Tektronix 2235 AN/USM 488 - Diagnosis Help - Page 1
Forum section: electronics / testgear

author: WhatsICC
author_rank: Contributor
timestamp: May 18, 2023, 06:17:42 am
content: Hi All,I've picked up this military version of the 2235 for my first scope...It was $200, probably waaay overpriced but there's no real used market in my area, and a better price than the cheapest brand new rigol or siglent DSO. Plus im really only using it for audio at the momentI got it from an electronics surplus store not too far from me that had a bunch from a military base somewhere nearby. It seems to be in great condition, besides the horizontal knob having a bit of a crack and the radiation mesh on the outside having a slight tear in it... id say im a noob but that's an understatement. Forgive me if this post is frowned upon, I didn't find too many topics specifically on the 2235 or USM488Everything seems to function as normal, both channels trigger in auto and norm. On the calibration point, it seems like it's hitting 500mv spot-on at .1v/div, the horizontal calibration seems a bit off but that's expected it doesn't look close to 1khz.However, there seems to be quite a lot of jitter in the traces, especially with the intensity turned up (this can be seen at 2:25 in my video.)When adjusting the vertical position the traces start to veer off "slightly" at the very top and bottom much worse at the bottom not as noticeable at the top. Could these be from the 2235 power supply issue I've read about? I'm also sure its got a few Rifas that need to go but it's working as of right now..Should I see if he'll swap it out for a different unit or is this one just a few tweaks away from glory days? I'd rather not, I'm not sure if this is the type of place that does returns at the least hopefully swap it out, he has 10... but if it is too far gone then it's worth a shot...I've included pictures and this is a quick 3 minute video:Any help would be great, besides the obvious return it for a DSO I feel like id learn much more from an analog.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 18, 2023, 11:42:41 am
content: While not directly addressing the issue you have, I have a video on my youtube channel on upgrading a bunch of things in these scopes that might help out a bit, especially on the PSU side of things.https://youtu.be/lrXCVg6T-ek

author: WhatsICC
author_rank: Contributor
timestamp: May 18, 2023, 05:03:23 pm
content: I actually found your video on a previous thread and had it saved!I've begun watching it, I think I may just tear it down, swap the Rifas out and at the least all the PSU electrolytics.However, If anyone out there has some insight into my funny trace behavior that would be amazing.

author: Old Printer
author_rank: Frequent Contributor
timestamp: May 18, 2023, 05:30:03 pm
content: One of thew things I like about Tek scopes is the great service manuals they wrote. I have a 2225 and a 475 and the first thing I did was buy the manuals. Artek has high res scanned copies that are usually searchable and in pdf format. I just looked and both the operator and service manuals are available as a download for $7.00 total. You might find them free on the web but often the scans are poor, particularly the schematics.

author: WhatsICC
author_rank: Contributor
timestamp: May 18, 2023, 05:41:56 pm
content: Awesome!!! Very glad you mentioned that. I didnt know about Artek, I found some free manuals on the TekWiki but havent downloaded yet. If they look rough ill absolutely go the Artek way. Thank you very much:)

author: LarryR
author_rank: Contributor
timestamp: May 18, 2023, 06:05:26 pm
content: You can find a lot of info on the Tektronix group at:https://groups.io/g/TekScopes2I own a Tek 2230 digital storage scope and it's a solid piece of equipment.I also just bought a Hantek DSO2x1x to play with (for the protocol decoding) - there has been quite a bit of hacking info provided on this forum.What surplus store did you get your scope from?

author: WhatsICC
author_rank: Contributor
timestamp: May 18, 2023, 06:14:39 pm
content: Thanks! I heard so much about the Tek Yahoo Group until they did away with groups, ill make good use of that link.The store is - Electronic Surplus Industries in North York.I had to drive a little over an hour from where I am, but this store is just filled top to bottom with testing equipment. Barely any room to walk (in a good way haha)... you can spend hours upon hours browsingI'm excited to own this 2235, I've read mixed reviews about it but if i can get the traces dialed in ill be more than happy. I hope its just my probe compensation ill have to go out and fiddle with it some more but I dont think it is

author: David Hess
author_rank: Super Contributor
timestamp: May 18, 2023, 06:24:44 pm
content: The screen over the CRT is for electromagnetic shielding and was an option on many Tektronix models. I think if you remove the CRT bezel, then the screen can be easily removed.My guess is that the power supply voltages, including the acceleration voltages, are sagging under load. When the beam is at either vertical extreme, one side of the differential output stage draws more current through the load resistors.If this is the case, then it should show up with a voltmeter measurement as the beam position is changed and the intensity is adjusted. The service manual gives details about checking the power supply voltages.Before getting too involved, run the same tests on the 2nd channel, and with DC coupling selected instead of AC coupling.

