Tektronix 475 oscilloscope - help needed - Page 1
Forum section: electronics / repair

author: F3nn3c
author_rank: Newbie
timestamp: May 20, 2018, 05:26:06 pm
content: Hello, everyone:I have acquired a Tektronix 475 scope (Option 4, with DM43), advertised as "working".Good news - itdoesdisplay a base trace, anddoestrigger (sort of) off of the internal calibrator.The bad news - everything else. Here is what I am experiencing:- The trace, when displayed, isextremelyjittery, and floats up, down, left, right, or even running left to right- A slightest touch of ANY control results in triggering being lost, and trace disappearing in a random location. Takes some bumping on the case and A trigger level control rotation to bring the traces back... For a bit.- No matter what I do, the trace is rather thick, even when "in focus"- DMM displays garbage numbers, and zeroing works only one time out of 50, if thatSince I am very new to this, what would be the proper troubleshooting/repair sequence? DeoxIT controls and switches? Reseat transistors and ICs? Check the voltages per Service manual afterwards? I hope some of you have seen these symptoms before, so maybe it is something more basic?Any advice of where to start would be greatly appreciated.Dmitri

author: particleman
author_rank: Regular Contributor
timestamp: May 20, 2018, 05:49:13 pm
content: Push switches can be deoxited. Cam switches are gold fingers that have to be cleaned with IPA and clay free paper. Re-seating transistors and socketed stuff can never hurt and you may find other "things" while you are doing that. Voltages are the first place to start. Get the manual. Did you also adjust astigmatism for the thick trace issue?

author: tautech
author_rank: Super Contributor
timestamp: May 20, 2018, 08:14:22 pm
content: Quote from: F3nn3c on May 20, 2018, 05:26:06 pmPSU supply railsmustmeet specs found in the service manual. Period.General guidance however the SM is your bible.http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

author: Electro Detective
author_rank: Super Contributor
timestamp: May 21, 2018, 12:56:56 am
content: FWIW, the last time I had these symptoms on a few Tek beasts, I didn't have any service info handy nor decent meter kit with meso I got out thin tough skin rubber gloves, protective glasses and preferred contact cleaner lubricant (Phillips xxx something at the time)and went through the switches, a few at a time slowly, with unit OFF and checking with unit ON as I went, and twiddling the adjacent pots too,till the issues went awaySometimes you get lucky and it's the first few switches and or scratchy/seized pots, and or the ones that cause a response to the issue when worked,and if the unit works great after sorting those, job done, close it up...no need to keep going into possible destructive preventative maintenance territory, or risk oops shorts hurriedly checking PSU test pointsHad a good run with those repairs, didn't even need a re-CAL, only one failed years later due to leaking capsGood luck

author: rhb
author_rank: Super Contributor
timestamp: May 21, 2018, 01:40:03 am
content: Quote from: Electro Detective on May 21, 2018, 12:56:56 amDO NOT SPRAY TUNER CLEANER ON THE ATTENUATOR SWITCHES!!!They warn about this in the manual, but I only got a manual *after* I had done it. So I spent a lot of time cleaning repeatedly with IPA. Apparently this is very bad for the capacitors in the attenuator.Insert a piece of bond paper between the contacts and pull it back and forth instead.Very likely you have bad solder joints and if it's a socketed transistor model, you'll need to pull all the transistors part way out and then reseat them. A retired Tek service guy taught me to whack the corners of the frame with a small stick to check for intermittent connections.. Doubtless someone will pop up to say that will damage the CRT guns. That is BS. Any impact that would damage the gun electrodes would demolish the scope. What *will* destroy the gun electrodes is fatigue caused by vibration near their mechanical resonant frequency.I bought an 8560A CRT based spectrum analyzer. It had been poorly packed and dropped in shipment. Fortunately, the only damage was a bent carry handle. I took it off, took it to my shop and pounded it back into shape. It's ~1/8" aluminum and inch or so wide. Bending that took a *lot* of force as did straightening it.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 21, 2018, 02:35:20 am
content: Quote from: rhb on May 21, 2018, 01:40:03 amAgreed, I should have specified I was referring to the basic spring loaded square/selection mode switches, with lube access in the rear, they dry up badly and affect a lot of operationsAttenuator and Time Base switches best left for last, and of course do those differently (and patiently)

