Posted by
Don Wilhelm-3 on
Nov 01, 2006; 5:55pm
URL: http://elecraft.85.s1.nabble.com/KX1-KXB3080-Q7-voltages-tp394979p394980.html
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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