Products > Test Equipment
A broken HP 33120A
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vaualbus:
ya is white as I suspect because when I remove the vacum displat to check voltage on the front panel I saw two cracks.
Now I need to find a new display vacum tube. It seem hard to find.
vaualbus:
IU 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 meseard them using the gnd as ground I meseare +15 and -15.
The strange think is that all the bypass cap are grounded using the agnd.
What is going on?
What I should use as ground agnd, or gnd?
(The worst think could happen is that near the caps that burn and that have the demage the pcb, the two ground plane have shorted).
Anybody can meseuare the resistence between GND and AGND?
vaualbus:
So i find somethingh strange; 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 volatages 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 explnation 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).
I'm right? It seem a good explanation to me.
Best regards, ALberto
vaualbus:
10 years later and I still am perplexed on this! I still own the instrument and never repair it. I burned like 3 or 4 DACs. I still thinking that the grounds should be the same but again have no way to check it! (I now own way more interesting stuff anyway  :box:)
Inherently enough in schematics I see that the grounds are connected at JM1006 but that part seems no where to be found in the PCB layout. What I think is that the part is the big middle mounting hole where the plastic insert is mounted.
HighVoltage:
Wow, it has been 10 years, time flies fast, crazy!

What if you connect the grounds to bring them all to the same zero level?

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