[Boatanchors] IT SINGS THE OLDIES!

[Philip Atchley]

Hi All,

Remember that SP-600 that arrived here, literally a basket case, with a
totally stripped chassis, tube sockets and IF cans in baggies etc, along with
my intent to rebuild it from scratch?

Well, today it got to the preliminary testing stage.

Today, after quickly buzzing it out to make sure there weren't any B+ shorts
or anything, I got brave and applied power.  First without the 5R4 rectifier
in place (to check bias and filaments etc).

All ok there, so I plugged the 5R4 in, put an antenna on it and checked around
the BCB.  Alas, no stations, but no smoke either ;-)  I tested the audio
section for buzz when I touched the volume control with a test lead, that was
ok! Not even normal IF background noise though, indicating either the detector
or an IF stage was dead.  I connected a lead to the 455KC output on the other
SP-600 figuring I'd use it as a signal source, starting at the detector and
working my way back.

I heard it at the detector, so kept moving back through the circuit, I could
poke the signal all the way through the IF strip, but couldn't determine if
the stages had any real gain (it doesn't yet have an "S" meter installed).

Then I thought, hmm. . .  Perhaps the signal from the good SP-600 IF output
was
just too strong.  So, I turned it's RF gain down, the volume all the way up,
and then turned the RF gain up just barely enough to hear a weak signal in the
speaker.  Again I went through the IF strip, this time as I went back a stage
I could hear the differences in level.  I peaked each stage as I went, most
were way out.  When I got finished I removed the test signal, and Voila!  I
had RF noise!  Putting an antenna on the radio it came to life with signals
all across the BCB.  The selectivity switch worked as expected.

Right now I have it over on the other side of the room playing the oldies
station through a headset, just to burn it in awhile.  I think the AGC section
needs more work as AGC voltage seems a little lower than I think it should be.
That could be a bad tube or other component (or a wrong wire somewhere), or
something else dragging the gain down, but all tubes were tested prior to
installing them.

But at least there is life in the old carcass, and I'll bet that is the first
time in many, 'many' years that any electrons have flowed through any of those
parts
too!  ALL resistors, paper caps, lytics etc are new.


73 de Phil,  KO6BB
991 Different NDB's heard to date.
SWL QSL GALLERY: http://photobucket.com/albums/y123/KO6BB/
THE BEACONEER'S LAIR:    http://www.geocities.com/ko6bb/
Merced, Central California, 37.3N  120.48W  CM97sh