[Elecraft] [K2] TX Issue in Part III checks

[Brian Handy]

Don,

You're absolutely right, I checked again across R98 just to confirm and you are 
on the money....5.6V at the drain.  It seems possible this part may be working 
fine.

I understand the voltages are going to be different, the whole feedback loop 
isn't there.  I'm just hopeful to identify the problem before nuking another set 
of finals ... !

I'll keep looking.  Thanks!

Brian AE7JW




----- Original Message ----
From: Don Wilhelm <w3fpr at embarqmail.com>
To: Brian Handy <holy_tire_iron at yahoo.com>
Cc: elecraft at mailman.qth.net
Sent: Sun, January 9, 2011 11:27:58 PM
Subject: Re: [Elecraft] [K2] TX Issue in Part III checks

  Brian,

Looking again at your Q24 measurements, I think you reversed the leads.  
Check against the J310 basing diagram on the Schematic Key page and 
remeasure.

5.6 volts on the drain is well within reason when there is no output - 
VALC will go to its maximum value when there is no output sensed and 
tries to make Q24 conduct harder so more BFO signal is delivered to the 
Transmit Mixer.  That is how the power control works - the conduction of 
Q24 is the controlling element.

The voltages listed in the DC voltage chart are for a properly working 
K2 at 5 watts output.

73,
Don W3FPR

On 1/8/2011 10:44 PM, Brian Handy wrote:
> Hi All,
>
> I've been working on my K2 and I'm on the home stretch.  I had to dabble a bit
> to find a cold solder joint in the Part II checks, but eventually got a
> home-brewed test oscillator to reach through on 40m!  Quite happy with things 
>so
> far, and learning a lot.
>
> I still have a lot to learn about the transmitter side though, that doesn't
> work.  When I first hit 'tune' in the Part III checks, I noticed high current 
>on
> the power supply, high current on the front panel, and changing the power 
level
> didn't help, it was always high draw.
>
> It didn't take much of this to zorch the PA finals and short things straight 
to
> ground.  Nothing worked until I pulled T3, and while I had the soldering out I
> took out the PAs and started looking around and reading the mail archives.
>
> I noted one suggestion from Don to someone else and checked the voltages on 
>base
> on the finals - both are around 0.65V, which I think is OK.
>
> I realize things won't be quite the same without the PAs there and proper
> feedback happening, but ... I then started going through all the voltages on 
>the
> RF board with TUNE on.  Everything looked pretty much fine except for Q24 ~
>
> Leg - Expected - Measured:
> G    - 0V - 5.6V
> S - 1.2 - 2.5V
> D - 13V - 0V
>
> That doesn't look right at all to me.  Exploring further, I started chasing 
>VALC
> down the rabbit hole.
>
> Pin 2 of U8 (V PWR) seems to vary between 3.8-5V as I turn the PWR knob in
> TUNE.  I decided that was OK.
>
> Pin 1 of U10A doesn't move around a lot, only up and down about 0.1V.  Finally
> the emitter of Q8 (V ALC) seems stuck at 7.4V.
>
> I was firmly confused at this point, so went back and started checking Control
> Board voltages per the page of expected readings in the back of the manual 
with
> CAL FCTR turned on.  Everything seems turned off when I do that and I'm seeing
> pretty much 0V everywhere.
>
> I'm a bit saturated at the moment, but thought I would ask if this sounds like
> I'm on the right track, or if I have to get to poking elsewhere ...
>
>
>
> Cheers and thanks in advance,
>
> Brian Handy
> AE7JW
> Tokyo, Japan
>
>
>
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