Philco Model 38-12 Problems

3/17/2009 9:09:09 AMGeorge F

Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a coulple to out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.

Thanks George F.

3/17/2009 12:28:13 PMRene

Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.

:Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a coulple to out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
:
:Thanks George F.

3/17/2009 1:04:30 PMGeorge F.

Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.

George F.

:Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
:
:
::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a coulple to out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
::
::Thanks George F.

3/17/2009 10:05:07 PMRene

If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.

:Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
:
:George F.
:
::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
::
::
:::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a coulple to out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
:::
:::Thanks George F.

3/18/2009 12:46:56 PMGeorge F.

Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?

:If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
:
::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
::
::George F.
::
:::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
:::
:::
::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a coulple to out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
::::
::::Thanks George F.

3/18/2009 1:41:44 PMTonyJ

If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.

:Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?
:
::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
::
:::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
:::
:::George F.
:::
::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
::::
::::
:::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a coulple to out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
:::::
:::::Thanks George F.

3/18/2009 3:22:42 PMGeorge F.

Thanks for your response, Yes I have another 38-12 where the oscillator coil was open, repaired it and it does play fine. This must be a weak spot for this radio. The one I am working on has low voltage at the screen connection between 6A7 (DET/OSC) and the first IF (78), should be 90V I was getting about 45V. Just changed the tuning condenser and low and behold I got some sound out of the speaker and I was able to tune some stations. A voltage check shows about 62V now still low but seems I (we) are making progress.

George F.

:If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.
:
::Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?

George F.
::
:::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
:::
::::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
::::
::::George F.
::::
:::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
:::::
:::::
::::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
::::::
::::::Thanks George F.

3/18/2009 4:27:28 PMTonyJ

I'm a bit confused on why the tuning condenser would bring the voltage up but it sounds like it made a difference. You said you checked the resistors as I would say that resistor 11 could have drifted upward. Is it possible something in one of those two tubes is drawing the supply down?

:Thanks for your response, Yes I have another 38-12 where the oscillator coil was open, repaired it and it does play fine. This must be a weak spot for this radio. The one I am working on has low voltage at the screen connection between 6A7 (DET/OSC) and the first IF (78), should be 90V I was getting about 45V. Just changed the tuning condenser and low and behold I got some sound out of the speaker and I was able to tune some stations. A voltage check shows about 62V now still low but seems I (we) are making progress.
:
:George F.
:
:
::If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.
::
:::Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?
:
:George F.
:::
::::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
::::
:::::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
:::::
:::::George F.
:::::
::::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
::::::
::::::
:::::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
:::::::
:::::::Thanks George F.

3/18/2009 5:08:46 PMGeorge F.

Hi Tony, It was the only thing left that I did not change, possibly one the 2 variable caps were going to ground, not sure and it did not solve the problem, just seemed to help some.

:I'm a bit confused on why the tuning condenser would bring the voltage up but it sounds like it made a difference. You said you checked the resistors as I would say that resistor 11 could have drifted upward. Is it possible something in one of those two tubes is drawing the supply down?
:
::Thanks for your response, Yes I have another 38-12 where the oscillator coil was open, repaired it and it does play fine. This must be a weak spot for this radio. The one I am working on has low voltage at the screen connection between 6A7 (DET/OSC) and the first IF (78), should be 90V I was getting about 45V. Just changed the tuning condenser and low and behold I got some sound out of the speaker and I was able to tune some stations. A voltage check shows about 62V now still low but seems I (we) are making progress.
::
::George F.
::
::
:::If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.
:::
::::Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?
::
::George F.
::::
:::::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
:::::
::::::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
::::::
::::::George F.
::::::
:::::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
:::::::
:::::::
::::::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
::::::::
::::::::Thanks George F.

3/18/2009 5:59:15 PMrob

so you did try new tubes?

:Hi Tony, It was the only thing left that I did not change, possibly one the 2 variable caps were going to ground, not sure and it did not solve the problem, just seemed to help some.
:
::I'm a bit confused on why the tuning condenser would bring the voltage up but it sounds like it made a difference. You said you checked the resistors as I would say that resistor 11 could have drifted upward. Is it possible something in one of those two tubes is drawing the supply down?
::
:::Thanks for your response, Yes I have another 38-12 where the oscillator coil was open, repaired it and it does play fine. This must be a weak spot for this radio. The one I am working on has low voltage at the screen connection between 6A7 (DET/OSC) and the first IF (78), should be 90V I was getting about 45V. Just changed the tuning condenser and low and behold I got some sound out of the speaker and I was able to tune some stations. A voltage check shows about 62V now still low but seems I (we) are making progress.
:::
:::George F.
:::
:::
::::If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.
::::
:::::Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?
:::
:::George F.
:::::
::::::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
::::::
:::::::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
:::::::
:::::::George F.
:::::::
::::::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
::::::::
::::::::
:::::::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
:::::::::
:::::::::Thanks George F.

