HP1741A mains humm - Page 1
Forum section: electronics / repair

author: uvos
author_rank: Newbie
timestamp: November 18, 2024, 07:15:56 pm
content: 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 preamp3. Dose not change at all by changing the channel4. Dose not change at all by operating the "mag 5x" button1. 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 point5. 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) 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.

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 19, 2024, 03:47:20 am
content: You diagnosed almost everything by yourself.The noise is 50 Hz and superimposed on the high voltage output stage.Do you have other diagrams?

author: uvos
author_rank: Newbie
timestamp: November 19, 2024, 11:47:32 am
content: 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

author: doktori
author_rank: Newbie
timestamp: November 19, 2024, 02:38:31 pm
content: Could something be producing a magnetic field near the scope. Maybe try it in a different location.

author: ondraN
author_rank: Regular Contributor
timestamp: November 19, 2024, 02:45:11 pm
content: There could be a faulty filter capacitor in the power supply.

author: jdragoset
author_rank: Frequent Contributor
timestamp: November 19, 2024, 05:17:11 pm
content: Is it possible that the 50Hz is from a change in the magnetic shielding of T1 or the CRT?

author: uvos
author_rank: Newbie
timestamp: November 19, 2024, 07:04:02 pm
content: Quote from: ondraN on November 19, 2024, 02:45:11 pmThat was my first thought too but unfortunately i have checked all the caps and the rails in the low voltage PSU and all the rails are clean and the caps mesure fine on the lcr, well besides the ones i replaced already with no improvement to the hum.Quote from: doktori on November 19, 2024, 02:38:31 pmThats an interesting idea, but unfortunately moving the scope to another room made no difference.Quote from: jdragoset on November 19, 2024, 05:17:11 pmSo T1 runs at 30 ish khz not mains frequency, so it cant be that. The unit has not seen any physical shock or anything recently, the shealding around the crt is firmly in place.

author: edpalmer42
author_rank: Super Contributor
timestamp: November 19, 2024, 08:05:37 pm
content: One thing you could try is to go through the scope and give the small transistors and ICs a good wiggle. Although they look like they're soldered in, some, if not all, are actually in single-pin sockets. Grab them and move them around to clean the connections in the sockets. My 1744A was dead one day and that was the fix.Obviously, do that with no power connected.Ed

author: ondraN
author_rank: Regular Contributor
timestamp: November 20, 2024, 01:01:57 pm
content: 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.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 20, 2024, 09:43:16 pm
content: 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 !

author: uvos
author_rank: Newbie
timestamp: November 20, 2024, 09:55:00 pm
content: Quote from: ondraN on November 20, 2024, 01:01:57 pmAs 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 pmI 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.

author: ondraN
author_rank: Regular Contributor
timestamp: November 21, 2024, 01:42:32 pm
content: 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

author: uvos
author_rank: Newbie
timestamp: November 29, 2024, 11:00:29 pm
content: 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.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 30, 2024, 03:12:54 am
content: That would have been a head scratcher!Good job