author: F3nn3c
author_rank: Newbie
timestamp: May 21, 2018, 06:10:55 pm
content: OK, thank you all for the suggestions. I will start with a contact leaner only on the push-button switches, and use IPA for the rest. BTW, after warming up overnight (with intensity turned off), there is a very marginal improvement in stability, but still far from useable...

author: oventech
author_rank: Contributor
timestamp: May 23, 2018, 12:45:49 am
content: Found this on a 475.https://www.amplifier.cd/Test_Equipment/Tektronix/Tektronix_other/475.htmOn repairs done, calibration. The site is in german. Used Firefox and translated the page. Good luck with the scope.

author: Ray Gianelli
author_rank: Contributor
timestamp: May 23, 2018, 07:15:15 pm
content: +1Voltages in oscilloscopes are tight spec wise. Regulated supplies should have VERY little ripple, usually just millivolts.These scopes are at the age now where the filter caps are bad and will need to be replaced.A lot of the voltages in these scopes are referenced from one of the 50 volt supplies, can't remember if it was -50 or +50.

author: deepskyridge
author_rank: Regular Contributor
timestamp: May 24, 2018, 12:31:41 pm
content: I just repaired a tek 465, replaced all 5 big can caps in the power supply. 110v rail had 22 volts of ripple. -8v rail had 9 volts of ripple. Works great now. I would replace these first.Good LuckGary

author: F3nn3c
author_rank: Newbie
timestamp: May 24, 2018, 01:11:29 pm
content: Oh, boy... OK, I guess after I finish the initial light cleaning, I will replace the PS filter cans - just as a precaution... Should I chose the lowest ESR available? Any suggestions on the make/source? BTW, the serial number of this unit is B264214, which puts its date of manufacture... around mid-80's ?

author: bd139
author_rank: Super Contributor
timestamp: May 24, 2018, 01:25:44 pm
content: Don't worry about low ESR.You won't get replacement cans that fit the holes perfectly now so some creative work might be required. Be aware that they have three pins and bridging two of those pins is required on the board.I use Rubycon, Vishay, Panasonic, EPCOS ones which are same rating both in capacitance and voltage (go higher on the voltage if you need to but never lower). DO NOT increase the capacitance value if you can help it. This causes more inrush current when you fire it up and might finish off the bridge rectifiers.But BEFORE you do this, measure the ripple on the voltage rails. I have a 1974 unit and the caps are still perfectly fine. They are extremely high quality ones in these scopes. There's absolutely no reason to replace them if you can eliminate this problem through measurement first.

author: PaulAm
author_rank: Frequent Contributor
timestamp: May 24, 2018, 05:03:11 pm
content: Removing the filter caps can be difficult at times without damaging the PCB, so if the caps are still good, I'd leave them in place. Let the specs be your guide here, if the ripple is within specs, leave the caps in place.For replacement I like to use 105C snap caps soldered to an adapter board that matches the PCB footprint and takes care of tying the three ground pins together. There's a thread around here somewhere about fixing a 7000 series I think that has links to gerbers. That makes for a very clean repair. If you can etch boards locally, you can make up a pattern with any art program.

author: Old Printer
author_rank: Frequent Contributor
timestamp: May 24, 2018, 09:11:39 pm
content: Nothing technical to add, but that scope appears to be in very nice shape. I have a 475 I picked up around 2005 and it still runs fine. 100% original components as far as i can tell. I don't use it much, but every time I fire it up I hold my breath

author: F3nn3c
author_rank: Newbie
timestamp: May 25, 2018, 12:26:01 pm
content: On a slight tangent - does anybody know the part number / source for what appears to be a front cover for 4xx series scopes, equipped with a DM option?