I have a Model 118 with a significant steady hum from the moment it is turned on. The electrolytics in the power supply have been replaced but I am not sure if the polarity is correct. There are two 8 mfd caps in parallel with the positive ends connected to each end of the choke coil and a .25 mfd cap the negative ends are coneected to each other and to the positive end of a 10 mfd cap with the negative end connected to ground. Two friends who know more about this stuff than I do suggested that the 10 mfd unit is part of a line that provides negative bias to the 2nd detector and the the driver and should be reversed with the positive side connected to ground. Most of the tubes with the exception of the 42 audio tubes and the rectifier appear to have high plate voltages compared to those indicated in the pages that accompany the schematic. The original cans are in place (disconnected) but do not give me any insight as to polarity. Thank you for any insight you can provide.
Hi JT
Assuming you have the 118 schematic...
Condensers (74) and the 8 uF section of (73) are the main filters in the radio. The positive leads of each connect to either side of choke (71). with a 0.25 uF condenser, (72), connected in parallel with the choke (71). The negative leads of the two 8 uF condensers connect to B-.
There is also a 10 uF, 25 volt section to condenser (73). Its negative lead connects to B-, and its positive lead connects to ground.
Your voltage readings will be somewhat higher for two reasons:
1. Today's line voltage is higher than it was 73 years ago; and
2. Modern multimeters are much more sensitive than the standard of 73 years ago, which was 1000 ohms/volt sensitivity. Those older voltmeters tended to load down the circuit under test, making the readings then appear to be lower than they actually were.
Now if your plate voltages are significantly higher than those in the Philco voltage chart, then you need to look for a problem.
Another point. There are four more electrolytic condensers used in Model 118, the four-section can (46). It contains three 1 uF sections and one 2 uF section. In this can, all four negatives connect to chassis ground so no confusion there. If you have not replaced this can...do so. This could very well be another source of hum. Other problems include (but are not limited to) heater to cathode shorts in tubes, defective field coil and/or filter choke, and defective condenser (72).
ALL electrolytic AND paper condensers need to be replaced, for reliability and safety.
Thank you Ron, You confirmed for me that the 10 mfd cap in the power supply is wired backward and you anticipated my next question which was about the 4 unit electrolytic bypass cap (#46). Any suggestions on how to wire in 4 new caps to replace 46? It is on the underside of the chassis. Should I remove it to create more working space? Is there a preferred method for grounding the new caps to the chassis? Thanks in advance for sharing your wealth of expertise. I do not take for granted the considerable time and effort that it has taken for you to acquire that expertise and it is invaluable to those of us who are at the lower end of the learning curve.