[ARC5] M.C. Jones Micromatch 517B

[Kenneth G. Gordon]

On 14 May 2015 at 17:45, Roy Morgan wrote:

> ARC-5- folks,
> 
> I have unearthed an M.C. Jones Micromatch 517B SWR meter.
> 
> It has N connectors and the usual controls:
> CALIBRATE
> FOR - BACK - CAL - VSWR
> and is marked 40 WATTS
> Watts scales 0-4 and 0-12
> SWR scale 1 to 100
> 
> The coupler is just over 3 inches long and has two 1N23B diodes.
> 
> I have tried the thing at HF - the needle does not budge
> With my 2 meter HT, the needle moves, but barely.
> 
> Is this thing meant for UHF?

Hi, Roy:

In my experience, the shorter the path between the two connectors, the 
higher the frequency for which such devices were designed....but not always.

Have you checked the resistors in it? There should be at least two identical 
ones and those would determine the impedance for which the device was 
designed.

Also, as I remember it, 1N23s were subject to deterioration such that over 
time they became nothing so much as resistors themselves.

But I could be wrong...

If that was mine, I think the first thing I would do, after checking the 
"impedance setting" resistors for value, would be to replace the 1N23s with 
something like 1N914s (silicon), or 1N34s or 1N91s (Ge) (if you can find 
them) even temporarily, then retest.

I have some NIB "radar" diodes around here somewhere which you could try 
too, if you wanted them. Those encapsulated jobs with the gold-plated ends 
on them.

Ken W7EKB