User's Manual

POWER AMPLIFIER HEATSINK ASSEMBLY
TSM21Q-365 Rev 0 July 17, 2009 PA Assembly 28
operating frequency band, also is equivalent to a dead short at lower frequencies where it loads the device so
heavily so that the stage will not become unstable.
Note that fuses are provided for the voltage supplied to the FET drain connections. The intent of these fuses is to
protect the surrounding circuitry in the event of a device failure. The normal failure mode of active devices such as
these is short-circuit, and the fuse will blow in this case, isolating the defective device from the rest of the module
and transmitter power supply, allowing the remaining devices to keep operating normally. A blown fuse can serve
as a valuable troubleshooting aid, when trying to identify failed devices.
4 TWO-WAY SPLITTER/COMBINER
The two-way splitter/combiner consists of a quadrature hybrid mounted coupler assembly When used as a
splitter the input signal is split two ways in equal amplitude signals 90° out of phase with one another. When used
as a combiner the two input signals, which are at 90° out of phase with one another, are combined to produce an
RF output.
5 ADJUSTMENT OF BIAS VOLTAGE TO ESTABLISH PROPER QUIESCENT FET
BIAS CURRENT:
Important: 50 Ω input and output terminations are necessary to achieve consistent results and prevent damage
to the devices when testing modules. Supplemental cooling is not required when performing bias adjustments or
low power sweep of the PA modules.
1. Remove all fuses from the module to be tested. (There are 8 fuses in total).
2. Adjust all bias pots to maximum resistance, for minimum bias voltage. (Again, there are 8).
3. 3.Use a clip lead to short the junction of C5, R6, and R7 to ground. This shuts off side B of the amplifier so
it will not interfere (thru L9) with measurement of quiescent current from side A.
4. Terminate the RF input and output into a 50 Ω load.
5. Apply +50VDC from the front panel test point on the transmitter, through an ammeter, to the positive
copper bus bar, and its negative to chassis. Caution: Observe polarity!
6. Check the voltage on the bias terminals, it should be 39 V ±2 V. (The bias terminals are connected
together via insulated bus wire).
7. Read the current drawn by the VSWR board and bias regulator. Next, install a fuse in side A (nearest the
panel) of amplifier #1; adjust the corresponding bias pot for a 500 mA increase in the power supply
current; this increase corresponds to an idling bias current of 0.5 A. Remove the fuse. Remove the side
B bias short and place it on side A at the junction of C6, R2, R3 and R9. Place the fuse in side B. Adjust
the side B bias pot for the proper current.
8. Move the fuse to the remaining fuse holders, one at a time, and adjust each companion bias
potentiometer in the same manner for the proper bias current.
9. Install remaining fuses and remove the bias short after all bias adjustments have been made.