[Elecraft] K2 PLL reference oscillator problems (DAC side!)

Wed May 29 04:56:22 EDT 2019

Hi,

I have replaced the J310 and the oscillator now runs over the full voltage range of the DAC. Many thanks. After this, the PLL calibration routine completes successfully. The EEPROM appears to be ok here. 

So at a high level the issue appears to be:

1. The J310 or circuit was at fault. Either the J310 device spread is suspect or the circuit is device spread sensitive. Having built a few VXOs over the years some problems are almost inevitable here so it's expected really.
2. The default EEPROM contents without PLL calibration have no calibration data so it's not possible to tune the oscillator down into a running state, not that it'd help potentially based on point 3.
2. The default DAC value of full scale pegs the varactors at full swing resulting in the oscillator not starting.
3. The PLL calibration routine will not run if the oscillator is not running which is fair enough.

Diagnostic steps to positively identify the issue:

1. Reset K2 (456+power up) to clear cal data. This will wedge the DAC at full scale.
2. Hook scope/counter to TP3 (internal counter is fine). If there is a signal here at around 12MHz, then the oscillator works. No further diagnostics required.
3. Lift one end of R19 to disconnect the DAC from the PLL oscillator.
4. Hook bench supply to RP2 pin 2 or 4. Set on 0V. 9v battery with 1k pot across it and DMM on wiper terminal will do if you don't have a bench supply.
5. Sweep bench supply across 0-8V and ensure that there is a signal at TP3 across the entire range.
6. If there isn't replace Q19 (J310) and retry until there is oscillation across the entire range.
7. Reinsert and solder lifted end of R19.
8. Complete PLL calibration.

As a test, I also tried an NOS On Semiconductor J310 (which arrived the same time as Elecraft's replacement!) and that worked consistently across the board. I have however substituted the Elecraft supplied replacement which matches the On Semi one now as well.

Thanks to Don for landing on this issue straight away. Next steps in progress now.

Best regards,

Chris M0XTE

On Tue, 28 May 2019, at 11:22, Chris Smith wrote:
> Hi,
> 
> Received replacement J310 and EEPROM today. I will install the J310 this evening and see where it goes, failing that the EEPROM.
> 
> Big thanks to Don and Elecraft's excellent support.
> 
> Best regards,
> 
> Chris M0XTE
> 
> On Tue, 21 May 2019, at 23:37, Don Wilhelm wrote:
> > Chris,
> > 
> > If it is a EEPROM data problem, then a Master Reset should load it with 
> > default values - hold the 4, 5, and 6 buttons in while turning power on.
> > 
> > I have not found that the EEPROM has been the source of the problem, but 
> > rather Q19.
> > Q19 should oscillate with anywhere between 0 volts and 8 volts on the 
> > varactors.
> > 
> > Before assuming it is a possible EEPROM problem, add the capacitor I 
> > indicated and see if it works. You should still replace Q19.
> > 
> > To further check the EEPROM and U5 DAC
> > 
> > On 5/21/2019 1:21 PM, Chris Smith wrote:
> > > Hi,
> > > 
> > > Thanks for the reply. Much appreciated.
> > > 
> > > That’s a bummer. Is there anything I can do here with the current EEPROM? I have a programmer available. I assuming erasing it and letting the K2 reinitialise it a small per earlier steps won’t resolve this?
> > > 
> > > Based on looking at it from the outside naively, it feels like theres something up with the VCO linearisation as it’s sending the control word on VFO steps but the same data. I’ll get a bus captures if it helps.
> > > 
> > > Assume that performing the VCO linearisation steps later won’t fix it either?
> > > 
> > > Hope you enjoy your vacation - sounds nice!
> > > 
> > > Best regards,
> > > 
> > > Chris M0XTE
> > > 
> > > On Tue, 21 May 2019, at 18:11, Dave Van Wallaghen wrote:
> > >> Chris,
> > >>
> > >> I have worked on a few late model K2'S of late with the exact same problem. The folks in support are currently looking into it to determine the source of the problem.
> > >>
> > >> My solution was to replace the Control board EEPROM with a known properly initialized one and everything came back to life. I even discussed this with Wayne in Dayton. We're looking into whether we have an EEPROM issue or a possible programming anomaly with the main PIC MCU.
> > >>
> > >> I'm currently on vacation with my wife 8 miles off the coast of Belize. I have limited WIFI but will continue to check in.
> > >>
> > >> Stay tuned...
> > >>
> > >> 73,
> > >> Dave, W8FGU
> > >>
> > >> On May 21, 2019 10:28:00 "Chris Smith" <me at m0xte.uk> wrote:
> > >>
> > >>> Hi,
> > >>>
> > >>> I've just completed my K2 up to the PLL reference oscillator.
> > >>>
> > >>> Initially when I assembled it the PLL reference oscillator would not oscillate at all. No reading on internal counter. I popped it on my external Racal counter and still dead. After tracing the problem out I found that the voltage on input to the varactors was pegged at 7.07V which was too high for the oscillation to carry on. I lifted one end of R19 on the output of U6A and the thing oscillates fine isolating this to a control voltage problem.
> > >>>
> > >>> Next I checked voltages on the thermistor board and input of the opamp. All good. However the input to U6A pin 3 from the LTC1451 DAC is stuck at 4.095V (top end of DAC range by the looks of it) which is forced up by the opamp. Popped U5 DIN on the scope and spun the VFO and it sends the same data word to the DAC every time regardless of VFO position. The CLK and CS lines are doing their thing correctly.
> > >>>
> > >>> Anyone got any ideas why the control word is the same? VFO reading on display is changing fine.
> > >>>
> > >>> What can I do to get out of this hole?
> > >>>
> > >>> Any help appreciated.
> > >>>
> > >>> Best regards,
> > >>>
> > >>> Chris M0XTE
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