The Star Roamer poor man's BFO

[d***@drumheller.org]

I just finished restoring a Knight-Kit Star Roamer shortwave
receiver. It's a mediocre performer, but fun to play with. Actually,
it sounds pretty good using headphones.

Anyhow, it has a "selectivity" control that allows reception of
sideband and Morse signals. After a two-minute analysis of the
schematic I concluded that this control makes the 6HR6 intermediate
frequency (IF) amplifier tube function as a tuned-grid/tuned-plate
(TGTP) oscillator. This certainly isn't a sophisticated
implementation of beat frequency oscillator (BFO), but is works,
albeit with lots of fine adjustment to the bandspread and,
surprisingly enough, the antenna trim capacitor.

QUESTION: What other sets from the past have used this technique to
implement an inexpensive, but marginally functional, BFO?

-Dave Drumheller, K3WQ

[Ed Engelken]

Post by d***@drumheller.org
I just finished restoring a Knight-Kit Star Roamer shortwave
receiver. It's a mediocre performer, but fun to play with. Actually,
it sounds pretty good using headphones.
Anyhow, it has a "selectivity" control that allows reception of
sideband and Morse signals. After a two-minute analysis of the
schematic I concluded that this control makes the 6HR6 intermediate
frequency (IF) amplifier tube function as a tuned-grid/tuned-plate
(TGTP) oscillator. This certainly isn't a sophisticated
implementation of beat frequency oscillator (BFO), but is works,
albeit with lots of fine adjustment to the bandspread and,
surprisingly enough, the antenna trim capacitor.
QUESTION: What other sets from the past have used this technique to
implement an inexpensive, but marginally functional, BFO?
-Dave Drumheller, K3WQ

======================
The Hallicrafters S-38A and later versions and the National SW-54 come
to mind. There were probably others. --Ed

[Michael Black]

Post by d***@drumheller.org
I just finished restoring a Knight-Kit Star Roamer shortwave
receiver. It's a mediocre performer, but fun to play with. Actually,
it sounds pretty good using headphones.
Anyhow, it has a "selectivity" control that allows reception of
sideband and Morse signals. After a two-minute analysis of the
schematic I concluded that this control makes the 6HR6 intermediate
frequency (IF) amplifier tube function as a tuned-grid/tuned-plate
(TGTP) oscillator. This certainly isn't a sophisticated
implementation of beat frequency oscillator (BFO), but is works,
albeit with lots of fine adjustment to the bandspread and,
surprisingly enough, the antenna trim capacitor.
QUESTION: What other sets from the past have used this technique to
implement an inexpensive, but marginally functional, BFO?
-Dave Drumheller, K3WQ

The advantage wasn't just "one less tube" but that it could improve
selectivity a tad.

It's regeneration, as seen in the regenerative receiver and in q-multipliers.
Positive feedback with a control to allow adjustment. On the verge of
oscillation, the selectivity gets very narrow (though lousy skirt
selectivity), which can be useful. Move it up a notch, and it goes
into oscillation, and you get your BFO.

Making an IF stage oscillate is one way. Far more common was a q-multiplier,
an extra stage that did the same thing, though it was connected to the plate
of the mixer. It was rarely used as a bfo there, it not quite being the right
place but also because it was usually an add-on for the purpose of
selectivity, and most cheap receivers did have a bfo.

Someone mentioned the Hallicrafters S-38. I seem to recall the company
also made a ham band only receiver that put an IF stage into regneration,
though that one may have still had a bfo.

Michael

[Bill Cohn]

Post by d***@drumheller.org
I just finished restoring a Knight-Kit Star Roamer shortwave
receiver. It's a mediocre performer, but fun to play with. Actually,
it sounds pretty good using headphones.
Anyhow, it has a "selectivity" control that allows reception of
sideband and Morse signals. After a two-minute analysis of the
schematic I concluded that this control makes the 6HR6 intermediate
frequency (IF) amplifier tube function as a tuned-grid/tuned-plate
(TGTP) oscillator. This certainly isn't a sophisticated
implementation of beat frequency oscillator (BFO), but is works,
albeit with lots of fine adjustment to the bandspread and,
surprisingly enough, the antenna trim capacitor.
QUESTION: What other sets from the past have used this technique to
implement an inexpensive, but marginally functional, BFO?
-Dave Drumheller, K3WQ

The Heathkit GR-91 and GR-64 as well as the AR-3. I think the equivalent
Eico receivers did this as well. This was common practice for the
typical AA5 type SW receiver. It saves a tube at the expense of making
it more difficult to tune an SSB signal. You needed to turn off the AVC
on these receivers when you made the IF oscillate or the sensitivity
would decrease.

