[onlyrick]

Hello can anyone help me out please ,I have a Philco model #20 , I want to change all caps and resistors except the long wire wound one, I have checked all the tabs on the long wire wound resistor and have found all seem fine except numbers 6 & 7 , they should be 975 ohms but check at zero ohms , for some reason there is a wire soldered across the 6 & 7 tabs , I am thinking some one did a repair on this set and did not have a 975 ohm resistor to put in here so they just soldered a pice of wire across the 6 & 7 tabs , can anyone tell me if maybe it came this way from the factory and is ok , or should I cut this wire and put in a 1k resistor and if so what watt should I use , I have attached the schematics and a photo of the wire wound resistor and also have attached a drawing of it with the readings I got , would be thankful for any help I won't change this until I am sure , all the very best have a great day Rick

PS I will send the schematic in another post had some problems                

[morzh]

Rick

We usually do not spawn threads that relate to the same radio.
Please move the link here, so I could delete the other thread.

[morzh]

http://www.nostalgiaair.org/PagesByModel...029582.pdf

The sch

[morzh]

As to the 6/7 pins shorted, it would be prudent to desolder that short and see if the section is open and someone decided to fix it that way.

[MrFixr55]

Hi Rick,

Here is a link to the schematic for the Model 20. It is found in our Philco Library under Documents under early schematics.
https://philcoradio.com/library/index.ph...chematics/


Yes, definitely cut the wire and add a resistor of 970 Ohm +/- 20%. A 1K 5W or preferably 10W resistor will do fine. Having that resistor section shorted raises the voltage across the rest of the divider and puts undue strain on the other resistors in the voltage divider- the subsections of this taped resistor, and definitely the volume pot which is part of the divider. The volume control has 2 sections. One section, connected to points e and F adjust volume by varying the cathode bias. The other section of the volume pot controls the strength of the signal entering the first RF amp. This set has no AVC. Too strong a signal will saturate the 1st and 2nd RF Amp and cut off sound. This is not evidence of an issue with the set.

The filter caps in this set are not electrolytics. they are paper and foil. However, they should be replaced, along with all the other caps. Removing the caps form the can is not very easy, but is doable.

The other caps and some resistors are enclosed in Bakelite "blocks" sealed with tar. Below is a link to an excellent article by Phorum member "Morzh" on how to hollow out the blocks and replace the parts inside without having to desolder the blocks from the set:
https://philcoradio.com/library/index.ph...soldering/

Also attached is a great article on issues with the audio "interstage" transformer, how to "bridge" an interstage transformer with an open primary (a common problem) and how to rewire a good transformer to capacitively couple the primary to the 1st AF stage, removing DC from the primary, extending the life of the transformer and greatly improving fidelity:
https://philcoradio.com/library/index.ph...nsformers/

This is a "TRF" (Tuned Radio Frequency) receiver, which performs rather well. This was a seminal set, as it was introduced in the throes of the Great Depression. This set ushered in the "Cathedral" style, and sold for around $50.00, providing great value for the price.

I am about to crash and burn for the night.

Hope this helps

[onlyrick]

Hey fellows thanks so much for the help , I feel better when I ask you fellows instead just going ahead changing things without knowing for sure , take care all the very best and moving on with this restore I may have a few more questions to ask , again thanks so much all the very best Rick

[onlyrick]

Hello again fellows, just wanted to ask another question on number 8 Bakelite condenser block it has a .05 Uf cap and in the same block it has a 250 ohm resistor winding , can I use a 2 watt metal oxide resistor or should I use a 5 watt power resistor also would it be better to mount the resistor on the outside of the Bakelite block so it can cool better , the reason I ask because on one of the data sheets I seen it was a winding resistor sounded to me like a power resistor , thanks in advance for any help , all the very best Rick

[morzh]

You can use 1/4W resistor and hide it inside.

[morzh]

https://philcoradio.com/library/index.ph...chematics/

This is a better quality sch for the late 20 model.

[Steve Davis]

The resistor is just a hank of wire, if not burned, you can't do any better, just reinstall it. If the .05 capacitor (also in block 8) has become shorted, the resistor will be burned to a crisp as it becomes a direct path from B+ to the HV center tap.

Steve

[morzh]

When I rebuild these, I use 1/2W 250 Ohms, but I only use them because this is what I have.
These do not dissipate anything at all under normal circumstances.

