Electronics > Repair
HP1741A mains humm
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uvos:

--- Quote from: ondraN on November 20, 2024, 01:01:57 pm ---I would check the signal on the vertical deflection plates. This will show if the hum is from the vertical amplifier or some other source. If a vertical amplifier does it, then follow the signal from the end to the point of origin.

--- End quote ---

As mentioned on the first post i have scoped both deflection plates, the signal there is clean and i can see a signal fed into the scopes input even when the deflection caused by that signal is less than the hum, so i would for sure see the hum on the plates if it where present but it is not.


--- Quote from: lowimpedance on November 20, 2024, 09:43:16 pm ---I would check for suspect interconnections, (plenty in these scopes), and all ground /chassis screws etc are tight and no corrosion.  Rivets were used extensively in HP gear of this era and have caused issues in at least the 3456a dvm needing a service note issued to replace with screw / nut.
Just thoughts, given you have not found actual hum directly at the vertical plates !

--- End quote ---

I dont recall any rivets but i will check for that. I will also scope all the grounds of all the boards to see if there is some ground bounce somewhere due to a failed connection. The unit did have some broken solder joints in the (distant) past.
ondraN:
And disconnect vertical deflection plates, grounded them? I would try this despite checking the voltage on the plates with an oscilloscope.
Then disconnect Trace rotation coil?
And then ...I have no idea  :P
uvos:
Ok i figured it out, and its a pretty interesting fault.

So the transformer is right below the neck of the tube in this scope. One of the bridge rectifiers had gone bad, in a strange manner, essentially it became a 250ish volt zenner on one side, so the transformer saw essentially a short at the top end of the wave, creating a pulsing magnetic field that ultimately affected the beam.
lowimpedance:
That would have been a head scratcher!
Good job  :-+
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