Regards,

Bill Cohn - N9MHT

[d***@drumheller.org]

Post by Bill Cohn
The Heathkit GR-91 and GR-64 as well as the AR-3. I think the equivalent
Eico receivers did this as well. This was common practice for the
typical AA5 type SW receiver. It saves a tube at the expense of making
it more difficult to tune an SSB signal. You needed to turn off the AVC
on these receivers when you made the IF oscillate or the sensitivity
would decrease.
Regards,
Bill Cohn - N9MHT

Yup--turn off the automatic volume control (AVC). That's how the Star
Roamer works, and it makes sense. The oscillating IF injects a large
carrier that swamps the AVC circuit. I didn't know this when I first
tried using the selectivity control, and my reaction was, "Gee, this
doesn't work well." Then, purely through trial and error, I switched
off the AVC and could suddenly copy single sideband (SSB) on 80
meters.

I notice that weak SSB signals are clear, whereas the strong ones tend
to have a raspy sound. I assume this is to due the IF stage being
driven non-linear when the signals are strong enough. I get some
relief by attenuating the signal by detuning the antenna a bit, but in
all cases the tuning is quite finicky.

-Dave Drumheller, K3WQ

[Michael Black]

Post by d***@drumheller.org

Post by Bill Cohn
The Heathkit GR-91 and GR-64 as well as the AR-3. I think the equivalent
Eico receivers did this as well. This was common practice for the
typical AA5 type SW receiver. It saves a tube at the expense of making
it more difficult to tune an SSB signal. You needed to turn off the AVC
on these receivers when you made the IF oscillate or the sensitivity
would decrease.
Regards,
Bill Cohn - N9MHT

Yup--turn off the automatic volume control (AVC). That's how the Star
Roamer works, and it makes sense. The oscillating IF injects a large
carrier that swamps the AVC circuit. I didn't know this when I first
tried using the selectivity control, and my reaction was, "Gee, this
doesn't work well." Then, purely through trial and error, I switched
off the AVC and could suddenly copy single sideband (SSB) on 80
meters.
I notice that weak SSB signals are clear, whereas the strong ones tend
to have a raspy sound. I assume this is to due the IF stage being
driven non-linear when the signals are strong enough. I get some
relief by attenuating the signal by detuning the antenna a bit, but in
all cases the tuning is quite finicky.

It's likely more that the BFO isn't strong enough in comparison with
the incoming signal.

That sort of receiver was mostly a AM/CW receiver. With CW, the bfo
doesn't really have to do anything but cause a beat tone in the speaker.

But SSB, you need the BFO to be strong in comparison with the incoming
sideband. So all the early books and articles about receiving SSB would
tell you to turn up the audio gain and reduce the rf gain, so the BFO
was stronger in comparison with the incoming sideband signal.

Michael

[d***@drumheller.org]

Post by Michael Black
It's likely more that the BFO isn't strong enough in comparison with
the incoming signal.
That sort of receiver was mostly a AM/CW receiver. With CW, the bfo
doesn't really have to do anything but cause a beat tone in the speaker.
But SSB, you need the BFO to be strong in comparison with the incoming
sideband. So all the early books and articles about receiving SSB would
tell you to turn up the audio gain and reduce the rf gain, so the BFO
was stronger in comparison with the incoming sideband signal.
Michael

Yes--that's a better explanation. And consistent with what I have to
do when receiving strong signals: detune the antenna. This provides
the attenuation necessary to keep the voice peaks below the carrier
level.

SSB reception is quite an adventure on the Star Roamer. The main
tuning, bandspread and antenna controls are all involved, each with
its own influence over the frequency. Some might find this much knob
twiddling annoying, but, oddly enough, I enjoy it. It keeps me
engaged with the radio.

-Dave Drumheller, K3WQ

[exray]

I recently wrapped up working on a Hallicrafters SX-140. It also uses
a regenerative IF scheme. Basically works as a Q-multiplier for
selectivity (hence the knob marking) on AM signals and provides a beat
note for CW albeit non-variable. With this 140 the AVC is
automatically cut off when you select CW and of course you have to
ride the RF gain control a bit like many rcvrs from the good old days.

It works as advertised. Mine had a problem regenning and AM signals
were VERY wide until that problem was resolved. I feel the same
sensation as if "bfo insertion" were low. For my 2 cents a real BFO
would have made it a much better receiver.

