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Stuck ISS Transmitter, MAI-75 - 15 July 2010



ISS Stuck Transmitter problem.
July 15, 2010

The Problem:

A few problems with the TX keying up there I think.
12.52utc UK pass - tx de-keyed before the 12.54utc image was complete.
14.27utc UK pass - tx keyed up half way through the 14.31utc image, the
carrier then remained up until the next image at 14.33utc after which it
dropped correctly.

David, G8OQW
***

Hi David:

Your transmitter observations are completely expected.

A problem was discovered in August 2006, during Pavels V. mission on ISS.  It appears that a last minute design change to the VOX box is causing RF on the power leads to cause the Transmitter Op-amp to get Stuck in the Transmitting mode.
A second VOX box was sent to the ISS in 2008, which also succumbed to the same RF on the power lead problem.

What should happen:

SpaceCam sends out a short DTMF tone, to start the TX session, followed by a CW ID.
The SSTV Audio from the Laptop keeps the VOX box in TX mode.
A few seconds after the end of the SSTV image, the silence will cause the VOX box to drop the Transmitter signal.

Here is what is really happening:

SpaceCam sends out a short DTMF tone, to start the TX session, followed by a CW ID.
The SSTV Audio from the Laptop keeps the VOX box in TX mode.
The VOX box picks up RF noise from the Power leads, which are attached “Inside” the specially modified Kenwood TM-D700.

When the SSTV audio Stops, the RF noise is still saturating the Op-Amp in the VOX box.  This causes the transmitter signal to be Stuck on the ON condition and keeps the Kenwood transmitting.

The Kenwood TM-D700 has a built in “WatchDog” timer, which will force the transmitter to turn off after a few minutes (usually set to a 3-minute value).

After each First-Start SSTV image, the TM-D700 transmitter may Stick on for a full three minutes or until the time value of the WatchDog timer is set for.  
The WatchDog timer will be reset when the Transmit control lead is finally released.

Based on your observations this is what is happening. 
If the Slide Show timer is set for 1 minute and the Watch Dog timer is set for 3 minutes, and Martin-1 is being used the following should be happening.

First start, Successful Martin-1 image.
Total time for CW ID and Martin-1 image, approximately 120 seconds.
The TM-D700 transmitter will stay ON, even though there is No new Image.

After a 1-minute delay, the Slide Show will start sending a second Martin-1 image.
When the WatchDog timer reaches 3 minutes, it should abort the transmitter, which will cause the SSTV image to abort prematurely.


A fix for this problem has been presented to ARISS on multiple occasions.  No action has been taken.  Volunteers welcome.


http://www.marexmg.org/documents/PSUCUSBProjectPub1.doc



http://www.marexmg.org/fileshtml/futureprojects.html


73 WF1F

www.marexmg.org


--- On Thu, 7/15/10, David Barber <david.barber@dbelectronics.co.uk> wrote:

> From: David Barber <david.barber@dbelectronics.co.uk>
> Subject: [sarex]  MAI-75 - 15 July 2010
> To: "'SAREX'" <sarex@AMSAT.Org>
> Date: Thursday, July 15, 2010, 11:05 AM
> A few problems with the TX keying up
> there I think.
> 
> 12.52utc UK pass - tx de-keyed before the 12.54utc image
> was complete.
> 
> 14.27utc UK pass - tx keyed up half way through the
> 14.31utc image, the
> carrier then remained up until the next image at 14.33utc
> after which it
> dropped correctly.
> 
> David
> G8OQW
> 
> ----
> Sent via sarex@amsat.org.
> Opinions expressed are those of the author.
> Not an AMSAT member? Join now to support the amateur
> satellite program!
> Subscription settings: http://amsat.org/mailman/listinfo/sarex
> 


      


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