SB-610 coils

[wa1vwl]

I measured the inductance of the SB-610 vertical amplifier coils today. I
also dug up the manual to refresh my memory. The parts list in the manual
shows there are 3 coil selections in the plate and 1 at the grid. I didn't
have grid coil handy. It's used for 3 - MHz only. I've never used my SB-610
with Heathkit SB/HW receivers because it is of limited value to monitor
received signals with those rigs. The IF bandwidth is too narrow to show
what's really going on with the other fellow's signal. It is nice to see,
however. Especially when you have visitors in the shack. The equipment used
to measure inductance was an L/C Meter IIB, a product of Almost All Digital
Electronics. I checked its accuracy against the lab standard at work and
like they say, it's amazingly accurate with low inductance and capacitance
values.

40-746 Low frequency coil, 455 - 2475 KHz, nom: 534 uH  min: 250 uH  max:
580 uH

40-747 Medium freq coil, range: ? , nom: 47 uH,  min: 42 uH,  max 206 uH

40-748 High freq coil, 3 - 6 MHz, nom: 23 uH,  min: 12 uH,  max: 29 uH

The nom(inal) value is what the coil initially measured. Min was with the
slug almost all the way out and max with the slug adjusted to read the
maximum
value.

For use with my Drake R-4B, a 50 KHz IF, I used a parallel resonant circuit
at the plate of V2. It consists of a 44 mH toroid, 150 pF cap, a 5 - 60 pF
trimmer and a 100 KOhm resistor across the combination to kill the Q. The
circuit is too sharp without it and gives false indications. With this
circuit, the vertical gain is around 3 o'clock. I coupled to the last IF
stage with a simple FET follower near the IF transformer so I didn't have to
worry about the length/type of coax used or wether there was any coax
connected at all. When everything was hooked up, I simply adjusted the
trimmer cap for maximum response on the CRT. The circuit is built on a small
piece of perf board attached to the chassis with a bracket taken off a
terminal strip.

As for monitoring 455 KHz signals, I wonder if anyone has ever tried a
standard 455 KHz IF transformer. I don't see why it wouldn't work. If memory
serves me right, they're around 1 mH. So what if the impedance is high, all
we're trying to do is look at voltage. If oscillations occured, I would try
the same thing I did with the 50 KHz circuit: resistance across the coil to
dampen the Q. For other frequencies, I'd try winding a toroid and use a
trimmer cap. It's worth a shot. Waiting to find those missing SB-610 coils
might take a very long time (if ever) and involve a certain amount of good
luck.

73/Gerry/W1ID


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