[Elecraft] K2 PLL reference oscillator range is zero

[Don Wilhelm]

Dave,

Your measurement on SCK is correct - the manual is wrong for that one.  
If you look at it with a 'scope, you will see negative going pulses when 
the clock is active (turn the VFO knob to generate a few pulses.

I believe your problem is with the /DAC2CS signal which I just verified 
as 5 volts when measured as a DC signal, but like the SCK signal has 
negative going pulses when the VFO knob is turned.  Since your K2 
/DAC2CS is at zero volts, the DAC is being gated for data-in at all 
times, so when the data is not being clocked to the DAC, it thinks the 
data bits in the data stream are all inactive, and the analog output 
goes to zero volts.

Your readings at U6 pins 33 and 38 are in the range of normal readings.  
Pin 38 is active positive and is normally low with frequent positive 
going pulses, so the DC voltage measured can vary depending on the time 
constant of the meter used, and pin 33 is also active high, being 
normally low and having less frequent positive pulses than those on pin 
38 - so its DC average voltage will be less than that on pin 38 - how 
much depends on your meter.

I don't know if your digital skill level was adequate to follow all of 
that explanation, but the bottom line is that something is not working 
between the microprocessor (Control Board U6) pin 19 and RF Board U5 pin 
3.  Go back to CB U6 pin 19 and measure the voltage there - if the 
voltage is 5 volts there while being 0 volts at RF U5 pin 3, there is a 
break in the path - that includes the CB to RF board connectors - find 
the break and fix it (you can verify the fix with a continuity check on 
your DMM).
OTOH, if the voltage is zero at both places, the problem is *either* a 
bad microprocessor output or a shorted RF Board U5 pin 3.  The best way 
to determine which it is requires removing U5.  If the voltage goes to 5 
volts with U5 removed, the microprocessor is good and U5 is bad - but if 
it stays at zero volts, then the microprocessor needs to be replaced.

There is another way to check if you do not have de-soldering equipment 
and cannot easily remove U5.  Remove Control Board U6 from its socket 
and insert the ends of a resistor (470 ohms to 2,2k will do nicely for 
this check) between U6 pins 11 and 19 (count carefully).  Then power the 
K2 on (do not expect a display or anything else to happen), then measure 
the voltage at RF Board U5 pin 3 - if it is 5 volts, U5 is OK, get a new 
K2 firmware chip - but if the voltage is zero at U5 pin 3, replace U5.

73,
Don W3FPR


David Johnson wrote:
> 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
>>     
>