Re: K2 low pre-driver

Posted by Don Wilhelm-4 on Jul 11, 2010; 10:04pm
URL: http://elecraft.85.s1.nabble.com/low-pre-driver-tp5280916p5280980.html

Robert,

OK, the Pre-driver output is low - next step is to check the DC voltages
for Q5 - but that must be done during transmit.  It is difficult (almost
impossible) to check the emitter without removing the heat-sink.  
WARNING - even though the power output is low, do not operate the K2
without the heatsink attached.
So during transmit, check the collector voltage - should be close to
12.4 volts
And check the base voltage which can be reached at the end or either R40
or R41 - should be 1.3 volts.
If all was well for those checks, remove the heatsink and measure the
resistance between the emitter of Q5 and ground - if it is more than 27
ohms, check the values of R42 and R43 and check their soldering.
If the resistance was correct, check the value of C117 (should be .047
uF) and reflow its soldering.   Also check C116 (33 pF), C118 (.01 uF)
and C115 (.01) and reflow the soldering for those too.  Then put the
heat sink back on and see if there is any change.

BTW - you can consider the division between the pre-driver and the
driver as being between the primary and the secondary winding of T1.

73,
Don W3FPR

Robert Greenwalt wrote:

> Hi,
>
> When doing the 40-Meter Transmitter Alignment, I couldn't keep the output
> power in th 1-2 watt range.  Turning the power all the way up only got up to
> .7 watts.  I checked the output across the dumy load and it was practically
> non-existent (.007Vrms) though key-down was noticeably different than
> key-up.
>
> I walked through the Transmitter signal tracing.  Everything was within
> tolerances until I got to the pre-driver output.  I expected about .132Vrms
> but got .07 Vrms.
>
> I've checked all the components in the pre-driver section up through T1 (not
> sure where pre-driver ends and driver begins) and they all have the right
> markings.  I verified T1 and reheated the connections.
>
> On qth.net I only found references to high pre-driver issues.  Suggestions?
>
> Transmit mixer output: .016 Vrms
> Buffer output: .216 Vrms
> Bandpass filter output: .035 Vrms
> TR switch output: .033 Vrms
>
> Predriver output: .072 Vrms (expected .13 )
>
> Robert, K6RJG
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