One of the problems Ive seen that users of ex-military Clansman radio have, is that of inadvertently leaving the set on, and ending up with a flat battery.
Clearly, in its original role, the radio would rarely be unattended. Either an operator would have it on his back and be wearing 'phones, or it would be in a position where the low battery alert (breaking of the squelch three times a second) would be heard.
Not so in amateur use. Besides, in amateur use, its unlikely that any other form of power indication, a light for instance, would cause any hassle, as it might under battle conditions. So, it seems sensible to look at the possibility of providing these radios with a 'Power On' indication.
Electrically, such a thing is about the simplest circuit you can think of! An LED, a series resistor, and a source of power that is constant when the set is switched on. Physically, this could be a challenge! It will involve finding a suitable place on the case of the radio to drill a small hole and mount the LED. It also requires finding where the connection can be made.
Although the PRC-350 is likely to be the easiest of the radios to add a power LED to physically, I dont have the service manuals and EMERs to hand for it! But I do have them for the PRC-351, so have had a look...
There are several supply lines in the PRC-351 - 3v, 6v, 9v, 17v and 100v. The 3v and 6v lines seem likely to be quite sensitive, and the 17v and 100v too high. So the 9v line would seem to be the supply of choice. I dont want it to take too much current though, but limiting the current to much less than the LED manufacturer specifies will only result in a dimmer light, which is likely to be preferable anyway. Ideally, a low current 3mm LED would be used, to minimise the loading.
I suspect that a usable brightness can be had from a modern 3mm red LED for very little forward current, but I will rig one up on the bench with a 9v supply and a potentiometer and find an acceptable brightness, then measure the pot to find what series resistor is needed.
On the PRC-351, the main source of the 9v line is module 19 pin 3. But, this line can also be found on several other modules - 20 pin 23 (DC switching), 13 pin 10 (Tx AF), 7 pin 9 (Rx AF) and 8 pin 2 (Non linear amp). Which to select would be the one that is nearest to the LEDs position!
I do need to open my -351 up at some point to perform the L/W volume mod described earlier in this blog (to make the Tx modulation level the same in both settings), and to finally add the rubber boot to the 10m conversion switch, so will look at adding a power LED at the same time.