"Joerg" posted on Fri, Mar 20 2009 6:06 pm

ken scharf wrote:
JIMMIE wrote:
On Mar 18, 9:24 pm, Robert casey wrote:

I once saw an article in 73 magazine showing a HB receiver that used a
re-worked turret tv tuner as a band switch. The coils were re-wound
onto the original forms, but some have just replaced the forms with some
of the smaller sized toroid cores. I have a bunch of old tv tuners in
the junk box, but over the years the contacts have gone bad and now show
a high resistance. Maybe they could be cleaned up, but it no longer
seems worth the effort. My new idea is to use miniature relays to
switch the circuits. I recently found nearly a gross of small relays
for free so why not?

Relays will most likely bring some grief over the years with contacts
not conducting 100% and such. Better use band switching diodes or PIN
diodes.

I disagree based on some experience in environmentally testing relays
ranging from high-power to low-power to 'choppers' in so-called
stabilized
(DC) amplifiers for military avionics, then doing comparative testing
against the cheaper commercial relay designs on the market over 50
years
ago.

About the only "grief" is one can get is failure to wire it correctly
or
not understanding what relay contacts acually do or how to power their
actuating coils.

Power relays CAN result in contact pitting and poor resistance as a
result
of many multiple activations while carrying inductive loads of many
Amperes of current with resulting very high voltage back-EMF
('flyback')
conditions on contact separation. That is NOT the case with receiver
bandswitching applications. Not even close.

There are some unusual contact effects in so-called "dry" circuits
carrying femtoAmpere currents or less but those won't affect high-
impedance input and output circuits common to vacuum tubes. One of
the
better choices for no-nonsense, easier to implement bandswitch
conversions without much cost is to use modern, easy-to-get small
relays
requiring only 100 to 150 mW coil powers, available in coil operating
voltage increments of 6, 12, 18, 24 VDC...or, if salvaging older tube
equipment, the "plate relays" of a half century ago designed for
higher
12 to 30 VDC, comparable powers for a few mA of plate current in
coils.
[much harder to get now given all electronics is firmly in the solid-
state era starting over 40 years ago] Some relay makers, such as
Omron, are widely used in electronics, especially for automotive
applications and are found listed in all major distributor catalogs.

Relay contacts can easily withstand 200 to 400 VDC stock of now v. old
(half-century ago). The make or break contacts do NOT involve any
unusual RF impedance characteristics other than a few pFd of
capacitance
equivalent to ordinary point-to-point wiring or the lead-length
inductance of some short point-to-point wire inductance in the low
nanoHy range. It has an excellent open or closed contact condition
with excellent isolation between actuating coil and contact set.

By contrast, conventional rotary bandswitch structures designed around
60 to 70 years ago, are open to all sorts of contamination, including
OXIDATION of contacts plus wear of typically cheap contact
construction
of a bygone era. Sealed or semi-sealed rotary switch wafers of the
post-WWII era have much longer life than the old open-to-everything
wafer designs that may look good in their first half decade, then
degrade from oxidation after that, before normal wear effects show up.

Based on over a decade of using the TV channel "turret tuner" (such as
the cheap products of Standard Coil tuners of long ago) types just for
TV channel selection, I wouldn't try to convert those to anything else
than scrap. I'm being kind on that evaluation. The MECHANICS of such
turret tuners may look good, but VHF-UHF circuits don't work on either
mechanics or appearance. HF circuits might squeak by, but the typical
contact set of turret tuners was never optimized for either wear or
contact resistance. It was 'optimized' entirely for maximum profit
from
lowest-possible cost of components in a highly-competitive TV market
of
long ago.

Today there are a number of inexpensive small relays used in HF radio
equipment, especially in automatic antenna tuners (used for switching
banks of binary-progression-values of inductors and capacitors) from
direct microcontroller control of actuating coils. Those work very
well
and show no degredation running with 100 W of RF or more at 50 Ohm
impedances. Millions of automobiles on the road today are tooling
around
with little packages of relays controlling everything from headlights
to
sensor selection in adverse temperature ranges and high vibration, no
real problems from those relays.

In short, small semi-sealed low-power relays are FINE for vacuum tube
circuit switching in homebrew electronics, properly applied. They can
easily replace most rotary bandswitching applications in layout with
less stray circuit series-inductance and parallel-to-ground
capacitance
than with point-to-point wiring.

73, Len AF6AY