TEKHIND 2235 (Tektronix India) - Page 1
Forum section: electronics / testgear

author: abhi1981
author_rank: Regular Contributor
timestamp: July 05, 2023, 07:12:01 pm
content: Hello,Just got aDEAD (Tekhind) Tektronix 2235 scope.Nothing happens when I plug it in. The power led doesn't light up.No visible damage other than some burnt rosin.Where do I start??Please help.Fuse is OK.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 06, 2023, 07:08:52 pm
content: Cap C902 is blown. Most probably this is the culprit.

author: Per Hansson
author_rank: Unknown
timestamp: July 06, 2023, 07:41:28 pm
content: Quote from: abhi1981 on July 06, 2023, 07:08:52 pmThat's just for line filtering, it will not have any effect, just make sure there is continuity through that board and it will be fine if you remove those two Rifa caps for now.I would be allot more worried about the state of the main PCB, it looks like it was stored improperly and something leaked and corroded it?

author: abhi1981
author_rank: Regular Contributor
timestamp: July 06, 2023, 07:58:53 pm
content: Thanks for your help Hansson. I'll remove the caps for the time being. Should they be replaced with exactly the same type of film cap or other material cap can be used ? Replacement options are limited in India.Not much damage under the high voltage cover. But there is a lot of corrosion around U555. The traces have become discolored. I pretty sure someone has done resoldering with low quality flux paste, hence the burnt flux everywhere. I'll resolder all the corroded points.

author: Dave Wise
author_rank: Regular Contributor
timestamp: July 07, 2023, 01:45:18 pm
content: You should join the "tekscopes" mailing list at groups.io, all the experts are there.

author: Per Hansson
author_rank: Unknown
timestamp: July 08, 2023, 03:33:03 pm
content: Quote from: abhi1981 on July 06, 2023, 07:58:53 pmThey don't have to be exactly the same but they need to be class Y safety rated because they go from live and neutral to ground (chassis) so a short there would make the chassis live at mains voltage (Y class caps are guaranteed to fail open, not shorted).P.S: You should replace all "Rifa" brand capacitors in the unit if one has failed the others are soon to follow...Quote from: abhi1981 on July 06, 2023, 07:58:53 pmI don't really agree, look at the first photo you attached around the screw hole in the PCB: there is corrosion underneath the soldermask.That is something that usually only happens when a battery leaks, but there is no battery in the unit which is why I said something must have contaminated it.Otherwise I don't know how this could happen...Anyway trouble-shooting is straightforward if it is complete dead: just download the service manual and follow the mains input.The failure could for example be C904 having failed shorted (it is another Rifa cap).

author: abhi1981
author_rank: Regular Contributor
timestamp: July 08, 2023, 04:14:20 pm
content: There is indeed corrosion under the solder mask, and I tried resoldering some of the corroded solder, but they are not melting easily due to the oxidation.The voltage upto the power switch is 220V, but at the junction of diodes CR903 and CR901 its falling to 118V. Are the diodes faulty?Checked them with DMM on diode mode, and they testing OK.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 08, 2023, 05:38:19 pm
content: Someone may have used acid-flux to do a repair?The corrosion damage looks pretty extensive, I'd be inclined to scratch the solder mask back wherever it's damaged and give it a really good clean to get any of whatever damaged it washed away. A fiberglass brush can help polish the traces clean too.Then repair the traces by tinning with solder and/or soldering bare tinned copper wire along the traces if any are open circuit. Use lots of rosin flux to help the solder flow properly.Seal it up again with conformal coating when you are finished and hopefully it will be ok..If you leave it, the corrosion will probably just continue to get worse..

author: Per Hansson
author_rank: Unknown
timestamp: July 08, 2023, 05:53:06 pm
content: Quote from: abhi1981 on July 08, 2023, 04:14:20 pmSounds like you are maybe measuring over only one of the diode pairs, measure the DC voltage on the large filter capacitor C906 instead, should be around 310VDC.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 08, 2023, 06:05:42 pm
content: Just switched on the scope to test voltage across C906, and behold, the scope suddenly powered on. The power led was on, but no trace. Beam find showed a bright dot. There was a faint whining sound and then there was pop. I switched it off immediately. No magic smoke but something definitely popped. I'll check further what blew up.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 08, 2023, 06:12:17 pm
content: Quote from: TERRA Operative on July 08, 2023, 05:38:19 pmTotally agree with you. "Technicians" here still use paste flux, which leaves a burnt residue and they don't care to clean itI'll clean the corrosion damage, but first it needs to power on properly.

author: floobydust
author_rank: Super Contributor
timestamp: July 08, 2023, 06:14:32 pm
content: Spreadsheet list of 2235 capacitors to replace when doing a recap:https://www.eevblog.com/forum/repair/tektronix-an-usm-4882235-high-8-6-v-ripple/msg2539461/#msg2539461At this age, I replace all electrolytics in the PSU to prevent drama.

author: tunk
author_rank: Super Contributor
timestamp: July 08, 2023, 06:17:09 pm
content: Ian Scott Johnston recently looked at a 2215 where he replaced 3-4 ofthe Rifa caps. Your scope looks similar, maybe it has more of those caps.https://youtu.be/MW_p8HzVAlY

author: abhi1981
author_rank: Regular Contributor
timestamp: July 08, 2023, 06:42:21 pm
content: Can the Rifas be replaced with ceramic caps?

