I have a Heathkit SB-610 monitor scope that I refurbished a few months
back. I connected it to my SB-301 receiver by tapping the secondary of
its last intermediate frequency (IF) transformer (T4 in the 301's
schematic) using a 1pf capacitor and RG62 cable as recommended in the
610's manual. After adjusting (“peaking”) the receiver IF transformer
and the coils in SB-610, the scope worked, but the trace didn’t have
much vertical deflection, even with the 610’s vertical gain control
fully rotated clockwise.

My remedy for the small trace was to build a tuned pre-amplifier using
a 2N3904 transistor. The extra 20dB of gain it provides produces a
trace that’s about 1 inch high for an S9 signal and the vertical gain
control at 12 o’clock. All is well.

So here’s my question: Building the pre-amp was a fun and useful
project, but is my scope’s performance without the pre-amp typical?

-Dave Drumheller, K3WQ

David wrote:
So here’s my question: Building the pre-amp was a fun and useful
project, but is my scope’s performance without the pre-amp typical?

I have one of those...And yeah, sounds like your gain is down a
bit from normal.

Jeff

--
“Egotism is the anesthetic that dulls the pain of stupidity.”
Frank Leahy, Head coach, Notre Dame 1941-1954

http://www.stay-connect.com

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

David wrote:

So here=92s my question: Building the pre-amp was a fun and useful
project, but is my scope=92s performance without the pre-amp typical?

No. Something is wrong. With the gain control set halfway, S9 signals
go way over the top of the CRT with my Collins R-390. This is using
log scale, too.

If your problem is the same in both linear and log, that's one thing to
rule out.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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

On Dec 25, 10:04*am, (Scott Dorsey) wrote:
David wrote:

So here=92s my question: Building the pre-amp was a fun and useful
project, but is my scope=92s performance without the pre-amp typical?

No. *Something is wrong. *With the gain control set halfway, S9 signals
go way over the top of the CRT with my Collins R-390. *This is using
log scale, too.

If your problem is the same in both linear and log, that's one thing to
rule out.
--scott

--
"C'est un Nagra. *C'est suisse, et tres, tres precis."

Scott,

The SB-610, which is crude oscilloscope, doesn't have a log scale.
Are you confusing it with the SB-620 pan adapter ("Scanalyzer") which
presents a frequency domain view of signals?

-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

In article
,
David wrote:

I connected it to my SB-301 receiver by tapping the secondary of
its last intermediate frequency (IF) transformer (T4 in the 301's
schematic) using a 1pf capacitor

Dave-

What do you mean by "tapping"? If you did not connect the 1pf capacitor
to the top of the coil, then that may be causing your reduced signal.

Fred
K4DII

On Dec 25, 12:43*pm, Fred McKenzie wrote:
In article
,

*David wrote:

I connected it to my SB-301 receiver by tapping the secondary of

its last intermediate frequency (IF) transformer (T4 in the 301's
schematic) using a 1pf capacitor

Dave-

What do you mean by "tapping"? *If you did not connect the 1pf capacitor
to the top of the coil, then that may be causing your reduced signal.

Fred
K4DII

Fred,

Sorry for using misleading terminology. I did just what you suggest--
I connected the 1pF cap to the top of the IF transformer's secondary
coil.

-Dave Drumheller, K3WQ