K2 Control Board Issue

[Glenn Haffly-2]

Doing something I should have not done , I managed to blow up Q11on the
control board. I fixed that with a replacement from my junkbox. All
voltages are correct on control board U3. When tapping display I get a
voltage of 23.3 volts indicated from a 12 volt supply and zero current.
Further checking shows R7 on the control board is zero ohms instead of
1.78K. How important is it to replace R7
with a 1% resistor rather than something from the junkbox?
Everything else on the K2 works fine , TX and RX are OK.
Hopefully a replacement R7 will then display the correct voltage and
current.
Any input from the reflector is appreciated.
Glenn , K5ZE
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[Ross Primrose]

Resistors almost never fail shorted, I've never seen it happen. Check
for solder bridges, and look at the schematic and see what else could be
causing a 0ohm reading.

73, Ross N4RP

On 5/9/2014 1:14 PM, Glenn Haffly wrote:

> Doing something I should have not done , I managed to blow up Q11on the
> control board. I fixed that with a replacement from my junkbox. All
> voltages are correct on control board U3. When tapping display I get a
> voltage of 23.3 volts indicated from a 12 volt supply and zero current.
> Further checking shows R7 on the control board is zero ohms instead of
> 1.78K. How important is it to replace R7
> with a 1% resistor rather than something from the junkbox?
> Everything else on the K2 works fine , TX and RX are OK.
> Hopefully a replacement R7 will then display the correct voltage and
> current.
> Any input from the reflector is appreciated.
> Glenn , K5ZE
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
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--
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[Don Wilhelm-4]

Glenn,

R7 has no association with the voltage display - it does however have a
lot to do with the current display.

The voltage detection is done by the voltage divider consisting of R9
and R10 on the control board.  The voltage measured at the junction of
R9 and R10 (or U3 pin 5) should be 20% of the 12 volt input (as measured
at P7 pin 2).

I would wager you have a bad solder connection at R10.  Measure the
resistance from U3 pin 5 to ground - it should read 196k.

As for R7 reading zero ohms, I would suggest you look for a solder
bridge.  If you find none, then remove U6 and remeasure.  Should the R7
resistance come up to normal, then U6 will have to be replaced - and be
careful about taking proper anti-static measures.

Is there any chance you are using an old VOM or VTVM to check
resistances?  If so, know that those can cause solid state devices to
conduct and may even cause damage.  Use a DMM - those do not use high
voltages for resistance measurements.

73,
Don W3FPR

On 5/9/2014 1:14 PM, Glenn Haffly wrote:

> Doing something I should have not done , I managed to blow up Q11on the
> control board. I fixed that with a replacement from my junkbox. All
> voltages are correct on control board U3. When tapping display I get a
> voltage of 23.3 volts indicated from a 12 volt supply and zero current.
> Further checking shows R7 on the control board is zero ohms instead of
> 1.78K. How important is it to replace R7
> with a 1% resistor rather than something from the junkbox?
> Everything else on the K2 works fine , TX and RX are OK.
> Hopefully a replacement R7 will then display the correct voltage and
> current.
> Any input from the reflector is appreciated.
>

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