Tektronix an/usm-488  - Page 1
Forum section: electronics / repair

author: Pyral
author_rank: Contributor
timestamp: February 02, 2022, 11:33:10 pm
content: I just got my 2235 that I got of ebay. I few surprises, first I didn't get a regular model I got the government version, this shouldn't of surprised me since it was right there in the photos, but not in the description. A not so pleasant surprise was that the var holder knob broke off at the base, I didn't find it anywhere in the box either. Oh well, I imagine finding a replacement will be impossible.Also got a lot of strange accessories I wasn't expecting, mine still had the vinyl pouch mounted on the top and it contained a 016-0566-00 viewing hood, a 013-0191-00 IC grabber still sealed in its pouch, a Belden 3 pin power cable that I don't even think goes to my unit, and some moldy instructions for the P6122 probes that came with it (dated July 84). I also got probes, but they are the P6017.I haven't powered this thing on because of the Rifa caps. I'm a little concerned that the instructions looked like they got damp enough to get moldy, so hopefully nothing has corroded inside. I have to do take a test, but I'll start tearing into it the moment I get back since this unit has issues. But for now I have a few questions.Does anyone know why the an/usm-488 all have a white screen with a red reticle, while the normal versions have a blue screen? Was this something the government requested or did the blue filters just get lost when they were in field use?How many rifa caps do these contain?And what did the P6017 probe that came with my unit actually go to?

author: bdunham7
author_rank: Super Contributor
timestamp: February 03, 2022, 01:18:29 am
content: You just need a blue filter. I don't know whether it was omitted or removed later. The P6017 is a very old probe using screw-on accessory tips, the type you would see on old 500-series scopes with plug-ins. Unless they are particularly nice and have all the accessories, you might want to just set them aside. Can you post a closeup photo of the broken knob (or where it went)? I'll have a look in my junk when I get a chance.

author: CaptDon
author_rank: Super Contributor
timestamp: February 03, 2022, 02:51:18 am
content: Not sure what timeframe when Tek went from the red graticle lines to the black lines. With the blue filter the red lines look black anyway. Funny that many scopes that used an external plastic graticle with grid lines used red colored lamps to illuminate the graticle. The Tek CRT's have a different suffix on the base number which can be cross-referenced to indicate a tube with red or black internal graticle lines. The late 1970's SC501, SC502 and SC504 were produced in both red and black versions. The SC502 and SC504 had blue filters making the lines appear black in either version. The SC501 with red lines were an oddity and SC501's did not come with filters. If you want to see some real oddballs, the SC502 and SC504 could be ordered with the slow P7 radar phosphor and had ORANGE filters!!! The SC502 15Mhz and SC504 80Mhz oddly enough use the same CRT.

author: Pyral
author_rank: Contributor
timestamp: February 03, 2022, 06:31:58 am
content: Okay, I've started tearing in. Worryingly it seems like someone went in at some point, because the black rifa shield wasn't screwed in all the way.I hoped to turn the scope on, but the ultra cheap soldering iron I bought (as a stand in) started smoking more than I remembered the last one doing. Shame Ever Start, shame. So I don't know if I'll be able to remove those rifa caps yet which there are 3.Few more things found. The channel 1 volts per division knob is missing the ring around it with the printed numbers. The channel 2 knob also has a crack on its number ring too. Not much I can do about that for now. The body of the oscilloscope looks a lot better, it looked really beat up but that was just residue from tape and adhesives.I'll get some photos soon.Alright here are the photos. I got my solder to stop smoking and I removed the two rifas on the filter board. The third rifa that is under the power switch (I think) seems to have been replaced and I don't feel like getting to it. I also got the thing powered on, and surprisingly I got a beam and the CRT is showing something, have no probes to measure a signal with.I don't know whats causing the scan line to jump up like that.Orange backlight causes the reticle too look orange. It is dark red without the backlightIn the corner you can see where the Var holdoff knob was snapped off. The volts/div knob for both the channels aren't in great shape either.That is the parts list book that was in the vinyl pouch. I don't think its original because the wiki says this model wasn't produced until 1985.Here is the old probe that doesn't go to my unit. Its in okay shape, it should clean up nicely.Here is the IC probe in its original packaging. The old tek logo makes me think it is from a different model.There are more photos that can be found here, but they aren't super important.https://imgur.com/a/tKYQDgq

