Electronics > Repair

HP1741A mains humm

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uvos:
I have a HP1741A scope that has been in my primary scope for over a decade. Recently it has developed some mains hum, that i cant seam to figure out the cause of.

So the hum is fairly small and only y+ direction (see image) and has the following properties:

1. only exists in vertical direction (no hum in x direction in xy mode)
2. Dose not change at all by changing the amplification of the vertical preamp
3. Dose not change at all by changing the channel
4. Dose not change at all by operating the "mag 5x" button
     1. this changes the amplification factor of the final amp before the defelection plates (see vertAmp.pdf)
     2. since this dose not increase the amplitude of the hum it cant be introduced before this point
5. the hum exists in storage and in conventional mode.

Trouble shooting steps already performed:

1. I checked all voltage rails and found the 153V and the +100V rail to have high ripple (caps that went high ESR)
     1. Unfortunately changing those caps eliminated the ripple but this didn't change the behavior at all.
     2. all other rails look fine, and i checked all the psu's caps with a lcr meter for good measure.
2. I scoped the vertical deflection plates with another scope (had to build one out of the display section of a 809-2a spectrum analyzer for the occasion ;D) and the hum is not there either.
3. I disconnected the flood/erase guns of the storage tube - no change (well besides storage not working)

at this point i am at a loss, maybe someone has some ideas, the service manual is readily available from key sights website.

Harry_22:
You diagnosed almost everything by yourself.
The noise is 50 Hz and superimposed on the high voltage output stage.

Do you have other diagrams?

uvos:
Sure attached is the hv psu schematic.

Unfortionatly i lack the equptment to probe any of the higher voltages there.
Im also not sure how mains hum could appear here and somehow only affect the beam in y direction, seams really strange to me.
Im am admittedly also not very versed on crts.

The whole sevice manual can also be grabed here:
https://www.keysight.com/mx/en/assets/9018-05945/user-manuals/9018-05945.pdf

doktori:
Could something be producing a magnetic field near the scope.  Maybe try it in a different location.

ondraN:
There could be a faulty filter capacitor in the power supply.

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