-Bill

[Jim Mueller]

I don't know about the other receivers, but the AR-3 does not use IF
regeneration. It has a separate BFO, of sorts. It uses the first audio
amplifier as the BFO. So this tube is the detector, AVC rectifier, noise
limiter, first audio amplifier, and BFO. Quite a lot to pack into one
12AV6!

You can download the schematic (and the rest of the manual) here:
http://bama.edebris.com/manuals/heath/ar-3/
--
Jim Mueller ***@nospam.com

To get my real email address, replace wrongname with eportiz. Then replace
nospam with sacbeemail.
<snip>

Post by Bill Cohn

Post by d***@drumheller.org
QUESTION: What other sets from the past have used this technique to
implement an inexpensive, but marginally functional, BFO?
-Dave Drumheller, K3WQ

The Heathkit GR-91 and GR-64 as well as the AR-3. I think the equivalent
Eico receivers did this as well. This was common practice for the typical
AA5 type SW receiver. It saves a tube at the expense of making it more
difficult to tune an SSB signal. You needed to turn off the AVC on these
receivers when you made the IF oscillate or the sensitivity would
decrease.
Regards,
Bill Cohn - N9MHT

[ken scharf]

Post by d***@drumheller.org
I just finished restoring a Knight-Kit Star Roamer shortwave
receiver. It's a mediocre performer, but fun to play with. Actually,
it sounds pretty good using headphones.
Anyhow, it has a "selectivity" control that allows reception of
sideband and Morse signals. After a two-minute analysis of the
schematic I concluded that this control makes the 6HR6 intermediate
frequency (IF) amplifier tube function as a tuned-grid/tuned-plate
(TGTP) oscillator. This certainly isn't a sophisticated
implementation of beat frequency oscillator (BFO), but is works,
albeit with lots of fine adjustment to the bandspread and,
surprisingly enough, the antenna trim capacitor.
QUESTION: What other sets from the past have used this technique to
implement an inexpensive, but marginally functional, BFO?
-Dave Drumheller, K3WQ

IIRC the circuit put a pot in series with the suppressor grid of the if
stage to ground. I think the effect of that would be to make the
pentode behave as a tetrode and oscillate due to secondary emission.
Varying the resistance sets the operation point to control the feedback.

This worked because the Star roamer ran at low plate voltage and the
screen and plate were near the same voltage. If the IF stage ran the
tube at 250v plate and 100v screen it wouldn't work.

[exray]

Post by ken scharf
IIRC the circuit put a pot in series with the suppressor grid of the if
stage to ground. I think the effect of that would be to make the
pentode behave as a tetrode and oscillate due to secondary emission.
Varying the resistance sets the operation point to control the feedback.
This worked because the Star roamer ran at low plate voltage and the
screen and plate were near the same voltage. If the IF stage ran the
tube at 250v plate and 100v screen it wouldn't work.

Hmmm, thanks for that explanation, Ken. While I was trying to
troubleshoot my SX-140 it never was clear to me WHY the thing was
supposed to oscillate. My problem turned out to be a case of low gain
in the circuit that was preventing it from taking off. After taking
care of some mica problems in the IF cans it jumped into oscillation
upon getting a good peak on the xfmrs.
Sounds like Dave's radio is working ok and the analogy to a TPTG is a
good one.

-Bill

[d***@drumheller.org]

Post by ken scharf
IIRC the circuit put a pot in series with the suppressor grid of the if
stage to ground. I think the effect of that would be to make the
pentode behave as a tetrode and oscillate due to secondary emission.
Varying the resistance sets the operation point to control the feedback.
This worked because the Star roamer ran at low plate voltage and the
screen and plate were near the same voltage. If the IF stage ran the
tube at 250v plate and 100v screen it wouldn't work.

Ken,

Yes, this is exactly what I was thinking when I looked at the
circuit.

When the sensitivity control is off, the wiper (center tap) of
potentiometer R4, which is connected to the suppressor grid of the
6HR6 IF amplifier tube, is grounded. In this configuration, the tube
is biased for normal operation as a pentode. When the control is on,
R4 places a resistance between the suppressor and ground, reducing
grid's ability to repel secondary electrons emitted from the plate,
thereby inducing a tetrode negative-resistance characteristic to the
tube. Varying R4 changes the potential of the grid (which draws a
small amount of current) and therefore its ability to repel
electrons. This provides control over the amount of negative
resistance. Furthermore, when the sensitivity is switched on, it
places bypass capacitor C10 across the tube's cathode resistor. This
reduces the amount of resistance in the circuit at the resonant
frequency (455 KHz), promoting oscillation.

-Dave Drumheller, K3WQ