green band AW-23 issue

[David C. Poland]

A known problem for the 1935-7 Scott 23 tube Allwave Hi-Fidelity model: the pair of NE-42 Voltage Regulators go dark when tuning the low end of the green band, say, below 12 MC. This is attributed to a parasitic oscillation problem stopping reception.

E H Scott Labs advised service reps of a grounding issue of the 76 tube oscillator cathode. Over time the riveted lug can develop a very low resistance to ground causing the green band problem. Scott Lab solution is to solder the lug and rivet to the chassis.

I recently acquired a AW-23 7 knob with the this problem. Late production example having the micrometer dial, 2nd antenna post, high serial number - suggesting 1937 constructionAW-23. This chassis showed prior extensive recapping and displayed good workmanship. Despite no sign of rust, with my digital meter, it had a very small, but measurable resistance from oscillator cathode to ground. 

The current advice is not only to solder that ground lug and rivet to ground, but also to add a heavy stranded wire from the tube socket pin 4 to another nearby small terminal strip support lug and solder that to the chassis too. “Heavy" like de-soldering braid. This considerably improved my situation: now the VR tubes stays “on” with about half the usual glow. I found that installing the chrome shield top on RF amp 6D6 helped the VR tubes glow even stronger. There was yet another issue. - read on.

AW-23%20improved%20OSC%20ground.jpeg

ADVICE: when faced with an original part that does not match documentation, believe the part. Scott was a custom set builder in the 1930’s, building each radio upon receiving a purchase order. Scott made improvements during the run of a particular model radio, which improvements may not appear in documentation.

• In my receiver, the oscillator tube socket resistor was replaced by a more modern 100K resistor - see photo. Circuit diagrams state 75K. While the parts list figure 4 part #120 says 100K. Evidently, the restorer saw fit to replace the 75K resistor found with 100K. He even “corrected” his 1936 diagram to read 100K. 
• I replaced the 100K resistor to an 80K I had, further improving the VR tube dimming issue. And a tube shield top on the RF tube no longer mattered. My conclusion, the diagram’s 75K value was correct. 
• Initially, Scott was using .1 MFD 400 volt caps in the AW-23. But later 1936, Scott changed to .05 mfd 600 volt caps - and issued a Service Bulletin recommending replacing the many .1 MFD caps of any AW-23 receiver serviced. A notation with my set’s documentation: he reverted to 400 volt caps instead to the original .05 600 volt caps he actually found in this chassis. Indeed, this restorer missed the original 600 volt .05  Sprague wax cap still in the RF coil can by the 6A7. ADVICE- Believe the original part (subject to service bulletin advice).

I consulted with Kent King in recent days on this receiver. He recalled seeing a number of 100 K resistor replacements on the oscillator tube socket. All three AW-23 diagrams show a 75K resistor. Seems the parts list error mislead a number of restorations.