Philco Model 20 Troubleshooting - Printable Version
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Philco Model 20 Troubleshooting - mgambuzza - 01-11-2013
I was making big progress on this set - replaced all caps and resistors - what a difference in Rated vs Actual for the resistors. Difference shown below
Resistor 12 - 50kohm - tested 66k actual
Resistor 15 - 250k - tested 423k actual
Resistor 17 - 500k - tested 993k actual
Resistor 18 - 100k - tested 210k actual
resistor 20 - 500k - tested 1.3M actual
Now I powered up and no audio - replaced all tubes from another working set and same results.I then started to use a screwdriver and touch the 1st RF tube - nothing, 2nd RF - Nothing, finally when I touched the third which is the Detector tube - full audio and I can select stations!
Seeking the advice of the experts for assistance. Where should I look next? I am hoping I am one step or component from a success story - but it is finding that one defective part.
RE: Philco Model 20 Troubleshooting - Ron Ramirez - 01-11-2013
My first suspect would be a bad RF coil, and I would look first at (7) or (10).
http://www.philcoradio.com/tech/images/20b.jpg
If you have access to a multimeter, measure the voltage between the plate and ground of the first two 24 RF amp tubes. I would bet one - or even both - have no plate voltage due to a defective coil or coils.
RE: Philco Model 20 Troubleshooting - mgambuzza - 01-11-2013
Excellent suggestion - thanks!!
Question - looking at the schematic - I started with looking between G of 24 and Chassis Ground at the Detector , and saw continuity. I then looked between G of 24 at and Chassis Ground - and saw continuity (I believe this is the Second RF). Finally - when I looked at G of 24 and Ground of #1 - and saw open. I would then presume that the First RF coil is open - which would be Schematic part number 7?
Let me know if my thinking/thought process seems logical - I am presuming that going from the Grid connection to chassis ground would be testing the integrity of the coil which terminates to the ground or chassis.
RE: Philco Model 20 Troubleshooting - Ron Ramirez - 01-11-2013
mgambuzza Wrote:which would be Schematic part number 7?
Are you asking which coil is (7)? If so, merely look at the link in my previous post. (2) is the antenna coil, (7) the first RF coil, (10) the second RF coil.
When you are measuring between the grid and ground, you are measuring the continuity of the coil's secondary. It is important that you measure between the plate and B+ - in other words, what I asked you to measure in my previous post. I asked this because it is usually the primary of these coils that is prone to failure.
RE: Philco Model 20 Troubleshooting - mgambuzza - 01-12-2013
Did some testing - Secondaries on part number 10 good - Primaries - open. Ordered some wire tonight - hopefully be here early next week and we will have a success story.
RE: Philco Model 20 Troubleshooting - mgambuzza - 01-17-2013
WOOHOO!! Success!!
I ordered the wire from TEMCo on Sunday, arrived Thursday morning - not bad from CA to NY.
I began to unwind the old primary - which fell apart multiple times - but once I had some consistent wiring was able to verify approximately 50 windings (fell in spec with documentation). I replaced the separating film, started the winding, and after the fourth time -10, then 38, then 45, finally 50- I had a finished coil (one time too much overlapping, the second the wire reel dropped, the third my hand cramped and I dropped the coil). I tested the secondaries each time - when I unreeled the primary, and after winding the new and still saw continuity. When I soldered the primary winding I tested and found continuity. Replaced coil and tested radio and found the volume and stations worked much better, but I suspected the other coil was defective. Unsoldered, removed, tested and found the primary on this was open as well. This time the process went much quicker and the winding process was much quicker. Once installed and powered up the radio came alive!
Thanks to the advice given here! Much appreciated!
RE: Philco Model 20 Troubleshooting - Radioroslyn - 01-21-2013
I hate when my hand cramps!!!!! 50 not to hard it's when you get up into the hundreds. Glad to hear you get it running again.
Terry
RE: Philco Model 20 Troubleshooting - Eliot Ness - 01-21-2013
Good job! I think Ron has spent more time rewinding Philco coils than Superman spent in a phone booth
RE: Philco Model 20 Troubleshooting - mgambuzza - 02-05-2013
Trouble in paradise. While conducting long term testing on radio the volume/signal quality was fading in and out. When I touch the top of the center 24 tube, reception comes strong again. Swapped tubes, resoldered wires. All with no improvement. I did rewind the primaries of the second and third RF transformers. Any other suggestions? I looked, desoldered, and removed the first RF transformer and did not see/test any opens like I did with the Second and Third RF transformer.
RE: Philco Model 20 Troubleshooting - Radioroslyn - 02-05-2013
Well a couple of thing to have a look at.Check for the heater the the tube in the bad stage. Have seen an intermittent heater in the tube it self. Check plate and screen grid voltages.
Terry
RE: Philco Model 20 Troubleshooting - mgambuzza - 02-05-2013
Thank you. Found a last few spare 24A tubes and began swapping. I did find one that so far in 2hrs of operation has not popped, dropped in volume, or faded. The one thing I do notice is when initially powered on volume is high then settles. Not noticeable if at lowest setting when powered up and wait until set warms up. If you have volume at half setting and power up it is really noticeable.
RE: Philco Model 20 Troubleshooting - mgambuzza - 02-06-2013
Thank you. Found a last few spare 24A tubes and began swapping. I did find one that so far in 2hrs of operation has not popped, dropped in volume, or faded. The one thing I do notice is when initially powered on volume is high then settles. Not noticeable if at lowest setting when powered up and wait until set warms up. If you have volume at half setting and power up it is really noticeable.
RE: Philco Model 20 Troubleshooting - Arran - 02-06-2013
Try resoldering the #24 tube pins and the grid caps, that may fix the problem on the original tube.
Regards
Arran