K2 suddenly stops RX while on 160

[Steve Jackson-5]

Gee, it's been almost eight years of nearly continuous, flawless operation!  

But my K2 --just now-- decided to stop working on 160.

I was doing a little S&P, looking for mults in the contest, and all of a sudden, the receiver stopped working.

 Doesn't transmit, either.

It's fine on other bands, and comes back for a few minutes when I power-cycle the radio, or when I change bands
momentarily.  Then it quits again, after a minute or so.  Just sort of fades out.

VFO shows it's trying to tune while it's out to lunch, but nothing happens.

Of course, I can fix it, but before I go and start, anybody else ever see this happen?

tnx

Steve KZ1X/4
owner of the World's Most Experienced Elecraft
... which seems right now to be wanting a siesta




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[Don Wilhelm-4]

Steve,

I have not seen anything that I would term 'fading out', but --

One of the first things I would do is to check the frequency of the VCO
while it is failing.
That can be easily done without dragging out your counter, just put the
internal probe in TP1 and use CAL FCTR - compare with the chart on the
Schematic Key page.

Right now I can't think of anything else that would cause a problem like
that.  I guess it might be a relay contact that develops resistance that
builds up until failure, but that would be a rare occurrence.

Since both transmit and receive fail, that points to something common -
the oscillators, the bandpass filter or the low pass filter - most other
things are not common to transmit and receive.

73,
Don W3FPR

Steve Jackson wrote:

> Gee, it's been almost eight years of nearly continuous, flawless operation!  
>
> But my K2 --just now-- decided to stop working on 160.
>
> I was doing a little S&P, looking for mults in the contest, and all of a sudden, the receiver stopped working.
>
>  Doesn't transmit, either.
>
> It's fine on other bands, and comes back for a few minutes when I power-cycle the radio, or when I change bands
> momentarily.  Then it quits again, after a minute or so.  Just sort of fades out.
>
> VFO shows it's trying to tune while it's out to lunch, but nothing happens.
>
> Of course, I can fix it, but before I go and start, anybody else ever see this happen?
>
> tnx
>
> Steve KZ1X/4
> owner of the World's Most Experienced Elecraft
> ... which seems right now to be wanting a siesta
>
>
>
>
>       ____________________________________________________________________________________
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>
>  

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[Steve Jackson-5]

Don

Yes, it really does 'fade out' - but it does so pretty quickly, like, in 5 seconds or less.  Kind of neat,
actually, it would be hard to design a circuit that does this on purpose!

Any band change or power cycle brings it back, and, it even comes back all by itself, after a while.

The RX is still 'on' ... although I don't know what it's listening to.  Seems like it's actually going to some
out-of-band frequency and staying there, as if the varactor in the master VFO became impotent.  I hear
the AGC pumping, as if there was a clicky Brand-Y running a gallon up the band a bit.

I guess I will have a look-see in the morning.  I wanted to get a few more multipliers tonight.  Too
tired to work on radios now.  Gee ... never thought I'd ever say that ... must be age.  Or wisdom.

I'll post here what I find when I find it.  -gn-

73 Steve KZ1X/4



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[Jukka Tarvainen-2]

>
> Yes, it really does 'fade out' - but it does so pretty quickly,
> like, in 5 seconds or less.  Kind of neat,
> actually, it would be hard to design a circuit that does this on purpose!
>
> Any band change or power cycle brings it back, and, it even comes
> back all by itself, after a while.
>
> The RX is still 'on' ... although I don't know what it's
> listening to.  Seems like it's actually going to some
> out-of-band frequency and staying there, as if the varactor in
> the master VFO became impotent.  I hear
> the AGC pumping, as if there was a clicky Brand-Y running a
> gallon up the band a bit.
>

Similar thing happened to my K2 last time during CQWW CW. And it has
happened occasionally before. Also some other OH-elecrafters report same
issue. I thought I have some soldering issue, but after reheating RF-board
several times and re-doing suspect toroids, it still happens sometimes.
Power cycle, band change, preamp/att cycling or small tap to case brings it
alive. S-meter shows signal level, but audio fades away.

I think some of us would be glad to hear how it can be cured.

