12-26-2023, 09:55 PM
I am trying to get a F-1540 back operable for a friend with a 1938 Ford. The radio came separately with the car purchase and the previous owner had no status on the radio condition. Cosmetically it is pretty decent shape.
I am a semi retired technician and worked with high power tube transmitters for several years but I have been away from that era for a long while. I have been reading till my eyes bleed on this site and took the good advice of changing out all the dripping wax caps. I have recently dealt with the power supply 4/8 uf can by putting a couple of caps on the bottom side … I don’t like the aesthetics but it brought the B+ voltage to about 211 volts. Not sure if that is correct as I don’t really see any documentation with typical tube voltages. I’d have changed a few resistors also that were off center value by quite a bit.
Today I was able to half step and inject 260 kHz in the If producing a nice clean tone. I eventually got to the point of injecting a 1MHz signal at the grid of the first RF tube and tuned around a bit eventually finding the signal a few hundred KHz off the dial indicator. The sensitivity I am seeing is around -50 dBm and that is not real crisp. I can inject into the antenna port also with a bit more loss.
As I am just getting started in this extracurricular activity I don’t have a supply of tubes but may order a 78 and 6A7. I am pretty encouraged about being able to jam a signal through now it may be just a matter of getting it stronger and cleaning the fuzz out.
I am probably not knowledgeable enough to ask the right questions yet but there is a couple that come to mind.
What kind of sensitivity should I expect out of this auto radio?
I’m pretty convinced the dial is not tracking right so maybe an alignment would help? Would a front end tube swap be useful before I went hog wild with a tuning tool.
Another symptom is the radio seems to produce a good amount of irritating buzzing but when I get the signal tuned in it reduces a lot but a but fuzzy. The IF to Audio stages were real clean sounding.
I would welcome any comments from the brain trust here as to my next steps.
Regards,
Bill
I am a semi retired technician and worked with high power tube transmitters for several years but I have been away from that era for a long while. I have been reading till my eyes bleed on this site and took the good advice of changing out all the dripping wax caps. I have recently dealt with the power supply 4/8 uf can by putting a couple of caps on the bottom side … I don’t like the aesthetics but it brought the B+ voltage to about 211 volts. Not sure if that is correct as I don’t really see any documentation with typical tube voltages. I’d have changed a few resistors also that were off center value by quite a bit.
Today I was able to half step and inject 260 kHz in the If producing a nice clean tone. I eventually got to the point of injecting a 1MHz signal at the grid of the first RF tube and tuned around a bit eventually finding the signal a few hundred KHz off the dial indicator. The sensitivity I am seeing is around -50 dBm and that is not real crisp. I can inject into the antenna port also with a bit more loss.
As I am just getting started in this extracurricular activity I don’t have a supply of tubes but may order a 78 and 6A7. I am pretty encouraged about being able to jam a signal through now it may be just a matter of getting it stronger and cleaning the fuzz out.
I am probably not knowledgeable enough to ask the right questions yet but there is a couple that come to mind.
What kind of sensitivity should I expect out of this auto radio?
I’m pretty convinced the dial is not tracking right so maybe an alignment would help? Would a front end tube swap be useful before I went hog wild with a tuning tool.
Another symptom is the radio seems to produce a good amount of irritating buzzing but when I get the signal tuned in it reduces a lot but a but fuzzy. The IF to Audio stages were real clean sounding.
I would welcome any comments from the brain trust here as to my next steps.
Regards,
Bill