author: pbarton
author_rank: Frequent Contributor
timestamp: February 03, 2022, 08:57:00 am
content: Does anyone know why the an/usm-488 all have a white screen with a red reticle, while the normal versions have a blue screen? Was this something the government requested or did the blue filters just get lost when they were in field use?US military contract AN/USM-488. (Army Navy/United States Military (marines?)-specification 488).Yes, I'm sure that the AN/USM-488 has Tektronix Option 1, EMC reduction on CRT screen.According toOption 1 emits less EMI radiation (from the CRT face).You are viewing the CRT screen through a built in metal mesh.Under certain backlighting conditions, the metal mesh is visible.Under normal lighting conditions you are not meant to see the mesh.So, reduce the backlight (scale illumination) and the beam intensity.Edit: I have just noticed that your first photo shows "OPT 01" at the bottom of the front panel, just below the serial number.So it's definitely EMI radiation reduction measures as specified/required by the military.

author: Pyral
author_rank: Contributor
timestamp: February 03, 2022, 11:16:20 am
content: That is interesting.So I noticed that my CRT has regions that illuminate whenever the scan line goes through the area (like blotches) whether they should illuminate or not.The scan line also warps and gets pushed off its beam pattern almost like there is some invisible field interfering with it. Does this mean my CRT is burnt in or damaged?This is with no signal, still waiting for my probes

author: pbarton
author_rank: Frequent Contributor
timestamp: February 03, 2022, 11:37:01 am
content: A photo(s) or a brief video of the anomaly would help.

author: bdunham7
author_rank: Super Contributor
timestamp: February 03, 2022, 03:18:47 pm
content: Quote from: Pyral on February 03, 2022, 11:16:20 amThat looks rough. If you set it up like you have it in your top photo and then move the trace up and down using the CH1 vertical position knob, does it's shape change? If so, put it at the top and scan slowly all the way down and see if it looks like it is scanning over a three-dimensional object. Then do the same with the other channel. If what I suspect has happened, both traces will have the same anomalies at the same places in the screen and that means CRT damage.

author: Pyral
author_rank: Contributor
timestamp: February 03, 2022, 05:19:54 pm
content: They have the same anomalies in the same places. I doubt the lack of filtering from the power supply or the removed case is my problem.I

author: bdunham7
author_rank: Super Contributor
timestamp: February 03, 2022, 06:02:48 pm
content: It's likely been dropped hard and there's damage to the CRT internals. Unfortunately, as is all too often the case these days, you have purchased an extensive repair project. The deals seem to have mostly dried up a few years back.

author: Pyral
author_rank: Contributor
timestamp: February 03, 2022, 06:03:17 pm
content: Here is the video. Changing the number of traces on the screen changed nothing.https://youtu.be/4FzaGIjBosc

author: pbarton
author_rank: Frequent Contributor
timestamp: February 03, 2022, 07:33:45 pm
content: The scope may have been dropped. There is an internal mesh inside the CRT which is used for "post deflector acceleration, where the electron beam has to pass though a mesh" before striking the anode/phosphor. Your internal mesh could be wrinkled. The X and Y plates appear to be undamaged.The internal mesh is different to the EMC radiation mesh, which could be on the face of the CRT.Your only hope is to replace the CRT. Probably cheaper to buy another scope.

author: Pyral
author_rank: Contributor
timestamp: February 03, 2022, 08:41:13 pm
content: So the shadow mask is wrinkled. No way to fix that. The similar replacement tubes go for 45-80$. I could get close to buying another scope, and I would have more parts to replace the broken switches.I'll look into the service manual, hopefully there is another cause for this.This unit has more problems. Controls can move the beam in ways they shouldn't, if I bump the table with a little force the beam disappears for a split second and the trigger LED flashes (anode might be loose). Also the neon indicators on the board only illuminate for moment when I turn the unit off, they aren't on if the scope is powered on which I don't think is normal.

