How not to build a transceiver

Primarily due to its flexibility, one of the most popular mobile rigs for amateur radio is the Kenwood TM-742A and associated models such as the 942, 741, etc. The TM-742A is a tri-band rig which can accept up to three band modules out of an available five; 10m, 6m, 2m, 1.25m, and 70cm. Interesting trivia; the MSRP in 1994 was $660. Today, clean TM-742A rigs can and do go for over $750 and rising as replacement parts and band modules become harder to find.

Every rig has its quirks, and a quirk of the TM-742A is that the 2m module is prone to failure. The 2m power amplifier is a Toshiba S-AV17 which is a set of power transistors and associated components soldered onto a beryllium ceramic substrate. Symptom of the failure is that the rig will transmit enough power to be heard on other close-by (within a few tens of feet) rigs but makes no power at the antenna. Most people just pony up the $65 and replace the S-AV17. Others have discovered that the failure lies in a microscopic crack in the ceramic that breaks one of the microstrip filter traces. The fix for this is to remove the S-AV17, pry off the plastic cover, and run a rapid thermal recovery soldering iron (like a Metcal or Hakko) over the crack area. A standard resistive heater iron will not work; because the ceramic module is designed to absorb lots of heat so the trace won't get hot enough to flow. It takes 15 minutes to disassemble the rig and 15 seconds to solder it. Thanks to Kevin W3KKC for his webpage discussing the problem and walking through the repair process; complete with photos.

(Disclaimer: beryllium is nasty stuff. You don't want to inhale it. If you're not comfortable doing this; don't have the right equipment; etc blah insert dire warnings here then pay the $65 and don't try to repair the amplifier!)

Interestingly enough, and relevant to the title of this post, is to examine why the module fails. The reason for the failure is excess heat. The stock configuration for the band modules is to have the 2m in the middle, which means that the 2m power amplifier is buried about as deep in the rig as it can be. At 50W the 2m module is also capable of the highest power output, so therefore it gets hotter than the other modules. The ceramic cracks and you get a dead S-AV17. I would accept this explanation readily enough except that every 2m module I've disassembled has had the same problem; the S-AV17 is mounted dry. Not one has used any form of thermal grease to promote conductivity into the heatsink and transceiver structural frame. This would be like a high-speed CPU being installed onto a motherboard without thermal grease; the CPU is essentially guaranteed to fail from thermal overload. This is (or more accurately was) a blatantly stupid move on Kenwood's part that has cost radio amateurs thousands (if not tens of thousands) of dollars in unnecessary repairs, replacements parts, shipping costs and downtime. Can you demand a recall of a 14 year old product?