| 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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