I have a Philco 38-12 that neede the #78 tube replaced to get working but had a hum, all caps have been replaced and radio still has a hum, any ideas what to look for? Dale

Have you checked all your tubes for heater to cathode leakage?

I assume the two electrolytics have been replaced and the speaker is the original one with a built in field coil.

Lou

:I have a Philco 38-12 that neede the #78 tube replaced to get working but had a hum, all caps have been replaced and radio still has a hum, any ideas what to look for? Dale
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:Have you checked all your tubes for heater to cathode leakage?
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:I assume the two electrolytics have been replaced and the speaker is the original one with a built in field coil.
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:Lou
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::I have a Philco 38-12 that neede the #78 tube replaced to get working but had a hum, all caps have been replaced and radio still has a hum, any ideas what to look for? Dale
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Hi Lou, yes the electrolytics have benn replaced and speaker is original, I will go trough the tubes and check, did not know this could be possiblebut does make sense, will an ohm meter suffice or will a tube tester be required, I have a Philco tube tester but have not tried it out yet, I am relatively new to repairing radio's and am enjoying briging them back to life, so far I have a 38-9, 37-610, 38-12, 38-7, and a PT-66, a 19l console with an 89 chassis wich will be a more difficult unit as it has a lot of bugs in those.Thanx for the great info and will be quite happy to rid this one of the hum,
Dale

There is an issue with wire lead dress as said in the Riders information about hum. The filter capacitors #28 & #26 do not share the same ground. The point of the original connections should be the same point the replacement filter capacitors connect. Remove the 75 tube if the hum is still there the problem is in the power supply, or the audio output. Check over all ground points make sure there is a good connection.

:There is an issue with wire lead dress as said in the Riders information about hum. The filter capacitors #28 & #26 do not share the same ground. The point of the original connections should be the same point the replacement filter capacitors connect. Remove the 75 tube if the hum is still there the problem is in the power supply, or the audio output. Check over all ground points make sure there is a good connection.
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Yes, speaker is original and filter caps replaced, You have given some very good ground to cover thanx guy's, I will let You know how it turns out.

::There is an issue with wire lead dress as said in the Riders information about hum. The filter capacitors #28 & #26 do not share the same ground. The point of the original connections should be the same point the replacement filter capacitors connect. Remove the 75 tube if the hum is still there the problem is in the power supply, or the audio output. Check over all ground points make sure there is a good connection.
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:Yes, speaker is original and filter caps replaced, You have given some very good ground to cover thanx guy's, I will let You know how it turns out.
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OK, I have looked things over the tubes all semm ok with an ohm meter, there has been work done on this before I got it, originally 2 seperate cans for caps 26 and 28, 4 and 12 farad, there was only one dual section can 8 farad and I followed where the wirs went to connect the new ones so they are both connected as should be according to scematic, I had aslso replaced the grid cap wire on the #75 tube and tried moving this wire away from the coil under the chassi but there is still some hum more noticable between stations and gets almost not noticeable whn tuned to a station, mabe some hum is normal on a field coil speaker?
Dale

The speaker field coil is also a filter choke in the power supply. This is the better design to help eliminate hum. Might try upping the value of capacitor 26 to 10 MFD. The grid of the 75 tube is where hum is most likely being picked up. Keep that wire away from heater wires. The radio being worked on before is where I would look for a possible error. Check over that work to make sure it's as the schematic shows. Again any RF signal path wires should be kept away from AC voltage. Original ground points are there for a reason, ground loop problems can result.

:The speaker field coil is also a filter choke in the power supply. This is the better design to help eliminate hum. Might try upping the value of capacitor 26 to 10 MFD. The grid of the 75 tube is where hum is most likely being picked up. Keep that wire away from heater wires. The radio being worked on before is where I would look for a possible error. Check over that work to make sure it's as the schematic shows. Again any RF signal path wires should be kept away from AC voltage. Original ground points are there for a reason, ground loop problems can result.
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Hey, problem solved, the output tube was the culprit,I gess this is my first lesson on bad tubes, filament is good and works but why the hum? mabe bad filter cap in the past had dammaged it, tube has some black flasing a the bottom,I remembered that I had another 41 and a 75 from a parts radio o I tried and hum is gone!!!!!Thanx for the help.