Mobilinkd interface with TM-942A / Shack

A while back I bought the excellent Mobilinkd TNC - a device that interfaces amateur radio with smartphones and PCs via Bluetooth.  There are a ton of useful applications for this, not the least of which is being able to place my radio where I want it, and not be cable-tethered to it.  I've used the Mobilinkd TNC with a Kenwood TH-F6A as a mobile device, and that worked very well - I can leave my radio and the TNC in my backpack and type messages on my smartphone.  If I'm doing something mobile/portable like parade/race support, I put the radio/TNC and a battery pack up where it can get good reception and use my smartphone or PC as the interface - as long as I'm in Bluetooth range, I'm good. 

I've long been a fan of the old Kenwood TM-742A and TM-942A tri-band mobile radios.  I own several of them, and have become fairly well-versed in the art and science of repairing them.  I've always been curious about a somewhat unique feature of the radio, which is the Receive Data (RD) line on the microphone 8P8C connector - it's a direct feed of the receiver audio intended for packet radio that bypasses the final audio shaping/amplification stage, with a 100 mV pk-pk signal across a 10k load. 

I run a TM-942A (which is just a TM-742A with the 1.2 GHz band module in third slot) in my shack, which is interfaced to an MFJ-1263 microphone switch.  I thought it would be interesting to wire the Mobilinkd into the MFJ-1263 switch, so I could use the TM-942A for audio or APRS/packet at the flick of a switch.  The RD line makes this really simple to wire up.  Critical for APRS/packet applications, the RD line is tapped off before the squelch and CTCSS circuits - so I can monitor the audio for debug, or mute it as needed, without having to worry if the squelch and CTCSS settings are screwing with the RD line.  (Of course, this requires me to run DCD on the TNC software, otherwise it will never transmit while it sees the channel as "busy".)  Volume control also has no effect. 

I started off by ensuring that the MFJ-1263 was jumpered properly to bring the TM-942A connector out to the switch's front panel 8P8C jack without re-ordering the pins.  I put this onto the switch's A-side, and left my voice microphone on the B-side.  I had previously built a 3.5mm (1/8th inch) TRRS cable with breadboard pins soldered to the ends for my test kit.  I also have an 8P8C breakout board in my test kit.  Both pieces were jumpered together on a small breadboard.  Once I had the wiring confirmed, I made the cable permanent.  It might sound like overkill to proceed this way, but the TM-942A's microphone jack also has an 8 VDC (@ 100 mA) voltage source, and I wanted to make sure I wasn't injecting a voltage into my TNC. 

I'm really happy with this set up - I can use the Mobilinkd TNC at home, or take it mobile by simply unplugging the TRRS connector and USB power cable.  My Mobilinkd stays charged via USB when it's in the home configuration.  My only gripe is that the Receive Data line from the TM-742A is a fixed level which is slightly lower than the Mobilinkd's software wants to see, but it doesn't appear to be affecting decode on APRS. 

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?