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

[Dave Olean]

Hello Ray

On many meters, gunk and filings or dirt can get in the way inside the 
magnetic structure to prevent the meter movement from moving. A non 
metallic tool can be used to fix such things but it is difficult and 
somewhat tedious if the particles happen to be metal as they tend to 
bind in the magnetic field there. Paint flecks are another problem. 
Warped meter face or bent needles can cause a problem. If the meter is 
hermetically sealed usually all bets are off if you cannot open it 
easily. At my age I would hesitate to try to fix a meter as my hands are 
a bit shaky and that can spell disaster in such a tight space with such 
delicate components in very close proximity. The springs on the front 
and back of the meter assy that control the pointer are very easily 
damaged. I used to make my own meter faces but now I would not even try 
at my age!  One thing that may help is to pulse the meter with 5 or 6 ma 
over a long period of time in hopes that whatever is hindering movement 
travel gets dislodged. It is a long shot, but I have fixed a few meters 
that way. Possibly there might be a bad connection that only fixes 
itself at higher current. Not much to be done in that case. Does the 
meter internal resistance read OK with a DVM that uses very low current 
on the ohms scale?

Dave K1WHS

On 9/19/2025 10:00 AM, Bob Camp wrote:
> Hi
>
> Best guess:
>
> There is a mechanical issue with the internals of the meter. It binds 
> here or drags there. A classic example is a meter with a bent pointer. 
> There are other things that can eventually happen inside there ….
>
> Unless you can somehow tear open the meter, there really aren’t many 
> ways to fix this sort of issue. Even if you can tear it open, it can 
> be a really crazy thing to fix.
>
> Bob
>
>> On Sep 19, 2025, at 9:18 AM, 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
>> *From:*milsurplus-bounces at mailman.qth.net<milsurplus-bounces at mailman.qth.net>*On 
>> Behalf Of*Brooke via Milsurplus
>> *Sent:*Thursday, September 18, 2025 8:16 PM
>> *To:*milsurplus at mailman.qth.net
>> *Subject:*[Milsurplus] mystery diode solved! I was the problem
>> 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 <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.
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>
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