[SixClub] Amplifier Questions...

Fri Jun 4 14:20:34 EDT 2010

You might need a new pair of MRF648s.  They're $48 each from RF Parts
(ouch!)

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 11:15 AM
To: World Wide Six Meter Club
Subject: Re: [SixClub] Amplifier Questions...

Thanks for the info!

Checked the relay.. All is well there, probe shows rf into and out of
the 
relay, and rf at the input of the MRF646 driver, little or no output
from 
the driver.  Checked all the chokes and caps, nothing looks burnt
either. 
All solder connections look good but I did reflow some just to be sure.

Another anomaly is that the amp was drawing 10a with power applied,
power 
switch off.  Lifted the Base of both finals and the current dropped to
mA, 
so it appears the finals are shot no matter what, they are shorted from
base 
to collector, tested after removing with VOM.

Looks like I need to start ordering some parts..  I love a good fixer
upper 
project!

73,

Scott

--------------------------------------------------
From: "Steve Katz" <stevek at jmr.com>
Sent: Friday, June 04, 2010 12:29 PM
To: "World Wide Six Meter Club" <sixclub at mailman.qth.net>
Subject: Re: [SixClub] Amplifier Questions...

> Comments in general about this kind of amp:
>
> I've worked on a lot of these.  The transistors can and do fail.  But
> many other things fail that can create "zero output:"
>
> -Bad relay at RY2 or RY3 (or just bad contacts on them)
> -Solder connections that became so hot at Q7 and/or Q8 that the solder
> crystallized and became an open or nearly open circuit (in these
cases,
> re-heating and removing all existing solder, then re-soldering with
new
> solder -- and doing it correctly -- can actually bring a "dead amp"
back
> to life)
> -Open or shorted capacitors, many locations
> -Open, burned or disconnected RF chokes at L7, L8, L9...
>
> The list is long...
>
> 73
>
> 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 10:22 AM
> To: World Wide Six Meter Club
> Subject: Re: [SixClub] Amplifier Questions...
>
> There is no output...
>
> Some more specifics... it's a 10w input 100w output amp.  Driving with
> 10w
> there is ZERO output.  I think the driver, an MRF646 might also be
bad.
>
> There is no measureable output with even an RF Probe.
>
> 73,
>
> Scott
>
>
>
> --------------------------------------------------
> From: "Peter Chase" <pdcn1j at msn.com>
> Sent: Friday, June 04, 2010 11:55 AM
> To: "6meter club" <sixclub at mailman.qth.net>
> Subject: Re: [SixClub] Amplifier Questions...
>
>>
>> Just check the input to each final and compare it to its output.  You
>> should have a notable  increase at the output of each .  You should
> look
>> up the RF device so you know what to expect for power output and what
> the
>> normal parameters are.  this can be done with basic test equipment.
> Hope
>> this helps.
>>
>> Regards
>>
>> Pete N1JHF
>>
>>> From: kb0nly at mchsi.com
>>> To: sixclub at mailman.qth.net
>>> Date: Fri, 4 Jun 2010 11:45:27 -0500
>>> 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.
>>>
>>> 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?
>>>
>>> 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.
>>>
>>> 73,
>>>
>>> Scott
>>>
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