Chris said:
I'm wonder if there is a simple answer to this question.
I get a lot of my parts from tearing apart old pieces
of electronics equipment.
I'm looking at the guts of an old Bearcat scanner.
It has three small metal-can variable inductors.
I'm thinking these are probably IF transformers of
some sort(?).
On the top each one says 'TOKO RCL' and on the
sides they have printing that says (for one example)
TB93
301
MALAYSIA
8020
My question is: is there some place I can look up
and find out the properties of these devices?
Thanks!
Chris
p.s. I know if I had an inductance meter I could
just measure the thing. Or if I had a grid-dip oscillator
and a fixed value capacitor I could get close. But
unfortunately I don't have either of those.
Thanks again!
I don't know about a list (did you do any searches?), but when taking
parts out of existing equipment, often you can get information from
the circuit.
By looking at the circuitry nearby, and even doing some tracing, you
can find out some things about the components. You could have
uncovered the purpose of these coils by seeing if they were close to
the antenna jack, or actually in the IF circuitry. If they were
near an IC, look up the IC number, and see how the coils are
connected to it. Even when transistors are the active elements,
you could tell if they were IF transformers by figuring out where
they lie in the circuit, ie after a mixer or two.
If there is a ceramic filter nearby, and they usually are marked
with frequency, then that would tell you the frequency the coils
are operating at (as long as you could ensure that both are connected
in the same signal path), as well as give you additional information
that these are indeed IF transformers.
If you look at crystals in the unit, you might get some data. It would
be a likely guess that this is double conversion, so a crystal lying
by itself might be a second conversion oscillator. Do some math, and
you might come up with some good possible IF frequencies.
The circuit also provides information beyond the coils. An IC
without any information likely isn't too useful, unless you can extract
the whole circuit, but if you just get a pinout (like if there is a match
in the NTE replacement line), tracing the circuitry around the IC
would actually show you how it's all supposed to be wired up.
Likewise, if you look at the circuitry you can figure out the required
termination of any ceramic filters, or any other odd wiring they require
(I've pulled ceramic filters out of AM radios and when they have more
than three pins the pinout is not always obvious).
Sometimes you'll come across modules, and the main circuit board
even labels the pins. I've seen this with tuner modules I've pulled
out of VCRs. Or, if the number of pins are low, you can figure them
out by doing a bit of tracing; which pin goes to ground, which to a
supply voltage, etc.
You can find parts by knowing where in the circuit they might be.
I've pulled germanium diodes off boards, not only because they tend
to have a distinctive look but because once I noticed they were appearing
in certain types of boards and in certain functions, it was easy to
watch for them.
Sometimes, it's worth keeping some boards intact until you have a need
for the parts. That way, you have the "schematic" with the uncommon
components. You can ask yourself "where would I find a ceramic filter?"
and then remember that cordless phone you pulled out of the garbage.
Pull the board out, look at the resistors at the input and output,
and then get the filter off the board.
It is worth harvesting common parts in batches, but when you
get a really neat part that isn't common, if you just pull it off
the board with the common resistors and transistors, you will be
missing some of the information. I find that even when I take
the time to trace out some things before I pull a part, by
the time I want to use the part the tracing may not be with
the part.
Michael
