Author Topic: DTV60 scope ground level unstable  (Read 722 times)

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Offline mhilton2013

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DTV60 scope ground level unstable
« on: July 03, 2016, 08:58:54 am »
Hi, this is my first post on the EEVblog forums. Hopefully my next one will be more useful to others :) .

I recently acquired a Farnell DTV60 scope but it has an issue with the Y axis. The ground level doesn't seem to be stable but moves up and down slightly and sometimes goes off  the CRT entirely for a short while. This can be observing when the channels are set to ground, DC or AC. Interestingly it seems to affect channel one more than channel two. Also I now recall that it would make a short sizzling noise on occasion when powered up. I looked inside the scope and I can't see any visible damage apart from a little leakage on the power supply's capacitors. I'm new to repairing test equipment so any pointers as to where the fault could possibly be would be greatly appreciated.
 

Online tggzzz

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Re: DTV60 scope ground level unstable
« Reply #1 on: July 03, 2016, 09:18:15 am »
PSU capacitors are a frequent source of problems.

Google for the scope's manual, and replace the electrolytics with new equivalents. Rules of thumb for aluminium electrolytics and tantalum beads are to ensure the rated voltage is double the working voltage, and for aluminium electrolytics that the capacitance is doubled. If it is a switching PSU, get a capacitor rated designed for switching PSUs, e.g. Nichicon PW series.

There are many threads on this topic.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline mhilton2013

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Re: DTV60 scope ground level unstable
« Reply #2 on: July 03, 2016, 09:22:45 pm »
Ok thanks, I'll see if I can get new caps for the PSU. I made another observation today that it seems to work fine without any issues at all for about 5 mins, then it starts bouncing around the place.

I'll post back when I have changed the caps, though it may be some time. Thanks :)
 

Offline mhilton2013

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Re: DTV60 scope ground level unstable
« Reply #3 on: July 09, 2016, 01:50:48 am »
I haven't got my soldering iron back yet so I haven't been able to replace the PSU caps but I found the circuit diagrams and probed the power rails and they seem very stable even when the oscilloscope is malfunctioning. I seem to be able to cause the trace to start bouncing around by giving the oscilloscope a sharp knock.

It's particularly annoying how this thing is impossible to take apart without desoldering lots of bodge jobs :( . Could it be something loose thats getting knocked about? CRT kaput?
« Last Edit: July 09, 2016, 01:56:15 am by mhilton2013 »
 


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