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

[Don Wilhelm]

  Brian,

The PA transistors will normally take some abuse - for a short time, so 
put them in and do a short test to see what happens - just be aware of 
the conditions and keep the transmit times as short as possible while 
still allowing some measurements to be taken.  Everything in life is a 
compromise, including this situation - TX time vs. potential abuse must 
be compromised to come to the resolution of the problem.

73,
Don W3FPR

On 1/9/2011 7:01 PM, Brian Handy wrote:
> 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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