author: WhatsICC
author_rank: Contributor
timestamp: May 18, 2023, 07:23:47 pm
content: I did notice in the service manual the screen does look removable, I'm thinking I may do that at some point.I'll run the same tests with DC coupling and see if there's a difference.If not, I'll look into the manual for checking the supply voltages and go from there.I appreciate the tip. Thanks!

author: Old Printer
author_rank: Frequent Contributor
timestamp: May 18, 2023, 08:19:43 pm
content: The 2200 series were solid scopes and mostly repairable by average people. Later on Tek scopes got pretty involved with hybrid IC's and stored calibration info dependent on built in batteries. Parts for repairs can be a problem, but Tek made a bunch of the 2235's and parts mules are readily available if you can swallow the shipping. When I got my 2225 the front control pots were badly worn and jumpy. I was able to source a late model board from Qtech in Greece and for $50 and a couple hours work my scope was like new..except for the aging electrolytic's that will start to go one of these power ups, but that's not a big deal. Hope you get yours humming.

author: Arts
author_rank: Regular Contributor
timestamp: May 19, 2023, 02:06:53 am
content: Quote from: WhatsICC on May 18, 2023, 06:14:39 pmMy own 2235 started acting up a while back. It has always been a great little summer-scope ( I use my ancient Tektronix 547 as my combination winter-scope/shop heater) so I decided to give it a proper tune-up; all of the electrolytics, IEC line filter and associated caps, focus resistors, delete MOSFET connector etc. Good as new nowAll you need to know is right here:https://w140.com/tekwiki/wiki/2235That store sounds like a place I'd like to visit, but it would be a bit of a drive from Montreal..

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 19, 2023, 05:03:21 am
content: My scope is rock solid and great to use, it had some problem with the focus due to drifted carbon composite resistors (a common but easily resolved issue) but after the upgrades in my video linked above, it's working great.It sits just at a nice point between simplicity of use and good specifications for day to day use.Re. Arkek, the owner recently passed on so his wife and other staff are picking up the pieces, so while they are still operating AFAIK, be understanding if there are delays on delivering the manuals.

author: WhatsICC
author_rank: Contributor
timestamp: May 19, 2023, 04:32:07 pm
content: Quote from: David Hess on May 18, 2023, 06:24:44 pmMuch better with DC coupling, I swore I had tried it in DC the night before and it was acting up even setting my ground point to zero then back to dc the 1khz signal was falling well below 0.However, tried again last night both channels working great in DC! However some funny business going on in the Horizontal Mode and in AC... I'll have to fiddle with it some more, I'm waiting on my signal generator.I think for now I'm gonna replace the Rifas at the minimum and do a full refresh down the line.Quote from: Old Printer on May 18, 2023, 08:19:43 pmThanks for the vote of confidence! The more I look into bringing it back into spec the less worried I get. It seems very straightforward just gotta watch those high voltagesQuote from: Arts on May 19, 2023, 02:06:53 amThank you, and for listing what you swapped! I'll make note, between TERRAs and your info I think Im setQuote from: TERRA Operative on May 19, 2023, 05:03:21 amI watched your video and am glad I did, great job with that by the way. Very informative and helped give me the confidence to bring another 2235 back to its glory days! hahaSad to hear about the Artek Owner... but, kudos to his family for stepping up

author: David Hess
author_rank: Super Contributor
timestamp: May 19, 2023, 06:06:52 pm
content: Quote from: WhatsICC on May 19, 2023, 04:32:07 pmThe difference between AC and DC coupling does not explain why the slope was changing with position and why the intensity control had such an effect. When results are weird, record the amount of weird.Using a signal generator in place of the probe calibration signal should reveal a lot more about what is going on.

author: WhatsICC
author_rank: Contributor
timestamp: May 19, 2023, 06:30:37 pm
content: When I get my signal generator I will do a more quantitative test and take record of what's going on.My first test in AC coupling was done in a quick 20-30 minutes. I'll wait for the scope to fully warm up and do some more involved testing.I am buying my signal generator from the same location, just waiting for him to get back to me with what he has, I may just do a simple online signal generator and see where it's at. If it's too far off I may see if he'll take it back and work a deal out.I appreciate the input once again, will update further with more detailed measurements

author: Arts
author_rank: Regular Contributor
timestamp: May 20, 2023, 03:03:39 am
content: Here's some of the less "generic" items I used, With Mouser part numbers's & qty's:IEC AC power entry, 1pc:https://www.mouser.ca/ProductDetail/631-FN9233-1-06HV rated focus resistors, 6pcs:https://www.mouser.ca/ProductDetail/594-HVR3700005103FR5Safety cap, 1pc:https://www.mouser.ca/ProductDetail/80-R46KN31504001KSafety cap, 3pcs:https://www.mouser.ca/ProductDetail/80-R413F12200000KReplacement for original 840µFd, 10pcs (for the price break):https://www.mouser.ca/ProductDetail/661-EKZN160E102MJ16S