[SixClub] Amplifier Questions...

[Jerry W9FS EN61]

Be sure to check the size of the metal clad micas they gome in different 
physical sizes. You were correct in the 30 =  30pf.

Jerry W9FS
----- Original Message ----- 
From: "KB0NLY" <kb0nly at mchsi.com>
To: "World Wide Six Meter Club" <sixclub at mailman.qth.net>
Sent: Friday, June 04, 2010 1:17 PM
Subject: Re: [SixClub] Amplifier Questions...


> 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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