[SixClub] Amplifier Questions...

[KB0NLY]

Thanks for the details!!

Mica, that's what I couldn't remember... I'm not dumb just memory 
challenged... Yes metal clad mica's, found them in the catalog.  Boy I wish 
my memory worked.

Because of which I'm printing off your email so I remember how to set the 
bias when I get there... HI HI

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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