ELECRAFT K2 BUILDER ALERT #22
KPA100 Cap Kit Upgrade
23 June, 2005
APPLICABILITY
This builder alert applies to ALL KPA100s shipped until June 13, 2005. If your
KPA100 was shipped after June 13, 2005, or if it puts out 100W on 20M and 17M
without drawing much more than 20 amps at 13.8V, it does NOT need this upgrade.
BACKGROUND
A small number of builders have reported difficulty in reaching full power
(100W) on the 20 and 17 meter bands. In other cases, while full power was
reached, the current drawn by the K2 was abnormally high when operating in the
20 and 17 meter bands. The high current draw is an indication of lower than
normal gain in the KPA100.
It was found that this condition can occur if C80 and C81, 4,700 pF 10% disk
ceramic capacitors, are at the low end of the capacitance tolerance range. A
sampling of recent stock revealed that most of the capacitors were in the 4,200
to 4,300 pF range. It was found that replacing C80 and C81 with 5,600 pF 5% poly
capacitors returned the KPA100 to normal operation. The improved tolerance and
higher nominal value will assure that this problem will not reoccur.
The newly available kit, "KPA100 CAP KIT" is available free for the asking to
customers owning an applicable KPA100. This kit provides the parts needed to
install the upgrade and restore your KPA100 to normal operation. (Please do not
request this kit if your KPA100 is not exhibiting the power problems noted
above.) This mod kit is available for order on-line via our Parts and Mod Kits
page. ( http://www.elecraft.com/order_form_parts.htm#KPA00 Parts)
MODIFICATION INSTRUCTIONS
1. Turn off the K2, turn off the power supply, and disconnect all rear-panel
cables.
2. Remove the six screws retaining the KPA100 module and lift it out carefully,
disconnecting all internal cables.
3. Remove the seven zinc-plated (bright) screws holding the KPA100 shield to the
PC board. Do NOT remove the three black screws that hold the grounding clips to
the shield (applies if you have the newer shield).
4. Locate capacitors C80 and C81 on the top (or component) side of the board,
found near power transistors Q1 and Q2.
5. Using heavy duty wire cutters, cut the capacitors in half and gently bend the
two halves away from each other.
6. Using your soldering iron, heat the leads of the clipped capacitors at the
solder pads in the pc board and use small pliers to pull them off the top of the
board.
7. Use solder wick to clear the holes from any remaining solder.
8. Locate the two new 5,600 pF capacitors that are provided with the kit.
9. Pre cut the leads for the new capacitors so they will just protrude out the
bottom of the board (about one-eighth inch long).
10. Insert the capacitors from the top (or component) side of the board into the
holes for C80 and C81 and solder the leads from the top of the board.
11. Reinstall the shield. Tighten the hardware to ensure good contact at both
ends of the ground strap.
12. Turn the KPA100 assembly over carefully and install it in the K2,
re-connecting all internal cables.
13. Check the PA transistor hardware, tightening the screws as described in your
assembly manual. This is VERY IMPORTANT, as loose hardware can lead to damaged
transistors.
14. While putting the side-panel screws back in, please be very careful not to
over tighten them as this will strip the screws!
15. Make sure the left and right spring clips are touching the side panels, and
that none of the interconnecting cables are pinched.
This completes the KPA100 cap upgrade.
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