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.

Scott Townley NX7U Gilbert, AZ DM43di mailto:nx7u@arrl.net http://members.cox.net/nx7u