Probe adj square wave duty cycle  - Page 1
Forum section: electronics / beginners

author: sarakisof
author_rank: Newbie
timestamp: March 27, 2024, 05:13:17 pm
content: Recently found a Tektronix 2213 in nice condition. Was working fine, just during first operation after 30' released the magic smoke. The obvious Rifa Y2 mains suppression paper cap exploded & failed open.Easy fix, took the opportunity and replaced every Rifa Y cap there (the mains exploded one & other four 2,2nF on the rectifier section) with fresh modern MKTs alongside with all the low voltage electrolytics with fresh Panasonics.Left the big aluminum Sprague filter power HV cap intact as it looked well & from my little experience they rarely fail & on every replacement they measure fine for both capacitance & ESR.All fine, oscillo runs smooth except one thing I noticed. During probe adjustment test I cannot get 50% duty cycle (top & bottoms are not equal). Note that this didn't happen before my slight repairs.Is this normal?

author: Ian.M
author_rank: Super Contributor
timestamp: March 27, 2024, 07:00:54 pm
content: I see no difference except the timebase setting.The probe adjust signal is for adjusting the probe compensation for squarest rising and falling edges, i.e. to adjust out any overshoot or undershoot. It is not required to be 50% duty cycle or have an accurate period.

author: sarakisof
author_rank: Newbie
timestamp: March 27, 2024, 07:13:31 pm
content: EDIT: The only thing I "made differently" during electrolytics replace was that I replaced C965 (270uF) with a 330uF one? I don't think it could be from that though.

author: sarakisof
author_rank: Newbie
timestamp: March 27, 2024, 07:15:27 pm
content: Thanks Ian. What could cause that before caos replacement it was 50% and then changed though? What parameter this has to do with?

author: Ian.M
author_rank: Super Contributor
timestamp: March 27, 2024, 07:56:24 pm
content: If you look closely at your first photo, its not 50% duty cycle. Replicate the timebase setting in that photo and you will have a very similar waveform, if not identical.

author: radiolistener
author_rank: Super Contributor
timestamp: March 27, 2024, 09:36:00 pm
content: Probably this is an issue of calibration source itself than oscilloscope input.

author: sarakisof
author_rank: Newbie
timestamp: March 27, 2024, 10:43:30 pm
content: Yeap I just confirmed that by using my cheapo DSO138 (had forgot that I own it). Feeding it with 2213's probe adjust output (500mV/1khz).Duty ~ 46%@Ian: yes you are most probably right. Actually I slightly remember having noticed it during first power on but then "booom" came up. Then spent a lot of time sourcing the right Y2 mains cap. Long story, local online shop had labeled "Kemet" cap using a pic of a new grey MKT kemet one, but I received NOS paper Rifa again -same with one used inside machine - so it took time to return & backorder it. Shop owner's fault, don't forget that Rifa was back then became Kemet..So yeap , I'm ready to go I guess.

author: Jwillis
author_rank: Super Contributor
timestamp: March 28, 2024, 07:49:01 pm
content: Quote from: sarakisof on March 27, 2024, 10:43:30 pmFor your ease of mind you can find out about the probe adjust operation in the service manual.https://elektrotanya.com/tektronix_2213.pdf/download.html. Specifically you want to check PDF pages 19, 31, 85, 158 and 181.The square wave generator uses a simple 741cp op amp (U535) on the front panel board. The duty cycle doesn't matter as long as the period is 1ms giving a 1kHz square wave.Used for probe compensation only. It is not used to verify vertical gain or for time base calibration.