Tektronix 422, channel amp issues. (Now with pics) - Page 1
Forum section: electronics / repair

author: apelly
author_rank: Unknown
timestamp: July 20, 2014, 11:39:38 pm
content: So, the other day I bought a Tektronix 422. The price was fine and the seller was pretty honest about it's condition. In the modern scheme of things it's a pretty crap scope, and I don't need it, but, frankly, it's as cool as fuck! I'm stoked. I could look inside this baby all day.Andthere's a reasonable chance it's older than me. It's by far my oldest bit of tech anyway. Serial number is a bit over 22000 I think. Haven't seen any dates inside apart one board with a '65 (c) notice in the copper. That's consistent with the design date, and I haven't looked for other revisions of that board.Anyway, I'm kind of reluctant to ask, because I'd sort of like to troubleshoot and fix it myself, but then it might be a year before I get around to it. I've had a quick peruse of the (extensive) docco online, but certainly haven't properly applied myself to finding solutions to it's issues yet. Perhaps some of you more experienced guys wouldn't mind gently nudging me in the right directions?The issues I've noticed so far:The trace appears too far to the left on the screen. It doesn't make it all the way to the right. Maybe short by about half a division.One of the graticule lights is blown. Haven't looked for a replacement yet. Should be simple enough.The pots and switches are pretty scratchy. Not quite sure the best way to resolve this. There is a lot of stuff to unplug to get easy access.There is something afoot with channel 2. Might be best to re-check after cleaning the switches, but its amplitude fluctuates, preferring to be too small.Edit:Issues above resolved by working the pots, and applying deoxit to the switches.New issues:Something(s) are overloading the power supplyInverting channel 2 is not symetrical about the centre lineEdit: Add some pr0n

author: tautech
author_rank: Super Contributor
timestamp: July 20, 2014, 11:52:41 pm
content: QuoteIn some sort of order:Get the TEK faultfinding pdf from the pinned thread at top of the Repair board.Pots should improve with use, but they may need cleaning IPL etcDirty switches I suspect on CH 2.

author: xwarp
author_rank: Frequent Contributor
timestamp: July 21, 2014, 01:25:46 am
content: I've got 2 of these 422 scopes. One has the battery option and I use that one for looking at signals to and from the ECM in my truck because I can reference ground to the truck chassis.Being that it's a 15mhz scope, it's nowhere near being a crap scope and I certainly have no issues with either one of mine displaying up 50mhz waveforms.That said, for basic electronics where you need something cheap and reliable, and especially the prices of them, you can't beat the price.In regards to the pots and switches, get yourself a can of DeOxit D5 and work the switches and pots.

author: apelly
author_rank: Unknown
timestamp: July 21, 2014, 10:34:24 pm
content: Quote from: tautech on July 20, 2014, 11:52:41 pmNice guide. Thanks.This looks worth investigating:

author: apelly
author_rank: Unknown
timestamp: July 21, 2014, 10:35:51 pm
content: Quote from: xwarp on July 21, 2014, 01:25:46 amIdidqualify that comment. But fair enough.Quote from: xwarp on July 21, 2014, 01:25:46 amI will do that. Never heard of the stuff before you mentioned it. People seem to rave about it all over the internet.I haven't looked closely, but the pots appear to be sealed. Do you think I should carefully open them?

author: xwarp
author_rank: Frequent Contributor
timestamp: July 22, 2014, 06:58:40 am
content: I don't recall at the moment what the pots looked like, but the switches should be easy to get to with the tube on the nozzle.

author: xwarp
author_rank: Frequent Contributor
timestamp: July 22, 2014, 07:05:05 am
content: Oh, one other thing, this page:http://w140.com/tekwiki/wiki/422has lots of info on the 422.If you by chance need any readings from a working scope for troubleshooting yours in the future if the need arises, let me know.The 1st one I picked up worked fine and was a/c only.The 2nd, with the a/c-d/c battery pack, came from ebay and had an issue with the display. Turned out a zener on the vertical deflection board was bad. Very strange symptom considering it appeared to be a timbase issue.

