Dead Kenwood TM-742


Chris Petersen:
Have you ever seen this problem? I have a remoted TM-742.

Here's what works:
Power on/off - radio lights up and displays frequencies.
Change frequencies/bands
When I key the mic, the fan starts

Here's what doesn't work:
No beeps from the speaker; when buttons are pressed
No RF when transmitting (although indicator shows full output)
No Rx
No Rx noise even with squelch open
Same problem on all 3 bands

The problem comes and goes with no action on my part. I suspect that the serial data stream from the control head is not making it to the radio. Naturally, it always seems to work when I have it opened up on the bench.

I would greatly appreciate it if you have seen this problem and could pass along some clues. It's driving me nuts!

Chris, K9EQ

Charles E. Harris:
Mine wouldn't work remoted and had similar problems, but was OK when I put the control head back on the unit with the short control cord.

In my case it was a bad cord or connector, but I have also had problems in mobile installs with excessive voltage drop when things quit working intermittently. Kenwoods are not very tolerant of low voltage. If the main unit is remoted to the trunk, if the leads need to be extended over 10 feet, go up two wire gage sizes to AWG10 to minimize the voltage drop.

The AGC type fuse holders on the Kenwood cord are also a problem and the fuse connections become intermittent under high current loads when the fuse connectors get hot. If changing to a heavier gage wire anyway, keep the fuse holder on the hot lead attached to the pigtail on the radio, but replace the flimsy AGC fuse holders next to the battery with the blade type which provide a better electrical and mechanical connection.  These  are current on most cars today anyway, unless you drive an old '87 like me!

I've also had mic problems with some keys not working in remote control mode, though I could control those functions OK by using the front panel controls.  In my other 742 the mic PTT would key the transmitter, but it would pass no voice or DTMF audio.  Problem in both cases was in the internal connections from the mic cord to the circuit board, caused by poor strain relief and tugging on the cord when mobile.

In my experience over six years owning several Kenwood mobiles, currently a TM251, two TM255 and two TM742s mic connection durability is a reoccurring problem.

I now use the Gregoire headset boom mics on my Kenwood VHF / UHF radios used in the shack, freeing up their hand mics for spares!

Hope this helps.  

pse delete this profile:
Hi Chris!

I do not use th-742 remote but i have exactly the same problem with the rig. It started one day with slowly disappearing audio output during receive and a high drain from the power supply. Switching the rig off and on again helped for a while.

Then i gave the rig to dealer to repair it. But he sayed the micro processor is dead and sent me the rig back without fixing the problem.

Now it is totally dead like in your story. Did you fixed the problem?

It was an expensive but very good rig. Now it is worthless for me. What a pity....

Bernd, dl4nde

Dale Edwards:
This was sent to me it might be of help. kn6ok

Repair of the Toshiba S-AV17
(and similar) RF power module
By Kevin Custer W3KKC
Edited by Mike Morris WA6ILQ    

Note from WA6ILQ:
The Toshiba S-AV17 is a 50w high band RF power module is used in the Kenwood 241, 741 and many other radios.   Clones of it are used in a large number of other radios, both amateur and commercial (land mobile).   I have found that many of the newer Kenwood radios are being shipped with none of the white heat sink compound.   If you have the radio open for some other reason you may what to check for that...   The RF power module is supposed to be heat sunk to the chassis and /&nnbsp;or the heat sink, but without any heat transfer compound it overheats and warps, making the heat transfer even worse.   Most of the time the module lasts long enough to be out of warranty by the time it fails.   The failure mode is frequently a crack caused by overheated substrate.   Sometimes the crack can be repaired, and this repair is the topic of this article.

Photo by Tony Lelieveld VE3DWI

Kevin Custer W3KKC wrote:
I recently started having trouble with my Kenwood TM-742 that I have in my work truck with not reliably putting out power on the 2 meter band. I have had this problem in the past and I always just called up RF Parts Co. and spent the big bucks to get a new module. I always check the two PIN diodes as these are sometimes the cause of module failure (and receiver deafness). This particular radio has had this module changed in the past, and this time didn't have any PIN diode failure.

Someone posted to the Yahoogroups LMR list the ability to locate a surface fracture and hence the culprit of why the module makes no power. I decided to open the S-AV17 to see if this was the case in my module - I had nothing to lose as the module was dead anyway. I tested the transistors with the diode function of my Fluke DVM.... all read ok. The bases of the transistors are connected to ground via a choke (stripline type) so they will read a dead short to the emitter. Both of my transistors were ok, so I started looking closely at the stripline circuit (you may need a magnifying glass). Immediately I spotted what looked like a crack. This spot was located in the stripline going from the control voltage pin (#2 from left with the pins toward you) to the collector of the first transistor (the driver.) The ohm meter didn't read a short circuit, but instead looked nearly like it had diode action. I took my x-acto knife and put it directly on this spot and the meter immediately beeped constantly and the reading went to a dead short. I soldered this spot over by blobbing solder over it, or you could use a strand of very thin wire (i.e. wire wrap wire). It is somewhat difficult to solder due to two things. The substrate is laminated to the heat spreader / mounting flange and heat conduction is very good, and secondly, the solder used originally looks like it is heavy silver bearing (real shiny.) I probably should have used silver bearing solder, but the tin / lead was convenient. I replaced the module into the radio using a small amount of new heat sink compound. The radio now makes full power and is no longer intermittent.

I'm uncertain who originally posted the repair possibility to the list, but I wanted to say "thanks" and remind the rest of you that it is a possibility that it may work for you too. I have had modules that toasted the transistors inside and repair of them is impossible I suppose. I have successfully jumpered a bad output transistor in the modules to get about a watt or so. Bad transistors can usually be identified by looking into the goo that is daubed on the guts of the transistor. If this looks nice and clear, with no bubbles, the transistor is probably ok. If it has a burnt spot or the goo has bubbles inside, it may be bad and the module may not be totally repairable.

BTW: The plastic lid (or cover) of the S-AV17 just simply pops off by carefully prying up on the capturing tabs.

Kevin Custer W3KKC


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