KX-1 Stopped working - Help Needed

[Larry Wright-4]

 KX-1 Stopped working - Help Needed


In June of 2006 I had finsihed building the KX-1 (40m and 20m only), and
after aligning it, worked about 8 or 10 stations with it.  Both receiver and
transmitter worked normally.  Then while listening, the received station
went down in signal strength about 10 or 20 db and the KX-1 would not
transmit.  Without any intervention on my part a day or so later it began
working again and would receive and transmit ok.  Then the received signal
again decreased substantially, and the transmitter would not work.

Joints were re-soldered, but the transmitter still had no output.
Continuity was checked where possible, and nothing was found.   I put the
KX-1 on the shelf and then a few days ago decided to look at it again.
There was no signals at all on receive, and the transmitter would not
transmit.  Using  a general coverage receiver, I was able to pick up the
4.913 Mhz BFO oscillator and the 3.995 Mhz oscillator.   I tried bypassing
the half wave input filters and connected the antenna directly at the RF
gain pot, but still no signals.  The external receiver, however would pick
up the KX-1 on transmit with a very weak signal.  Using the external
receiver as an RF probe, I determined that the Q4 and Q5 were amplifying the
signal, but Q6 (final) was not.   Measuring the transmit signal on the
collector of driver Q5 revealed a signal of only 25 to 30 millivolts.  The
signal on the base of Q6 was of course even smaller and not nearly large
enough to cause Q6 to conduct.

Voltage measurements on Q4, Q5, and Q6 were as listed in the DC voltage
chart in the manual.  (The manual has an error in the emitter voltage of Q5.
It should be about 1.4 volts - not 2.24 volts as listed).  Voltage
measurents on Q! were not in complete agreement with the manual, but are
probably not a problem.  They were gate voltage of 0v instead of 0.34v, and
source voltage of 1.3v instead of 1.77v.  The drain voltage was the expected
13.5 volt.

In summary, the DDS is generating the signal, and the signal is being
amplified by Q4 and Q5, but is way too small to turn on Q6.  What controls
the amplitude of the signal coming from the DDS?  Or does this simply mean
the DDS is not working properly?

At this point, voltage measurements, using a 100k ohm per volt VOM, were
taken on all ICs and transistors.  Q7, Q8, and Q9 measurements agreed with
the manual voltage chart readings.  Q2 and Q3 readings  do not agree.  Both
Sources were the expected 4.4 volts.  However, both Drains were 2.2v instead
of 4.4v, and both Gates were 0v instead of 3.4v.

Diode voltages are all ok.

U3, U4, U5, U6,  were all ok.

U1, U@ and U7 readings were not all what they should be.

On U1, Pin 6 read 0v instead of 3.4v.  Pin 14 read 4.8v instead of 0v.  All
other pins read correct voltage.

On U2, Pin 1 read 0v instead of 1.15v.   All other pins read correct
voltage.

On U7 Pin 4 read 0.8 v instead of 1.17v (VOM loading?).  All other pins read
correct voltage.

It seems to me that perhaps this is what could have happened:  Initially
there was a bad connection that caused the loss of received signal and no
transmit.  Resoldering the connections fixed this, but then there was an
early IC failure in UI and\or U2.

Any comments or suggestions would be greatly appreciated.

As background, I have built half a dozen Heathkits including a SB-100, and
in the last two years have  built the SST, and MMR-40, transceivers and the
KD1JV Stand Alone Transmitter.  All worked well.

Thanks in advance for any help you can provide.

Larry
N4QY



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