Another TS-950SDX problem

[RX3AFV]

My 950SDX worked perfect but suddenly appeared short (2-5 sec) brakes in TX. They appeared more and more often. Finaly the rig stop to switch to the TX mode. Rig does not switch to the tx mode even when I push TX knobe, it continue to receive signals but with some changes in sound. Seems part of rig switched to TX, another part continue to RX:) Sure, defect is not serious but do not know, where I have to find it! Maybe somebody know this very interesting rig well?

[N1GPT]

It's very hard to diagnose these kinds of problems remotely: you really need to see the problem first hand in order to analyze the symptoms properly, because very often when someone just tries to describe what's wrong with their radio, they leave out many details which they decided weren't important. (For example, you didn't tell us what modes you tested. Did you try SSB only? What about AM, FM or CW? Is the behavior the same in those modes or different?)

BTW, if you don't already have it, I recommend getting a copy of the TS-950SDX service manual. You can find one here here:

http://people.freebsd.org/~wpaul/ts-950sdx/firmware/TS-950SDX_Service_manual.pdf

Oh, also: it would be interesting to know the first 3 digits of the radio's serial number. It should be on the back of the rig.

As it is, I can only offer a guess. One common problem with the TS-950SDX seems to be a failure in the RX bus and TX bus voltage switching due to poor solder joints on the TRX unit. The problem is described in detail here:

http://www.voodoo-labs.com/index/sdxmods_data/trxmod.htm

The RX and TX voltage buses (marked as RXB and TXB in the manual and on various boards in the rig) provide power for various parts of the radio depending on whether you're receiving or transmitting. The RXB line should be 0 volts when transmitting and +15v (I think) when receiving. The TXB line should be 0 volts when receiving and +15v when transmitting.

Apparently there's a fair amount of heat that builds up in the area of the TRX board, and some of the parts are not very well soldered from the factory. This causes the solder joints on some of the parts on the TRX board to become progressively more intermittent.

I didn't have this particular problem with my radio, but I re-flowed the solder on some of the parts as a precaution.

What I would do is find a place where you can test the RXB and TXB voltages with a volt meter. In particular, find one of the TXB test points and test it while pressing the 'SEND' button on the front of the radio. It should switch from 0 to +15v. If it doesn't change, then the RXB/TXB switching is most likely your problem.

The only thing is that the TRX unit is mounted on the control unit, which is right behind the face of the radio and which has many plugs connected to it. You need to get the control unit out in order to re-flow the solder on the TRX unit, and that can be a bit of a challenge. Just go slowly and be careful not to damage any of the plugs.

If that isn't the problem, then the issue could be more complicated to track down. There are different parts of the radio that need to be switched on or off depending on what mode you're in (AM, FM, LSB, USB, CW, FSK). Some of this is done by the main computer in the radio, but there is also some switching logic on the signal unit (X57-4130-00). There are some small digital switching transistors on this board that handle this logic. At one point I was attempting to analyze the FM squelch circuit (which is also on this board) and I managed to zap one of the switching transistors. This put the rig into a weird state where it was stuck halfway between modes. Luckily it's easy to get replacements for these transistors (I got mine from DigiKey) but figuring out which one is bad can be tricky.

I would start by checking for the RXB/TXB switching problem first.

-Bill, N1GPT

[KI6USW]

Copy down all of your settings and then give the radio a full reset.
If that doesn't work and if you are unwilling to fix it yourself; then it may need to be sent off to the repair shop.
Very nice radio. . .