author: apelly
author_rank: Unknown
timestamp: July 29, 2014, 01:10:46 am
content: So, as advised, I cleaned the switches with deoxit and worked the pots a heap. That pretty much resolved the obvious issues. Thanks for the tips xwarp & tautech.I set about calibrating it as per the manual. Someone has had a crack at that (badly) before I reckon. It was starting to look good. Then it stopped.Inspecting the power supply revealed r661 (+55v) and r641 (-110v) had burned and been replaced. r641 was well toasted again. Replaced it. r651 (1r, +12v & scale illumination) was toast too, by design, it appears to be a fuse. Replaced with a jumper and the scope is back to life.The unregulated voltages I measure are on the high side. Mains here is 230v, and that is how the transformer is wired, but the manual does have details for wiring it for 250v. I wonder if there would be any benefit adapting it to 250v? The regulated voltages are pretty good.I was having a browse through the schematics to see if I could see anything that might obviously overload those rails. Nothing stood out. Advice appreciated on this front.I also noticed that r475 on the sweep generator was badly burned, this is a divider for the external blanking input I think. D476 seems fine, so someone may have just (well) overloaded it in the past. I will replace it at some stage.Now, I have another issue. This might be ignorance on my part, but channel 2 does not seem to invert symmetrically. With channel 2 ground coupled, trace positioned on the centre line, the invert switch moves the trace by a whole division. This might go away after I calibrate channel 2, but I'm not sure I want to go to all that trouble only to do it again if this issue is somewhere on the input amp. Again, advice appreciated.I updated the first post with my current issues.I have to say that having the schematics is quite a treat. So far this has been fun; and the scope still makes me smile.

author: tautech
author_rank: Super Contributor
timestamp: July 29, 2014, 01:51:19 am
content: I am not sure re the 422, but look for bead tantalums on the power rails....my first suspect.I would not worry about Ch 2 until all the other little issues are fixed and some sort of cal is done.

author: apelly
author_rank: Unknown
timestamp: July 29, 2014, 02:21:37 am
content: That sounds like good advice. Will do. Thanks tautech.Edit: No tantalums, but will check caps anyway.

author: apelly
author_rank: Unknown
timestamp: August 23, 2014, 01:42:53 pm
content: Well, I sat down today for another couple of hours to finish the calibration. The power supply stood up OK, but I got distracted:Contrary to advice I started investigating why channel 2 inversion and therefore subtraction didn't work symmetrically. Didn't learn anything.I was troubled by the signals leading from the channel 2 input amp to the vertical amp. They are in phase. The same signals from channel 1 are 180 degrees out of phase. I wondered if some transistor might not be switching.Checking some, it looks like either most of the 2N4275's in the whole unit are broken, or so leaky it's crazy. I've read about doping migration in early transistors. Maybe this is what I'm seeing here.Anyway, if anyone knows an equivalent for a 2N4275 would you please let me know? Cheers.Edit: Fix bjt name. It was late when I typed this.

author: apelly
author_rank: Unknown
timestamp: August 24, 2014, 03:25:03 am
content: Some pics of her with her gear off added to the 1st post; for those interested in that sort of thing.

author: tautech
author_rank: Super Contributor
timestamp: August 24, 2014, 01:04:24 pm
content: My book lists: BSV91 or 92, BSX92 or 93, 2N2368 or 9.NPN, High speed switching, 30V, 100mA, 400 MHz2N2369 datasheet attached, just note it was version 2 on my database, change the name.In the past I have sourced similar through RS, Farnell or the like to ensure quality devices for equipment repair.TO-18 will look much better in your old Tek than any new-fangled TO-92.

author: xwarp
author_rank: Frequent Contributor
timestamp: August 24, 2014, 10:22:08 pm
content: Here is an NTE replacement, if you like those:http://www.weisd.com/test/WEISD_TBL_view.php?editid1=NTE123A