author: bdunham7
author_rank: Super Contributor
timestamp: February 03, 2022, 09:02:46 pm
content: Quote from: Pyral on February 03, 2022, 08:41:13 pmIf you decide to dive in, go slowly and ask questions! Many have trodden the same path before you...The beam 'disappearing' when the trigger LED flashes is probably because the scope is in AUTO/PP trigger mode and when it sees an actual trigger event, it triggers normally and then pauses before resuming the AUTO trigger. So it is simply responding normally to some abnormal noise in the input. If it responds to tapping, I'd suspect either a loose connection or more likely, dirty attenuator switches. These can be cleaned with IPA (Isopropyl alcohol, not beer) and dust-free paper, but they are very fiddly and easy to damage if you aren't very careful. Just try rotating them a few time for now.The operation of those neon bulbs (they aren't indicators really, they're part of the HV circuitry) is normal.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 03, 2022, 09:22:59 pm
content: Sometimes this company has the correct CRT and reasonable prices:https://www.sphere.bc.ca/test/index.htmlMaybe e mail them if you do not see it on their site

author: Pyral
author_rank: Contributor
timestamp: February 04, 2022, 02:07:17 am
content: Will the tube from a normal 22xx series swap into one with option 1. Upon closer inspection of the board I noticed that the C180 hole running off the U180 was unpopulated.The C180 comes up under the Channel 2 high frequency compensation, the U180 is the micro circuit vertical preamp.You can Cntrl + F this document to check it outhttps://download.tek.com/manual/2235_sm.pdf

author: jdragoset
author_rank: Frequent Contributor
timestamp: February 04, 2022, 03:05:21 am
content: https://www.ebay.com/itm/303745369005?hash=item46b8a283ad:g:G8YAAOSwyYdgYl-rlike this on ebay, many parts mules in stock.If you are running on 120 VAC, worry about the RIFA caps after you have the rest up and working ok.

author: bdunham7
author_rank: Super Contributor
timestamp: February 04, 2022, 04:24:19 am
content: Quote from: Pyral on February 04, 2022, 02:07:17 amAFAIK the mesh should be just loose in front of the tube and the tube should be Tek p/n 154-0861-00. It may be possible to use a different CRT as well.groups.io/g/TekScopes/topic/info_on_replacing_tek_2235/7632344I actually have the 154-0861-00 CRT, but there is some internal damage as well--it is minor and not nearly as bad as yours--and it might be useful just as a test unit to make sure you can get everything else working. You'd also get some practice at disassembling/reassembling, which actually is a bit of a chore on those at first. It's yours for shipping if you want it.I've no idea why C180 would be missing--does it look like it just hasn't ever been there? Is C130 present? Perhaps there was a revision at some point. I'm sure it isn't causing you any current issues as its function is pretty minor.

author: Pyral
author_rank: Contributor
timestamp: February 04, 2022, 05:34:03 am
content: Wish I could buy that scope, but its 100$ in shipping. I'm leaning towards buying one sold as working for slightly more and selling my current one as non-working broken CRT. I'd love to fix it, but the price of a CRT financially totals the scope unless you have the replacement part. A new volts/div knob here, a NOS CRT there and then suddenly I'm paying the same or more money then I would have if I had just bought it working.A word of cation, if your on Ebay right now looking for tek scopes the person I bought mine from still had over 8 available. I'm not saying they are all like mine, but if they are coming from the same place you got to assume the scopes might of lived similarly rough lives.I cant get a good view of the part on the board because its directly under the tube. I think it might be a test point, but the flux from C180 sort of spread out and made it look like the unpopulated hole was a test point. The c130 is present

author: bdunham7
author_rank: Super Contributor
timestamp: February 04, 2022, 05:58:52 am
content: Quote from: Pyral on February 04, 2022, 05:34:03 amI bought, fixed and sold a few of these but I'm not doing it any more because there isn't enough difference between the price of a tested, working and guaranteed unit which I sell and a beat-down wreck that I would buy for parts or repair. Not that there was any real money in it in the first place.Yours may have been part of this lot:https://www.ebay.com/itm/303745369138?nma=true&si=UcguXpLCSPBdpygcCGCKO5PSwdE%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557Is there any particular reason you want the 2235 specifically?

