[SixClub] Amplifier Questions...

[KB0NLY]

Steve,

One last note.  I did remove the MRF648's and tested them out of circuit, 
that's how I got the readings of the base and collector being shorted, they 
aren't testing right with a VOM.  So the first step I think will be buying 
new components!

Thanks for all the suggestions and help guys.  Off to order some parts and 
wait for their arrival.  Worst part of these projects, waiting for the 
parts!!

73,

Scott


--------------------------------------------------
From: "Steve Katz" <stevek at jmr.com>
Sent: Friday, June 04, 2010 12:18 PM
To: "World Wide Six Meter Club" <sixclub at mailman.qth.net>
Subject: Re: [SixClub] Amplifier Questions...

> 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!!
>
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