Philco 39-25 -- No reception
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Hello everyone
I am currently performing an electronic restoration of a Philco 39-25.
The radio powers on (everything seems normal but there is no reception. I have rechecked the wiring against the schematic which I traced out with a highlighter while comparing to the actual wiring. I thought it might be a oscillator coil problem but also rechecked for continuity which was also fine. Did the same with the broadcast and shortwave coils as well. I connected an outside antenna to see if that would help but there is no change. I also have touched the top of the caps on 6A8G, 78, and 75 to see if I could hear any hum . Nothing.
Any help will be appreciated
I have attached a link to the schematic
http://www.nostalgiaair.org/pagesbymodel...013286.pdf
I have listed the pin voltage readings that I got to help:
6A8-
PIN 1 = NO CONNECTION
PIN 2 = 6.67
PIN 3 = 114
PIN 4 = 12
PIN 6 = 12
PIN 7= 0
PIN 8 = 0
78-
PIN 1= 6.6
PIN 2= 189
PIN 3= 4.5
PIN 4= NEG 3
PIN 5= 0
PIN 6= 0
75
PIN 1 = 0
PIN 2 =211
PIN 3 = NEG 2.47
PIN 4 = 0
PIN 5 = 0
PIN 6 = 6.5
41
PIN 1 = 6.5
PIN 2 = 202
PIN 3 = 208
PIN 4 = NEG 13
PIN 5 = 0
PIN 6 = 0
84
PIN 1 = 6.5
PIN 2 = 292
PIN 3 = 292
PIN 4 = 300
PIN 5 = 0
ELECTROLYTIC (C17 ) = 191
ELECTROLYTIC (C18 ) = 208
ELECTROLYTIC (C41 ) = 302
I have noticed that I can get static when touching pin 4 of the 41 tube.
Kirk
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Have you replaced all the old caps and bad resistors yet?
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(03-02-2016, 10:31 PM)Warren Wrote: Have you replaced all the old caps and bad resistors yet?
Yes
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1. If you could, it'd be nice to have pins functions instead of numbers. A, K, SG, SUP, G, H (anode or plate, Cathode,screen, suppressor, grid, heater).
2. Have you heard ANY sound at all? Scratch, pop, hum? If not, have you looked at opens in output xfmr or feld coil?
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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City: Joppa, Maryland
6A8-
PIN 1 = NO CONNECTION
H = 6.67
P = 114
G3/G5 = 12
G1 =12
G2 = 12
H = 0
K = 0
78-
H = 6.6
P = 189
G2 = 4.5
G3 = NEG 3
K = 0
H = 0
75
H = 0
PT =211
PD2 = NEG 2.47
PD1 = 0
K = 0
H = 6.5
41
H = 6.5
P = 202
G2 = 208
G1 = NEG 13
K = 0
H = 0
84
H = 6.5
PD2 = 292
PD1 = 292
K = 300
H = 0
Only sound I am hearing is a very faint hum just enough to know that it is on. I can get a scratching sound when I touch G1 with the probe of my multimeter. Field coil is reading 2000 ohms. Output xfmr is a new Hammond 125ASE.
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To clarify, the sound I get comes from G1 of the 41 tube.
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Looks like your SG voltage is very low on the 6A8 and 78 tubes. Check values of #14, 15 and 16. Plate voltage on the 75 is high, may have a weak tube or may resolve it self when you get the SG problem sorted out.
Terry
(This post was last modified: 03-03-2016, 12:46 AM by Radioroslyn.)
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Have you tried touching the volume control center connection with your meter lead or a hot soldering iron? You should get a loud buzz in the speaker when you touch one of those connectors if the audio amp section is working. Sounds like your output stage is working somewhat. Have you tested your tubes? A weak tube could send you in circles hunting for ghosts. I also know the 6A8 is a little odd, so you may not be oscillating. Might have to sub it to check, since it may check fine on the tube tester.
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Tim
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Two quick test. First put your finger on the grid cap of the 75 tube. You should hear a hum. If not, you have a problem in the output section. Second, place a working radio near the 39-25. Tune the 39-25 to the low to mid part of the dial say 700. Tune the working radio 470 kc above 39-25. Rock either dial, you should be able to hear the oscillator. By doing these two tests you have tested about two thirds of your radio.
Steve
M R Radios C M Tubes
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Steve, he did say he touched the cap on the 75 and got nothing so that would be a good place to start looking. I'd do the two radio check as well though.
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Hello to all and first let me say thank you to everyone who has responded.
Radioroslyn - Did check values of 15 16 and 17 - all were ok.
Madmurdock - Did get a slight hum when touching center connection of volume control.
Steve Davis - I did try the 2 radio check - rocked dials on both - heard nothing in either set.
I did try touching the cap again on the 75 and I did hear a very faint hum. I must have missed it the first time I tried it.
Thanks again for the suggestions and info.
Kirk
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Well something is screwed up as your 6A8 voltages are too low. Grid 1 should be -3 or -4 vdc. Grid 2 (which acts as the oscillator plate) should be up around +150vdc or better. Screen grid (p4) at 80vdc. Plate should be about 200vdc. No way your local osc is going to be running with these voltages you have.
The screen grid of the 78 should be much higher too. If those resistors check ok then either the 6A8 is shorted or there is a problem with one of those caps.
Voltages seem ok over in the audio stages. I'm suspecting that you have a bad connection between the plate or cathode pin and socket on the75 tube or the tube bad. Voltage is good but tube isn't drawing any current. I'm assuming that the heater in the 75 is lit. Try swapping the 6A8 with another and recheck the voltages to see if they come up.
GL
Terry
ps. Grew up a little south of you in Sandy Spring.
(This post was last modified: 03-04-2016, 11:04 PM by Radioroslyn.)
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(03-03-2016, 10:42 PM)Radioroslyn Wrote: Well something is screwed up as your 6A8 voltages are too low. Grid 1 should be -3 or -4 vdc. Grid 2 (which acts as the oscillator plate) should be up around +150vdc or better. Screen grid (p4) at 80vdc. Plate should be about 200vdc. No way your local osc is going to be running with these voltages you have.
The screen grid of the 78 should be much higher too. If those resistors check ok then either the 6A8 is shorted or there is a problem with one of those caps.
Voltages seem ok over in the audio stages. I'm suspecting that you have a bad connection between the plate or cathode pin and socket on the75 tube or the tube bad. Voltage is good but tube isn't drawing any current. I'm assuming that the heater in the 75 is lit. Try swapping the 6A8 with another and recheck the voltages to see if they come up.
GL
Terry
ps. Grew up a little south of you in Sandy Spring.
Did swap out 6A8 and came up with the following voltages:
H =6.67
P = 188 (Can also hear scratching noise thru the speaker when I touch probe to the pin)
G3/G5 = 75
G1 - NEG 3
G2 = NEG 1.5
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Well that's better but grid 2 needs to be at 150vdc. So try disconnecting #7 And see if the voltage goes up. If not check the 1/2 ohm section of the osc coil may be open. Measure the voltage on both side of winding should be the same on both sides around 150vdc.
Terry
(This post was last modified: 03-05-2016, 06:51 PM by Radioroslyn.)
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