[Elecraft] ELECRAFT K2 BUILDER ALERT #21

[Eric Swartz - WA6HHQ, Elecraft]

ELECRAFT K2 BUILDER ALERT #21
Internal freq. counter may fail on TP3
Oct  19, 2004

APPLICABILITY
Recent K2 kits with blue colored MPS5179 transistors for the frequency 
counter preamp at Q9, Q10 on the control board.

Background
Some late serial number K2 kits have been supplied with MPS5179 
transistors that may not function correctly. The transistor may be 
identified by its flat side being a metallic blue color.  This 
transistor may fail to allow the K2's internal frequency counter to 
count at higher frequencies. It may work OK at BFO test point TP2, but 
not at PLL reference oscillator test point TP3 or at VFO test point TP1 
at higher VFO frequencies. Transistors that are all black in color do 
not have the problem. (Even these transistors can be damaged by ESD, 
however. To protect Q9 from ESD, always touch a grounded metal surface 
before installing or removing the counter probe.)

The problem may mislead you to believe the PLL reference oscillator or 
the VFO is not working. You can use an oscilloscope if available to 
check for the 12 MHz signal at TP3;   use the included RF probe in the 
kit; or use another receiver or transceiver that can tune to 
approximately 12.1 Mhz.  Hold a wire from the antenna input of the other 
receiver near the PLL area and tune around 12.1 Mhz to see if a signal 
is detected. You can also try the K2's counter probe at the junction of 
Q19's source and RFC14, on the underside of the RF board. Remove the 
left side panel if necessary for better access. If Q19 is oscillating, 
the DC voltage at the junction of RFC14 and R20 should be about 0.5 to 1 
VDC. If there is no oscillation, the DC voltage will usually be higher 
than 1 VDC. Also check RFC15 that supplies 5V to pin 16 of  PLL IC U4 is 
not open, damaged, or missing.

Changes Required
Q9 and Q10 on the control board are the affected parts, and  they should 
be replaced with a new MPS5179 transistor from us or another source. 
Email parts at elecraft.com for a replacement if necessary.  Other 
high-frequency NPN transistors may work in this circuit as well. If 
you're in a hurry, try a 2N4124, PN2222A, MPS2222A, etc. If these work, 
there's no need to use the 5179's.
 
Remove Q9 and Q10 on the control board and replace them with the new 
devices. It's often better to cut the leads on the original part  to 
make removal easier, and to prevent damage to the PCB. Once the original 
transistors are removed, clean out the PCB pads and any remaining wire 
or solder using preferably a vacuum desoldering tool, or desoldering 
braid, using care to not overheat or damage the PCB. Clean off any flux 
residue or dirt with a cotton swab moistened (not dripping) with 
denatured or isopropyl alcohol or acetone.  Use care to not damage any 
plastic parts or paint.  Install the new transistor about 1/8-1/4" from 
the PCB and clip its leads flush. Test the frequency counter at TP3, 
using the CAL FCTR menu procedure shown on page 60 of the current 
revision F manual.