08-28-2023, 07:04 PM
Hi BrackenMan,
When you aligned the IF, where did you inject the signal? Was it modulated? If you inject 455KHz into the antenna input, does it pass through?
Measure across Cap #5 with a DVM. There should be a DC Voltage if between 0 and about -5V. It should get quite negative if you put a modulated 455KHz signal through the IF stage.
This is an interesting issue, but this is a rather simple radio. One other issue may be that the 7A8 is oscillating but not converting. Apparently, Loctal tubes and sockets are known for poor connection. I don't know why this is an issue with Loctal tubes but not with 7 or 9 pin miniatures. Diameter of pins? The Shop Talk section of the Philco Library discusses Loctal tube issues. It is reproduced below with thanks to the anonymous author:
***********************************************************************************************************************************
From the Philco Library-> Shop Talk-> Techniques
Having replaced all of the paper and electrolytic capacitors in a Philco 41-605, power was applied for the first time. The radio played well with good volume – only it had a very loud hum!
The next day, the radio suddenly began to make loud static noise, like the sound of a close thunderstorm. Remembering an experience with the Type 75 tube (which can be notorious for producing loud static even when it tests good), the decision was made to go after the 7C6 second detector/first audio tube. Wiggling this tube not only made the static stop, but also ended the hum.
Thoroughly cleaning the tube socket with contact cleaner, and replacing the 7C6, cured the trouble.
Bad loctal tube socket contacts can produce a multitude of symptoms, including intermittent or no reception.
Loctal tube socket contacts should always be cleaned; in this case, they had not been. Many repairmen use a very tiny file, found in welding supply shops, to carefully clean the contacts. Another approach is to carefully spray each contact in the tube socket, accompanied by the use of a tiny brush intended to clean between your teeth, in each socket contact hole. Spray contact cleaner into all eight holes of the socket, one at a time, then insert and remove the brush several times to work the contact cleaner in before it evaporates. Do this to every loctal socket in the set.
Important: Wait a while before you apply power again, however, to allow the cleaner residue to completely evaporate. This is especially important if you use DeoxIT instead of regular contact cleaner.
While you have the tubes out of their sockets, carefully inspect the pins. If they look blackened, then carefully scrape this corrosion off each tube pin, or use sandpaper sparingly on each pin, until they are all shiny. The corrosion can also interfere with making proper contacts between tube pins and socket contacts.
When you aligned the IF, where did you inject the signal? Was it modulated? If you inject 455KHz into the antenna input, does it pass through?
Measure across Cap #5 with a DVM. There should be a DC Voltage if between 0 and about -5V. It should get quite negative if you put a modulated 455KHz signal through the IF stage.
This is an interesting issue, but this is a rather simple radio. One other issue may be that the 7A8 is oscillating but not converting. Apparently, Loctal tubes and sockets are known for poor connection. I don't know why this is an issue with Loctal tubes but not with 7 or 9 pin miniatures. Diameter of pins? The Shop Talk section of the Philco Library discusses Loctal tube issues. It is reproduced below with thanks to the anonymous author:
***********************************************************************************************************************************
From the Philco Library-> Shop Talk-> Techniques
Having replaced all of the paper and electrolytic capacitors in a Philco 41-605, power was applied for the first time. The radio played well with good volume – only it had a very loud hum!
The next day, the radio suddenly began to make loud static noise, like the sound of a close thunderstorm. Remembering an experience with the Type 75 tube (which can be notorious for producing loud static even when it tests good), the decision was made to go after the 7C6 second detector/first audio tube. Wiggling this tube not only made the static stop, but also ended the hum.
Thoroughly cleaning the tube socket with contact cleaner, and replacing the 7C6, cured the trouble.
Bad loctal tube socket contacts can produce a multitude of symptoms, including intermittent or no reception.
Loctal tube socket contacts should always be cleaned; in this case, they had not been. Many repairmen use a very tiny file, found in welding supply shops, to carefully clean the contacts. Another approach is to carefully spray each contact in the tube socket, accompanied by the use of a tiny brush intended to clean between your teeth, in each socket contact hole. Spray contact cleaner into all eight holes of the socket, one at a time, then insert and remove the brush several times to work the contact cleaner in before it evaporates. Do this to every loctal socket in the set.
Important: Wait a while before you apply power again, however, to allow the cleaner residue to completely evaporate. This is especially important if you use DeoxIT instead of regular contact cleaner.
While you have the tubes out of their sockets, carefully inspect the pins. If they look blackened, then carefully scrape this corrosion off each tube pin, or use sandpaper sparingly on each pin, until they are all shiny. The corrosion can also interfere with making proper contacts between tube pins and socket contacts.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
