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K2 Q7 and Q8 Short to Ground

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Mark Yergin

K2 Q7 and Q8 Short to Ground

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?

Mark W8EWH
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Don Wilhelm

Re: K2 Q7 and Q8 Short to Ground

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?
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Mark Yergin

Re: K2 Q7 and Q8 Short to Ground

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 <[hidden email]> 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?
>

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Don Wilhelm

Re: K2 Q7 and Q8 Short to Ground

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 <[hidden email]
> <mailto:[hidden email]>> 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?
>