[Ron Ramirez]

A December 15, 1936 "Changes In Models" sheet issued by Philco gives the following info for Model 37-690:

ELIMINATING NOISY RESISTORS IN POWER UNIT

To eliminate the noise caused by the 80 and 325 ohm sections of resistor (177), (referring to Fig. 6 and the schematic diagram of Bulletin No. 267), two flexible resistors, Part No. 33-3027, 75 ohms and Part No. 33-3121, 300 ohms, are now used in place of these sections. To replace these resistors proceed as follows:

First install a wiring panel (Part No. 03103) under condenser (175) mounting screw. (Be sure condenser lug is grounded after panel is in place.) Then disconnect the green and white wire, from terminal 4 of resistor (177) and connect it to the wiring panel lug. Remove wire between terminal 5 of resistor (177) and terminal 6 of power cable socket. Now connect resistor 33-3121, 300 ohms between the terminal panel lug and terminal 6 of power cable socket. Then add the 75 ohm resistor, Part No. 33-3027 between the terminal panel and terminal 3 of resistor (177). After doing this disconnect the brown wire from terminal 5 of (177) and connect it to any ground on the chassis.

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Part (177) is one of those infamous Candohm resistors. In addition to the above changes, Ghirardi (in his "Radio Troubleshooter's Handbook" recommends replacing the 3000 and 2,240 ohm sections of this Candohm with a new unit. Of course you can no longer buy Candohms, so replace with new individual resistors.

Other possibilities of noisy operation in the 37-690:

• faulty 1000 ohm, 1/2 watt resistor, part (71); replace with 1 watt unit;
  
• faulty bass amplifier audio choke, part (104);
  
• loose connections in 10-contact plug that connects upper chassis to lower chassis.
  

For the Model 680, Ghirardi mentions these possibilities.

• defective 6A3 tubes (even if they test O.K.);
  
• defective 300 ohm resistor, part (151);
  
• noisy or intermittent resistors ( 148 ), (151) or (53).

Hope this helps.