4 MHz oscillator is too high in frequency (K2)

[Sverre Holm (LA3ZA)]

My aging K2 (#2198) has developed a curious symptom in that the 4 MHz oscillator cannot be adjusted lower than about 4,000,000.55 kHz. It has probably been like this for a while and my BFO settings have probably been done with the incorrect reference. I discovered it by observing that the indicated frequency was about  0.75 kHz off.

Questions to the group:
1. Has anybody experienced anything like this, and can it be blamed on aging of e.g. the crystal or on some of the capacitors in the circuit? (BTW I have the KI6WX mod of the oscillator circuit http://mailman.qth.net/pipermail/elecraft/2003-August/022687.html which has worked well before)

2. How can I do a quick fix? e.g. by reinstalling C21=33 pF instead of the 10 pF of the mod above - or some other idea for a fix?

73
Sverre

[Sverre Holm (LA3ZA)]

Sverre Holm (LA3ZA) wrote

My aging K2 (#2198) has developed a curious symptom in that the 4 MHz oscillator cannot be adjusted lower than about 4,000,000.55 kHz.

This should be 550 Hz too high, that must be 4,000.55 kHz (I think!)

[Don Wilhelm-4]

Sverre,

You could add additional capacity across C22 to adjust it into range -
you would have to experiment with the value.  The range of C22 is likely
6 to 50 pf, so adding 40 pF at a time would retain the C22 bandspread
and should allow it to tune.  i.e., I would suggest first putting 39 pF
in place of the 10 pF - if that does not bring it down far enough, try
68 pF.

Of course, the easiest fix is to change C21 back to 33 pF and removing
the 10 pF across C22, but you will find the C22 adjustment much more
'touchy'.  Even then, you may need some padding across C22.

I believe the aging of the crystal is to blame.

73,
Don W3FPR

On 4/21/2014 10:36 AM, Sverre Holm (LA3ZA) wrote:

> My aging K2 (#2198) has developed a curious symptom in that the 4 MHz
> oscillator cannot be adjusted lower than about 4,000,000.55 kHz. It has
> probably been like this for a while and my BFO settings have probably been
> done with the incorrect reference. I discovered it by observing that the
> indicated frequency was about  0.75 kHz off.
>
> Questions to the group:
> 1. Has anybody experienced anything like this, and can it be blamed on aging
> of e.g. the crystal or on some of the capacitors in the circuit? (BTW I have
> the KI6WX mod of the oscillator circuit
> http://mailman.qth.net/pipermail/elecraft/2003-August/022687.html which has
> worked well before)
>
> 2. How can I do a quick fix? e.g. by reinstalling C21=33 pF instead of the
> 10 pF of the mod above - or some other idea for a fix?
>
>

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[Sverre Holm (LA3ZA)]

Hi Don,

Thanks - heating up the soldering iron now!

- Sverre