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 1VDC. If there is no oscillation, the DC voltage will usually be higher than 1 VDC. Also check RFC15 that supplies 5 V 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@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.