[Jay Thomas]

Just completed restoring a beautifully preserved Model 20 Grand Cathedral radio (late model with maple trim).  The radio seems to work fine, absolutely no hum and plenty of volume, however, the sound is not real clear as I would expect.  It is kind of garbled, especially listening to music.  Talk shows are a little better with less garble.  I spent a lot of money replacing tubes, thinking maybe a gassy tube, to no avail.  Before I started restoring the radio, I installed new electrolytics, tested tubes and did a visual.  I fired it up (I bought it as a working radio). It had the same garble sound before I replaced all capacitors, resistors and rebuilt the Filter Condenser Bank.   I have restored many radios, but this is my 2nd from the early 1930's.  My first restoration was a model 81.  The sound is excellent on that one. On the model 20, I also temporarily connected another speaker to the lugs on the existing speaker.  It sounded the same. 

My question for anyone, since this 1930 radio is basically the beginning of great things to come, is my problem normal for this model radio? 

By the way, thanks to those in the Phorum for providing excellent tips some of which I used to rebuild this model 20.

Thanks in advance. 

[Ron Ramirez]

Jay

Yes, the problem is normal.

Model 20 uses what is called a power detector which offers greater signal to the audio circuits of your 20, but at a sacrifice of higher distortion.

More Phorum links dealing with this subject:
http://www.philcoradio.com/phorum/showth...?tid=12812
http://philcoradio.com/phorum/showthread.php?tid=2210
http://philcoradio.com/phorum/showthread.php?tid=5189

[radiohenry]

There are no electrolytics in a model 20.Just the filter block and none there.
                                                        Good luck.
                                                               Henry

[Mondial]

The model 20 is really not a great sounding radio. Aside from the stiff speaker, it uses a plate detector which creates a lot of distortion, especially with the high levels of modulation generated by modern AM stations.

[Jay Thomas]

There are no electrolytics in a model 20.Just the filter block and none there.
                                                        Good luck.
                                                               Henry

I was incorrect.  I am working on another radio where I replaced electrolytics.  I did not install any electrolytics in this radio.

Thanks!

Jay

[morzh]

Besides 20 plate detector is used in 70, 90 and maybe some others.

The problem can be remedied by using much smaller cap in the plate detector cathode but that will come at the expense of the gain and thus the sensitivity.

[Arran]

 It is always my first order of business with any of these pre war sets to replace all the paper caps, filter caps (paper or electrolytic), and any resistors that have drifted more then 20% off value. Before I do this I usually test the tubes first, shorted, dead, or weak tubes will usually be smoked out on any decent tube tester. However I don't usually going into swapping out tubes that otherwise test alright until I electrically overhaul a set and am still experiencing problems. The Philco pie pan style speakers all have a common handicap in design, they used a solid center mounted spider that really reduces the frequency response and fidelity compared to something like a a Rola or an Atwater Kent speaker from the same era. The plate or power detectors do introduce some distortion, but even with diode detector these sets would still sound crappy compared to a Majestic or RCA from the same year, also the distortion from the detectors was probably less of an issue at the time since the average radio station put out between 10 and 15 KW of power compared to about 50 KW today.
Regards
Arran

[Jay Thomas]

[attachment=8307]Besides 20 plate detector is used in 70, 90 and maybe some others.

The problem can be remedied by using much smaller cap in the plate detector cathode but that will come at the expense of the gain and thus the sensitivity.

Hi Morzh:

On the attached schematic, are you referring to cap #19?  If so, what size cap would you suggest I try to see if I can reduce some of the distortion?

Thanks again,
Jay

[morzh]

Where do we have any attached schematics?
On the Early set sch on this website is #14, 0.5uF.

[Jay Thomas]

Where do we have any attached schematics?
On the Early set sch on this website is #14, 0.5uF.

Sorry.  I guess I haven't quite figured out how to attach a file.  However, thanks for pointing out the capacitor in question.  The newer model 20 schematic shows #14 as a .25uf and that is what I installed in the aluminum can.  I have both the early version and the newer version schematics.  I am assuming from Ron Ramirez' Philco Radio book that the model I have with the maple curly front is a later 1930 model.  The tuner in mine is a three gang tuning condenser.  Am I correct that the trimmer capacitors noted on the early schematic is the tuning capacitor?  If I am incorrect, maybe changing the .25uf to a 0.5uf may help?
Thanks for your patience and assistance.

Jay

[Ron Ramirez]

Phorum 101 - How To Use Your Phorum
http://www.philcoradio.com/phorum/forumd...php?fid=27

How To Attach Files To Your Posts
http://www.philcoradio.com/phorum/showth...p?tid=4407

Overview: How To Add Images To Your Posts
http://www.philcoradio.com/phorum/showthread.php?tid=29

Add Photos To Your Posts Using Photobucket (a bit outdated, but still helpful)
http://www.philcoradio.com/phorum/showth...p?tid=4371

[Ron Ramirez]

Jay, you would not have the early Model 20 unless someone stuck an early chassis in your Deluxe cabinet.

Common Model 20 schematic:
http://www.philcoradio.com/tech/images/20b.jpg

For comparison, early Model 20 schematic (only covers the first few weeks of Model 20 production):
http://www.philcoradio.com/tech/images/20a.jpg