Dave,
Yes, do check U5, but ignore the voltages on pins 1, 2 and 3 because
they are digital and the active state may not correspond with the
voltage measured with a DMM.
U5 pin 8 must have 5 volts DC for the chip to work - if that is not
true, stop (all other tests are futile).
Next monitor the voltage at U5 pin 7 while tapping BAND+ and BAND-. the
voltage should go to 4 volts or more when BAND+ is tapped and to nearly
zero volts when BAND- is tapped. If that is true, then U5 and its
driving digital signals are OK.
Next thing to check is U6 - check at pin 1. The voltage when BAND+ is
tapped should be near 8 volts and when BAND- is tapped should be near
zero volts. If U5 pin 7 was OK, but this test fails, the fault is
likely with U6 or the thermistor board.
To give a quick check on the thermistor board, measure the voltage at U6
pin 5 - it should be 4 volts (or more precisely, 1/2 of the 8 volt
regulator voltage.
If the U6 pin 1 output is correct, there is not much more to check. R19
should have a similar voltage swing (between BAND+ and BAND-), and the
voltage on varactors D16 and D17 cathodes should be similar.
If all checks out so far, blame X1 or L31.
Remember that since this K2 worked properly before, there should be only
a single failure. Your challenge is to find the source.
73,
Don W3FPR
David Johnson wrote:
Hello and Happy New Year to everyone.
I have owned K2 serial no. 3980 for about five years and it has worked flawlessly. A couple of days ago it stopped tuning properly. Instead of tuning across a signal, it now acts like a crystal controlled radio that tunes in discrete steps of approximately 10 khz as the knob is rotated.
This same problem was described on this list just last month by Jon, KB1QBZ,
and he said that he solved it by redoing the alignment and test steps, part II.
So I began by redoing the 4 MHz Oscillator Calibration, which gave me a
frequency of 12.086 Mhz.
The next step is the PLL Reference Oscillator Range test, but I got no change
in the 12.086 Mhz indication by tapping either the Band + or Band - switch.
So I measured the voltages on U6, and got 0 volts on pins 2, 3 and 4 at all
times, with no change as I tapped the Band + and Band - switches. I also got a
steady 2.0 volts on pin 7. I know these voltages are supposed to change as you
tap the Band + and Band - switches, but is this something that you can see on a
digital voltmeter, or is it something instantaneous that you need to view with
an oscilloscope?
I backed up and measured voltages on U5. I got 5V on pin 1, and 0 volts on
pins 3 and 4, all opposite of what the voltage table lists. I also got 0 volts
on pin 7 continuously even when I tapped the Band + and Band - switches.
Finally I looked at the thermistor board, but I wasn't sure quite how to investigate it. I have 1.2 volts at the junction of RA/Rt/RB and 0 volts at the junction of RB and RC.
Are my symptoms consistent with a bad thermistor, or is this more likely to
be a failure within U5 or U6?
Sorry for taking up all this bandwidth but I am not sure what my next step
should be.
Thank you for any and all suggestions on how to proceed.
-- Dave WB4JTT
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