author: abhi1981
author_rank: Regular Contributor
timestamp: July 08, 2023, 06:48:59 pm
content: Or these

author: tunk
author_rank: Super Contributor
timestamp: July 08, 2023, 07:08:16 pm
content: I think they are safety caps (X or Y) and should be replaced with the same type.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 09, 2023, 01:26:12 pm
content: Yeah, only replace with the appropriate X or Y rated caps!Also, here's a video I made on restoring and upgrading my 2235. There are some mods to the PSU section to make it more reliable.https://youtu.be/lrXCVg6T-ekAlso, you may find some of your problems are due to the corrosion eating away traces, I'd start by scraping back the soldermask to check, or at least do continuity tests on all the corroded traces to be sure that none are open circuit...

author: abhi1981
author_rank: Regular Contributor
timestamp: July 09, 2023, 02:57:41 pm
content: Quote from: TERRA Operative on July 09, 2023, 01:26:12 pmI was watching your video yesterday. Very good video.The main problem in India is procuring the parts. You easily ordered all the parts from Digikey, but I cannot do so here. The postal system is very bad, packages from ebay etc. frequently get lost in transit. Only reliable service is Amazon, but not many electronic parts in stock.I checked some local websites, but none of them have Y rated caps. X rated are available and at ridiculous prices.Ceramic and mylar ones are easily available. Electrolytics are available, but not good brands such as Nichicon or Rubycon.Regarding the corrosion, I'm also starting to think that the problem is not the caps but corrosion. There may be a loose connection somewhere, that is making intermittent contact. That's why the scope suddenly powered on yesterday. I'll scrape the traces clean, resolder and then seal them with green UV mask.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 09, 2023, 02:58:57 pm
content: Quote from: floobydust on July 08, 2023, 06:14:32 pmThanks. The spreadsheet was really helpful.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 09, 2023, 02:59:49 pm
content: Quote from: tunk on July 08, 2023, 06:17:09 pmWatched the video. Very informative.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 09, 2023, 07:28:57 pm
content: Resoldered all the dirty points. Now the scope powers on. No trace. Beam find shows a bright dot.The focus, position and intensity seem to work.Checked the voltages on points W961, W968, W960, W954 and W956 - all OK.Nothing has been changed on the board. One Rifa C902 is removed.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 09, 2023, 08:33:32 pm
content: IT'S ALIVEMy bad. The scope settings were wrong. Just followed the instructions on this posthttps://www.eevblog.com/forum/repair/tektronix-2235-no-trace/and it works.Channel B is noisy though.But, how it started working suddenly is till a mystery.Now to change those rifas.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 10, 2023, 01:20:47 am
content: For the cost of shipping, I can see what I have in my stock of parts and send them over. A small box from Japan to India shouldn't cost too much.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 10, 2023, 02:20:43 am
content: Quote from: TERRA Operative on July 10, 2023, 01:20:47 amThat's very kind of you Terra, but please don't send anything. The international postal services is very bad here. The package will surely be lost in transit.Regarding the replacement for the rifas, should the capacitance be exactly the same or a higher value can be used.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 10, 2023, 11:11:57 am
content: It should be as close to original as possible, but a slight variation in capacitance wouldn't be terrible.More important is that you don't use a lower voltage rating, but higher is fine (and potentially better for longer life).

author: abhi1981
author_rank: Regular Contributor
timestamp: July 10, 2023, 02:55:27 pm
content: Will this work? This one is the closest one I got.

author: Per Hansson
author_rank: Unknown
timestamp: July 10, 2023, 03:08:07 pm
content: No they have to be Y safety rated, it is better to leave the capacitor out than to install a non-safety rated capacitor!The reason is that its failure could in worst case kill the user of the oscilloscope!It is very easy to understand why: they are connected directly between live and ground.So if the scope is not grounded and the capacitor fail shorted the whole chassis, with BNC connectors on the front panel would be at full mains potential!I suggest you can scavenge these capacitors from some old switch mode power supply from a PC, TV or monitor.Just make sure the capacitor has safety agency marks on it.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 10, 2023, 04:16:44 pm
content: Quote from: Per Hansson on July 10, 2023, 03:08:07 pmYikes!! That won't be a pleasant thing.

author: abhi1981
author_rank: Regular Contributor
timestamp: July 12, 2023, 08:09:59 am
content: Have been hunting capacitors, online and offline. Nearly all the capacitors available here are cheap low quality Chinese or fakes.I also opened up a Nichicon.I've narrowed down to one brand Keltron. This is a reputed Indian brand and quality is quite good. I have attached the spec sheet. What do you think??I noticed that the scope has some Indian caps - Punsumi, Philips and even a Rubycon Lic. India cap. (light blue one in pic)

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 12, 2023, 11:27:28 am
content: The badcaps forums might have more info on specific brands, I just stick with Nichicon, Rubycon, Panasonic, United Chrmicon, etc so I don't have experience with the less known brands.I would advise checking data sheets of the caps you remove and make a list that includes impedance/ESR and ripple rating of each cap and make sure your replacement caps meet or exceed those ratings (lower impedance/ESR and higher ripple current rating)Also, check the pitch (distance between the legs) and height/width too, to make sure the new ones will fit...

author: abhi1981
author_rank: Regular Contributor
timestamp: November 03, 2023, 06:34:50 pm
content: Sorry for the delay, was busy in another project.Finally its done. Changed all the capacitors. Not high quality caps, but fair enough. The rifa replacements were the exact ones as recommended in the spreadsheet.Now the scope is running perfectly. No noise. The switched may need cleaning, but that will be future project.Thanks everyone for your help.