.  Since I = E/R, a 3.3k resistor will regulate the diode current 
to ~4 mA.  Since P = EI, the resistor will dissipate ~0.05 W (50 
mW).  So, a 1/4 W resistor is sufficient.

Use appropriately-sized heatshrink tubing at the resistor (run it 
right down to the PCB, completely covering the resistor) and on each 
LED lead (again, run it right to the body of the LED).  Route the 
wires neatly and use tie wraps as appropriate.

Of course, do all the work with the radio off, and check it visually 
for correct polarity and for solder bridges or other shorts before 
you turn it on.

Note that there are several regulated voltages in the 320 (+5 
digital, +5 analog, and +10).  I did not suggest using them for two 
reasons: first, the lower voltages increase the uncertainty of the 
voltage across the resistor, and thus the uncertainty of the LED 
current.  And second, there is no need to add a further current 
burden (with increased power dissipation) to the regulators.  If you 
measure the drop of the particular diode you use, you can calculate 
the resistor for the desired current with the lower voltage, and such 
a small additional load will probably not adversely affect the 
regulator, so you could probably use a regulated voltage if you 
prefer.  If you do, I'd use the +5 V digital supply.

Best regards,

Charles