TEK TDS 320 ch2 trace is off after 2 minuts of POWER ON

[jalves]

Hi,

I bought a used Tektronix TDS320 and it worked well from the box.

I was happily learning how to set it up to "look" over a video PAL signal (I am trying to implement a video telemetry for my hobby) using dual trace when suddenly the channel 2 trace oscillate and goes up in the display and disappear, the triggering for this channel becomes inoperative and the channel useless.

Now, after a power off and some hours of rest, when I turn on the Tek the channel 2 is operative as usual, but only for about 2 minutes, after that it goes away.

Channel 1 works (so far) with out problems.

Running some diagnostics in the scope, it yields 6 Acq errors:
 
diagAcq_fisoShortPipe
diagAcq_fisoCell
diagAcq_peakDetectAcq
diagAcq_holdoff
diagAcq_ch2Trigger
diagAcq_ch2SigPath

I open the scope only to find a very clean boards and little dust inside, look for bad caps but couldn't see any that look like in the need of replacement.

I hope there are some Tek TDS scope users that can give me an hand, because I am clueless about what to next.

[tom66]

My bet would be a cold joint on one of the analog supplies for that channel. (If it's a very low noise unit, it'll use one supply for each channel.) I had a HP 54501A which would do a similar thing, except all the traces would go off screen and the unit would reset/power cycle after some time. The -12V was dropping out due to a bad solder joint on the power supply connector.

[jalves]

in this case CH1 works flawlessly.

CH2 work for some time and then the trace goes up and disappear.

It seems the logic unit of the scope is receiving a erroneous level signal from CH2 input stage.

The question is how can I debug this input area of the scope, this is a "hybrid" ceramic board TEK US 165 2500
http://www.qservice.tv/N_PICS/PICS_TEK_SPARES/ALL_8/EX_LARGE/DSC02534.JPG

[jalves]

The problem is solved!

You can read here how: http://www.edaboard.com/thread271343-2.html

Thanks.