author: Pyral
author_rank: Contributor
timestamp: February 04, 2022, 07:13:17 am
content: The one you linked is gone but that is not it. I'll link it below.Few reasons I'm went with this scope. I work on analog audio equipment, mainly cassette decks, and an analog scope is better for calibrating them because of its low latency (there are easier ways to calibrate), and it would be helpful in general repairs. The main reason is that when I buy anything like this, I prefer to buy it broken and fix it myself to get an idea of how it works; newer electronics don't have the nice service manuals anymore.I'd probably be fine with any of the 50MHZ and up scopes.https://www.ebay.com/itm/154441423663?hash=item23f56d1b2f:g:iBoAAOSwMkpgkwXy

author: scopeman
author_rank: Frequent Contributor
timestamp: February 07, 2022, 07:15:24 pm
content: I saw the picture of the trace in your post. Have you checked all of the power supplies for ripple on the output? If you short the two leads together that go to the vertical plates and the trace goes to a flat line then the problem is not the CRT. You can also disconnect the vertical plate pins and you should have a straight horizontal line. If you do have a straight horizontal line after doing either of these then the problem is not the CRT.Go here and get this excellent free bookhttps://www.smcelectronics.com/DOWNLOADS/TEK%20TROUBLESHOOTING.PDFSamW3OHM

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2022, 07:40:34 pm
content: Quote from: Pyral on February 04, 2022, 07:13:17 amYou might get a better deal going with a less popular model. I have one of these and it is a pretty decent scope. A bit different than Tek, with pros and cons. Never needed to service it so I can't comment on that.https://www.ebay.com/itm/393720394335?hash=item5bab90725f%3Ag%3AF0sAAOSwG4NhnUR6&nma=true&si=UcguXpLCSPBdpygcCGCKO5PSwdE%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557

author: Pyral
author_rank: Contributor
timestamp: February 12, 2022, 11:10:33 am
content: Alright got one that isn't f'd up, no messed up CRT. This particular scope is almost a year newer with its warranty expiring in 87. For the most part its intact, but part of the clear number ring on it cracked and fell away as I was checking it. The plastic Tek used for all the dials and knobs sure is brittle.The intact knob has problems too. The red calibration dial will just spin and spin, and its lost the ability to turn off the "uncal" LED. I hope that there is fix for this. Does anyone 3d printed replacement knobs and switches for these scopes? It would be nice since they all seem to have similar issues (and my sample size of 2 has only one intact dial).The only new problem the newer scope has is the back plastic panel has a small crack. I could always swap it off my old deck, but it has all the serial number and calibration information and I'd like to keep that with the prospective scope. Does anyone know have any suggestions on re-fusing the plastic?

author: jdragoset
author_rank: Frequent Contributor
timestamp: February 17, 2022, 10:01:17 pm
content: https://www.ebay.com/itm/175018433585?epid=1970974464&hash=item28bfe95c31:g:tlYAAOSw79BgLGE0

author: bdunham7
author_rank: Super Contributor
timestamp: February 17, 2022, 11:25:19 pm
content: Quote from: Pyral on February 12, 2022, 11:10:33 amThe vertical cal knobs are connected to potentiometers on the back of the attenuator board by long, plastic rods that go through the attenuator assembly. If the calibration knob spins but doesn't pull out, the set screw is likely loose on one end or the other. If it pulls out, the rod is broken or has become completely disconnected. Worst case is the potentiometer is broken, although I haven't seen that yet.The plastic can be reworked to a degree with a hot-air gun and a very small nozzle. Start at 120C and work your way up 10C at a time until you can manipulate it with a pick or small spatula. If it were me I'd just put the other panel on. How is the information attached to the panel? I haven't seen that on other models.

author: Pyral
author_rank: Contributor
timestamp: February 18, 2022, 02:54:12 am
content: The information is on a sticker on the back panel. The best result you could hope for is a sticker that curls up at the edges, so I'm not gonna try swapping them. Some plastics like ABS can be melted and fused relatively easy, other materials like G10 (and probably FRN) burn before they ever melt.Don't know what plastic they used, maybe it has a molding mark of some sort