[Elecraft] K2 PLL reference oscillator range is zero
David Johnson
wb4jtt at yahoo.com
Tue Jan 6 16:11:53 EST 2009
Don and the group,
I have not yet solved this problem, but upon further troubleshooting a recurrent theme seems to be that the clock signal (SCK), which appears at pin 18 of MCU U6, pin 6 of EEPROM U7 and pin 10 of U8 on the control board, as well as pin 1 of DAC U5 on the RF board is constantly 5 volts when, according to the voltage charts, it should be 0 volts.
Further, the /DAC2CS value at pin 19 of the MCU U6 (control board), which is also at pin 3 of U5 (RF board) is 0 volts when it should be 5 volts.
The only other anomalous voltage values on MCU U6 are 0.1 volt on SRDIN pin 33 instead of the listed 5 volts, and 0.2 volt on /SR RD pin 38 instead of the listed 1.2 volts in the chart.
So my question is, what controls the SCK value on pin 18 of the MCU U6? In other words, I assume that there is another pin on U6 that controls whether pin 18 is high or low. If not, should I assume that the MCU is bad because it always reads high?
Many thanks to anyone willing to wade through all this!
-- Dave WB4JTT
--- On Wed, 12/31/08, Don Wilhelm <w3fpr at embarqmail.com> wrote:
> From: Don Wilhelm <w3fpr at embarqmail.com>
> Subject: Re: [Elecraft] K2 PLL reference oscillator range is zero
> To: wb4jtt at yahoo.com
> Date: Wednesday, December 31, 2008, 1:53 PM
> Dave,
>
> Yes, you should suspect U5 (or one of the digital signals
> going into it). No need to look at pin 7 with a scope - the
> output at pin 7 is analog and should remain a steady
> voltage.
>
> Check the soldering of U5 and the /DAC2CS signal path back
> to Control Board U6 pin 19 (there are connectors in the path
> that could also be suspect. The inputs on pin1 and pin 2
> are shared with other digital devices on the Control Board,
> and if you have checked for things like the presence of
> sidetone, those signals (SCK and SDO) are working at their
> source so should be OK although there are also the RF Board
> to Control Board connections to consider as possible faults.
>
> 73,
> Don W3FPR
>
> David Johnson wrote:
> > Don,
> >
> > Thanks for the quick response!
> > I do have the 5 volts on pin 8 of U5, but a constant
> 0 volts on pin 7 even when tapping the Band + and Band -
> switches. But I'm just looking at it with a Digital
> Voltmeter, not a scope so maybe I'm missing it.
> >
> > Pin 5 of U6 is 4 volts, so at least that data point
> is OK.
> >
> > Should I suspect U5 at this point? What can I do to
> verify that U5 is being properly triggered at its input when
> I tap the Band + and Band - switches?
> >
> > Thanks again.
> >
> > -- Dave WB4JTT
> >
> >
> > --- On Wed, 12/31/08, Don Wilhelm
> <w3fpr at embarqmail.com> wrote:
> >
> >
> >> From: Don Wilhelm <w3fpr at embarqmail.com>
> >> Subject: Re: [Elecraft] K2 PLL reference
> oscillator range is zero
> >> To: wb4jtt at yahoo.com
> >> Cc: elecraft at mailman.qth.net
> >> Date: Wednesday, December 31, 2008, 2:56 AM
> >> 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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