K9AXN SERVICE NOTE 009B
SR-2000 FINAL TUNE UP PROCEEDURE
The Hallicrafters SR-2000 transceiver was designed using the class AB1 circuit configuration requiring very low drive power. This enables the circuitry to fit into a much smaller space than class AB2 systems. The AB1 system achieves superb linearity by prohibiting grid current; presenting a consistent load to the drivers throughout the power curve. Most transceivers that use the 8122 final will use the class AB1 design because of space and power requirements. The only issues that become apparent are: slightly less power out than the AB2 configuration (1100 watts AB1 vs. 1200 watts AB2); and neutralization must be implemented, both insignificant issues.
The final circuit configuration and how it works:
Both the Hallicrafters SR-2000 and SR-400 series transceivers implement the class AB1 design. This allows the control grid to be driven anywhere below zero volts but intervenes when the drive system attempts to exceed zero volts in the positive direction.
The mechanism that controls this behavior consists of up to 15db of ALC compression, and a 10K grid resistor. ALC detection is derived from audio level rectification of any control grid current. The grid resistor imposes a negative grid voltage in the event the 15db ALC compression is overwhelmed by excessive drive resulting in grid current.
With these notions in mind, the behavior of the SR-2000, while executing the tune up procedure, should make a bit more sense.
1) Power the SR-2000 up and allow it 180 seconds to warm up; not the 90 seconds allowed by the delay tube. The 90 second delay is inadequate when using parallel 8122’s, especially when the SR-2000 heater voltage is 12.6 rather than the 13.5 the tubes were designed to use.
2) Peak the pre-selector in receive mode.
3) Pre-set the final tuning and load capacitors per manual.
4) Set the transmit RF gain to “0”
5) VERIFY THE ANTENNA CONNECTED HAS AN SWR NOT EXCEEDING 2/1 !!!!
6) Go to transmit/tune mode and verify 200ma of plate current and if not adjust the bias.
7) Switch meter to the screen current position.
8) Increase the transmit RF gain control to slightly move the plate current off of the 200ma mark.
9) Peak the output using an external power meter, not the RFO meter switch position.
10) Monitor screen current throughout the tune-up process. Do not allow it to exceed 20ma. You will find this quite easy to accomplish after a few tuning exercises.
11) Re-peak the output using both the pre-selector and plate tuning control. There should not be any screen current at this point.
12) Carefully increase the transmit RF drive, always being vigilant of screen current, until the screen current reaches a maximum of 15ma then begins to drop off. If it threatens to exceed 20ma it is too lightly loaded and If it will not reach 15ma, it is too heavily loaded. Adjust the load control and re-tune until the transmit RF gain control, when increased, will not exceed 15ma. You will usually see the screen current, plate current, and power out reach a peak then drop a bit as the transmit RF drive is increased. The drop off is caused by overdriving the 8122’s resulting in grid current which flowing through the grid resistor will generate a negative bias. You can expect 450 to 550 watts out at the peak.
NOTE: If you don't note a drop in screen or plate current at some point as you increase the RF drive, the radio should be checked for appropriate drive or use the following step as a temporary solution.
13) The final check for the tune up is to switch to USB or LSB mode and whistle into the mike while monitoring screen current. If it exceeds 15ma, return to step 12 and load the finals a bit more heavily; then whistle it up again watching for 15ma peak reading.
14) If you follow this procedure your 8122’s will last for years.