My answer applied to your suggestion to check the fixed capacitor(s) in parallel
to the Pi-network loading capacitor.

As to the other issue you have raised, it is not easy for me to fully
appreciate - without actually looking at the diagram - whether an open inductor
could really yield some change in the neutralization bridge, and not just a loss
of drive signal, but it could well be.

Anyway, it now comes to my mind that also the capacitor byassing (though not
completely) the cold side of that inductor to ground could be responsible for
the noticed effect. As a matter of fact, in typycal diagrams, the neutralization
partitioning circuit is just formed by that capacitor and by the high-voltage
low-capacity variable capacitor directly connected to the tube plate.

Yes, on 80 meters the grid / plate capacitance is probably be too low for
triggering an oscillation, but we are just talking of a modest offset the
neutralization bridge that can only be detected when tuning the pi-network plate
capacitor.

Anyway, let us read the tests results and then comment further.

73

Tony I0JX
Rome, Italy

I had understood that you suggested to check
"Kenneth Scharf" ha scritto nel messaggio
...
I find it hard to imagine that the final could exhibit symptoms of being out of
neutralization on 80 meters. The grid / plate capacitance is simply too low to
cause feedback on this band.

The Heath SB series used an unusual pre-selector / driver tuning circuit. On
80 meters a single coil with a coupling link is used in the tank circuit on
both the input and output of the driver. It is tuned by variable capacitor,
and tracking is adjusted with a variable powered iron slug. On the other
bands additional slug tuned inductors are switched in parallel with the 80
meter inductor. These inductors are connected in series and are shorted out
one by one between 40 to 10 meters by the band switch. On 80 they are
disconnected from the circuit. IF the 80 meter inductor were open the circuit
would not tune correctly on that band, but the series connected inductors
might still resonate close enough on the the other bands, on 10 meters the
open 80 meter inductor would not even be noticed. Since this circuit is part
of the neutralization bridge it would be worth checking.



On 07/16/2011 04:55 AM, Antonio Vernucci wrote:
"Kenneth Scharf" ha scritto nel messaggio
...
If the problem is ONLY on 80 meters then the issue may be with the
output loading capacitor. On 80 meters the three gang variable loading
capacitor isn't big enough so the band switch adds an additional fixed
cap in parallel. Maybe that cap has gone bad or the switch is not
making contact on that band. Try putting a 500pf 500v cap in parallel
with the output loading capacitor and try it on 80 meters.

Should the output Pi network capacitance be too low (bad additional
fixed capacitor or faulty switch), one would just see a dull plate
current dip and get low RF output power, due to the Pi network
overcoupling to the antenna. I see no reason why it should also cause
the final stage to become non properly neutralized (as evident from the
fact that the plate curent dip does not occur as the same plate
capacitor setting as for maximum output power)

73

Tony I0JX
Rome, Italy

OK, here are the test results:

I connected additional bypass capacitors across the cathode resistors
and screen grid, just as Tony suggested. Nothing changed. I also
checked the fixed tuning capacitors in the pi network, just as Kenneth
suggested. Nothing found.

OK, all the caps were fine. So what else was wrong?

Well, I noticed that in CW and TUNE modes, the peak RF output was less
than the peak audio output in LSB or USB. Strange. So I poked
around and determined that the diode-based balanced modulator was not
sufficiently “imbalanced” for CW and TUNE. I think this was a
combination of carbon resistor R402 having drifted higher in value
(it’s specified at 68K Ohms) and a bias supply voltage running a bit
low (should be +250 VDC, but runs at +230 VDC). So I soldered a 220K
Ohm resistor across R402 and now get full power in CW and TUNE because
diodes in the modulator are now sufficiently forward biased.

So why was low power a problem?

Well, the peak-power-at-plate-current-dip rule really only applies at
full output power when finals are running class B. When I was tuning
in CW or TUNE, and didn’t have full power, that rule didn’t apply and
it looked as if the finals weren’t neutralized. However, the apparent
neutralization problem didn’t show up when I tuned-up the SB-401 by
applying a pure tone to the mic input in LSB or USB.

-Dave, K3WQ

On 07/17/2011 09:17 PM, David wrote:
OK, here are the test results:

I connected additional bypass capacitors across the cathode resistors
and screen grid, just as Tony suggested. Nothing changed. I also
checked the fixed tuning capacitors in the pi network, just as Kenneth
suggested. Nothing found.

OK, all the caps were fine. So what else was wrong?

Well, I noticed that in CW and TUNE modes, the peak RF output was less
than the peak audio output in LSB or USB. Strange. So I poked
around and determined that the diode-based balanced modulator was not
sufficiently “imbalanced” for CW and TUNE. I think this was a
combination of carbon resistor R402 having drifted higher in value
(it’s specified at 68K Ohms) and a bias supply voltage running a bit
low (should be +250 VDC, but runs at +230 VDC). So I soldered a 220K
Ohm resistor across R402 and now get full power in CW and TUNE because
diodes in the modulator are now sufficiently forward biased.

So why was low power a problem?

Well, the peak-power-at-plate-current-dip rule really only applies at
full output power when finals are running class B. When I was tuning
in CW or TUNE, and didn’t have full power, that rule didn’t apply and
it looked as if the finals weren’t neutralized. However, the apparent
neutralization problem didn’t show up when I tuned-up the SB-401 by
applying a pure tone to the mic input in LSB or USB.

-Dave, K3WQ
Because at low power the effective load resistance of the finals is much
higher than it is at low power, and the PI network does not match the
load to 50 ohms. The Q of the network at resonance is off, I'm not sure
if it is higher or lower .... depends on where the loading capacitor
ends up. Also on 80 meters the range of the capacitors is limited to
begin with....

On Jul 19, 7:17*pm, Kenneth Scharf wrote:

Because at low power the effective load resistance of the finals is much
higher than it is at low does not match the
load to 50 ohms. *The Q of the network at resonance is off, I'm not sure
if it is higher or lower .... depends on where the loading capacitor
ends up. *Also on 80 meters the range of the capacitors is limited to
begin with....

Yup. I'm sure the difference in load resistance was at play.

Since my last post, I permanently replaced the aforementioned 68K Ohm
resistor (R402) with a 200K Ohm in parallel with a 100K Ohm. The
original carbon composition resistor had a resistance of 380K Ohms--
over five times its original value. That's the largest amount of
drift I've ever encountered.

Also, I now get grid current during key-down in CW and TUNE.

Thanks for all the help, guys.

-Dave, K3WQ

David wrote:
I have a Heathkit SB-401 transmitter I refurbished a few years back
that exhibits a couple of peculiarities during tune-up:

1) First, on 80 meters (and only 80 meters) when I tune the final
amp’s plate circuit, the plate current dip point does not correspond
to maximum output.


Check the neutralization of the finals.