11-11-2016, 01:07 PM
OK, I am making it into separate thread so it is not stale. After it ends we might consolidate it with main 16B thread. Sorry in advance, admins, but please let it hang around awhile.
So. I am having that hum in my 16B cathedral 121.
What I know.
1. The hum is more pronounced with large volume and disappears with low volume.
2. The hum is greatly diminishes when I touch the chassis and disappears when I ground it.
3. The hum is picked up at 1st Audio, a 78 tube. Even if the grid cap is removed it is there. Grounding grid cap removes it.
4. Moving hand anywhere near the wire from the volume pot wiper or the detector output that comes to it (it is a large hub of various parts), or the 1st audio grid cap, or the DC blocking caps in the way increases the hum.
5. Ground-shielding the wire from the volume pot wiper did not help much.
6. Shorting and streamlining the wires to tone control did not help.
7. Do not see any GND loops.
8. Putting a voltmeter to the 1st audio 78 Plate and GND diminishes the hum with no tone change, and with touching the chassis by hand at the same time removes it fully.
9. Putting resistor of 300K from CG of the 1st audio to GND does not help.
10. restoring the volume ctl to the old 300K (well it is about 500K now) and no capacitor between the detector output and the top pin of the pot (and removing the 300K shunt from Det output to GND) does not help (I thought it might be the result of much higher resistance).
11. Playing with GND to Rect Neg decoupling cap (10uF) did not help.
It is there no matter what I do.
The last resort is extracting the chassis from Peter's radio and doing comparative analisys (it does not seem to hum). But it is not really restored plus it is cumbersome - I would have to situate another chassis nearby plus extract the speaker and there is always a chance to damage something, so I am trying to not resort to it.
PS. Oh, and the shadowmeter is not there and a 3K resistor is in its place (came with the radio, seems to be a Philco though is not supposed to be there).
So. if any of you has any ideas, I could use some input.
Me.
So. I am having that hum in my 16B cathedral 121.
What I know.
1. The hum is more pronounced with large volume and disappears with low volume.
2. The hum is greatly diminishes when I touch the chassis and disappears when I ground it.
3. The hum is picked up at 1st Audio, a 78 tube. Even if the grid cap is removed it is there. Grounding grid cap removes it.
4. Moving hand anywhere near the wire from the volume pot wiper or the detector output that comes to it (it is a large hub of various parts), or the 1st audio grid cap, or the DC blocking caps in the way increases the hum.
5. Ground-shielding the wire from the volume pot wiper did not help much.
6. Shorting and streamlining the wires to tone control did not help.
7. Do not see any GND loops.
8. Putting a voltmeter to the 1st audio 78 Plate and GND diminishes the hum with no tone change, and with touching the chassis by hand at the same time removes it fully.
9. Putting resistor of 300K from CG of the 1st audio to GND does not help.
10. restoring the volume ctl to the old 300K (well it is about 500K now) and no capacitor between the detector output and the top pin of the pot (and removing the 300K shunt from Det output to GND) does not help (I thought it might be the result of much higher resistance).
11. Playing with GND to Rect Neg decoupling cap (10uF) did not help.
It is there no matter what I do.
The last resort is extracting the chassis from Peter's radio and doing comparative analisys (it does not seem to hum). But it is not really restored plus it is cumbersome - I would have to situate another chassis nearby plus extract the speaker and there is always a chance to damage something, so I am trying to not resort to it.
PS. Oh, and the shadowmeter is not there and a 3K resistor is in its place (came with the radio, seems to be a Philco though is not supposed to be there).
So. if any of you has any ideas, I could use some input.
Me.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
![[-]](https://philcoradio.com/phorum/images/bootbb/collapse.png "[-]")