3/18/2009 7:50:36 PMGeorge F.

Hi Rob, Tested all the tubes the only weak one was #78 replace it and tried the set, thats when I found the low screen voltage. Have since taken a set of tubes out of an operating radio (same kind) and tried them, same result......I suppose its these kind kind of problems that teach us so much. It is not that radio you bought and put a set of new caps in it a plays just fine, you don't really learn much.

George F.

:so you did try new tubes?
:
::Hi Tony, It was the only thing left that I did not change, possibly one the 2 variable caps were going to ground, not sure and it did not solve the problem, just seemed to help some.
::
:::I'm a bit confused on why the tuning condenser would bring the voltage up but it sounds like it made a difference. You said you checked the resistors as I would say that resistor 11 could have drifted upward. Is it possible something in one of those two tubes is drawing the supply down?
:::
::::Thanks for your response, Yes I have another 38-12 where the oscillator coil was open, repaired it and it does play fine. This must be a weak spot for this radio. The one I am working on has low voltage at the screen connection between 6A7 (DET/OSC) and the first IF (78), should be 90V I was getting about 45V. Just changed the tuning condenser and low and behold I got some sound out of the speaker and I was able to tune some stations. A voltage check shows about 62V now still low but seems I (we) are making progress.
::::
::::George F.
::::
::::
:::::If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.
:::::
::::::Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?
::::
::::George F.
::::::
:::::::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
:::::::
::::::::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
::::::::
::::::::George F.
::::::::
:::::::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
:::::::::
:::::::::
::::::::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
::::::::::
::::::::::Thanks George F.

3/19/2009 8:45:27 AMRene

Try calculating the screen current for those two tubes. You know the screen resistor value...measure the voltage drop across the resistor and use ohm's law to determine the current. Then compare with tube data sheet info.

:Hi Rob, Tested all the tubes the only weak one was #78 replace it and tried the set, thats when I found the low screen voltage. Have since taken a set of tubes out of an operating radio (same kind) and tried them, same result......I suppose its these kind kind of problems that teach us so much. It is not that radio you bought and put a set of new caps in it a plays just fine, you don't really learn much.
:
:George F.
:
::so you did try new tubes?
::
:::Hi Tony, It was the only thing left that I did not change, possibly one the 2 variable caps were going to ground, not sure and it did not solve the problem, just seemed to help some.
:::
::::I'm a bit confused on why the tuning condenser would bring the voltage up but it sounds like it made a difference. You said you checked the resistors as I would say that resistor 11 could have drifted upward. Is it possible something in one of those two tubes is drawing the supply down?
::::
:::::Thanks for your response, Yes I have another 38-12 where the oscillator coil was open, repaired it and it does play fine. This must be a weak spot for this radio. The one I am working on has low voltage at the screen connection between 6A7 (DET/OSC) and the first IF (78), should be 90V I was getting about 45V. Just changed the tuning condenser and low and behold I got some sound out of the speaker and I was able to tune some stations. A voltage check shows about 62V now still low but seems I (we) are making progress.
:::::
:::::George F.
:::::
:::::
::::::If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.
::::::
:::::::Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?
:::::
:::::George F.
:::::::
::::::::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
::::::::
:::::::::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
:::::::::
:::::::::George F.
:::::::::
::::::::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
::::::::::
::::::::::
:::::::::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
:::::::::::
:::::::::::Thanks George F.

3/19/2009 12:44:03 PMGeorge F.

Hi Rene, The Resistor is item 11 at 39,000 ohms and the voltage drop across it is 164 VDC based on this data the current would be about 4mA. I do not know where you are getting your current flow data form, the pages I down-loaded from this site does not show any current flow results.

