On Dec 25, 8:52*am, "COLIN LAMB" wrote:
Hi David:

It is hard to tell from the facts you gave us. *It might be useful to feed a
signal generator into the scope and compare to factory specifications. *The
1 pf coupling capacitor could be less capacitance, and simply let less
signal through. *Or, your RG-62 could be lossier that expected.

I assume the input to the SB-610 is properly tuned to the correct frequency,
since there were different "front ends" available for the SB-610. *If the
front end is not tuned properly, then the indications of the SB-610 will be
misleading, even though you have increased the gain.

73, *Colin *K7FM

Colin,

Thanks for the reply.

I had to wind my own coils for the grid and plate tank circuits of the
SB-610’s vertical amplifier that’s based on a 6EW6 tube. The tank
circuits are peaked at 3.395 Mhz, the IF frequency of the SB-301. I
used no. 30 magnet wire and ferrite slugs from what were probably 455
Khz IF cans littering my junk box. I suppose these coils could have
less Q than what the original Heathkit coils had, but I doubt it.

So, yes, the “front end” is properly tuned and the RG-62 patch cables
are constructed from stock coax I’ve used in other applications
without trouble.

Maybe the SB-301 IF strip has less gain than usual, but the receiver
doesn’t seem to be insensitive. One remedy might be to use a coupling
capacitor larger than 1pF, as you suggest, but I calculate that this
will make little difference, as the SB-610’s input impedance is 100K
Ohms.

Perhaps I should ask a different question: For an S9 signal and the
vertical gain control set to 12 o’clock, what’s the height of the
trace on your SB-610?

-Dave Drumheller, K3WQ

It is possible that the powder used at 455 kHz is quite lossy at 3.395 MHz.
Usually, a quick and dirty way to tell is if the peak is sharp. The broader
the peak, the lower the Q.

Will try to get my manual out to review it for clues. I suspect your SB-610
sensitivity is below par.

73, Colin K7FM

On Dec 25, 10:35*am, "COLIN LAMB" wrote:
It is possible that the powder used at 455 kHz is quite lossy at 3.395 MHz.
Usually, a quick and dirty way to tell is if the peak is sharp. *The broader
the peak, the lower the Q.

Will try to get my manual out to review it for clues. *I suspect your SB-610
sensitivity is below par.

73, *Colin *K7FM

Colin,

Regarding the possible low Q, the pre-amp has the same tank circuit as
the 6EW6 grid circuit: a 330 pF cap in parallel with a 6.7 uH coil.
The coil is comprised of about 25 turns. When I bench tested the pre-
amp, I recall computing a Q of 30 for the tank circuit. These seemed
reasonable at the time, although your explanation about the lossy core
material plausible.

Your help is much appreciated. Let me know what you think.

-Dave Drumheller, K3WQ