[Amps] rotary switch - shorting/non-shorting

[Tom Rauch]

> I just fried a rotary switch in my little MFJ-901B 
> L-Tuner.
> The switch in that tuner is shorting the "lower portion" 
> of inductor to the
> ground.
> Usual situation from manual - "bla-bla-bla, never switch 
> the band while
> applying full RF power..."
> In my case full power was just a 100 W - pretty funny 
> number in AMPS
> reflector :)

Alex, the single biggest operating problem in tuners is 
always how we set the capacitors.

The same T- network tuner that blows up at 100W into a load 
with the capacitors set at a low capacitance value will 
often handle several times that power when the user adjusts 
the tuner correctly.

If MFJ upgraded that manual it should have a clear warning 
to always use the minimum inductance and maximum possible 
capacitance that allows a match.  If it wasn't upgraded and 
doesn't warn you of that, that is still what you should do. 
Always use the absolute maximum capacitance that allows a 
match on any band. You will have minimum loss, maximum power 
rating, and maximum bandwidth without retuning.

> I was looking (at mouser.com) for replacement and looks 
> like I found what I
> need, but I have no idea what kind of switch I suppose to 
> use there -
> shorting or not?

Amplifier tank or tuner, you NEVER want a shorting switch. 
( I never did get that fully drummed into the people at Amp 
Supply.)

A shorting switch means the contact, as the switch is 
rotated, picks up the next contact before releasing the last 
contact. This is unnecessary in a tuner or amplifier. 
Shorting switches almost always have significantly less 
breakdown voltage than non-shorting switches of the same 
type.

The confusion comes from the fact we often want a 
pick-up-and-hold switch, which some people mistakenly call a 
"shorting switch". The other less common name for "pick up 
and hold" is "progressively shorting", but that is NOT the 
same as a "shorting" switch.

You never want to order a switch called a "shorting" switch. 
If you do that the leading rotating contacts will come 
positioned closer to the next open contacts so it "makes" on 
the new contact before the old contacts are released or 
before the detent locks the switch. You will have a switch 
that is intentionally built with close spacing to the next 
open contact, and we never want that.

What we do want is either progressively shorting (rarely 
used as a description in catalogs) or pick-up-and-hold or 
"holding" type switch. A conventional switch can be wired to 
do that if it has an extra rotary contact. Generally all we 
need to do is grab the"half-way" tap and tie it to the 
common on higher bands, although it sometimes is better to 
pick up and hold larger inductance contacts as inductance is 
switched out.

73 Tom