The PHILCO Phorum

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.

Is there any tube shield''s ??? And,, are they in Place over the tube???

Something I've done on hi gain homebrew stuff is rectify the filament voltage. Just a simple half wave and a big cap. If you can do it near the pt it will eliminate ac radiation of the filament wiring. I realize this isn't quite what you're looking for but may give you some insight to what is the real issue. Something else how about the line bypass caps, if you remove those does it change things? Or replacing the 1st audio tube?

Terry

To answer the two last questions:

All bypass caps are as specified by the sch and are bough from Mouser, mostly Panasonic. Adding extra on top of them (tried) changes exactly zilch.
Yes did replace the 1st Audio, have just bought cheaply bunch of good 78 and 77 tubes (needed a couple but at $3 on average bought 4 of each). Same result.

Yes, rectification is not the way out for me, as I know that the other radio does not seem to exhibit that so it should work the way it is (or the way it should be in case something is botched). Plus, it is about 3A in filament currents without the 80 tube so the diodes will be pretty hefty.

I also checked all the GND-ing screws on the bypass caps, and on RF cans (though these do not behave this way). And everywhere I could. I poked the GND-ing rivets.
I tried extra decoupling Neg-GND and even cut the cap I use to try short-lead cap in another place. Nope.

One more thing......I have installed the 0.5uF cap in parallel to the filter choke, as per sch. I am not sure it was there when I started to work on the radio. It supposedly should compensate a it of the ripple that comes through the choke as it should come 180 degrees opposite. What if it is not 180 degrees or the value is off....
Then again, the only affected tube is the 1st audio.

And, last thing, I checked the filaments grounding through the centertap, it is Grounded.





Ken:

Yes all shield (they are over every 78, 77 and the oscillator tube), the only unshielded ones (by design) are the AVC 78 tube and three 42, the driver and the output.

Does the 16B speaker have a hum bucker coil? Any chance it's wired backwards or otherwise broken?

Found some good advice by a guy named morzh here:
http://philcoradio.com/phorum/showthread.php?tid=10714

Nathan

There is of course a chance of that, though I do not think it has a numbucker simply because the field coil in 16 does not double as the DC filter. But even if it has the humbucker, it then shows all the time and does not disappear with GND-ing chassis and such. Here we are likely dealing with a noise pickup.

Unplug ALL of your switching mode p/s!!!! Darn things are NOISY you know!
Had to state the obvious.

Nope did not guess it
Plus have none in the basement and cannot make the whole family to stop using computers or watching TV.

FOUND IT!

While checking the grounding bolts on backelite block and measuring them to GND I found missing connection in the backelite holding the Y-caps of the MAINS filter. The reason: they used this for the solder post to bring one of the power wires in and connect to the power transformer. The post has insulating base effectively isolating the bolt from the block's power lug, so the 10nF caps have no center GND point.
After that the hum went WAY down. A little remains but no worse than the rest of my radios. And this is when plugged through the isolation transformer; when plugged into the outlet straight, no hum.

Terry, remember the discussion about these caps one day here? Well....they do matter

WOW,,,I would have never check that,,,,GGreat Catch,,,,,Cheers,,,,,