02-06-2018, 02:51 PM
Jcassity;
First I looked at the plates of the two diodes of the 7C6, which are tied together. It showed simultaneously both the 455KC IF signal and the Audio signal. Then moving onto the junction of the 3.3M ohm resistor and the .06uF to the volume control you see the detected audio. From there you can trace it to the signal grid of the 7C6 and then its Plate viewing the audio signal and its amplification at its Plate. From there it can be also traced through to the Plate and Cathode of the 6J5 Phase Inverter and on to the signal grids of the two audio output tubes.
A good scope is a very valuable tool to use. It can show you both the RF signals including the local Oscillator then the signal level increases through the IF circuits. It makes it easy to spot problems along the way. I am using a Tektronix 2335 oscilloscope which is a 100mHz capable scope. Even a scope capable of say 10 or 15mHz would be fine for working on most AM/SW radios of this vintage. I also like to work on vintage AM/FM tuners and receivers, so the extra bandwidth is useful in those cases. I originally bought the scope for $80 and have had to replace some capacitors in it, but still my overall outlay is less than $100 for a very useful tool. I am not saying everyone can find a good scope for such a low price, or manage to repair it if it fails, but with good searching and help from people here you can likely put together a very good set of equipment to help you in your repair and restoration projects.
Joe
First I looked at the plates of the two diodes of the 7C6, which are tied together. It showed simultaneously both the 455KC IF signal and the Audio signal. Then moving onto the junction of the 3.3M ohm resistor and the .06uF to the volume control you see the detected audio. From there you can trace it to the signal grid of the 7C6 and then its Plate viewing the audio signal and its amplification at its Plate. From there it can be also traced through to the Plate and Cathode of the 6J5 Phase Inverter and on to the signal grids of the two audio output tubes.
A good scope is a very valuable tool to use. It can show you both the RF signals including the local Oscillator then the signal level increases through the IF circuits. It makes it easy to spot problems along the way. I am using a Tektronix 2335 oscilloscope which is a 100mHz capable scope. Even a scope capable of say 10 or 15mHz would be fine for working on most AM/SW radios of this vintage. I also like to work on vintage AM/FM tuners and receivers, so the extra bandwidth is useful in those cases. I originally bought the scope for $80 and have had to replace some capacitors in it, but still my overall outlay is less than $100 for a very useful tool. I am not saying everyone can find a good scope for such a low price, or manage to repair it if it fails, but with good searching and help from people here you can likely put together a very good set of equipment to help you in your repair and restoration projects.
Joe
