12-28-2017, 03:08 PM
Thank you for the suggestions.
I tend to measure everything when I take electronics apart, because it's easy to do so when it's apart and not so much when it's all assembled. You're probably right that I will never need most of the measurements, but they are occasionally useful.
As for the capacitors, I thought it would be a good idea to at least measure the leakage current near their rated voltage. I've seen plenty of capacitors which measure fine in terms of capacitance and ESR fail when a higher voltage within their rated ranges is placed on them. Just wanted to be certain this wasn't going the case. The other measurements were gratis since I already had the caps out of circuit at that point.
There is a strong AM radio station here at 1410 kHz. When I tune it up on another radio, there is no noticeable hum on it. However, when I tune it on the Philco, there is a distinct 120 Hz hum. This is distinct from typical EMI. In particular, looking at a real-time power spectrum of the audio output shows a significant peak at 120 Hz on the Philco -- something that is not present on another radios tuned to the same frequency in the same room.
I suppose many would just live with this annoyance, but I can't believe this is normal behavior. If it is, then I suspect there are modifications that can be made to reduce it to a more acceptable level. In any event, it's a learning opportunity for me to familiarize myself with this old tech and maybe expand my collection if it continues to be an enjoyable experience.
If I understand the schematic correctly, the plate of the 7C6 is the detector output, and is biased to 100 Volts through a 220k resistor. Given that hum increases and decreases in proportion to the volume, and disappears completely when the volume control is at minimum, doesn't that rule out the detector and audio amplifiers as the source of the problem?
I should probably go over all of the work my repairman did before pulling any more hair out trying to track down this problem -- it would be unfortunate to discover the problem is caused by using the wrong-valued component somewhere or incorrect wiring.
--
Jason
I tend to measure everything when I take electronics apart, because it's easy to do so when it's apart and not so much when it's all assembled. You're probably right that I will never need most of the measurements, but they are occasionally useful.
As for the capacitors, I thought it would be a good idea to at least measure the leakage current near their rated voltage. I've seen plenty of capacitors which measure fine in terms of capacitance and ESR fail when a higher voltage within their rated ranges is placed on them. Just wanted to be certain this wasn't going the case. The other measurements were gratis since I already had the caps out of circuit at that point.
There is a strong AM radio station here at 1410 kHz. When I tune it up on another radio, there is no noticeable hum on it. However, when I tune it on the Philco, there is a distinct 120 Hz hum. This is distinct from typical EMI. In particular, looking at a real-time power spectrum of the audio output shows a significant peak at 120 Hz on the Philco -- something that is not present on another radios tuned to the same frequency in the same room.
I suppose many would just live with this annoyance, but I can't believe this is normal behavior. If it is, then I suspect there are modifications that can be made to reduce it to a more acceptable level. In any event, it's a learning opportunity for me to familiarize myself with this old tech and maybe expand my collection if it continues to be an enjoyable experience.
If I understand the schematic correctly, the plate of the 7C6 is the detector output, and is biased to 100 Volts through a 220k resistor. Given that hum increases and decreases in proportion to the volume, and disappears completely when the volume control is at minimum, doesn't that rule out the detector and audio amplifiers as the source of the problem?
I should probably go over all of the work my repairman did before pulling any more hair out trying to track down this problem -- it would be unfortunate to discover the problem is caused by using the wrong-valued component somewhere or incorrect wiring.
--
Jason
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