ken scharf wrote:
Joerg wrote:
ken scharf wrote:
JIMMIE wrote:
On Mar 18, 9:24 pm, Robert casey wrote:
The design used plug-in coils for the osc and rf stages, and
they were double conversion designs, with a first IF of
1600 kc and a second IF of either 100 kc or 85 kc (when
using surplus ARC-5 IFTs.) The 1800 mc xtal you bought
your dad was used for the second coversion osc. to convert
the 1600 kc IF down to 100 kc. Many hams deviated from
the exact original IF frequencies (i.e. strong local BC
station on 1600 kc) which might explain why the xtal

was chosen for 1800 kc instead of 1500 or 1700.
We have a local mid power station at 1600KHz, WWRL, at my parents'
house,
so my father might have wanted to avoid problems with it.

His radio had a bandswitch instead of plug in coils, and he'd receive
various broadcast SW stations. He wasn't a ham at the time just yet.

I had a friend who was a retired engineer with GE turned TV tech who
built
one with a band switch and he later modified it to
be more of a general purpose SW receiver. His name was Olin Griggs.


Jimmie.
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.

There is a trick with relays, they must conduct a minimum amount of
current or the contacts won't self clean. Using relays to switch
receiver input circuits that are only passing micro amps will eventually
cause problems. But, if you bias the circuits with some resistors so
the relay must switch at least a few milliamps they will last longer.
Another idea if the relays are computer controlled would be to switch in
a high current source (few hundred ma) before opening or closing the
desired relays, then switch off the high current source. Relays should
not be any worse than switches as far as contacts go, however rotary
switches would tend to be self cleaning due to the wiping action.
Relays are often used in switching low pass filers used after solid
state finals. Here they ARE carrying high current (rf output) but not
switching it.


Yeah, I wish the designers of the NRD-515 here had known that as well.
Every few months a relay will become "sticky". The S-meter drops to
zilch, rock the attenuator switch 5-10 times, and it's back :-(

Relays with some current are ok but I prefer the electronic method. PIN
diodes with lots of current and plenty of carrier lifetime usually work.
One just has to make sure there aren't any rectification effects that
can cause IM or harmonics in the presence of strong signals. FET
switches are sometimes a good deal as well but since the SD5000 and
SD5400 have gone towards high-priced boutique status not so much anymore.

--
73, Joerg