The PHILCO Phorum
Philco Model 20 escessive hum - Printable Version

+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Philco Radio Discussions (https://philcoradio.com/phorum/forumdisplay.php?fid=5)
+--- Forum: Philco Electronic Restoration (https://philcoradio.com/phorum/forumdisplay.php?fid=8)
+--- Thread: Philco Model 20 escessive hum (/showthread.php?tid=11909)

Pages: 1 2

RE: Philco Model 20 escessive hum - radiohenry - 03-28-2015

    Why not use the values and types of capacitors Philco used to begin with? I don't understand why so many people go down the wrong path to begin with on the model 20. There is plenty of room in the old filter can for everything to fit.
                                            Henry

RE: Philco Model 20 escessive hum - morzh - 03-28-2015

Henry


There is a mantra, which is not that untrue, especially today, "the more capacitance the better".
Unfortunately for certain circuits, especially with tube rectifier, it is not so.

RE: Philco Model 20 escessive hum - radiohenry - 03-28-2015

 It seems like the first everyone wants to do is put in 10 uf  electrolytics  in instead of the 1 or 11/2 uf non polarized caps originally used.  It makes me crazy.
                                           Henry

RE: Philco Model 20 escessive hum - klondike98 - 03-28-2015

Glad to hear you tracked down the problem! Thanks for the update on it.

RE: Philco Model 20 escessive hum - radiohenry - 03-28-2015

      Thanks for the update .
                   Henry

RE: Philco Model 20 escessive hum - Tubesforme - 03-30-2015

(03-27-2015, 01:01 AM)Arran Wrote:   I don't have a model 20 (as yet) to play with so I'm not familiar with the circuitry, however the most common reason I have encountered for a set to have excessive hum, even though new filter caps were installed, is that someone made an error in connecting the negative ends of the filter capacitors. What throws people off is that most sets built from about 1939 onward use what was known as cathode bias, sometimes called self biasing, in the audio output stages, most sets built between the late 1920s and 1938 used fixed bias in the audio output stages by using a large wire wound resistor, and sometimes the speaker field coil as well, in series with the center tap on the power transformer. So one filter cap will be connected directly to the center tap of the H.V winding, then there will be a wire wound resistor in series with the tap, the the next filter cap will follow that resistor, sometimes there will be another bias resistor and another filter cap following that with it's negative lead connected to the chassis ground. It's a long shot but there could also be a heater to cathode short in that 227 tube you pulled, have you tried substituting another?
Regards
Arran
Arran, I tried four different supposedly good used 227 tubes. I did find that one of them did quiet down the hum, considerably. I have also taken the main filter cap apart and rechecked the polarity of the capacitors. They are correct. I did replace the 10uf's that I originally installed with 1uf @ 630VDC. I did leave one of the 10uf @ 450VDC in place of the 1.5uf, and yes, the polarity is correct, because it does filter the audio B+ and I have been told that many repairmen add a 10 uf, externally (watch polarity), connected to point "B". See schematic in first post. My main problem is still to high of B+ Point "D" is 295VDC. Plates of the 71A's is also 295VDC. Should be 250VDC. Thanks

RE: Philco Model 20 escessive hum - Tubesforme - 03-30-2015

(03-28-2015, 08:47 PM)klondike98 Wrote:  Glad to hear you tracked down the problem! Thanks for the update on it.

Bob, I didn't quite fix the problem. By changing out the filters and a 227 tube the hum is lower, but still too high. The main problem is the high B+. There is 285VDC on the 71A plates and it should be 250VDC.

RE: Philco Model 20 escessive hum - morzh - 03-30-2015

What meter are you using, a DMM? This will increase reading quite a bit, as the numbers from the manual were obtained with old 1k/10k per volt meter.

RE: Philco Model 20 escessive hum - Tubesforme - 03-30-2015

(03-30-2015, 01:27 PM)morzh Wrote:  What meter are you using, a DMM? This will increase reading quite a bit, as the numbers from the manual were obtained with old 1k/10k per volt meter.

I have use and both read the same.d an older VOM and a newer DMM

RE: Philco Model 20 escessive hum - morzh - 03-30-2015

An oldER VOM still can be high impedance. See what it lists (often written on the case), and multiply by your range where you measuring (say 10k/V at the range of 500V will make it 5M that does not load the circuit much, whereas A totally old one with 1K/V at the range of 500V makes it 500K and this might load it it enough).

RE: Philco Model 20 escessive hum - Radioroslyn - 03-30-2015

Interesting Tube data sez 180v max on the 71A. Hope it's got a lot of grid bias. 71A was mostly used in battery sets as the output tube. Single tube good for about 3/4 of a watt of audio.
Terry

RE: Philco Model 20 escessive hum - morzh - 03-30-2015

On all my 20s it is close to 250V. I measured with a DMM.

If it works I would not worry much about it as your field coil is the same and the power transformer is too.

RE: Philco Model 20 escessive hum - codefox1 - 03-30-2015

Is this the set?

http://www.nostalgiaair.org/Resources/582/M0029582.htm

If so, you should know what to do.

RE: Philco Model 20 escessive hum - Tubesforme - 03-31-2015

(03-30-2015, 03:21 PM)codefox1 Wrote:  Is this the set?

http://www.nostalgiaair.org/Resources/582/M0029582.htm

If so, you should know what to do.

Thanks to everyone. I have the problem resolved. It was a combination of things, filter caps, 227 AF tube and a problem in the 224 detector circuit. Also a good cleaning of all the tube sockets helped. The radio is repaired now and working. By the way, my old 260VOM and my newer Flukle digital meter both read very close to the same voltages.