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Philco Model 20 distortion - Printable Version

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Philco Model 20 distortion - 93foxconvert - 04-15-2012

Hi all. This is my first post on the phorum. I have been restoring radios for about 2 years and this was my first Philco. I wouldn't have gotten as far as I did without the help of the phorum.

I was given this Philco 20 as the owner did not want to spend money getting it restored. Otherwise, at this stage of my experience, I likely wouldn't have tackled this little gem.

The 20's sound is distorted. Using my signal tracer, I can hear the distorted sound at the detector.

Here's what I've done:
replaced all the tubes (most tested bad), replaced all caps (including those in the large can) and resistors, cleaned all the tube pins and tube sockets. All the while learning new things like Philco bakelite caps and such. I double checked my wiring.

The radio is very sensitive with just a 10 foot wire antenna but even at low volumes, the sound is distorted.

When I began on the radio, someone had left the large cap can wired into the radio but added 1 20uf electrolytic. It hummed and didn't receive any signal. As I progressed replacing caps and resistors, I got it to receive. The 27 tube tested very poor and when I replaced that, it really came to life, albeit with a hum and distortion.

After finishing with all caps and tubes, the hum is virtually gone but I'm left with distortion/garbled sound.

What is my next step?







RE: Philco Model 20 distortion - Radioroslyn - 04-16-2012

(04-15-2012, 11:34 PM)93foxconvert Wrote:  Hi all. This is my first post on the phorum. I have been restoring radios for about 2 years and this was my first Philco. I wouldn't have gotten as far as I did without the help of the phorum.

I was given this Philco 20 as the owner did not want to spend money getting it restored. Otherwise, at this stage of my experience, I likely wouldn't have tackled this little gem.

The 20's sound is distorted. Using my signal tracer, I can hear the distorted sound at the detector.

Here's what I've done:
replaced all the tubes (most tested bad), replaced all caps (including those in the large can) and resistors, cleaned all the tube pins and tube sockets. All the while learning new things like Philco bakelite caps and such. I double checked my wiring.

The radio is very sensitive with just a 10 foot wire antenna but even at low volumes, the sound is distorted.

When I began on the radio, someone had left the large cap can wired into the radio but added 1 20uf electrolytic. It hummed and didn't receive any signal. As I progressed replacing caps and resistors, I got it to receive. The 27 tube tested very poor and when I replaced that, it really came to life, albeit with a hum and distortion.

After finishing with all caps and tubes, the hum is virtually gone but I'm left with distortion/garbled sound.

What is my next step?

Hi & Welcome!!
Check the HV to see if it is too high. If that 20mf cap is tied to the filament of the 80 your HV will be significantly higher. Replace filters with the same mf rating as originals on this set.
Best I remember there is a mica cap on the plate of the 27 may want to replace it too.
Keep us posted
Terry
ps Does it have it's tube shield??

RE: Philco Model 20 distortion - morzh - 04-16-2012

The mica cap is on the last 24's plate (detector). A 250pF.

RE: Philco Model 20 distortion - Chuck Schwark - 04-16-2012

(04-16-2012, 01:42 PM)morzh Wrote:  The mica cap is on the last 24's plate (detector). A 250pF.

Be sure ALL resistors in the set are within a 20% tolerance of their nominal values. I'll bet you have a grid bias problem on the 71A's or insufficient plate or screen voltage(s) somewhere. Carbon comp resistors usually go way up in value with age. Also check the BC resistor sections for correct values.

Chuck

RE: Philco Model 20 distortion - morzh - 04-16-2012

The output stage (71As) relies on the big wire-wound resistor for its biases.


The sections are 1-2 and 2-3 (1.4 KOhm and 187 Ohm respectively). Rarely changes value, but does burn.
In my 20 the 1-2 section (1.4K) was burned out. I put a 10 Watt resistor in there.

RE: Philco Model 20 distortion - 93foxconvert - 05-06-2012

Long story short.....after reviewing the schematic and checking all my wiring, i found i missed a connection to ground on the detector. That solved the distortion. Yet, I still had a hum. Traced that to a bad replacement filter cap. It plays like new again. No hum, no distortion. Now I just have to repair some veneer and it's ready for service again. Thanks for all your input. It helped.