K2 VCO Voltage Instability

[John Huffman-3]

Guys -

This topic was KPA100 Behaving Badly, but at Tom's N0SS suggestion, I
tried the KPA100 on my working K2 and all was well.  I also tried the
control board of the sick K2 in my working K2 and all was well.  So,
it's not the KPA100 Behaving Badly, it's the K2.

I'm back to the original problem -  watery CW signals on receive,
frequency shifts on receive and loss of all signals.

I suspect the VCO is not getting the correct consistent voltage.  I
attempted to do the VCO Alignment in Alignment and Test - Part 2.  I
tuned to 4mhz and easily adjusted L30 for 6.0v as shown on R30 on the RF
board.

But, as I tuned toward 3.5mhz, the voltage decreased steadily until
about 3.8mhz where it dropped to zero.  Tuning back up to 4mhz, it
remained at 0.  Turning the radio on and off while tuned to 4mhz
restored the voltage to 6v, but the same thing happened when I tried to
tune below 3.8mhz.

Turning the radio on while it is set for 3.5mhz give a reading at R30 of
6.8v.  Tuning higher increases the reading to 8v where it stays at all
higher frequencies.  Tuning back down, the voltage goes to 0v around
3.8mhz and remains there at all frequencies.

The voltages on these pins of U6 (RF Board) remain constant - pin 5 -
4v, pin 6 - 4v, pin 8 - 8v.  Would a bad U6 explain these problems?

It there somewhere else I should be checking?

Any and all help greatly appreciated.

73 de K1ESE
John
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[Don Wilhelm-4]

John,

That R30 voltage is changing too rapidly on 80 meters

I doubt U6 is the problem, but more data is required to say yes or no.
The voltage at pin 5 should be a steady 4 volts coming from the
thermistor board.  As long as this is steady, there is no problem with it.
The voltage at pin 6 will vary depending on the voltage coming out of U4
pin 13.  That variation will result in  a corresponding (but larger)
variation at pin 7 (which is also the left end of R30).  Since you do
get some variation at R30, I would say U6 is likely working properly.

Your problem is the VCO locking - this is a PLL loop involving the VFO
area and its feedback loop to U4 - the output of U4 then drives U6 which
in turn drives the voltage to the varactors in the VFO to alter the
frequency.  At the same time, there is data (from the EEPROM) being sent
to the PLL chip (U4) to tell it what divisor to use in syncing with the
incoming PLL Reference frequency - so any of these things could cause
incorrect operation.

First things first - set to 4.000 MHz and check to see that you do have
voltage at R30 - Adjust L30 until it is near 6 volts.  Then use the
internal counter probe and CAL FCTR to measure the frequency at TP1 (the
VFO frequency)   The frequency at TP1 should be close to 8.915 MHz.

Move the frequency down while monitoring the voltage at R30 - when the
voltage gets to 1.5 volts, note the displayed frequency and again
measure the frequency at TP1 - subtract 4.915 from the measured
frequency and compare that to the K2 displayed frequency - there should
be a close match (forget anything less than 1 kHz, it is not significant
at this time).

If there is a match, the PLL and VCO are likely working properly.  Next
step is to assume that there is bad data in the EEPROM - try doing a CAL
PLL to see if that sets things better.  If not, then write down the menu
items including the filter settings and do a Master Reset (hold the 4,
5, and 6 buttons while turning on power) - then after the Info 201 goes
away repeat the CAL PLL.

If anything in the steps above seem totally "out of whack", stop and let
me know what you are finding (it may be that we are looking in the wrong
places).

73,
Don W3FPR




John Huffman wrote:

> Guys -
>
> This topic was KPA100 Behaving Badly, but at Tom's N0SS suggestion, I
> tried the KPA100 on my working K2 and all was well.  I also tried the
> control board of the sick K2 in my working K2 and all was well.  So,
> it's not the KPA100 Behaving Badly, it's the K2.
>
> I'm back to the original problem -  watery CW signals on receive,
> frequency shifts on receive and loss of all signals.
>
> I suspect the VCO is not getting the correct consistent voltage.  I
> attempted to do the VCO Alignment in Alignment and Test - Part 2.  I
> tuned to 4mhz and easily adjusted L30 for 6.0v as shown on R30 on the
> RF board.
> But, as I tuned toward 3.5mhz, the voltage decreased steadily until
> about 3.8mhz where it dropped to zero.  Tuning back up to 4mhz, it
> remained at 0.  Turning the radio on and off while tuned to 4mhz
> restored the voltage to 6v, but the same thing happened when I tried
> to tune below 3.8mhz.
>
> Turning the radio on while it is set for 3.5mhz give a reading at R30
> of 6.8v.  Tuning higher increases the reading to 8v where it stays at
> all higher frequencies.  Tuning back down, the voltage goes to 0v
> around 3.8mhz and remains there at all frequencies.
>
> The voltages on these pins of U6 (RF Board) remain constant - pin 5 -
> 4v, pin 6 - 4v, pin 8 - 8v.  Would a bad U6 explain these problems?
>
> It there somewhere else I should be checking?
>
> Any and all help greatly appreciated.
>
> 73 de K1ESE
> John
> ------------------------------------------------------------------------
>
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>  

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