K9AXN SERVICE NOTE 040A
QUICK CHECK OFTRANSMIT DRIVE TO 8122’S
Tune the transmitter up in the band to be tested.
Once tuned up, vary the transmit RF drive from "0" to maximum while observing the plate current. If there is an increase then a drop as it is increased, there is adequate drive.
A class AB1 amplifier will use a grid resistor. Once grid current begins to flow, the resistor will generate negative bias reducing drive to the tubes. The SR-2000 and SR-400 series radios use AALC to provide 15db of compression in SSB mode and when the drive exceeds the AALC compression capabilities the grid resistor intervenes.
In CW and Tune mode, there is no AALC action leaving the grid resistor the only control agent to sustain some semblance of class AB1 mode.
1.Make a copper sleeve to slip over the 12BY7A. Buy some flashing or other copper sheet that can be wrapped around the 12BY7A running the length of the anode. We want a tight and consistent capacitive coupling to the plate of the tube.. We will use this measurement technique for other circuits throughout the radio because it is non-disruptive. Drill a hole at the top and insert a 2” copper wire using silver solder. This will be used for a connection point to your scope: through a 1Megohm 10X probe. The reason for the silver solder is the 12BY7A runs at nearly 400 degrees F. The wire keeps some heat away from your plastic scope probe.
2.Allow the radio to warm up for 10 minutes then set the controls to 14.2Mc; Receive mode; Peak the pre-selector; Set meter to AALC; and verify the HV is off and disconnected.
3.Go to transmit; Key the microphone; Whistle into it while raising the transmit audio drive until the meter reports AALC. The moment the meter begins to rise represents the point where the 8122’s are as fully driven as they can be in class AB1 mode.
4.Measure the voltage on the tube shield just as the AALC begins to appear. It will be somewhere around 20 volts PEV.
5.If it generated no AALC measure and record the maximum voltage attainable for later analysis.
6.If AALC was generated, use the measurement to compare to the voltage read while in tune mode with the XMIT carrier drive at maximum. If it is not the same or more than the measurement that you recorded just as AALC was generated, you need to get your alignment tools out. Covered in other service notes to come.
Kindest regards Jim K9AXN