[Elecraft] Question about K2's Q7 and Q8 Final Amp

[W3FPR - Don Wilhelm]

Tom,

Quite rare, but if you have a short with the heatsink off, look for a 
soldering problem with T4 or T4 might have an internal short (unlikely, but 
possible).  You might try lifting the 1 and 2 terminals of T4 to further 
isolate the collectors and confirm that T4 is the culprit.

Ask yourself 'What did I change?' - you said the K2 was working before you 
replaced the transformers, so it has to be associated with something that 
was touched in the process.

Since you said you are using the 'buzzer' of your DMM, it amy be informative 
to see what the actual resistance is - the collectors normally show 500 to 
600 ohms to ground, and if your DMM buzzes with that value, that would be 
misleading.

It is much more likely to have a short only when the heat sink is mounted. 
That is most commonly caused by a small bit of aluminum becoming lodged in 
the thermal pads.  The bits of aluminum come about when the toothed 
lockwashers are loosened.  SOLUTION - replace the thermal pads and be 
certain there are no bits of aluminum still clinging to the transistors or 
the underside of the heat sink.  As a further precaution, I usually do not 
use the toothed lockwashers for locking the heat sink screws and nuts, I use 
the split lockwashers instead because they are not as likely to leave those 
bits of aluminum behind when they are removed.

73,
Don W3FPR

----- Original Message ----- 
>
>  My K2 has been working fine but I decided to fix it anyway...hi.
>
>  Well actually I was a little short on the power output on 10 meters.  I 
> was getting about 7 watts out on 10 meters and Gary Suggested that I 
> replace T2 and T1.  Instead of winding them myself, I contacted 'The 
> Toroid Guy' and he sent me two beauties (along with a T-4) - the guy is 
> great, BTW.
>
>  So yesterday I installed the new T1 and T2 (I did not yet install T4) and 
> replaced the heat sink.  The K2 manual warns that whenever the heat sink 
> is removed and replaced you should verify that the collectors of Q7 and Q8 
> are not shorted to ground.
>
>  With the heatsink attached I check and sure enough, there was a short 
> between the collector (middle pin) and ground (the ground wire on the RF 
> board) - I used the continuity buzzer on my DMM for this.
>
>  I removed the heatsink and all of the hardware which connects Q7 and Q8 
> to the heatsink and then again checked for the sort.  It was still there.
>
>  In looking at the schematic, the section of the final which contains Q7 
> and Q8 seems pretty isolated from everything else, with just T3 and T4 
> seemingly getting into the action.  Try as a I might I couldn't locate any 
> shorts.  Using the schematic, I started at the collectors of the 
> transistors and worked to the right of the schematic.
>
>  The only options I have presently connected are the Noise Blanker and the 
> SSB module, both of which don't seem to be involved with the finals.
>
>  Obviously the problem must be in the area that I was working on.
>
> If anyone has any suggestions in tracking this down, I appreciate your 
> help
>
> Tom, WB2QDG
> K2 1103
>