Stewart Radio Corp. Style 320
Stewart Radio Corp. Style 320Author: Edgardo Castro Bruse
I bought this radio in January 2021.
What Kind of Radio is this One?
I traced the circuit carefully and made its diagram.
The fact that the Plate of the Detector's Stage was connected directly to the Antenna coil, and then to its Control Grid by means of the Grid Leak Resistance and its associated Condenser disturbed my mind.
I made an extensive web research and concluded that my radio was a unique commercial version of an ULTRA AUDION RECEIVER.
It is a very peculiar regenerative receiver.
Ultra Audion Features
It's most distinct feature is that there is a direct connection between the Detector's Plate and its Control Grid.
Therefore the easiest way to have SOME control of the regeneration is by varying the Filament Voltage.
Another method to control the regeneration is to increase the grid leak resistance and to decrease the grid condenser:
General Restoration ProcessThe restoration of this radio was done in two parts. First the Cabinet's and Second the Electronics.
To give the reader a description of the process, including the damages produced during transportation, I will show three Sets of Pictures.
Set 1 eBay seller's pictures. Set 2 Damages upon arrival. Set 3 the repair process.
Front Panel Restoration
Restoration of the Electronics
After visual inspection I proceeded to check the two inter-stage transformers. They checked OK!
Then I checked the Grid Leak Resistance. It was open! I replaced it with a 3.3 Mega ohm resistance.
It was evident that some of the wires dealing with the distribution of B+ voltage, B+ detector's voltage, B- and the Filament Voltage were missing.
At this point I decided to trace the circuit, in order to have its diagram. This diagram told me I had a radio I had never seen before: *AN ULTRA AUDION*
After completing the missing wiring, I found that the detector's rheostat was only 10 Ohms and the Audios' rheostat was 30 Ohms. This indicated to me that they had to be inter-changed.
Interchange of Rheostats
This process is self explanatory. Please see pictures.
Substitute Tubes for 01-A Tubes
Because I did not have the required 01-A tubes, I adapted some tubes I had in my Junk Box.They worked very well!
Pictures of the Radio after its Electrical Restoration:
Testing The Radio
I used a home-made power supply that provided 90V DC, 45V DC, 22.5V DC and 5.0V DC at 3 amperes.
First I tried this radio with a 5 meter long L shaped antenna and a ground connection provided by the mains home line. The results were modest sensitivity, and a regeneration difficult to control with the filament rheostat.
I also tried my Tuned Loop Antenna and the results were just a little bit better.
Improving the Radio
I wanted to keep the original circuit intact, as much as it could possible be.
Also wanted to have some control of the regeneration.
So I placed an estimated 1.5 mh RF Choke in parallel with a 10 K ohm rheostat between the Detector's Plate and the Antenna Coil.
I turned the radio ON and tuned to a station at 770 KHz (YSKL). I increased the detector's filament voltage until YSKL was heard, then I increased it even more until a vigorous oscillation was heard. At this point I began to reduce the resistance of the rheostat until the Audio Signal increased the most and the Oscillation was reduced to zero.
Eureka! it was working very well.
At this point I turned the radio OFF, and measure the resistance of the rheostat (2,5 K ohm). Then I connected a fixed 2,5 K ohm resistance in parallel with the Choke!
The radio works real well in the range of 720 to 970 KHz. For other ranges the Rheostat must be re-adjusted.
Please see picture. I include the diagram now showing the RF Choke and the 2,5 K ohm resistance.
The unconventionality of the Ultra Audion Detector pushed me to investigate and to experiment with it.
As a substitute for the 01-A, in the output stage, I first used a 3V4 in a triode connection circuit. It worked ok even without a C- voltage. I experimented connecting the 3V4 in pentode connection.
I used a screen grid dropping resistance of 33Kohms and a bypass capacitor of 22uf/160 volts.
I included a 9 volt battery to provide a negative 9 volts bias voltage to the grids of both audio tubes. This arrangement worked real well! With a very noticeable increase in volume!
I now present to you the modified diagram, and a picture that shows the C-bias battery of 9 volts. Please note that I kept the original Audion Detector as in the original radio. I used a 15 Meg grid-leak resistance and a 47 pf grid capacitor
Finally I encourage the reader to experiment with this interesting circuit and enjoy some interesting surprises!
Acknowledgements I wish to thank my friend Raúl Urquilla YS1UF for his continuous support, and for turning my Hand-made Diagrams into Computer-made Diagrams.