[ARC5] Solid State Conversion of a BC-779

[Richard Knoppow]

    There are a couple of types of pulling. One happens mostly in 
converter stages, that is  a stage that is both local oscillator 
and mixer in one tube. The presence of the RF can cause the 
oscillator to change frequency. Another kind of pulling is caused 
by the variation of voltage on the oscillator. That is the kind I 
am referring to. The B+ changes when the plate current in the IF 
and RF stages varied due to the bias changing. So, the local 
oscillator can change frequency when the RF gain changes, weather 
controlled by AVC or manually. In my receiver my modified LO used 
an electron coupled circuit, in which the plate and screen 
voltages counter-act each other. If the correct ratio is chosen 
the oscillator will not react to varying voltage. In my receiver 
I also used a VR tube. When I rebuild this set again I will use 
the VR tube but with the original triode LO since I will probably 
use the original mixer which needs the large LO voltage. This is 
close to the original circuit. The plate resistor on the LO is 
normally 12K but if a VR tube is used it should be reduced to 
about 8K. That keeps the voltage on the plate steady at +150 
volts, which is the correct value. That removes the pulling 
although it does nothing about the temperature drift. In the last 
of the series, the SP-400, about a 5pf N1500 ceramic cap was 
placed across the main tuning capacitor just under the cover of 
the tuning compartment. These are hard to find, normally an N750 
is the most negative of the TC caps. Even an N750 will reduce the 
warm up drift. There are probably other causes for temperature 
drift in the Super-Pro.
    Also, the circuit in the SP-210  which goes to 40 Mhz is 
different than the ones with 20Mhz higher limit. In the 20Mhz 
version the RF stages are series fed, in the 40Mhz version (and I 
think the 400) the stages are shunt fed to get the DC out of the 
cores and increase the Q. Any single conversion receiver with a 
455 Khz IF is struggling for decent image rejection much above 
about 10 Mhz. RCA obtained decent image rejection in the AR-88 by 
using very low loss formers for the coils. Still it can't hold a 
candle to a double conversion receiver.
     Hammarlund resorted to their very complex guillotine band 
switch to reduce loses, however, I think RCA did about as well in 
the AR-88 with a conventional bandswitch but using very low loss 
wafers and an attempt to reduce lead length. The best of all is 
the HRO which does not have a bandswitch. Nonetheless the HRO 
specs for images is no better than the RCA receiver.
     Of course Collins blew everyone out of the water with the 
75A-1, with its double conversion system.

On 11/4/2020 5:28 PM, Hubert Miller wrote:
> I thought 'pulling' had as much to do with difference between signal and HF osc. as AVC voltage. I think
> 'pulling' occurs even without a signal, doesn't it, as you for example, change the RF antenna trim control ?
>
> Interesting article. I have been saving such articles and schematics of receivers for years, mostly still
> puzzled about AGC circuits in the conversions. My preference would be to use tube-base plug-ins.
>
> Ben, did you publish your procedure with the Trio receiver?
> I would love to see that too.
>
> I bought a couple 'transistor conversion manuals' from a fellow who has some videos on Youtube. I do
> have some differences though. I see no need to patch in a new power supply  when there's already one
> right there. And I see no need to use a whole lot of 2.5 mH RF chokes. Nevertheless, I consider my money
> well spent when I get some information and perhaps gather some new ideas.
> -Hue Miller
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL