[SixClub] Amplifier Questions...

[Steve Katz]

See some stuff below...

73 de Steve WB2WIK/6


-----Original Message-----
From: sixclub-bounces at mailman.qth.net
[mailto:sixclub-bounces at mailman.qth.net] On Behalf Of KB0NLY
Sent: Friday, June 04, 2010 9:45 AM
To: World Wide Six Meter Club
Subject: [SixClub] Amplifier Questions...

I know this is OT, but there is a lot of smart cookies on here and the
other 
reflectors I have tried, like the AMPS reflector just don't get any
replies 
like here...

Questions..

1.  I have a UHF amp I'm working on, a RF Concepts 4-110.  I think the 
finals are bad, but need someone to verify my bad memory.  When testing 
these bipolar transistors the collector-base and base-emitter junctions 
should test as a diode correct?  I have a dead short from base to
collector 
so that seems to verify they are fried.  And the emitter-collector has
to be 
open in both directions, well of course its not because of the dead
short to 
the base.

>>Unless you lift the transistor tabs off the PC board, you can't make
any real determination like this.  If you want to non-destructively
(avoiding overheating) lift the tabs, loosen the bolts holding down the
transistors, first (not remove them, just loosen them a bit so they're
not tightly binding the transistors to the heatsink below them).  This
way it will take less heat to reflow the solder and lift the tabs.

2.  It's been a while since I worked on a solid state amp...  Whats the 
package type of the small capacitors used on the finals, those little
square 
metal jobs with a tab out of one side.  For the life of me I can't
remember 
the package number so I can go catalog search for some new ones, as I'm
sure 
they need replacement after the heating they took on removal.  Also, the

marking on them, in this case 30 and 40, I believe that's in pF correct?

>>Those are mica capacitors without any outer covering, and without any
leads; you could replace with high-Q RF ceramics (NPO) which will also
usually work.  Yes, the caps around Q7 and Q8 are 30pF and 40pF.

3.  This amp has a bias adjust for each transistor, what is the best way
to 
adjust the bias after replacing the transistors, just set for minimum
idle 
current or is there a number I should be shooting for?  No specs
available 
from RF Concepts unfortunately but I was able to actually find a one
page 
schematic at least.

>>Lift L7 from the base connection of Q7 and adjust the 100 Ohm bias pot
for Q8 to idle at 150mA; then re-connect L7 to the base of Q7 and adjust
its bias pot for a total idling current of 300mA.  It can be a bit
higher, e.g., 200 and 400mA or whatever (very little power there either
way).  The idea is to make them "equal."  Obviously, this is with no RF
drive applied!!