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Michael Black[_2_] · July 19th 14, 06:31 AM posted to rec.radio.shortwave

On Fri, 18 Jul 2014, wrote:

On Friday, July 18, 2014 12:29:27 PM UTC-4, Michael Black wrote:
On Fri, 18 Jul 2014, dave wrote:

On 07/17/2014 04:49 PM, Michael Black wrote:

On Thu, 17 Jul 2014, Scott Smith wrote:

SB-303 heathkit repairs, I bought a heathkit SB-303 radio off ebay,

all it gets are various tones when I tune the dial, any idea on

repairs, thanks in advance, email .

Does it have the crystals? If that's the ham band one (rather than the

one for shortwave), the crystals are the same frequency as used in the

full SB line of ham band equipment. Buying crystals these days has

become quite expensive, so if someone needed the crystals for whatever

reasons, they may have stripped a receiver, rather than buy the crystals

new, or find a set of crystals on the used market.

It's a receiver that tunes a 500KHz segement (I think around 3MHz) with

a crystal controlled converter ahead of it, a crystal needed for each band.

Michael

Anybody roll their own quartz crystals? You need an oven. Big bucks for

energy.

I gather the radio magazines did show how to make your own crystals, back

in the thirties.

I'm sure they started with a slab of quartz, and cut it down and started

grinding.

Michael

Very , very difficult. Even after it is close enough(!)in resonant
frequency- there are a few MORE extremely critical parameters involved
... This is why true "Crystal Filters" were so expensive,even when they
were mass produced ...

First, in the thirties, crystal holders were big, making it so easy.
Second, the need was for stable frequency, ao they just had to get it
within the ham band.

But, after WWII there were a lot of military surplus crystals on the
market. Crystlas are generally ground to order, but the military needed
so much they just churned out crystals on specific frequencies. That was
a bounty, lots of cheap crystals, which lasted decades. And they were so
spread out that usually you could find one that was close enough. If not,
you opened up the case, which back then was held together with a screw,
and actually ground the crystal a bit, to bring it up in frequency. Had
to be careful that you did it evenly or else the crystal might not
oscillate anymore. And if you wet too far, you could lower it back a tad
with some solder or pencil lead. Lots of people did that to get a
crystal where it was needed, starting with one as close as possible (and
you never could move them by much).

Yes, crytals had to be precise, so you'd send the circuit you were using
to INternational Crystal or one of the other companies and the frequency
you needed and they'd grind it to specs. But it was labor intensive,
which was the cost.

Witness all the commodity crystals that have come along in forty years.
It used to be you could get a 100KHz and maybe a 1MYz crystal out of the
catalogs, maybe a 3.58 subcarrier crystal. But that started widening when
digital circuits came along, some frequencies more useful than others
because they could divide down easily. And commodity crystals were always
cheap, they were made for certain specs and you had to live with it. Mass
producing probably helped keep prices lower, too.

Forty years ago, if you wanted to make a crystal filter, you'd have to
make do with what was surplus, and probably did have to grind them.
Unless you did a filter in the 9MHz range, where there were all those CB
crystals relatively cheap (since channel spacing was generally 10KHz, just
find a pile of crystals on adjacent channels, and go through them until
you found two matching crystals on one channel and two matching crystals
on another channel.

I'm not fully sure why more than one crystal filters were so expensive,
except they probably were never mass produced (that's relative, an SSB
filter wasn't going to see much use beyond amateur radio and some other
services).

Nowadays, everyone makes ladder filters, making use of those cheap
commodity crystals, just buying enough of them and sorting them out to use
the ones closest in frequency. So much easier than finding some crystals
that were about the spacing of the needed bandwidth. Ladder filters are
not a new thing, I remember reading about them in 1974, but it took time
till they properly propagated in amateur circules. But since they
existed, surely they were used for some commercial crystal filters,
probably making a more integrated set of crystals rather than a string of
crystals.

Michael

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