Posted by
Kevin Simmons-2 on
Nov 09, 2006; 4:36pm
URL: http://elecraft.85.s1.nabble.com/KX1-KXB3080-Q7-voltages-tp394979p394982.html
Unfortunately I am still having issues with this KX1...
I changed out Q7 and there was no change in voltage reading at Q7's
base. I asked Elecraft about this and they informed me that it can be
hard to measure accurately at this spot with a DMM do to RF. It makes me
wonder if that I am seeing inaccurate readings due to RF as PO seems
fine! If I turn R4 counter-clockwise I get the correct 0.7VDC. Meanwhile
I resolved the low power issue and everything seemed fine, both transmit
and receive, but I still had the negative voltage on Q7's base with R4
fully clockwise.
Well things have gone from (seemingly) bad to worse.
I looked at other components in the path. I replaced Q6, just because I
had a hard time getting it out to test it. No change. T1's windings
didn't look quite right so I redid the green windings, re-soldered,
looks good. Checked my soldering in general, looks OK. *Now* I have low
audio and low power out on 20 meters. Everything else seems OK. I
replaced K1 because it got hit with the soldering iron during repairs. I
completely redid T2 on the KX1 board. Putting a finger on T2 raises the
noise level greatly on 20M only and there is *no* peaking of the 20M
signal with trimmer 20A. 20B peaks as normal as do all the other
trimmers for their bands. LPF1 installation looks good. So, I am now
trying to resolve a low signal/audio/power out on 20 meters.
Tnx,
Kevin, KF0JX
Kevin Simmons said the following on 11/01/2006 10:30 PM:
> 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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