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. 

Persistence

A few years ago I started experimenting with microphone audio processing as a way to way to improve my HF signal.  I live in a fairly dense suburb and haven't been able to put up a tower, so running >100 watts isn't really an option.  Speech processing such as that described in this article seemed like a good idea.

I started out playing around with a PC app called Voice Shaper by Alex VE3NEA.  (YouTube demo of Voice Shaper.)  This worked fairly well, had all the features I needed such as RF envelope clipping/limiting, compression, and equalization.  The only downside to Alex's app is that there's a digital processing delay, and I like to monitor my transmitted audio with headphones so I can detect if there's any distortion or RFI on the signal.  Voice Shaper's delay was enough to send me looking for other solutions.

Reading around I found that a few hams are using equipment like Mackie tabletop mixers, parametric equalizers, etc.  The one that caught my attention was the dbx 286a, a rack-mount microphone processor for studio work.  I found one used for a decent price and figured I'd be on the air no problems.  As it turns out, I was starting a journey of discovery which would teach me a lot about RFI, filtering, ferrites, and ultimately signal impedance.

I made up an adapter cable from the mic processor to my Heil Pro-Set Plus and another to the mic input on my Kenwood TS-2000 and started testing on the air.  I quickly discovered that while some bands were OK, others were causing some RFI on the transmitted audio.  Depending on which antenna I used, the RFI ranged from barely noticeable to so bad that it blocked my audio completely.  A couple of cases were so bad that even keying the radio caused RFI feedback which continued until I unkeyed.

So I started experimenting with adjusting audio levels, adding ferrites, grounding and tuned grounding, etc.  I found that I could clear up some bands, but others got worse.  After a lot of work I was able to get most bands working, but it bothered me that I couldn't get all of them to work.  The question haunted me, and the dbx processor sat near my station unused, silently accusing me of being an idiot.  Why won't it work?  Other people had clearly made theirs work.  Was I just doomed to wander the earth for the rest of my life in search of a solution?

Every few months I would get an idea and try again.  Modern HF radios use BALANCED inputs, and so I went through and made sure that nothing was pulling the differential pair to ground.  I didn't find anything, but at least I had eliminated that as a possible cause.  Maybe I had faulty bought equipment?  I tested using some other audio gear and found that the problem shifted around; some bands got better, others got worse.  I gave up for about nine months after that.

My most recent attempt proved to be the solution.  I had been listening to Ham Nation while driving and Bob Heil was talking about ground loops.  Something he said made me realize my mistake.  He was talking about balanced microphone inputs (already knew that) but he also mentioned that most radio microphone jacks are expecting a LOW LEVEL audio signal, whereas the auxiliary in port usually wants to see line level signals .  Clearly the mic processor was emitting a line level signal.  What if I connected the mic processor to the AUX IN port on the radio?  This proved to be the solution!  Not a bit of RFI on any band, or on any antenna.

It's a little embarrassing to admit that it took me so long to figure this one out, but I'm really pleased to have finally resolved this one.  I'm looking forward to finally getting some use out of the mic processor I bought!

TK-890 Amateur Radio Mod

Over the past weekend a friend of mine asked if I would help him convert his Kenwood TK-890 mobile to work on the ham bands.  I wasn't sure how successful we'd be, since most every online search came up with at best little information or at worst flat out statements saying "Nope, can't be done."  As it turns out, it can be done.  Kudos to Tim K for his notes posted to Radio Reference which gave enough hints to make this happen. 

In general this is how it went.  My friend wanted his radio to work on the Bay-Net repeater system, which operates 443.225 with a +5 MHz TX split.  TX was fine, but RX was giving a steady "beep-beep-beep..." which indicates PLL unlock.

In the PLL section, under the copper foil, are three adjustment pots: A = TC302, B = TC303, and C = TC301.  (Don't ask why they're out of order.)  According to the Service Manual, Pot A sets the PLL for the low end of the receiver range, Pot B sets the high end of the receiver range, and Pot C sets the TX PLL.  The goal is to monitor test point CV with a voltmeter and adjust for minimum voltage during RX and TX.  This requires re-programming the radio's test frequencies to match the band of interest, so you'll need the KPG software and cable. 

Once we had the PLL voltages minimized for RX and TX, I found that the radio's TX frequency was way off, so a frequency alignment was needed.  This again required the KPG software - for some reason we couldn't get the radio into Panel Test/Tune via the control head.  It was easy enough with the KPG, once we realized you need to press "Enter" to lock the modified value. 

Other things like adjusting the BPF and checking deviations should be done.  In the end, the conversion was very easy and the radio is working well on the UHF amateur band.

Pimp My Rig

During the recent severe storms we lost power at home a few times. I had my radios on battery power monitoring various emergency frequencies but at times I was struggling to read some of my equipment with non-backlit displays. A small LED flashlight worked well in a pinch, but afterwards I started to think about how I might improve the lighting at my station without using wall-socket power and/or consuming a lot of my station's reserve battery power.

The answer came to me while I was wandering around a Target store. There's a company called WinPlus that makes a line of auto accessories under the brand name "Type-S"; they offer a lot of dashboard gizmos, LED map lights, gadget holders, etc. Their stuff is actually quite well-conceived, for the most part. One of the items they sell is a "Dash Mood Light Set" which consists of two small "LEDs in a barrel" with 3M adhesive-backed swivel-mounts and a "cigarette lighter" accessory plug. Setup was very simple; all I had to do was plug in the lights (using an accessory socket) and stick the swivel-mounts on the radio faceplates. The lights draw less than 10 mA of current and provide great illumination of the displays even in total darkness. The wires are very slender so I was able to tape them down with black electrical tape and now they're essentially invisible.

TS-2000 Remote Control w/ Audio

One of the fun but also useful capabilities to have in your shack is the ability to run your rig remotely. I decided to do this for my Kenwood TS-2000 since it's basically a radio wrapped around a computer and has a zillion interface options. Kenwood makes ARCP-2000, a remote display program which frankly isn't worth the $250 they ask for it. Ham Radio Deluxe (aka HRD) is a great program; it's feature-rich, highly configurable, and best of all free. You can run it over a screen-scraper like VNC, Remote Desktop, or even X-Windows. A better solution is to use the HRD remote access system; this essentially allows you to tunnel serial data to your rig and control it via a remote instance of HRD.

Two challenges exist here; publishing the HRD server over the Internet without open router firewall ports, and creating a high-quality and stable audio path.

Dealing with the server question first; I absolutely do not recommend opening holes in your firewall. There are a lot of solutions available which eliminate that need. I use Hamachi which has an additional benefit in that the clients are coordinated via a central server so a static DNS is not required. I simply point my remote HRD instance at the private IP assigned to my home system by Hamachi and I'm connected.

Getting quality audio across the Internet used to be a challenge, but after using Skype this year for business I think it's ready for prime time. I created a separate Skype client ID intended only for receiving inbound calls from me when I'm remote. I set Skype to auto-answer, and point the audio paths to my rig's sound interface.

Detailed diagram of my setup is available here.

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?