12-03-2017, 11:51 AM
To help NM put all this into context with his Saalburg 5170, in the text books it will show a tertiary winding as part of a ratio detector circuit and also an electrolytic capacitor connected across the output of the two diodes, it will say that the tertiary winding is tightly coupled to the last IF's primary winding and that the audio signal is derived from it, you can see these two components in the receivers schematic, L23 is the tertiary winding, it fits the bill as described in the books because it's tightly coupled to the transformers primary as indicated by the common tuning slug coupling it to the primary winding (L25) you can also see that the audio output on FM is taken from that via a simple filter network and ends up at switch contact 17.
The electrolytic capacitor is easy to see and it also fits the bill per the text books as it's connected across the output of the two diodes, the capacitor is C51 in the receivers schematic.
Given a decent signal the audio output at the switch contact 17 should be nice and clean, the voltage across C51 will be -ve DC with respect to chassis when a signal is received, the voltage typically can be between -ve 20 and -ve 30 volts in some receivers with a very strong signal and will be less as the signal strength decreases, that -ve voltage is used for the AGC and also as the control grid grid voltage for the magic eye tuning indicator.
So, from a troubleshooting point of view what we are expecting that's connected with the ratio detector is a good quality audio output and a functioning AGC voltage....just like an AM detector.
Lawrence.
The electrolytic capacitor is easy to see and it also fits the bill per the text books as it's connected across the output of the two diodes, the capacitor is C51 in the receivers schematic.
Given a decent signal the audio output at the switch contact 17 should be nice and clean, the voltage across C51 will be -ve DC with respect to chassis when a signal is received, the voltage typically can be between -ve 20 and -ve 30 volts in some receivers with a very strong signal and will be less as the signal strength decreases, that -ve voltage is used for the AGC and also as the control grid grid voltage for the magic eye tuning indicator.
So, from a troubleshooting point of view what we are expecting that's connected with the ratio detector is a good quality audio output and a functioning AGC voltage....just like an AM detector.
Lawrence.
