User's Manual

Ma in t ena n ce Bulle t in Topic: Fla n ged RF Am plifier Tr ansist or s
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This Bullet in covers t he procedure for t he replacem ent of Flanged RF Am plifier Transist ors. These
transist ors, bot h Bipolar and FET types, are m ounted directly t o heat sink assem blies using two
m ounting screws. This category of transist or covers a num ber of different types and part
num bers, but t he inform at ion covered in this bullet in is of a general nat ure and applies to all such
transist ors.
Caut ion: Som e of the transist ors m ay contain beryllium oxide, a toxic m at erial. I f a device
becom es dam aged and the inner m aterial is exposed do not handle the transist or.
I f a Flanged Transist or needs replaced due t o failure of the com ponent, the following procedure
should be com plet ed during the inst allat ion and initial operat ion of the new com ponent.
1. Rem ove t he volt ages t o t he device t hat you are about t o change. This is accom plished by
turning t he Transm it t er Off, by rem oving the power plug locat ed on the Opto Bias Board t o
that device or by rem oving the fuse located on the Am plifier Prot ect ion Board, Dual Bias
Prot ection Board or t he Fused Current Metering Board associat ed wit h the Transist or Device.
2. Rem ove t he t w o m achine screws t hat m ount the t ransistor t o the heat sink assem bly.
3. Using a soldering iron and solder wick, rem ove t he solder from t he tabs of the t ransistor w hich
are soldered to the board. Lift t he tabs from the board and carefully rem ove the t ransist or
from t he assem bly. Be careful not to peal t he t rack off the PC Board. I f there are chip st yle
capacit ors soldered t o t he t abs, they need t o be replaced along wit h t he t ransistor.
I m por t a nt : Not e t he posit ion of any chip capacit ors which are soldered t o the t abs of t he
transist or and before com pletely rem oving t he t ransistor from it s m ounting place,
note how t he t ransistor is m ount ed on the board, so that t he new t ransist or is
m ounted properly.
4. Check t hat the heat sink surface on w hich t he transist or was m ount ed is free from any debris
and is sm oot h. I f the surface is irregular, a piece of 440 sand paper or em ery clot h can be
used t o sm ooth it . I rregularities in t his surface cause poor heat transfer from the transist or t o
the heat sink.
5. Rem ove t he new transist or from the case, and apply a t hin coat of heat sink com pound t o t he
bot t om surface of t he t ransistor. Ca ution: Only a t hin coat of heat sink com pound is needed;
too m uch will cause poor heat transfer. Place the new t ransistor into the area from which t he
old device was rem oved. Using the m achine screws t hat were rem oved in St ep 2, m ount t he
new device in place, by first lightly tight ening one side then t he other. Repeat t he process
until bot h sides of the transist or are equally t ight.
6. Solder the Base and Collect or or Drain and Source Tabs to the board, as t he previous
transist or was soldered. Check that a good solder flow is achieved and t hat no solder is
shorted to ot her tracks. Solder new Chip Capacit ors, of the sam e value as rem oved, in t he
sam e locat ions as they were originally. Caut ion: Use a t em perat ure controlled soldering iron
and Do N ot Ove r h e at t he Chip Capacit ors as they are soldered in place.
7. Before applying power t o t he device, the Operat ing Current of the device m ust be t urned
down. This is accom plished by t urning the Bias Adj ustm ent Pot ent iom eter locat ed on the Opto
Bias Board, GaAs FET Control Board or in t he Drain Circuit of t he FET Device t o the full
Count er- Clockwise posit ion.
8. The I dling or Stat ic Current of the Device is initially set with no RF Drive applied to the Device.
Rem ove the RF Drive Signal input t o the Device or Module. St art t he Set up of the I dling
Current by first restoring the Supply Volt age to the Transist or by reversing the process used in