NX7U

Scott Townley
Gilbert, AZ

From AMSAT-NA mailing list, 6 Sept 2007
Date: Thu, 06 Sep 2007 17:41:13 -0700
From: Scott Townley <nx7u(AT)cox.net>
Subject: [amsat-bb]  IC-821H TX problem on 70cm--solved
To: amsat-bb(AT)amsat.org
Message-ID: <6.2.1.2.2.20070906170236.0258f000(AT)pop.west.cox.net>
Content-Type: text/plain; charset="us-ascii"; format=flowed

I had written the list awhile back regarding an intermittent TX problem
with my IC-821H on 70cm.

Someone pointed me to the eHam reviews, where PE3HMP mentions thermal
problems with Q43 causing the 70cm TX to drop out.

I opened the radio up, and found significant thermal browning on the sheet
metal and PCB in the left-side rear corner, where several voltage
regulators are.  Q43 was confirmed on the schematic as controlling the 70cm
TX, and is found on the underside of the MAIN board, sure enough in the
area of thermal browning.  Quick bench measurements confirmed that the uP
triggers for TX were all present up until Q43...so it was definitely the
culprit.

Q43 is an internally-biased PNP switching transistor in an SOT-23
surface-mount package (Toshiba RN2425), rated at 200mW and will switch
800mA.  Of course it's unobtainium nowadays.   Which you could argue is a
good thing, since it failed to begin with...

To replace it, I searched out a P-channel enhancement-mode MOSFET.  These
devices can be drop-in replacements for switching PNP transistors (same
polarity relationships between DGS and CBE).  The device I chose was a
Zetex ZXMP6A13F, rated at 625mW and will switch 1100mA.  Also the switching
performance was very good, with Rds(on)=0.4ohms.  I was happy to find an
up-rated device from the RN2425 too.  They are available for USD0.63 in
single quantities from Digikey.

A quick solder job (which finds my SMT skills deteriorating with age) and
the '821H was back on the air with reliable 70cm TX.

Just a further note, that today's MOSFETs are really incredible switching
devices, worthy of consideration in many applications.
ZXMP6A13F datasheet
Figure 1  Underneath the MAIN board.  Q43 is in the Closeup Area, located in the front left corner (note the e-mail excerpt refers to the wrong corner).
Figure 2  Closeup showing location of Q43, already replaced in this photo.