CRO Oscilloscope repair - Page 1
Forum section: electronics / repair

author: MrJack
author_rank: Newbie
timestamp: January 15, 2019, 09:12:14 pm
content: Hi all,New to the forum, shamelessly asking for help with my first post, please don't hate me! I'm just getting started with electronics, trying to learn more. I just brought my first oscilloscope, a Hitachi v-212 which I won for cheap from eBay, it was listed as untested but it was cheap and figured I could have a go at fixing it, as a bit of a learning experience.It arrived and I really like it, I'm sure it's only a beginners scope but I have a thing for old school electronics. When it arrived channel one worked perfectly, but channel 2 seemed really compressed, existing entirely within two of the small divisions of the display, exactly as described in this post here:https://www.eevblog.com/forum/testgear/hitachi-v212-ch2-amplifier-problem/I don't know if it's relevant but even the channel 2 vertical adjust could not move the line out of these two divisions.I found the service manual here:https://www.manualslib.com/manual/1171085/Hitachi-V-211.html#product-V-212and concluded that since channel 1 worked fine it must be somewhere before the 'diode gate'. At this point I may have made my first mistake and opened it up to see if i could visually spot anything looking obviously wrong. Most people suggest replacing electrolytic caps but they all look great, suspiciously good in fact, as though they have been previously replaced. The circuitry is spread out over two boards, it seems roughly along the lines of horizontal deflection on one board and vertical on the other. I removed the board with all the input preamp circuits to check for bad solder points on the back and started checking resistor values with a multimeter (stupidly not taking the components off the board first, like I said im a beginner).I couldnt find anything so I thought I would put the board back in thinking i would apply a signal and probe voltages along different points along the two preamp circuits and see if i could see any difference (I dont have another scope to observe signals). Unfortunately, and in typical fashion I might add, I seem to have made it worse. I now have no horizontal deflection at all, and when I apply a signal to the previously working channel one the dot doesn't move. (I assumed in the absence of a horizontal sweep I would just get a vertical line, but no). I find the lack of sweep particularly annoying as I did not even touch the board with sweep circuitry on at all. Another curious note is while I can't get any deflection from signals (or external triggers) both the X and Y position adjusts both seem to work (including the previously non working channel 2 adjust). So now I'm really confused, and I'm suspecting there's multiple faults now even though I was careful taking it apart.So what do you guys think, have I completely wrecked it or do you think it may be salvagable? Any suggestions on where faults may lie?PS. Sorry for the long post, but I figured the more details the better!

author: Armadillo
author_rank: Super Contributor
timestamp: January 15, 2019, 09:47:15 pm
content: Hi, of course you are aware that this thing carry lethal voltages. So careful and don't rush.Many of your country men will help you out, i trust.Just want to add, relax, check carefully that everything, wire connectors are put back correctly. Double check it again and again.Then make sure you are not in XY mode, rotate the sweep away to other sweep rate, put to auto-trigger for example.Read the manual would help.

author: MrJack
author_rank: Newbie
timestamp: January 15, 2019, 10:07:29 pm
content: Yes, but thanks for the warning. No i went very slowly with the dissasembly and did plenty of research into the thing to identify where all the high voltage parts are, and safety practices around working with high voltage. Someone once said to me that electricity is a thing that can kill you but the body has no natural way of detecting it, thus it deserves respect... words I live by! And no I have tried it at every sweep value but nothing.Cheers for the advice, I will keep at it, and check I put the wires back in the right place.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 16, 2019, 02:33:49 am
content: Quote from: MrJack on January 15, 2019, 10:07:29 pmProbably the most logical explanation for a completely unrelated problem appearing after you "messed with it" is a missing or incorrect connection.If, for instance, you plugged a connector in offset by one pin, you may have made a unintended connection, causing the observed fault.Worst case, you may have killed a power supply.If you can get it back to the original condition, there are two things to think about:-(1) A two channel 'scope with one faulty channel is still a quite useful "single channel" instrument.(2) Both channels are identical, so youcanget useful results by looking at circuit resistance without disconnecting things, just so long as you check, & compare, both the "good" & "bad" channels.

author: rhb
author_rank: Super Contributor
timestamp: January 17, 2019, 03:17:26 am
content: If a vertical channel is working, but not the horizontal, you can still use it to probe the horizontal sweep circuit. I *really* wish I'd been smart enough to realize this 30 years ago, but that's another story.With old age and humid conditions, solder joints tend to fail. If you didn't connect something incorrectly, then you are likely chasing bad solder joints. I had the experience working on a Tek 465 that I traced a delay line fault to a particular resistor. When I went to resolder the joints it fell off the board on to my bench! I'd melted the only joint still holding it in place.Start at the horizontal sweep oscillator and probe until you find the point you don't get a vertical line on the display. Study the circuit at that point and resolder all the joints in the neighborhood. The best approach is probably to get some liquid flux, paint the joints and reheat them. I didn't have any so I just applied more solder.You should also inspect the boards for flux residue. In humid climates the residue absorbs moisture and causes all manner of problems. I got a 34401A DMM from eBay which arrived completely wonky. Just random digits on all ranges. I opened it up and there was a 1 cm brown spot around a part that had been replaced. I cleaned it off with a soft toothbrush and isopropyl alcohol and it has worked flawlessly ever since and reads within a few ppm of my other 34401A.I have now made 7-8 repairs which consisted of nothing more than cleaning flux residue off boards. The 34401A is simply the highest value instance. The most recent instance was an $11 motion sensitive night light.The Hitachi just wants to know if you *really* love it

