[Elecraft] Update, re K3 loss of RX sensitivity

[Jim Garland]

A few days ago I posted a request for help with loss of sensitivity on my
K3's main and sub-receiver.  The problem was quickly identified by Van,
W1WCG as a known issue - failure of  PIN diode D5 on the KXV3 board.  D5 is
a pinhead-size 3-terminal surface mount device (actually two diodes in one
package) that routes signals to the main and sub-receivers during receive.
Failure of the diode is caused by electrostatic discharge (from
thunderstorms) or a strong RF field from a nearby transmitter (which was the
cause of my failure.)

 

It is possible to replace D5 either by sending the KXV5 board back to
Elecraft, or simply by replacing the diode in one's workshop, which is what
I did. The Elecraft part number is  E560047. Replacement diodes are also
available from Digikey (p/n 568-1932-1-ND) and Mouser p/n
(771-BAP64-04W-T/R), and the manufacturer's part number is BAP64-04W. The
cost is only $0.54 in small quantities, and it is advisable to buy a few
spares.

 

The PIN diodes in this device are rated at 100V reverse breakdown. Since
failures of these diodes are a known problem, it is better to replace them
with a more robust substitute. Fortunately diodes  having identical
electrical specifications are available at the same price, but with a 175V
breakdown rating. For these, the part numbers are Digikey: p/n
568-1931-1-ND, Mouser: p/n 771-BAP64-04-T/R, Manufacturer: p/n BAP64-04.

 

There is a small problem with ugrading D5 to the higher voltage version: the
package sizes are slightly different. The 100V diode has package SOT323,
whereas the 175V diode has package SOT23, which has marginally wider pin
spacing. Fortunately, there is enough overlap between the pins of the larger
device and the solder pads on the KXV3 PCB to ensure a reliable solder
contact. However the magin of error for alignment is reduced, and so if you
want to substitute the more robust diode, you must use particular care in
making the substitution.

 

Replacing SMT devices requires a steady hand and good eyesight. Although,
replacing D5 is not difficult, I don't recommend it unless you are
comfortable working with SMT components.

 

I also recommend that Elecraft  specify the larger PIN diode in any future
revision of the KXV3 board in order to increase reliability.

73,

Jim Garland W8ZR