[Milsurplus] mystery diode solved! I was the problem

[Brooke]

Hi Ray:

Can you use external test equipment (batteries and resistors) to check the meter in the radio?

If it moves, but in a strange way there might be a tiny bit of ferrous metal inside.  A stereo microscope is handy for a 
close look.

-- 
Have Fun,

Brooke Clarke
https://www.PRC68.com
axioms:
1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------
> Date: Thu, 18 Sep 2025 01:40:16 +0000
> From: Ray Fantini<RAFANTINI at salisbury.edu>
> To: MMRCG<mmrcg at groups.io>, Military Radio Collectors Association
> 	<mrca at mailman.qth.net>, List Milsurplus<milsurplus at mailman.qth.net>
> Subject: [Milsurplus] mystery diode solved! I was the problem.
> Message-ID:
> 	<DM8P220MB04728DCB3E255ED22070A919B816A at DM8P220MB0472.NAMP220.PROD.OUTLOOK.COM>
> 	
> Content-Type: text/plain; charset="windows-1252"
>
> After spending a couple days trying to sort the discriminator that?s used to drive the tuning meter on the AN/PRC-74 I have discovered that the problem is the meter itself, and not the discriminator in the power amplifier! Would like to think that I am not a complete moron and when I saw the tuning meter was not working one of the first tests was to read across the meter with the VOM in low power mode and the meter went full scale. But not a bit of movement when in tune or transmit. Tonight after reinstalling the PA assembly for the tenth time, I decided to take the discriminator output directly from the PA just to see if I was getting anything at all and discovered that I get good swing and readings up to 1.0 mA on an external meter. For some strange reason the tuning meter works at five or six mA but not below one mA where it's supposed to, no sign of any movement at all but good meter action using an external zero to one mA meter. Don?t have any idea how it can fail like t
>   hat, but it has. I did end up replacing CR-15 with a 1N914 and that works but the new problem will be where to come up with a miniature zero to one mA meter to replace M201?
> You can see the radio in the attached picture and everything works great except that meter will in no way fit in that radio.
> Thanks for all the help with the diode, although in the end I was the problem, still the first time I have ever seen a meter fail like this.
>
> Ray F/KA3EKH

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/milsurplus/attachments/20250918/12f3d671/attachment.html>