K2 Q7 unstable resistance

[MikeH-3]

Hello Group,

The build for K2 #4661 continues...

I am doing the final resistance check on p. 71 of the K2 manual (the RF
board is basically complete now), and I am getting unstable readings on
the Q7 collector. The readings are all above 500 ohms, as they should
be, but they are constantly changing. Hopefully this is normal? I get
the same sort of response on the Q8 collector (although it's not one of
the resistance checks). All other readings are ok.

DMM is an Extech Instruments 22-816, about 2 years old.

Thanks (again) for your thoughts.

Mike

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[Ron D'Eau Claire-2]

Mike asked:

The build for K2 #4661 continues...

I am doing the final resistance check on p. 71 of the K2 manual (the RF
board is basically complete now), and I am getting unstable readings on
the Q7 collector. The readings are all above 500 ohms, as they should
be, but they are constantly changing. Hopefully this is normal?
----------------------

Hi, Mike!

Is the resistance steadily increasing as you hold the probe on the
collector? (Just a guess).

There's a big 47 ufd capacitor in the collector circuit that must charge up
(C126). It'll take a while, depending upon how much current your DMM
provides in Ohms range. Modern DMM's really restrict the current to protect
transistors and diodes from damage, so the capacitor may take quite a while
to charge up and let the apparent resistance reading stabilize.

Ron AC7AC



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[ac0h]

MikeH wrote:

> Hello Group,
>
> The build for K2 #4661 continues...
>
> I am doing the final resistance check on p. 71 of the K2 manual (the RF
> board is basically complete now), and I am getting unstable readings on
> the Q7 collector. The readings are all above 500 ohms, as they should
> be, but they are constantly changing. Hopefully this is normal? I get
> the same sort of response on the Q8 collector (although it's not one of
> the resistance checks). All other readings are ok.

Hi Mike.

My K2, #4684 does the same thing and believe it caused by a cap charging
from DMM current. As long as you don't show a short you should be OK.
When you get to the next step of mounting the finals to the heatsink
double and triple check to make sure the tabs of the transistors are
flat and parallel to the heat sink and are slightly above the 2D
connector between them.

For me, the best way to make sure the thermal pads are installed
correctly and will stay in place while installing the heatsink is to
take the wide flat edge of the green alignment tool and gently "smooth"
the pads down onto the transistors. Slight pressure only as you don't
want to rip the pads.

Once you've got the heatsink installed recheck the resistance on both Q7
and Q8 collectors to make sure you don't have a short.

Good luck.



--
R. Kevin Stover ACØH

K2/100 #4684

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