[Heathkit] SB-303 no recieve HELP!!!

[k2cby]

I know you swore up and down that you checked all the plugs and cleaned all
the interconnects, but humor me by doing it again. Also check to be sure
that one of the solder connections on the plugs and sockets - including the
RCA plugs - hasn't come unglued. There are a lot of "joints" in the form of
mechanical plugs and sockets (rather than hard-soldered connections) in an
SB-303 and all it takes is a little corrosion on one of them.

Then check the bandswitch again - not only to be sure it is clean but also
to make certain that all of the wafers are--"indexed" - that is, they all
make contact at the same time. 

Finally, check and be sure none of the grounds from the circuit boards to
the chassis and the motherboard haven't come adrift.

The symptoms are a little sketchy, but the on-again off-again performance on
80 meters makes me think there is a crystal oscillator problem. While it
might be the crystal, it is more likely something else in the oscillator
circuit. (That is, for some reason - possibly the oscillator tuned circuit
-- the 80 meter crystal required more excitation than the others, and the 80
meter band therefore failed first.) I suspect that eventually the feedback
dropped off to the point where none of the crystals would oscillate.

Your best troubleshooting friend is a general coverage receiver. For a given
SB-303 receive frequency, figure out from the SB-303 tech manual what
frequencies the crystal oscillator and the variable oscillator are supposed
to be producing. Drape a short piece of insulated wire connected to the
general coverage antenna inside the SB-303 chassis Then listen for both
oscillators. If you don't hear both of them, you have found out where to
start troubleshooting.

Unless something is smoked, the problem is probably NOT a semiconductor.
They are hardy little devils and very seldom fail except from over-voltage
or over-current caused by something else. Once you have got the problem
isolated to a single board, look for a bad capacitor. Be particularly
suspicious of electrolytics and tantalum caps. If they fail short (and
that's the most common failure mode), they will definitely knock out the
circuit by pulling down the B+.

DO NOT TWEAK. However tempting it might be, STEP AWAY FROM THE ALIGNMENT
TOOL. This is definitely NOT an alignment problem, and if you diddle
something with the receiver dead you are will have to find and fix two
problems instead of one.

The lack of extender boards is a nuisance, but it is not fatal. You can make
a substitute with #14 insulated electrician's wire and the female contacts
from appropriately sized Molex sockets (used to be available from Radio
Shack). The only problem is that you will have a bunch of patches instead of
one.

/Miles, K2CBY

 

Miles B. Anderson, K2CBY
16 Round Pond Lane
Sag Harbor, NY 11963
Tel.: (631) 725-4400

Fax.: (631) 725-2223
e-mail: k2cby at optonline.net