author: MrJack
author_rank: Newbie
timestamp: January 17, 2019, 10:01:29 pm
content: Thanks for all the advice! I swallowed my pride and accepted there was a tiny chance I may have wired it back together incorrectly, and then very carefully consulted the wiring diagram. And to my surprise when I put it back together again it worked perfectly, both channels!I was so happy to have a fully working scope, however it was short lived as the next day it went back to how it was originally, with channel 2 not working. It actually happened while I was using it, nothing dramatic or anything, it was working fine and I was switching back and forth between channels and then I switched to channel 2 and it was dead again. No sparks or the signal going crazy, or happened when I did a particular thing, it just went

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 10:19:08 pm
content: Sometimes when you push those connections, the pin pushed out from behind instead of fully engaging in.Sounds like loose connections to me including cold solder joints or corrosion of joints by now.While you at it, do use contact cleaners to blast all those contacts again. And using a magnifying glass, go through all the solder joints, making sure they shine like watery glass.Like I said, relax and enjoy all the moments of opening it up again, slowly, one more time.

author: rhb
author_rank: Super Contributor
timestamp: January 17, 2019, 10:44:13 pm
content: You've almost certainly got a cracked solder joint.I spent each evening for a week tracing my way through the horizontal section of the Dumont. All of a sudden it worked. I left it running all night and it still was working in the morning. I powered it off, took a shower and powered it back up. The horizontal sweep was once again dead. That evening I went through and resoldered all the connections in the immediate area where I had been probing and in short order had it working.Trick I learned from a retired Tek service man that I took my 465 to for advice. He powered it up and then clapped his hands against the opposite sides of the case near the front. He then took a roughly 1 ft long stick (broom handle or such) and rapped on the corners of the front frame (and probably the back frame as well, This was 25 years ago). He said it was basically working, but had some bad connections I would have to locate and fix. Sold or gave me a knob or two. I think I paid him $35 for the diagnostic. Worth every penny. Once the Dumont was working I used it to find and fix the bad joints in the 465.When the Tek guy started beating on ths cope I was quite alarmed, but later realized the logic of it. It's a quick way to assess whether an instrument has bad connections whether it appears to work or not. It won't find them, but it will tell you if you have them. The other thing he did was press very gently on the PCB in key areas with a wooden dowel like a bamboo chopstick. The whole time he had a probe connected to the calibrator and was watching for changes in the display. That's a very useful way to isolate a cracked solder joint quickly without detailed tracing of the circuit.In case I didn't mention it, do *not* spray tuner cleaner into the attenuator section. Clean the contacts by closing them over a sheet of bond paper and pulling the paper through until it doesn't leave a mark. The cleaner residue is apparently very bad for the capacitors in particular. I sprayed the Tek before I read the manual section warning not to do that. I then spent a lot of time washing it with 99% isopropyl.You are well on your way to having the scope fixed and being able to say you did it. It's a very pleasant feeling of accomplishment, especially if you are a novice as I was.

author: MrJack
author_rank: Newbie
timestamp: January 17, 2019, 10:55:05 pm
content: Looks like its coming open again! although not tonight as its getting late. It sounds like I have some cleaning to do. When you say contact cleaner, what do you mean exactly? I did notice some solder spots that looked less than healthy but they seemed to work when I probed them. More rigour is required methinks.I only have a cheapo soldering iron so I might see if I can find something better, or perhaps one of those solder removers at the very least. What does flux residue look like? The board has a heatsink on and the areas around that have some sort of browning on them which I assumed as to do with heat but wasn't certain. Although I'm pretty sure this is fine as this is the final amplifier stage after both channels have joined together, so it stands to reason it works fine.

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 11:08:45 pm
content: My all time favorite, if you can reach it;https://youtu.be/l-ve0Jk6bGc?t=5

author: MrJack
author_rank: Newbie
timestamp: January 17, 2019, 11:15:59 pm
content: Thanks!On the bright side, at least i know none of the components are busted

author: rhb
author_rank: Super Contributor
timestamp: January 18, 2019, 12:16:49 am
content: The tuner cleaner/lubricant stuff. Great for lots of other things, but not scope attenuators.While you're in there and after you finish, make sure you clean up any traces of flux residue. It absorbs moisture from the air and will cause trouble. I got a wonky 34401A via eBay. Just random garbage on all settings. Opened it up and found a 1 cm brown spot where an LF357 had been replaced. Cleaned it with 91% isopropyl from Walmart and a large artist's hair brush. It now works flawlessly and is within a few ppm of my other 34401A.I've now done 7-8 repairs which consisted of nothing more than cleaning flux off boards. It's especially a problem with 1.8 & 3.3 V logic. But the last instance was a motion sensing LED night light. I had fished it out of the junk bin and taken it apart with the intent of stripping some parts. I saw some flux residue, so I decided to try cleaning it. Works perfectly now.You've got a good bit of work ahead of you, but I feel confident you'll enjoy it and learn a lot. After it's fixed, you'll need to learn to calibrate it. I highly recommend getting one of Leo Bodnar's 40 pS pulsers.http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=295