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A broken HP 33120A

Question asked by vaualbus on Jun 10, 2014
Recently I've bought a broken HP 33120A that have no output.
I've powerd up, as it was sell there no output but after a while it blows the fuse.
Open the instrument I've found a cuple of burned  tantalium caps, a shorted diode CR704 and a broken diode CR803. I replaced all and now the instrument turn on again.
Unfortunally during the repair somehow I've broke the vfd display.
Anybody know where I can find on of this display, it is mark obsolate on the agilent store? Anybody in this forum can sell it to me?
By the way the instrument is still have not output.

I have discovered something very very strange.
So If I measeaure the +/- 15V of the output amplifier using the agnd as ground I meseaure -20V an +9
but If I measured them using the gnd as ground I meseare +15 and -15.
What I should use as ground agnd, or gnd?
Anybody can meseuare the resistence between GND and AGND?
Apparently the two ground are not isolated, there about 800ohm of resistence between.
If I meseaure the voltage across the grounds it reads, a magic number, -5V. A little bit of math with the previous measaruments explain the solution.
-15V + -5V = -20V, 15V + -5V = 9V. So the voltages I read was right the only problem is that the ground is not a zero volt.
I think this is not normal, so looking for a explanation the only component in the entire instruments that are betweeen the grounds, is the semi custom DAC.
(Fortunally it seem to be avaible in ebay).
In spite of trying my idea, I've remove the DAC but the the problem isn't solved.
What is going on?
Apparently there is +/- 5V of difference If measaure the voltages (+15/-15) using AGND instead of GND.
What I should do?
Best regards Alberto.  

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