If the LM2940 is true of all Scouts (its was pin and mounting
interchangeable with the LM7810 that would guarantee that 12.8 volt
limit without fuse and connector drop).
1.1 volt drop through the connector and fuse is unreasonable. One should
be certain that the fuse is a 32 volt rated fuse rather than 250 volt
rating. The higher voltage fuse has to use a much longer fuse wire to
cause a much longer gap to interrupt the load properly and will always
have a greater voltage drop under load. It might be that the automobile
industry has learned about voltage drop since they have practically
universally changed from 1/4" cartridge fuses to fuses with blades that
fit into female spade connectors.
The round stamped Molex or Molex like connector pins universally used
for Ya-Com-Wood HF radios are known to occasionally cause voltage drop
problems. I suspect that may partly come from a manufacturer's
specifications that are a bit optimistic about voltage drop and partly
from looseness coming from a user not aligning the pins perfectly and so
squashing the male pin and expanding the female pin. I've always
preferred the Jones 300 and 400 series but their (perhaps honest)
voltage drop claims are higher than Molex stamped pin connectors so are
hard to justify based on manufacturer's claims alone. The Japanese
in-line fuse holders are often sources of far more than 1 volt drop,
likely from the use of high voltage fuses and from sprung brass springs.
Removable connections are a problem. The spade lugs in the old 6 volt
beetles were a constant maintenance problem. They often were just bare
brass and not very springy. When there was any electrical problem the
SOP was to remove and clean each one, squeezing the female side with
pliers to close the springs tighter. Today Ford and others apply
silicone grease to connector pins and lamp sockets to slow corrosion
(Ford pn F8AZ-19G209AA). Perhaps that would be a good idea for radio
fuse clips and connectors. I did that for all the lamp and spade lug
connections on my camper last June and so far they are all lighting
reliably.
Another possibility is the contact grease specified for aluminum power
connections. There are many brands, the grease is black and filled with
crystalline zinc which serves to break through surface oxides. I know
electricians who use that on copper connections (at least on mains) and
who think it cuts their call backs for heating connections. It can't
replace a good connection but might help the marginal connections. One
must be careful to not get it on insulators as it can be a conductor. I
used it on all the ground connections on my camper last summer. I
replaced those that depended on the spring pressure of a pop rivet
squeezing a connection through plastic, with solid metal with stainless
steel star lock washers and aluminum contact grease. I expect those
connections to stay for decades with no further care. Time will tell.
The Koolamp company makes a silver filled grease for improving marginal
connections in power panels. Last time I tried it, the electricians
globbed it on instead of using it sparingly as they should have. Its not
especially cheap and thick layers are not much benefit.
I notice the power connector dangling from my latest TM-262 mobile is a
pair of flat spades, not the stamped round connectors.
Tin contact surfaces will never be as good in the real world of
basements, cars, and outdoors as silver plated contact surfaces. Those
silver plated pins can be had for the connectors Tentec uses but cost
quite a bit more. Oxidized silver is still a pretty good conductor. Most
other metal oxides turn into semiconductors or true insulators (ala
aluminum).
One should not only use fuses rated at 32 volts but fuses made only by
Buss or Littlefuse, not something made in offshore. Buss and Littlefuse
parts have more attention to detail, such as voltage drop and the proper
metallurgy so that they blow at the predicted current than offshore
products sold by the corner convenience store or super import department
store.
When looking for the minimum voltage drop through a low voltage fuse it
might be worth looking at the fuses made for semiconductor protection,
if they can be had in 32 volt versions. The super fast fuse elements are
generally made of silver, not some lead based alloy and so should have a
lower fuse element voltage drop. But that may not be true of the 250
volt rated fuses compared to an ordinary 32 volt lead alloy fuse. The
silver element fuses will cost ten to 50 times as much as plain 32 volt
fuses, but will protect finals a little better. However my experience
with fuses still seems to be that the fastest, most expensive
semiconductor fuse is still primarily a costly and reliable indicator
that the semiconductor has shorted. Though the 15 amp buss ATC that I
use in the cable I made for my TenTec Corsair II has blown a couple
times before the finals have fried from high SWR loads.
I wonder if a Jones P402 plug would fit the back of the Tentec radios
without too much chopping?
73, Jerry, K0CQ
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