[Collins] Fuse blowing in Power supply on 32S-1

[Jim Isbell]

An interesting thing happened yesterday.  I recently bought 6 "Collins" 
mike plugs from Hosfelt electronics when I discovered that they had them 
for only $1.75 each.   So the work began, I removed the amplifier and 
put a Collins mike plug on the D-104 mike that I got at a swap meet a 
couple of months ago.  I also put a Collins mike plug on the SM-1 that I 
have had around for a while.  The SM-1 didn't do a thing.  No output. 
 But  the D-104 worked fine....well almost....for a while anyway. 
 Suddenly the fuse in the power supply blew.  I saw no connection 
between the fuse and the mike until I replaced the fuse and noticed that 
when I touched the mike there was a BIG increase in the grid current on 
the final even though I was NOT transmitting.  So I swapped the mikes 
and replaced the fuse.  No more problems.

I could not see how a mike could blow the fuse in the power supply, but 
decided to check the mike out.

I discovered that the grounded side of the mike (yellow wire) was 
connected to the ring on the plug and the high side of the mike (green 
wire) was connected to the ground.  I connected it that way because 
there was a GREEN wire and I assumed it was ground.  Also, the mike head 
didn't have a ground on any of the three pins so I didn't think it made 
any difference.  BUT....inside the stalk of the mike the yellow wire is 
somewhere connected to the mike stalk/base and thus becomes ground by 
default.  So when the mike was sitting on the desk all was well, but 
when I picked it up my hand was connected to the input of the audio amp 
which put a lot of 60 cps in to the input.  I noticed that the grid 
current of the final went way up when I touched the mike.

Anyway, the moral of the story is, "Green is not always ground"

The D-104 is now working fine and it seems to punch through where the 
Shure with the Heil HC-5 element doesn't, so I guess good looks isn't 
all that the D-104 has.  I think I will put the Heil element into the 
SM-1 for a back up mike.  The D-104 really looks good in front of an 
S-Line.  NOW I can take the picture I was wanting for my QRZ listing.

Jim W5JAI Isbell