Posted by
Kevin Simmons-2 on
Nov 02, 2006; 6:30am
URL: http://elecraft.85.s1.nabble.com/KX1-KXB3080-Q7-voltages-tp394979p394981.html
Don,
Yes, you are correct. Lifting one end of R28, Q7 still had a negative
voltage at its base during transmit. I'll change out Q7.
I'd like confirmation weather the KX1 circuit diagram on page 18 of the
KBX3080 manual is correct or not. It shows Q7's base connecting to R2, a
1Kohm resistor.
Thanks Don,
73,
Kevin, KF0JX
Don Wilhelm said the following on 11/01/2006 09:55 AM:
> Kevin,
>
> There is no change in the base circuit of Q7 - that is true whether the
> KXB30 or the KXB3080 is installed. You should be tracking down the source
> of the negative voltage at Q7 during transmit.
>
> Yes, the voltage at the base of Q7 should be about 0.7 volts during transmit
> (long explaination of why follows:) - The source of the 6 volts during
> transmit is at pin 7 of U3, then it goes through R26 and then through R28 on
> its way to the base of Q7.
>
> When the 6T signal becomes active (it should during transmit), among other
> things it is applied to the base of Q7 to saturate Q7 - effectively
> producing a short between the collector and emitter of Q7 - the reason is to
> protect the receiver input from the transmit RF. When a transistor is
> saturated the base to emitter voltage will be close to 0.7 volts - if it is
> anything else, either the transistor is bad or something else strange is
> wrong.
>
> There is not a negative voltage produced intentionally in the KX1 - I would
> guess you have a bad Q7 which is acting more like a diode and rectifying the
> RF on its collector to produce a negative voltage at the base. If you want
> to do a test to confirm my suspicion, lift one end of R28 and try a short
> transmit while measuring the base voltage of Q7 - I would guess that it is
> producing a negative voltage all by itself, if so, replace it (and put R28
> back in place).
>
> 73,
> Don W3FPR
>
>
>> -----Original Message-----
>>
>> While tracking down a low power output after installing the
>> KXB3080 into the
>> KX1, I was checking the voltages at Q1, Q4, Q5, Q6, and Q7 during
>> transmit into
>> a dummy load. Page E-6 of the KX1 manual states there should be
>> 0.7VDC at the
>> base of Q7 during transmit with a KXB30 (not KXB3080) installed.
>> I would get
>> between -1.5 to -2.7 VDC at the base of Q7 with the KXB3080. I
>> found no mention
>> of this change in the voltage in the supplied manuals. In fact,
>> looking at the
>> KXB3080 manual, page 18, figure 15, the circuit around Q7 is not
>> what I have
>> installed. The diagram shows the base of Q7 connecting to R2, a
>> 1Kohm resistor.
>> I don't find this change in the procedures. The KX1 manual shows Q7 base
>> connecting to R28, a 1.8Kohm resistor.
>> I resolved the low power output issue (yay!) and the
>> voltage at the base of Q7
>> during transmit is still ~-2.5VDC. The question is "What /should/
>> the voltage
>> be?" If it should now be something other than 0.7VDC, then there
>> should be a
>> reference to that in the KXB3080 manual. Do others think the
>> circuit diagram on
>> page 18 is inaccurate?
>>
>> --
>> Kevin
>> KF0JX
>>
>>
>
>
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