I would actually soldered them back (I usually save them as this is some sort of hi-ohm wire) but I would then have to pot it (as it is not rigid) plus I do not know how well this wire solders; this is why I just use axial small ones.

[onlyrick]

Hello fellows thanks again for the help , I will order the parts tomorrow, I will be changing all caps and resistors except the long wire wound resistor just the last section ,will use a 10 watt 1k resistor for that , thanks MrFixr55 for the input , I will use the hint you sent the one morzh posted on changing the block caps without removing all the wires and doing it on the bench , looks so much faster and easier to take the cap out well still in the set , will let you know how I make out , I will be taking my time seems I can’t stand at the bench too long now my back gets too sore , again thanks all the very best Rick

[onlyrick]

Morning fellows , I removed the jumper wire between #6 & 7 on the long wire wound resistor and tested it and got 950 ohms and that should be fine , but I can not finger out why it had a jumper across it , I did follow the wires coming off it and they go to the right spots on the radio from both leads off the resistor , do you think that when the set warms up the wires expand and it opens up ? , and do you think I should just replace it with a new 10 watt 1K resistor , kind of wish it tested open but I checked it several times with one leg of the resistor unsoldered and get 950 ohms , just looking for a reason that some one along the way put a jumper across it , what would you fellows do and can you think of a reason some one would do this , again thanks for the help and any input all the very best Rick

[onlyrick]

Afternoon fellows , I had two condensers on hand a orange dip .01 at 600 volt and a .05 at 600 volt the rest I have ordered , I took the #13 cap out of the radio its in a metal rectangle can .05uf ,out of the radio it tested zero dead I replaced that then I took out the #19
capacitor and it checked at .01uf, it was in a bakelite block , I replaced it as well , I did it the way morzh did it leaving all wires attached and still in the radio just heated it and removed the hold down screw and pushed out the tar block . hey this worked great and made it easy . I did cut one lead on #17 and #20 resistor attached to block #19 because I was going to replace them anyway , with one leg cut and out of circuit I tested those two and got one meg on#17 and 975K on #20 resistor , wow tried my other tester and got the same readings , can anyone tell me is this normal for dog bone resistors to drift that far out of spec , data sheet tells me those two and they are battle ship grey should ohm out at 500K , thanks for ay input all the very best thanks again Rick

[MrFixr55]

Hi OnlyRick,

Yes, they are notorious for drifting.  Several ways of dealing with them:

• If authenticity is not an issue, replace them with modern metal oxide resistors.  Non "voltage divider" resistors can be 1/2 Watt.  For an added safety margin, use 1 Watt resistors in cathode and plate circuits.
  
• If authenticity is an issue, search the Phorum for articles on reproducing dog bone resistors.
  
• You can use Ohm's law or simple math to determine the value of a resistor needed to bridge the resistor in question.  This bridge resistor can be soldered to the underside of the dog bone and hidden.  The issue here, is that the dog bone can continue to change value.
  

Resistors 1 -7, the 250 Ohm section of the volume control and the speaker field comprise the main "voltage divider" of the radio where all B voltages and bias for the output tubes are derived from.  If any of these resistors open, use 5 Watt or better yet, 10 Watt resistors to repair the break in the divider.

If you have the newer paper spider type speaker, this is a nice sounding set.  IMHO, the metal spider speaker is disappointing.

Re the 950 Ohm resistor, you can check by CAREFULLY measuring the various plate and screen voltages.  If they are off, the problem is the resistor that all voltages "above" it are too high and all voltages below it are lower than they are supposed to be.  I have measured the voltages on 3 Model 20s and on all 3, the detector plate is much higher than documented, bit all other voltages are close.

Although the Model 20 schematic is in our library, the Nostalgia Air schematic for the 20 has good pictorials and a list of voltages.

Philco 3 20 20A.pdf Size: 1.09 MB  Downloads: 6

BTW, check out the schematic for the Model 3 car radioat the end of the NA document. Early car radios actually used "B" batteries like the first radios. Deluxe models had a motor-generator for HV. It was only around 1933 or so that the Mallory Vibrator type power supply was introduced, eliminating the B Battery. With 4 45V batteries, this set put out 0.7W. How does one hear this over the engine and exhaust noises of your Model A Ford Roadster? With the depression, this radio was more of a luxury than the car itself!

Keep us posted, and hope this helps.  Good luck bringing this historically important radio back to health.