73 GL
Jukka OH4MFA
K2/100 #4293

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[Jukka Tarvainen-2]

Hi!

I got suggestion from Rick, KC0OV, to check VCO control voltages.

1800 kHz 3,5 V
1960 kHz 7,5 V
1970- kHz 8 V

Other bands are well inside 1,5-7,5 V.

I search archives and found case where it has been below 1,5 V and that was
cured with additional capacitance in C75. I test with some other big size
capacitors around 450 pF and found that VCO voltage can be lowered. I need
to buy several small caps to make suitable replacement for original 470 pF.

We will see if this helps.

73
Jukka OH4MFA
getting ready for Stew Perry


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[Don Wilhelm-4]

Jukka,

You are on the right path.  When the voltage is high, there is too much
capacity in the circuit and the VCO/PLL is trying to reduce it - but it
runs out of steam at about 7.5 volts and cannot change the capacity any
more.

C75 in your K2 needs less capacitance.  You can try several other 470 pf
capacitors (NPO disc ceramics or C0G mono capacitors) to see if you can
discover one that is on the low side of tolerance rather than the high
side (like yours), or you can replace C75 with a 390 to 450 pf capacitor
and add padder capacitors to bring the total capacitance up so the 160
meter VCO voltages are in the 1.5 to 7.0 volt range over the entire
tuning range.

73,
Don W3FPR

Jukka Tarvainen wrote:

> Hi!
>
> I got suggestion from Rick, KC0OV, to check VCO control voltages.
>
> 1800 kHz 3,5 V
> 1960 kHz 7,5 V
> 1970- kHz 8 V
>
> Other bands are well inside 1,5-7,5 V.
>
> I search archives and found case where it has been below 1,5 V and that was
> cured with additional capacitance in C75. I test with some other big size
> capacitors around 450 pF and found that VCO voltage can be lowered. I need
> to buy several small caps to make suitable replacement for original 470 pF.
>
> We will see if this helps.
>
> 73
> Jukka OH4MFA
>
>  

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[Jukka Tarvainen-2]

Hi Don!

Thank you for good advice once again!

>
> C75 in your K2 needs less capacitance.  You can try several other 470 pf
> capacitors (NPO disc ceramics or C0G mono capacitors) to see if you can
> discover one that is on the low side of tolerance rather than the high
> side (like yours), or you can replace C75 with a 390 to 450 pf capacitor
> and add padder capacitors to bring the total capacitance up so the 160
> meter VCO voltages are in the 1.5 to 7.0 volt range over the entire
> tuning range.
>

Today I got some NPO disc ceramic caps and tried different variations. With
330+150 pF in parallel measured VCO voltages are now inside 1.5-7.0 V:

1800 kHz 1.6 V
2000 kHz 4.0 V

These new capacitors are not small in physical size, bigger is 10 mm, about
0.4" in diameter. Does size matter in this circuit?

Best Regards
Jukka, OH4MFA
K2/100 #4293


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[Don Wilhelm-4]

   Jukka,
   Congratulations.   With those voltages, you could go even a bit higher
   in capacity, but it is OK as it is.
   Size  only matters to the extent of physical fit.  If you were able to
   get the larger caps mounted and the K2 panels still fit, all is well.
   73,
   Don W3FPR
   Jukka Tarvainen wrote:

Hi Don!

Thank you for good advice once again!

 

C75 in your K2 needs less capacitance.  You can try several other 470 pf
capacitors (NPO disc ceramics or C0G mono capacitors) to see if you can
discover one that is on the low side of tolerance rather than the high
side (like yours), or you can replace C75 with a 390 to 450 pf capacitor
and add padder capacitors to bring the total capacitance up so the 160
meter VCO voltages are in the 1.5 to 7.0 volt range over the entire
tuning range.

   

Today I got some NPO disc ceramic caps and tried different variations. With
330+150 pF in parallel measured VCO voltages are now inside 1.5-7.0 V:

1800 kHz 1.6 V
2000 kHz 4.0 V

These new capacitors are not small in physical size, bigger is 10 mm, about
0.4" in diameter. Does size matter in this circuit?

Best Regards
Jukka, OH4MFA
K2/100 #4293




 
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