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

 

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[LA0BY-2]

CONTENTS DELETED

The author has deleted this message.

[LA0BY]

Hi Jim,
I have a K3 that developed a similar problem:

My K3 was build by Elecraft in 2011 with the only extras added by myself later being the KXV3A transverter interface, some CW filter and the KDVR3 voice recorder. I have been using it perhaps once or twice each month only as IF unit for a 2 m XVTR. The setup worked very fine for almost 2 years but started then to exhibit intermittent drop-outs of the RX signal, after a TX phase. The path was not completely gone, just had extra attenuation of 30-40 dB. Transmission seemed never affected. Initially it resembled a hanging contact of a T/R-relay, and could be cured by pushing the PTT once or twice.

So I am not using the SO-239 rear socket. But during the first hilltop operation after the winter the problem was permanent. Back home in the shack the problem was still there but went away by itself after some hours of warm-up. I tried to identify the failing stage but never got to the bottom of it before it disappeared. But I found the extra insertion loss has to be way down into the receiver, probably at the second IF level. During a second hilltop operation the problem surfaced again. Back in the shack the failure was present only for some minutes before it disappeared. While it persisted, I could not detect any effect of tapping somewhere on the chassis.

There is no doubt that my problem is related to the K3. It was present on all HF bands even if not connected to the external transverter. I was then not using the same cables as with the XVTR.

Do you think my problem could be attributed to the same failing PIN-diodes?

73, Stefan (LA0BY)