:Try calculating the screen current for those two tubes. You know the screen resistor value...measure the voltage drop across the resistor and use ohm's law to determine the current. Then compare with tube data sheet info.
:
::Hi Rob, Tested all the tubes the only weak one was #78 replace it and tried the set, thats when I found the low screen voltage. Have since taken a set of tubes out of an operating radio (same kind) and tried them, same result......I suppose its these kind kind of problems that teach us so much. It is not that radio you bought and put a set of new caps in it a plays just fine, you don't really learn much.
::
::George F.
::
:::so you did try new tubes?
:::
::::Hi Tony, It was the only thing left that I did not change, possibly one the 2 variable caps were going to ground, not sure and it did not solve the problem, just seemed to help some.
::::
:::::I'm a bit confused on why the tuning condenser would bring the voltage up but it sounds like it made a difference. You said you checked the resistors as I would say that resistor 11 could have drifted upward. Is it possible something in one of those two tubes is drawing the supply down?
:::::
::::::Thanks for your response, Yes I have another 38-12 where the oscillator coil was open, repaired it and it does play fine. This must be a weak spot for this radio. The one I am working on has low voltage at the screen connection between 6A7 (DET/OSC) and the first IF (78), should be 90V I was getting about 45V. Just changed the tuning condenser and low and behold I got some sound out of the speaker and I was able to tune some stations. A voltage check shows about 62V now still low but seems I (we) are making progress.
::::::
::::::George F.
::::::
::::::
:::::::If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.
:::::::
::::::::Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?
::::::
::::::George F.
::::::::
:::::::::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
:::::::::
::::::::::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
::::::::::
::::::::::George F.
::::::::::
:::::::::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
:::::::::::
:::::::::::
::::::::::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
::::::::::::
::::::::::::Thanks George F.

3/19/2009 2:52:39 PMTonyJ

He probably is referring to the tube data sheets themselves. Here is a site and pulled up an RCA type 78 tube for example:

http://tubedata.milbert.com/sheets/021/7/78.pdf

:Hi Rene, The Resistor is item 11 at 39,000 ohms and the voltage drop across it is 164 VDC based on this data the current would be about 4mA. I do not know where you are getting your current flow data form, the pages I down-loaded from this site does not show any current flow results.
:
::Try calculating the screen current for those two tubes. You know the screen resistor value...measure the voltage drop across the resistor and use ohm's law to determine the current. Then compare with tube data sheet info.
::
:::Hi Rob, Tested all the tubes the only weak one was #78 replace it and tried the set, thats when I found the low screen voltage. Have since taken a set of tubes out of an operating radio (same kind) and tried them, same result......I suppose its these kind kind of problems that teach us so much. It is not that radio you bought and put a set of new caps in it a plays just fine, you don't really learn much.
:::
:::George F.
:::
::::so you did try new tubes?
::::
:::::Hi Tony, It was the only thing left that I did not change, possibly one the 2 variable caps were going to ground, not sure and it did not solve the problem, just seemed to help some.
:::::
::::::I'm a bit confused on why the tuning condenser would bring the voltage up but it sounds like it made a difference. You said you checked the resistors as I would say that resistor 11 could have drifted upward. Is it possible something in one of those two tubes is drawing the supply down?
::::::
:::::::Thanks for your response, Yes I have another 38-12 where the oscillator coil was open, repaired it and it does play fine. This must be a weak spot for this radio. The one I am working on has low voltage at the screen connection between 6A7 (DET/OSC) and the first IF (78), should be 90V I was getting about 45V. Just changed the tuning condenser and low and behold I got some sound out of the speaker and I was able to tune some stations. A voltage check shows about 62V now still low but seems I (we) are making progress.
:::::::
:::::::George F.
:::::::
:::::::
::::::::If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.
::::::::
:::::::::Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?
:::::::
:::::::George F.
:::::::::
::::::::::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
::::::::::
:::::::::::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
:::::::::::
:::::::::::George F.
:::::::::::
::::::::::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
::::::::::::
::::::::::::
:::::::::::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
:::::::::::::
:::::::::::::Thanks George F.

3/19/2009 4:14:03 PMGeorge F.

Thanks Tony, that helps. That data page shows the max screen current at 2.6mA, I measured close to 4mA, something is wrong here, I going back to work on it some more. It has become a challange to solve this problem.

