04-05-2017, 03:26 PM
Strange that the second RF amplifier is an EF50 when as far as I can understand EF54 would have been entirely suitable for the task. The first RF amplifier stage employs an EF54 and is biased to operate at max gain. The second RF is gain controlled in the normal manner by a variable resistor in the cathode circuit. Perhaps the EF50 has a better gain control characteristic compared with the EF54.
Anyway, back to the work done on the set this week. The Mazda octal valve socket used for the sound IF amplifier valve has been replaced with an international octal socket. The Mazda SP61 pentode will be replaced by an Marconi-Osram Z66 or, if I can get away with it a vari-mu KTZ61 or W61.
The attachment shows that the vision IF transformer have received some modifications by the mystery engineer. I'm not going to make any attempt at returning the transformers back to the original EMI designers specification.
It seems unbelievable that someone has gone to all the bother of altering the receiver with such complex modifications when a simple external channel 1 to channel 4 frequency shifter unit would have done the job.
Some mornings I look at this set and think should I give up on this strange receiver?
Geordie McBoyne.
Anyway, back to the work done on the set this week. The Mazda octal valve socket used for the sound IF amplifier valve has been replaced with an international octal socket. The Mazda SP61 pentode will be replaced by an Marconi-Osram Z66 or, if I can get away with it a vari-mu KTZ61 or W61.
The attachment shows that the vision IF transformer have received some modifications by the mystery engineer. I'm not going to make any attempt at returning the transformers back to the original EMI designers specification.
It seems unbelievable that someone has gone to all the bother of altering the receiver with such complex modifications when a simple external channel 1 to channel 4 frequency shifter unit would have done the job.
Some mornings I look at this set and think should I give up on this strange receiver?
Geordie McBoyne.
