Tektronix 2236 vertical deflection issue  - Page 1
Forum section: electronics / repair

author: bigredlevy
author_rank: Newbie
timestamp: April 01, 2022, 05:48:27 am
content: Recently, I noticed that the vertical scale on my Tektronix 2236 was reading incorrectly; showing a lower volts/div than it should be. Both channels were out of calibration by the same amount.I have been unable to correct the issue following the vertical adjustment procedure in the service manualhttps://drive.google.com/file/d/1Bov9bbMTNz2E5p89Vo06ZOQFHUtqk8Tj/view?usp=sharingFully rotating the gain pots (R145 and R195) does not provide enough vertical deflection to achieve calibration.Power supply rails are within spec, as per service manual.I haven't manage to find an EEV blog thread with this specific fault.Any help would be greatly appreciated.Note: As far as I'm aware, the 2236 is the same as the 2235 scope, but with an additional multimeter and frequency counter PCBA mounted in the chassis.UPDATE:Shorting +/- of the delay line produces a flat beam near the top of the screen. I suspect this means that the fault lies in the vertical amplifier.

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2022, 06:21:14 pm
content: How is the horizontal deflection? If the horizontal deflection is low by the same amount as the vertical deflection, then the overall deflection is low which is caused by high CRT cathode voltage or low CRT post deflection acceleration voltage. Could the high voltage multiplier be bad?Quote from: bigredlevy on April 01, 2022, 05:48:27 amI agree; the fault is after the vertical channel switch where the channels are combined into one.The usual failure is a resistor going open. If the gain is lower, then the open resistor has to be in an emitter network. Take a very close look at R233, R236, and RT236 between the emitters of output transistors Q230 and Q231.Update: Also check R218 and R219 in the delay line driver. If these drift high, then it will also lower the vertical gain.There are other things which could cause low vertical deflection, but the resistors above are the easiest thing to check so a good place to start.

author: floobydust
author_rank: Super Contributor
timestamp: April 01, 2022, 07:48:48 pm
content: I outright replace all electrolytics (and Rifa's) in the power supply, at this age- it's time and I see many that are failing in these Teks.Before going down the rabbit hole of troubleshooting a rail that is low or high-ripple etc. that so many people do, as fun as it is.recap spreadsheet for 2235, 2235AI would also confirm if H-defl is narrow as well, which would point to a CRT sensitivity issue (V/cm deflection) that happens if there are HV power supply problems. Just to narrow down what section is misbehaving.

author: bigredlevy
author_rank: Newbie
timestamp: April 02, 2022, 07:37:57 am
content: David,Quote from: David Hess on April 01, 2022, 06:21:14 pmHorizontal deflection seemed to be okay.Quote from: David Hess on April 01, 2022, 06:21:14 pmAll resistors in vertical deflection ouput section measured okay (including R233, R236, and RT236).The fault temporarily disappeared after momentarily shorting the base of Q256 and Q257.but after several minutes, the screen faded to black and has not shown any signs of life since, so yeah it might be the HV supply.Floobydust,Quote from: floobydust on April 01, 2022, 07:48:48 pmyep, I'll do a cap dump on the supply now.

author: bigredlevy
author_rank: Newbie
timestamp: October 29, 2022, 12:22:16 am
content: I did a cap dump, but the issue was not resloved.After shelving this for several months, I borrowed another scope. Using an octopus tester I discovered that one of the vertical deflection output transistors (BFR96) was failing.Replacing these transistors fixed the issue.

author: David Hess
author_rank: Super Contributor
timestamp: October 31, 2022, 12:42:26 am
content: Quote from: bigredlevy on October 29, 2022, 12:22:16 amThanks for the update. The transistors are not usually what fails unless abused.