[Elecraft] K2 finals

[John Zaruba Jr]

Well, I apparently mis-diagnosed my issue since after installing all the components in the K2PAKIT made little difference in the symptoms.

So, back to square one...

Initial conditions: base K2 with KAT2 and KSB2 removed. 7.1 MHz CW, 5 watts set on power control.

Transmit signal tracing:

Basic voltage checks - pass
Sidetone - pass
ALC - pass up to item 4, VPWR around 4.5 VRMS
Transmit mixer output - pass
Buffer output - 1.38 VRMS
Anode D7 - .46 VRMS
D7 / R5 junction - .4 VRMS
Junction C5 / C4 - .6 VRMS
L1 / C6 junction - .6 VRMS
C6 / L2 junction - 0 VRMS

Thinking I had the issue identified, I replaced C6 with a 4.7 pF NP0 disc ceramic. Still no voltage at the C6/L2 junction. Removed C6 again, check pad/trace continuity to the top of L3, DC checked OK. Re-installed C6, no change. 

No output RF output on any band noted.

I saw some questionable solder joints with T1 and T2 so I'm going to pull both transformers and rewind them. I have plenty of the correct magnet wire left over from previous K2 builds.

I'm stumped at this point, so any guidance would be appreciated.

Thanks and 73,

John K2ZA

Sent from my iPad

On Aug 10, 2013, at 0:04, Don Wilhelm <w3fpr at embarqmail.com> wrote:

> John,
> 
> What is the DC voltage at the base of Q7/Q8 during a TUNE?  If it is in excess of 0.64 volts, then Q11 and Q13 as well as Q7 and Q8 are toast.  Order and install the K2PAKIT which includes all those parts.  Q7 and Q8 are connected in push-pull output, so a failure of one will reflect on the other.
> 
> I don't know how that would change the behavior of the KIO2, so you may have multiple problems in your K2.
> 
> If you want to do a test, remove the KAT2 and try working into a 50 ohm dummy load.  If you then have good power output, the PA transistors are OK and the problem lies with the KAT2,
> 
> 73,
> Don W3FPR
> 
> On 8/9/2013 11:39 PM, John Zaruba Jr wrote:
>> I was using my new to me K2/10 (S/N 50xx) which has been working flawlessly for the last 2 months when out of the blue I started getting a P LO message from the KAT2. I checked the power out with an external meter and it was about .4 watts in general agreement with the K2 power display. No snap, crackle, or other dramatics, just loss of power. Coincident with the output power failure, the KIO2 stopped working, neither receiving or sending data. Coincidence does not imply causality, so I'm going to treat this as a separate issue.
>> 
>> I started signal tracing per the manual with a DSO scope (Tek TDS-210) and the RMS voltages I read on the scope were nothing like the voltage charts, so I was just looking at voltage gains from the various stages. I removed the KSB2 and installed the cap and 2 jumpers to eliminate that module, with no symptom change.
>> 
>> The one anomaly I think I've found occurred with Q7 & Q8, the voltage at each device's base lead was 3.2 VRMS (as measured by the cyclic RMS setting on the scope), but the collector voltages were around 500 mVRMS. Just to clarify my thinking, I would expect the collector voltage to be significantly higher than the base voltage.
>> 
>> As an additional question, if I have had a final amplifier failure, what might cause both devices to fail, rather than one or the other? Hopefully I can receive some wisdom from the list before I call the factory on Monday to order parts (to make sure I get the correct ones) I'm also thinking I should order new thermal pads and shoulder washers, if new finals are indeed required.
>