[Elecraft] [K2] Erratic power control with base cw K2

[Alastair Couper]

I have a base K2 sn 1056, with a to b mods, including cw shaping mod, and  
the latest firmware. I have removed all options to ease troubleshooting.  
Turning the power control gives erratic power output, varying widely and  
randomly as I turn the control. If I turn the power control to max, then  
power is also max, but other positions are random. This happens on all  
bands, into 50 ohm load, and also using the antenna tuner option. Max  
power out is normal at about 15w, more or less. The rig works in all other  
aspects. Power supply shows 13.0 to 13.5 while transmitting.

The display info smoothly follows the effects of turning the knob while  
receiving, so the info is making it into the CPU, and the pot is OK. If I  
push TUNE, then the power output level at the watt meter adjusts as I turn  
the control. But the level shown on the watt meter is quite variable with  
regards to the control setting. If I leave the control at half, one push  
of the TUNE will give 15w, and the next push will give 2w. Also the watt  
meter reading jumps to max intermittently as I turn the control back and  
forth from zero to max.

The voltage from VRFDET is making it back to pin 2 of the CPU, and it's  
value reflects the level that I see at the output on the watt meter at  
R66. The voltage at VPWR is erratic when turning the power control, as is  
the voltage at VALC, but those two voltages seem to be about equal, as  
best I can measure. (No scope). The VALC at R98 on the RF board is the  
same as at Q8 on the control board, so that signal is making it. As I turn  
the control in TUNE, I see from around ~.4 volts to ~1.2 volt at 15 watts  
out. Higher voltage on VALC does make more output power, so that potion is  
OK.

If I plug in the KSB2 board, then on SSB I can get what is apparently  
normal smooth power control, but the problem is still there in CW. Initial  
keying sometimes give HiCur readings. So the loop on CW is not working for  
some reason, and I am open to further suggestions.

I have been reading posts about R98, which is at 270 ohms. Not sure if  
that applies here.

tnx,

Al NH7O