Hitachi V-212 choppy CRT display - Page 1
Forum section: electronics / testgear

author: tsjwang
author_rank: Newbie
timestamp: March 14, 2024, 03:03:14 pm
content: Hi all,After plugging in a jigsaw / vacuum cleaner into the same outlet as my oscilloscope, things were never the same.My oscilloscope now has a chopped line on it's display.It seems to be that the CRT only lights up at 60 Hz (or some harmonic of it).This problem becomes more obvious the higher the time/div. If I measure high frequency signals, this isn't very noticeable.When I put the oscilloscope on line trigger, the dotted line stabilizes.It seems that my Horizontal circuit works fine (I can change time/div and it works properly)It seems that my Vertical circuit works fine (I can change amplitude and it works properly)I don't have much experience with CRT displays, so I'm asking for help.Here is the schematic to my scope, I have been looking at V-222 sectionshttps://xdevs.com/doc/Hitachi/Hitachi_V212_Oscilloscope_Service_Manual-Hitachi_V212_Service_Manual.pdfHas anyone had a similar problem before?Where should I start probing to debug?I think my heater/filament circuit may be at fault? Or maybe it is my Horizontal circuit? Perhaps some rectifier is not working properly so I get this 60 Hz choppiness?Please see attached images

author: tsjwang
author_rank: Newbie
timestamp: March 14, 2024, 03:05:38 pm
content: one more photo...

author: tsjwang
author_rank: Newbie
timestamp: March 15, 2024, 02:37:34 pm
content: SO I had some time to do some debugging.I just learned about Z amplifiers and blanking. It seems like the problem is there.I followed the guide and did some tracingIn the oscilloscope image to the far left of the manual, I should be expecting a 20us period signal (50Khz)But instead I'm seeing a 120 Hz signalThe two images with the red lines over them, instead of sine waves I see jagged lines at every period, then a flat line

author: tsjwang
author_rank: Newbie
timestamp: March 15, 2024, 04:42:41 pm
content: I think I've got it!The 120V input to the schematic in the previous post is not outputting a perfectly flat 120VDC. It's got that 120 Hz rippleI should have suspected it being a power supply related issue, being that something broke due to me plugging in a jigsaw to the same outlet as my scope.It seems that C1102 , a 47uF 250V cap, is bad.it's reading ~4uF (all over the place) and a 33% voltage loss.I'll keep you updated if this is the fix.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 15, 2024, 04:52:11 pm
content: The top half of that schematic is to generate highish voltages. If you are seeing 120hz there the most likely reason is that it is getting powered only intermittently at 120Hz. Given that you are in the USA with 60Hz AC supply I'd guess the smoothing capacitor(s) after a bridge rectifier are shot.Edit: I typed the above paragraph before seeing your post with the cap being tested. Its ESR is way too high too. Dollar to a dime that cap is at least part of the problem.

author: tsjwang
author_rank: Newbie
timestamp: March 15, 2024, 08:13:57 pm
content: It surely was the cap!I just replaced it. Works as it should now.Hopes this helps someone else out.As always, it's was the power supply caps

author: ebastler
author_rank: Super Contributor
timestamp: March 15, 2024, 08:26:52 pm
content: Thanks for sharing!I will definitely make a mental not of this. I still have the very same scope in the cupboard -- for occasional X/Y use at good speed and resolution, and because I bought it new as a student, spending a lot of hard-earned 1980s money on it.