Think I broke my scope trying to fix it. - Page 1
Forum section: electronics / beginners

author: pmcquay
author_rank: Newbie
timestamp: December 17, 2024, 03:46:34 pm
content: I recently purchased an old Agilent dso3062a with what I thought was encoder rot (on some of the encoders, the scope would change and then immediately change back when I turned one of the encoders one click, or it would double click, or nothing would happen. It wouldn't always happen, but a good amount of the time.)I followed this post:https://www.eevblog.com/forum/repair/rotary-encoder-replacement-in-bk-precision-2500-and-agilent-dso3000a-scopes/about changing them, and bought these parts:https://www.digikey.ca/en/products/detail/bourns-inc/PEC12R-4220F-S0024/4499653https://www.digikey.ca/en/products/detail/bourns-inc/PEC12R-4020F-N0024/4499644Removing the old encoders and installing the new ones went well, I just cut all of the legs of the old ones and desoldered everything afterwards. The new ones soldered in just fine.After reassembling everything, every encoder has the same problem, and some of the buttons don't work sometimes. Things seem to have gotten worse.I pulled it all back apart, and tested the flat flex cable (while connected to the mainboard, probed from the pad on the mainboard to the pad on the encoder board) for conductivity. It looks fine to me, but it could have just been in the right place for the cable to connect.I also reflowed all of the chips on the encoder board (I think this is where I messed up. I don't have a hot air station, but I've done some SMD work with just my iron, so I went for it). After putting it back together again, now turning the encoders changes the state of other buttons, and everything just goes crazy. I pulled it apart one more time and found and fixed one solder bridge, but it doesn't seem to have changed anything. I think I either cooked something inadvertently, or I have another solder bridge.My questions are:1. could it still be the cable or the connector on the mainboard?2. what other things can I check? I don't really have a lot of other test equipment.

author: jwet
author_rank: Frequent Contributor
timestamp: December 18, 2024, 07:49:20 pm
content: Responding to get a conversation going-Its likely the ribbon though this won't usually have one thing affect another. They usually fail open or intermittent.Is there a chance that a cable is mirrored (last pin reversed for first pin)- this can do weird things.You can do a lot with an ohmmeter. The swtiches are usually in a matrix to save port pins and interconnect wires. These button matrices are usually scanned at fairly frequencies with signals that come out of the logic- too fast for a DMM to see. You can buy a cheap logic analyzer on Ebay for about $10 that might help you look at these signals and see what's happening.If its any consolation, I probably ruin one in ten things that I set out to fix- don't feel bad, it wasn't working right before. There is a certain satisfaction about throwing some really tough repair mess into the dumpster.Good luck.

author: pmcquay
author_rank: Newbie
timestamp: December 19, 2024, 04:01:30 pm
content: The ribbon is not mirrored, I took pictures before, and I didn't desolder the button board side (there is no connector, its just soldered to the board)I don't really feel bad. Its a cheap old scope that I hoped to get some more use out of. If I have to pitch it I'm not going to be that miffed. I'm going to have another crack at it, I'll probably just buy anotherslightly longer ribbon cable and desolder the connector on the main board. That way it cant be the problem.Also I'd like to have a word with whoever designed this to have the mainboard connector facing the side of the case like an inch away, under the cooling fan, when the inside of this thing is 95% air. Could easily have used an upward facing connector.

author: pmcquay
author_rank: Newbie
timestamp: December 19, 2024, 07:25:55 pm
content: I tore it apart again, and found two solder bridges. One on the cable solder joint to the button board, and one on the other side of the board, between two of the legs of one of the encoders.Its now almost working. The knob to control the trigger level still does other things, primarily changing the timebase. When that knob is in a certain position, all of the other knobs appear to work correctly, otherwise they do not.I'm thinking either I cooked something, or I have another bridge that I haven't found. I've checked all of the diodes on the board, and they all appear to be good. I've compared the trigger level knob to another that has the same kind (just put my multimeter in conductivity mode and listened to the pulses between common and a, and common and b. Not particularly scientific, but it would detect a broken or permanently connected circuit).

author: thm_w
author_rank: Super Contributor
timestamp: December 20, 2024, 01:39:10 am
content: Photos might help.

author: pmcquay
author_rank: Newbie
timestamp: December 20, 2024, 12:54:56 pm
content: Here are these for a start, on the encoder side of the board, the bottom right encoder is the trigger level that I was talking about. I can get more localized pictures if there is a need.

author: jwet
author_rank: Frequent Contributor
timestamp: December 20, 2024, 02:46:10 pm
content: Likely a bridge. It doesn't take much to make a short at these impedances. Also look in ribbon connectors for debris.Good Luck.

author: thm_w
author_rank: Super Contributor
timestamp: December 20, 2024, 10:50:11 pm
content: Do a beep test on the ribbon cable, so from pin 1 to pin 2, pin 2 to pin 3, etc. making sure nothing is connected. If you haven't already. Sharp tips will help.Trace the lines from the trigger knob you are talking about, see which IC's those end up on. Then beep test for bridges there.It seems odd that it could affect another channel unless a short circuit exists. But, if nothing else is found might be time to start replacing ICs.

author: pmcquay
author_rank: Newbie
timestamp: January 07, 2025, 02:45:49 pm
content: welp. Did a beep test on the ribbon, traced some of the traces back from the encoders and didnt find broken traces or bridges or anything. Bought a cheap hot air station and removed all of the IC's, checked for anything underneath them, put them all back (turns out hot air makes this way easier!), checked again for bridges. Tested all the diodes with my multimeter. Aaaand its still broken the same way.I'm thinking this is an issue with one or more of the ICs here. I don't really have time or energy for parts shotgunning this. Perhaps I'll wait until I need a scope again and either try again or just snag something else cheap.