[Elecraft] [K2] Control board U4 8v regulator issue

[Don Wilhelm]

Mike,

The regulator is a 10% tolerance part, so any voltage between 7.2 and 
8.8 volts meets the spec for the regulator.  They have been running on 
the low side of the 8 volt nominal rating for some time now, and cause 
no harm other than the AGC Threshold voltage cannot be adjusted up to 
the level indicated in the manual.

Everything is proportional, and since the IF amplifier (which is 
controlled by the AGC) also runs off the 8 volt regulator, setting the 
AGC Threshold at its maximum setting works just fine.  The real 
consequence of low AGC Threshold voltage is that the S-meter setting get 
a bit 'squirrelly' when the voltage is lower.  If you are able to adjust 
the CAL S LO and CAL S HI for proper S-meter action, then all is well.

If you have trouble with the S-meter settings, add a 10k to 15k resistor 
across Control Board RP5 pins 3 and 4 - then readjust the AGC Threshold.

73,
Don W3FPR

Mike-WE0H wrote:
> Has anyone else had a 8v regulator (U4) on their control board show a 
> lower voltage than it should? Mine shows 7.61v with the board in the 
> radio or on the bench with a 14v supply connected to the input of that 
> regulator. All components and soldering look correct. The 5v regulator 
> is at 5.00v. All functions in the test procedure work as they should 
> except the AGC voltage couldn't be set to 3.80v because of the 8v buss 
> being at 7.61v, the AGC voltage changed as it should but was lower than 
> the 3.80v suggested for setting R1 to.
>
> Nothing runs hot or smells hot. What do you guys think? Bad regulator or???
>
> Many thanks,
> Mike
> WE0H
>   
>