In sthe summer of '99 Radio Shack discontinued
their ferrite core coil xtal set and introduced a new model #28-178.
The design is quite different from the old model, and the price is a bit
higher. For $9.99 plus tax, I picked up one of the new models and
brought it home. Larry Pizzella had already measured the coil and
capacitor at 330 uH and 265 pF respectively, saving me the trouble.
This should give you enough to cover the BC band. The coil is wound
with solid enameled wire on a 1 inch diameter tube, with no taps.
The set is very basic, consisting of a parallel tank circuit of the coil
and variable capacitor, with the antenna and 1N60 diode connected to one
end, and the ground and earphone return at the other. There are only
3 tie points, using springs pushed into the oversized, futuristic-looking
plastic chassis: Antenna, Ground, and Diode. The earphone connects
between the bitter end of the diode and ground. Total assembly time,
5 minutes, which included rooting around the drawer for a small Phillips
head screwdriver for mounting the capacitor and tuning knob. Okay,
so I didn't bother installing the cap on the tuning knob or the really
nifty stickers provided with the set to tell you it was made by RS.
On the air: I stuck the
earphone in my ear, and attached a nice 120 foot antenna to the antenna
connection. Yes, there is one of my bandmasters coming in.
Tuning the capacitor, I realize the other bandmaster, 50 kHz above the
first, is there also, and can't be separated. Moving down the dial,
I find local bandmaster #3, mid dial. Okay, time to attach my fine
water pipe ground. Uh-oh, everything went away except, what's that,
a cw navigation beacon from down in the LF region, and, yes, one of the
locals is still there, but I can't tune it in, nor the beacon out.
Switching antenna/ground connections has no effect.
Time to go upstairs; my daughter
has only a 60 foot antenna, and a compromise ground (a switch plate screw).
Results: same as above but a little weaker.
Now I'm starting to get irritated,
but I calm down and pull out one of the original RS xtal sets and return
it to its original specification configuration. Now it's time for
head-to-head. The Original behaves well with my antenna and ground,
and I can pick up my "big 5", which includes a 50 kw station some 70 miles
away and my fourth local. I can also almost separate the first two
bandmasters. Back upstairs, and the Original gets all but the fourth
local, and selectivity is about the same. Incidentally, both the
120 and 60 footers work best on the "short antenna" connection of the Original.
After a short pondering session
with a tall drink, I decide to make one small modification to the '99 model.
I disconnect the coil from the ground end of the tank circuit, attach the
antenna to the now free end of the coil, and the ground to the normal ground
connection. Now I am using a series tank circuit, with the detector/earplug
across the capacitor. This seems to work pretty well. I can
get 3 locals and the 50 kw station to the south, and separate them a bit
better than before. End of test.
What can I say good about this
set?
1.
It works; any kid should be able to hear something on it.
2.
Easy to put together.
3.
The components, what few there are, are of ok quality.
4.
Most Radio Shack stores carry it in stock.
What else can I say?
1.
No flexibility in configuration - no coil taps, and only 3 tie points (the
original had 2 taps on the coil and 9 tie points). I consider this
to be the most critical deficiency. Also recall that the Original
would pickup some HF by just pulling the ferrite core out of the coil.
2.
What's with the oversized plastic chassis?
3.
Cost. Paying more for less set. The new model probably costs
less to manufacturer, and the 30% price jump overcompensates for any inflation.
And don't even try to tell me about the "neat" little manual that comes
with it.
What I would have done had I
been the Tandy man:
1. Add
a low tap for antenna, and a mid tap for detector on the coil.
2.
Drill a couple more holes in the chassis and throw in springs to fit them,
one for each tap.
3.
Get the coil out of the "pit" so it can be used with loose coupling.
4.
Write a better set of instructions on the different ways this thing can
now be configured, and how to continue on, such as how to build a simple
antenna tuner.... When I was 10, I mailed a quarter to R.L. Allen and received
a mounted galena crystal and a very tiny booklet with about 20 different
xtal set designs, complete with instructions and comments, and general
xtal set philosophy. If I had that booklet today, I could offer it
to you for $2.00 and you would think it was a bargain.
I guess my real gripe here is that Radio Shack, with local stores everywhere, has turned down a golden opportunity to create an interest in basic electronics and communications for the next generation , choosing to offer another toy that will get its 15 minutes of fame under the tree before it goes to the back of the closet. For the same asking price, they could offer the start of a lifelong interest in electronic technology. If you look at the RS catalog, it is obvious that they want repeat business in not only new dumputers and boom boxes, but also in discrete components and hardware. Considering that the target customer of the xtal set is a 10 year old boy, letting a better designed set go at cost might be considered a sound investment.
Having had my vent, let's look at this set again and see what we can do with it. Time for more fun with Radio Shack.