KC9QQ wrote:
>
>
>
> Don Wilhelm-4 wrote:
>>
>> Fred,
>>
>> In cases like yours, first step is to connect a dummy load to the K2
>> just in case it is trying to transmit - it is not good to enter transmit
>> state without a load on the PA transistors. If it is a K2/100, remove
>> the KPA100 and connect the dummy load to the BNC antenna jack on the
>> lower rear panel.
>>
>> First thing is to check the state of the 8T and 8R voltage rails. Those
>> are easily checked at the anodes of D6 and D7. If D7 anode has 8 volts
>> on it while D6 anode has zero volts, then the K2 is in a transmit
>> state. If both D7 and D6 anodes have 8 volts present, power it only
>> briefly while making further checks lest some components overheat.
>>
>> If both 8T and 8R are present, you need to check the state of the RX
>> signal at the gate of control board Q4 as well as the TX signal at the
>> gate of CB Q3 - one should be on (about 5 volts) while the other should
>> be off (near zero volts). If both of these are on, the microprocessor
>> was zapped with the power surge, but if only one is on, the problem is a
>> shorted Q3 or Q4 on the control board.
>>
>> 73,
>> Don W3FPR
>>
>>
>>
>>
>
> Don,
>
> I pulled the control board again to reset the relays and then reinstalled
> it to make the measurements on D6 and D7. When I powered the unit,
> Elecraft was again shown and the frequency of 7100 was briefly displayed.
> When then display went out I measured the anodes and cathodes of both d6
> and d7 and measured nearly zero volts on both.
>
I tested the voltages on D6 and D7 anodes again when I powered up. I had to
power up twice, but each showed around 7 volts before they fell to near
zero. I have found that I can let the unit sit for a few minutes before I
re-power and I don't have to pull the control board to reset the relays.
This only gives me about 2 seconds to make the measurement.
The voltage on the gate of Q4 is 5 volts. The voltage on the Gate of Q3 is
close to zero for the brief period I have to make the measurement. After a
few moments the gate of Q4 drops to zero.
How do I tell which is shorted? I made the following measurements to
resistance measurements referenced to ground.
Q3
S - 0.4 M
G - 5.4 M
D - 8.1 M
Q4
S - 0.3 ohms
G - 3.9 K ohms
D - 8.1 K ohms
I am assuming the culprit is Q4. Am I correct? Is it possible the
something else is fried that may take out Q4 if I install a replacement? I
still have 4 boards that I have not built (SSB, DSP2, AT and NB). I
believe at least one of these has a 2N7000 I can rob for a temporary
replacement. I just don't want to install it until I am sure another
failure somewhere else upstream won't cause it to fail. Are there any other
items I should check before I replace the failed transistor?
Thanks,
Fred
Fred
Fred
--
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