03-08-2014, 07:57 PM
Many of the terminal strips and grounding points in older radios are riveted to the chassis metal. This type junction can become corroded at numerous points in the chassis and cause hum. Sometimes you can ground a clip lead to a suitable bright chassis point and get a good ground and one by one try grounding each ground lug throughout the chassis looking for change in the hum level. You will probably want to turn power off while making the connection to avoid shorting something out. Make sure you get the clip lead onto the suspect connection securely before turning the set back on. Work your way through the set section by section this way. If you find a ground lug that when grounded by the clip lead produces better audio with less hum, that ground lug needs to be drilled out and remounted with screw, nut and star washer. You may be able to find bad grounds if you have a very sensitive ohmmeter on RX1 scale.
Also some tubes can develop leakage or a short between the heater element and the cathode. It may or may not show up on a tube tester. Having another set of tubes to try that are known to be good is a good troubleshooting tool.
I have a Heathkit SG-8 RF signal generator and using a loop to feed a signal into a set with this unit is not successful as the loop usually just acts like a short to the RF. I added some resistors at the loop so that the total resistance was about 50 ohms and managed to get that method to work for AM and SW RF input circuits. The mixer grid connection via a coupling capacitor will work too as suggested above. That gets around dealing with tricky connections on some sets for aligning the IF circuits. For alignment of the AM front end or converter stage, as someone mentioned, tune as high in frequency as you can to adjust the trimmer capacitors for best reception there, then tune to the lowest station you can find and adjust the ferrite slug of any RF input coil to get best reception. Your information on the set should at least tell you which adjustments to work on for high and low frequencies on AM or SW. You may have to make several passes at it until you get it best across each band as the adjustments do interact some.
If you find that some stations do not appear at the right point on the dial, the oscillator circuit is where you need to concentrate effort. Again there is a trimmer to handle the high end of the band and a coil with slug and/or a padder capacitor to help with the low end of the dial. Often tracking is not ideal with any radio and a compromise has to be reached.
Good luck.
Joe
Also some tubes can develop leakage or a short between the heater element and the cathode. It may or may not show up on a tube tester. Having another set of tubes to try that are known to be good is a good troubleshooting tool.
I have a Heathkit SG-8 RF signal generator and using a loop to feed a signal into a set with this unit is not successful as the loop usually just acts like a short to the RF. I added some resistors at the loop so that the total resistance was about 50 ohms and managed to get that method to work for AM and SW RF input circuits. The mixer grid connection via a coupling capacitor will work too as suggested above. That gets around dealing with tricky connections on some sets for aligning the IF circuits. For alignment of the AM front end or converter stage, as someone mentioned, tune as high in frequency as you can to adjust the trimmer capacitors for best reception there, then tune to the lowest station you can find and adjust the ferrite slug of any RF input coil to get best reception. Your information on the set should at least tell you which adjustments to work on for high and low frequencies on AM or SW. You may have to make several passes at it until you get it best across each band as the adjustments do interact some.
If you find that some stations do not appear at the right point on the dial, the oscillator circuit is where you need to concentrate effort. Again there is a trimmer to handle the high end of the band and a coil with slug and/or a padder capacitor to help with the low end of the dial. Often tracking is not ideal with any radio and a compromise has to be reached.
Good luck.
Joe
