Problem with AW 23 Oscillator

[John Stafford]

Nearing completion of restoration of my AW 23 7 knob.

One significant problem remains. Something happens when tuning the lower frequencies on the red and green bands. As I tune towards the lower frequencies on these bands something happens to the oscillator circuit. Reception goes quiet after a big noise (loud pop) and enough current is being drawn that the neon regulator bulbs go dark. My scope shows that the oscillator is still running.Not sure if it is still on the correct frequency though.

The regulated Oscillator supply measures 129 VDC when all is good. As I tune lower the neon bulbs start to loose brightness slowly then they go out, there is an audible "pop" and things go quiet. This always happens on the Green Band, less often on the Red Band. It does not always happen and the onset point is not always the same. The White and Blue Bands work well from top to bottom.

Has anyone experienced this? I have inspected the tuning capacitor and have not found any issues with it. Looking for advice regarding what to check next.

[Norman S Braithwaite]

Parasitic oscillation.  Have you removed the tuner, loosened and re-tightened the screws holding the solder lugs to the stator sections?  If not, resistance in the machine screw connections to the stators is a likely cause.  Another cause is a poor ground connection at the riveted oscillator cathode ground connection.  Also, EH Scott Radio Laboratories recommended a change in the value of grid resistor but I cannot remember the values of the original and recommended replacement resistor.

Norman

[John Stafford]

Thanks Norman. I will take a look at those items. I have not removed the tuner so far.

[Thomas Day]

I had a similar problem with mine, Oscillator finally quit. Tube was bad. Replaced it and seems to be fixed. I have a second chassis that I have totally rebuilt. Also, a service bulletin recommending a direct connection of cathode to ground and .05 bypass caps on filament pins.

[John Stafford]

Problem fixed. As Norman suggested there were poor connections to the stators on the tuning capacitor. There was no corrosion present. Over time the phenolic insulators had yielded to pressure from the screws allowing the screws to come loose. All 3 screws were loose enough to remove without using a screwdriver.

This radio is already configured per service bulletin Thomas mentions.

In a related issue I noticed that the series resistor ahead of the neon voltage regulators measures 8K ohms. The schematic shows this resistor as a 10K. I don't think this is a case of value drift since it would be unusual for a wirewound resistor to go down in value and land on 8K exactly. Since I have found many deviations from the schematic in this radio I am assuming that this is just another running design change.

Thanks for the help. 

[Norman S Braithwaite]

EH Scott Radio Laboratories was continuously making minor changes to component values.  Phenolic outgasses and shrinks slightly with age.

Norman