[Milsurplus] mystery diode solved! The PRC-74 saga

[hwhall@aol.com]

May we assume you've attempted to swing the needle gradually from zero upward & from the pinned position downward? How did the needle perform under those conditions. Recommend an accurate VOM in series with the meter as that is done, to see if it's anywhere near accurate if/when it's nearer to mid-scale.

Wayne
WB4OGM

On Friday, September 19, 2025 at 07:18:49 AM MDT, Ray Fantini via Milsurplus <milsurplus at mailman.qth.net> wrote: 

Never saw anything like this before. The meter works but reads low on current. If I run five mA thru it the meter pegs, but below one mA nothing at all. The discriminator on the 74  puts out maybe one mA max but with that  input see nothing at all on the meter. Using an external VOM set to the 2.5 mA scale (PSM-37) see plenty of deflection and action below one mA and going a step further and jumping in an external zero to one mA meter that works just like it should. But the internal meter on the radio dose nothing? 

The meter on the PRC-74 is just about 5/8 of an inch in width and maybe a inch and a half deep and of course its sealed. Have found a replacement on line but its crazy expensive. This is not my radio but one that I am working on for someone else, if it were my radio would be tempted to mount the zero to one mA meter that I have been using it with on the bench in a little box and attach it to the radios case being its easy to see unlike the meter that’s on the radio but being its not my radio want to try to keep it original and right. Another thought would be to build a small one transistor current amplifier to drive the “weird” acting meter at a high current. Can do that with a 2N222 and a resistor or two but that’s not fixing the problem and have to wonder if the meter would change or become worse over time. Still also perplexed by why low currents don’t do anything at all on the original meter? But I have also seen these meters get wonkey before where you sometimes have to smack the radio to get them moving again. 

Ray F/KA3EKH