RadioBanter - View Single Post - SB-303 heathkit repairs

dave · July 19th 14, 03:05 PM posted to rec.radio.shortwave

On 07/18/2014 09:31 PM, Michael Black wrote:
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

3580KHz is the 80m psk band