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Electronics => Repair => Topic started by: dexters_lab on November 28, 2009, 05:28:08 pm

Title: Oscilloscope repair
Post by: dexters_lab on November 28, 2009, 05:28:08 pm
i have an old Gould 1421 Analog/Digital (circa mid 80s) scope thats gone faulty on me and wondered if someone could suggest an area of it's internals that would be a good place to start looking. Thankfully i do have a schema.

The problem is on the display, on both channels the trace seems to fold back into a central point on the crt that never moves. Looking past the funny visual i can see a signal being displayed and the storage side all seem to work so i suspect it's something with the crt driver side.

I have a short video of the fault here: http://www.mr2.net/media/gould1421fault2.wmv
Title: Re: Oscilloscope repair
Post by: qno on November 29, 2009, 09:55:35 am
It looks to me like some kind of power supply problem.

It looks like the x and y amp break down resulting in the beam ending up in the middle of the screen.

Because this is both x and y I would start in the power supply. Look for oscillating or constant switching on off.

The problem could be a capacitor that has lost all capacitance or something simple as a bad connector or a broken PCB track
Title: Re: Oscilloscope repair
Post by: Neilm on December 02, 2009, 07:29:12 pm
It is worth just reminding you that any CRT has some high voltages in it. So be careful when probing around and make sure that any caps are discharged.
Title: Re: Oscilloscope repair
Post by: dexters_lab on December 09, 2009, 08:03:44 pm
thanks guys, will check psu and see. It's certainly old enough for bad caps.
Title: Re: Oscilloscope repair
Post by: qno on December 10, 2009, 12:40:06 pm
Also check for broken power resistors.
Title: Re: Oscilloscope repair
Post by: walterdelbono on December 10, 2009, 09:45:36 pm
awesome video...

good luck...
 
:)