Tektronix 2246 "A sweep" issue - Page 1
Forum section: electronics / repair

author: yam
author_rank: Newbie
timestamp: June 21, 2023, 09:01:49 pm
content: Hello members,I know there are some people who deeply now the guts of tektronix scopes here so I come here to humbly ask for your help.I am currently debugging a Tektronix 2246 with several (most likely related, I think) problems about triggering and time-related things. I did some investigations with the help of the service manual but I am now in a situation I don't understand. Let's start with the symptoms first.Since the beginning I own this scope (bought used, of course) I always had issues with triggering. Initially, no message was displayed. I just kept using the B trigger as the A was ineffective.At some point in time, I decided it could be useful to run the self calibration routine. Since then, it keeps displaying a message "TIME SELF CAL FAILED".More precisely:1/ When running the "SELF CAL MEASUREMENTS", I get "SEARCH FAILED AT SWEEP SPEED 5µs"2/ When running the DIAGNOSE routine, I get a "TIME SIGNAL TOO SMALL AT A TRIGGER":3/ As stated in introduction, when out of the service menu and back to normal operation, I get a message "TIME SELF CAL FAILED" but only when using trigger B, which works:4/ When using trigger A, I get no message but the scope never triggers the sweep:5/ Finally, in some situations, the sweep does not go at the end of the screen. I have to increase the time/div knob and decrease it again to have a full picture. That may be normal or related to the issues above. Anyways that's definitely not the most critical issuePics with .2ms/.5ms/.2ms again below:According to the service manual (which corresponds to my s/n)https://transfert.free.fr/j7pkRMw:I am currently in getting another scope to check the signal on this pin but, in the meantime, just checking with a multimeter does show some changes when the exercise routing is running and when its not.Similarly, the "TIME SIGNAL TOO SMALL" error asks to check the input signal from the ramp generators. Not done yet but I bet they do work.What I fear is that the U421 used for the A trigger is dead. Since that's a tektronix specific part (234-02-04-20), I m afraid there is no luck in getting this thing runningagain.What I don't understand is that if we consider this U421 which is related to trigger A as dead (which is consistent with trigger A not working), why am I getting no message when using it and, instead, getting an error message when using trigger B which works ?Do you think my analysis is correct besides this question?Thank you guys !

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 23, 2023, 11:04:06 am
content: I do not know much about that model.Perhaps this group can help:https://groups.io/g/TekScopes2/messagesJoin and post your problem

author: yam
author_rank: Newbie
timestamp: June 23, 2023, 07:51:56 pm
content: Thank you for the pointer. I ll have a look

author: yam
author_rank: Newbie
timestamp: July 09, 2023, 09:15:22 pm
content: A few updates: I was able to borrow a scope and I came to the conclusion that U421 (234-0204-20) was faulty. Inputs are consistent but the output (pin 19) is erroneous. Apparently (according to a post on tekscopes @yahoo), old 2246 like mine suffered and issue with this part number which was replaced by another reference later. Similarly, I had an issue with the vertical amplifier CH3 (234-0203-20) which was drawing the line for CH3 way below the screen. Fortunately I was able to find replacement parts (with different part number this time). Expected soon. Hopefuly that will solve my issues !