:He probably is referring to the tube data sheets themselves. Here is a site and pulled up an RCA type 78 tube for example:
:
:http://tubedata.milbert.com/sheets/021/7/78.pdf
:
:
::Hi Rene, The Resistor is item 11 at 39,000 ohms and the voltage drop across it is 164 VDC based on this data the current would be about 4mA. I do not know where you are getting your current flow data form, the pages I down-loaded from this site does not show any current flow results.
::
:::Try calculating the screen current for those two tubes. You know the screen resistor value...measure the voltage drop across the resistor and use ohm's law to determine the current. Then compare with tube data sheet info.
:::
::::Hi Rob, Tested all the tubes the only weak one was #78 replace it and tried the set, thats when I found the low screen voltage. Have since taken a set of tubes out of an operating radio (same kind) and tried them, same result......I suppose its these kind kind of problems that teach us so much. It is not that radio you bought and put a set of new caps in it a plays just fine, you don't really learn much.
::::
::::George F.
::::
:::::so you did try new tubes?
:::::
::::::Hi Tony, It was the only thing left that I did not change, possibly one the 2 variable caps were going to ground, not sure and it did not solve the problem, just seemed to help some.
::::::
:::::::I'm a bit confused on why the tuning condenser would bring the voltage up but it sounds like it made a difference. You said you checked the resistors as I would say that resistor 11 could have drifted upward. Is it possible something in one of those two tubes is drawing the supply down?
:::::::
::::::::Thanks for your response, Yes I have another 38-12 where the oscillator coil was open, repaired it and it does play fine. This must be a weak spot for this radio. The one I am working on has low voltage at the screen connection between 6A7 (DET/OSC) and the first IF (78), should be 90V I was getting about 45V. Just changed the tuning condenser and low and behold I got some sound out of the speaker and I was able to tune some stations. A voltage check shows about 62V now still low but seems I (we) are making progress.
::::::::
::::::::George F.
::::::::
::::::::
:::::::::If you're not picking up stations I'd check the oscillator coil. I had a 38-12 with an open winding and the parts set I purchased had it's coil open as well. Ended up removing the outer winding and rewinding with a similar numbers of turns and all is well.
:::::::::
::::::::::Thanks for your reply, Checked the screen resistor and it is within the specs, in fact I checked all the resistors , replaced two, to bring them in line with the schematic. Checked the antenna transformer, seems OK. What are your thoughts on the tuning condenser being bad?
::::::::
::::::::George F.
::::::::::
:::::::::::If the cap is OK you might want to check that screen resistor. If it has gone up significantly in value then the result could be low voltage.
:::::::::::
::::::::::::Thanks for the answer, I relaced this cap (0.03 uF) easy to get at, maybe I should try another one, just in case.
::::::::::::
::::::::::::George F.
::::::::::::
:::::::::::::Something is pulling the voltage down. Did you change the .03 uF cap identified as component 10? Sometimes caps like this are buried in bandswitch assemblies and are hard to get at.
:::::::::::::
:::::::::::::
::::::::::::::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
::::::::::::::
::::::::::::::Thanks George F.

3/19/2009 5:58:15 PMGeorge F. Got IT!

Got It: finally found the problem, the second IF transformer seemed real erratic when I tried to adjust it, so I replaced it with another one (from a junk radio). Turned the radio on and it played real nice, checked the screen voltage and it measured around 85VDC, with a current of 3mA, a little high but reasonable. Many Thanks to everyone who helped, this is a great WEB site, thanks again.

George F.

:Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
:
:Thanks George F.

3/19/2009 9:13:43 PMRene

Good deal! I'd have to take a closer look at the schematic to figure out how that 2nd IF trans would cause low screen voltage at the converter. But hey, whatever works!

:Got It: finally found the problem, the second IF transformer seemed real erratic when I tried to adjust it, so I replaced it with another one (from a junk radio). Turned the radio on and it played real nice, checked the screen voltage and it measured around 85VDC, with a current of 3mA, a little high but reasonable. Many Thanks to everyone who helped, this is a great WEB site, thanks again.
:
:George F.
:
::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
::
::Thanks George F.

3/20/2009 1:20:03 PMrob

one side of the 2nd IF connects to one side of that resistor. if the IF was shorting or something inside it would drag the voltage down. but then you would also have low plate voltage and you said all were on target except the screens. im at a loss why the 2nd did it.

:Good deal! I'd have to take a closer look at the schematic to figure out how that 2nd IF trans would cause low screen voltage at the converter. But hey, whatever works!
:
::Got It: finally found the problem, the second IF transformer seemed real erratic when I tried to adjust it, so I replaced it with another one (from a junk radio). Turned the radio on and it played real nice, checked the screen voltage and it measured around 85VDC, with a current of 3mA, a little high but reasonable. Many Thanks to everyone who helped, this is a great WEB site, thanks again.
::
::George F.
::
:::Hi, I have been working on a Philco Model 38-12 for the last couple days. Have recaped the radio and relaced a couple of out of tolerance resistors, checked all tubes. When I turned on the radio just get a lot of static from the speaker. A voltage check shows everything to reasonable value except the screen connection between the 6A7 (DET/OSC) and the first IF (78) it should be 90 VDC, I get about 43 VDC. I would appreciate any help on this matter.
:::
:::Thanks George F.