The 560 pF capacitor you removed was the capacitor that enables the oscillator
in the 50398 to scan at a frequency around 4 KHz, which you could easily hear.
Removing that cap would get rid of your noise but likely kills the ability of
the chip to scan the preset data.
The band switch is what determines the first two digits of the Omni D readout
because this determines what preset is used in the counter to subtract or add 9
MHz IF to or from the LO frequency to get correct frequency on the display. All
bands below 10 MHz use one preset, 10 MHz uses another and the higher bands use
a third preset. Thus your problems on 10 through 29 MHz. This preset data is
not being read.
Put the 560 pF back in to determine if you can get back to proper operation of
the display.
Then look for decoupling capacitors, or improper grounds, that are not doing
their jobs of eliminating the annoying tone.
Killing the counter is not the solution…
73,
Gary
> On Jan 7, 2019, at 9:29 PM, <forum4ll@verizon.net> <forum4ll@verizon.net>
> wrote:
>
> Hi Carlos,
> Thanks for the link to the Corsair. Looks like the whining issue is
> resolved by just removing the 560pf cap between pin 16 and Vccon the IC4, the
> MK50398N. This cap was to enable the scan oscillator.I can't seem to find any
> explanation as to why this is needed butit must serve some function only an
> electrical engineer would know.
>
> I now have a different problem and that is the first two digits of
> thefrequency display is incorrect for 10, 14, 21, 28, and 29mhz. I willstart
> a new thread on that and describe the problem in more detail.
>
> LymanKK6LPW
>
>> Hi Lyman,
>
>> I don't know why do you have that funny reading on 10 MHz. I have
>> never studied the heterodyne VFO topology or which corrections is the
>> frequency counter making depending on the selected band.
>
>> My feeling is that the frequency counter module is applying the same
>> settings that correspond to the 10 meter band.
>
>
>> There is a brief description here on how the Corsair works (not
>> exactly your radio, but not a distant relative either)
>
>> http://www.radiomanual.info/schemi/Vari/Ten-Tec_Corsair_560_review_1984.pdf
>
>
>
> 73,
> Carlos VK1EA
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