AM-FM Philharmonic Bias voltage strip

[Tom Jardine]

During the restoration process of my AM-FM I noticed, rather late, a 500 ohm power resistor on the -27 v tap of the bias resistor strip to ground. The -27 tap has been measuring -26.6 volts and I didn't think much of it. The 500 ohm resistor is not shown on the AM-FM schematic. I then saw the 30 tube Philharmonic schematic shows a 300 ohm resistor to ground from the -15 volt tap. I'm wondering if the 500 ohm resistor was a "fix" years ago and should I look for problems elsewhere? By the way, the -27 tap measures -33 volts without the 500 ohms to ground.

Tom

[Norman S Braithwaite]

The 500-ohm resistor was used to bring the bias voltage into compliance with the circuit requirements.  Negative 33-volts is clearly to much bias.  If you search a bit, you will find that Scott recommended adjusting the value of the shunt resistor to improve sensitivity of the receiver if the customer desired.  This modification should considered carefully as it will cause the set to run hot and shorten tube life.

Norman

[Tom Jardine]

Norman,

Thank you for a very quick reply! The radio has been working very well I was hoping for what you described. After spending a couple of weeks working on this radio, I've documented several schematic omissions. By the way, removing the 500 Ohms was just a test after which the resistor went back into place.

When I took possession of this radio years ago, all the tubes were original Sylvania's with the 'leaf' trade mark. The tubes are still original except for magic eye's and power amp tubes.

Tom

[Scott Seickel]

The AM only schematic shows a 300 ohm resistor to ground from the -15v tap.  In all 3 AM only sets I have, the actual value installed by Scott is 1.5K for this resistor. 

Does your AM-FM have the wirewound variable resistor to ground from middle taps of the bias strip? 

If your voltages are within tolerance, leave it all alone.  Otherwise you can adjust these to get the bias voltages correct.  My AM-FM does have a power resistor to ground right off of the -27v tap.  Not sure of the value offhand. 

[Tom Jardine]

Yes, Scott, it has the variable WW 750 ohm resistor on the -6 volt tap. It's in the scratch filter circuit, but wasn't on the AM set. The 30 tube technical manual mentions the option of shunting the -6 volts with an appropriate resistor to bring scratch filter action to spec. Adding the shunt resistor to the AM-FM set made adjusting the scratch filter easy.