[Elecraft] K2 Q7 and Q8 Short to Ground

[Don Wilhelm]

Mark,

I trust you are using a DMM and not a VOM or other meter that applies a 
high voltage for resistance readings - those can destroy solid state 
devices.

The PA transistor collectors are connected directly to the +12 volt 
power rail, so a short anywhere on that power rail will be indicated as 
a short on the collectors of Q7 and Q8.
Note that this is the first time a resistance reading to the +12v power 
rail is done.

Normally, there is an electrolytic capacitor on that voltage rail which 
will show a resistance value that starts out rather low (about 500 to 
1000 ohms) and will climb with time as the capacitor charges to a higher 
value.  The initial resistance will depend on the voltage your DMM uses 
for measuring resistance.

So if you are seeing a direct short to ground, it is not necessarily a 
problem with Q7 or Q8.  To check the transistors themselves, remove T3 
which will remove Q7/Q8 from the +12 volt power rail.

73,
Don W3FPR

On 1/4/2020 10:39 PM, Mark Yergin wrote:
> Don - The PA’s have never seen any power nor have I tried to transmit 
> yet.  In the morning I’m going to take all the panels and boards off 
> the RF board and see if I still pass all the resistance checks.  I 
> don’t understand how I can pass the resistance check and then see this 
> behavior when the boards and panels are installed.
>
> Mark W8EWH
>
> On Sat, Jan 4, 2020 at 10:28 PM Don Wilhelm <donwilh at embarqmail.com 
> <mailto:donwilh at embarqmail.com>> wrote:
>
>     Mark,
>
>     The most usual cause of a Q7/Q8 short to ground is that the thermal
>     insulators are not installed properly.
>     However, your statement that you see the same with the heatsink off
>     indicates that the Q7 or Q8 transistors have been damaged by a
>     collector
>     to emitter short.  If the K2 was subjected to a transmit without the
>     heatsink installed, that could be reason for the demise of Q7/Q8
>     transistors.
>
>     Make certain that the PA mounting hardware is installed correctly,
>     especially the shoulder washers.
>
>     73,
>     Don W3FPR
>
>     On 1/4/2020 9:11 PM, Mark Yergin wrote:
>     > I'm in the final stages of my K2 build, but having a problem
>     with Q7 and Q8
>     > shorting to ground.  When I did my Resistance Checks on pg 74, I
>     had 3.66K
>     > ohms to ground at the Q7 collector.  So that was fine. But when
>     I put all
>     > the boards and panels on, then attached Q7 and Q8 to the heat
>     sink, I see
>     > about 135 ohms between the Q7 and Q8 collectors and ground.  I
>     assume
>     > that's unacceptable.  I see the same with the heat sink off. 
>     Suggestions?
>