Tektronix 475 sweep and triggering problems - Page 1
Forum section: electronics / repair

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 22, 2025, 12:43:14 pm
content: Hi all,I've got a Tek 475 that has multiple problems (vertical atten contact issue, horizontal sweep and/or triggering issues etc) but while troubleshooting the unit it silently went off.Checked the fuse and it's good and then I measured all rails to ground discovering that I have a dead short (0 ohms to gnd) on the 50V rail.Thing is that I can't seem to find where the short is.I was thinking to post here to get help with the sweep/trig(?) issues I was having but now I have to fix this first in order to proceedI checked all the tantalum capacitors on the 50V rail on the bottom board and on the triggering board but I didn't find any shorted ones.Any clues on what I should check next? What can become short circuited without any signs of smell etc?The unit went off silently no smell, no bangs not anything..

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 22, 2025, 06:03:59 pm
content: Okay I managed to find the shorted tantalum it was C1428 the first one right on the 50 rail! But I somehow missed it on my first pass.Anyway the scope is back and running as before..I get no trace on almost half the timebase settings meaning from XY up to mS. From the μS mark and on I can get a trace.I don't know if it's a sweep or a triggering problem.I suspect it's a triggering problem since if I mess with the trigger level I can randomly get a trace at the lower timebase settings..The B channel is somewhat in better shape than the A one but that might just be the vertical attenuators so I think that for troubleshooting purposes I would say both channels behave about the same.I know about leaky transistors on this scope and I might just replace all of these at some point but I just want to make it working again if possible.Logic says that if both channels are somewhat equal in their triggering, since they have the same triggering circuits for each one the problem might lie elsewhere?I really hope it's not the Tektronix sweep IC..How should I proceed from now on.I noticed that on the triggering board a point marked -8.2V is more like -8.52V I don't know if this is a problem..

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 25, 2025, 10:19:12 am
content: Update on this:With the help of the TekScopes mailing list I was able to fix the issue on some timebases not showing a trace (open resistor in the timing board).No I'm still left with two major faults that I'd love some help with.1) I can't get a proper trace from the 10mS setting and below, the signal is far right out of the screen and compressed to one graticule and I have to bring it to the left with the h. position knob in order to see it otherwise it's just a glow on the screen2) I can't get sweep below the 50uS mark. Without a signal input I can get a flat trace showing when in AUTO mode and all trig sources in the upper position only when the timebase setting is at the 50uS mark and above, the trace vanishes.I don't know if those issues are interconnected. I posted part of the schematic of the timing board with highlighted parts thatI have issues, on the green settings I can get a trace when I input a signal but no sweep without it, and on the yellow settings I don't get a correct signal with or without a signal input.Any help would be appreciated!

author: ondraN
author_rank: Regular Contributor
timestamp: August 25, 2025, 12:05:12 pm
content: I have encountered faulty time base capacitors (10uF, 1uF, 0.1uF) on several 4XX series oscilloscopes. Try measuring them. They should have accurate capacitance and very low ESR. The problem may also be with the switch contacts. I also recommend measuring their contact resistance.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 25, 2025, 01:05:14 pm
content: Ι can't find any connection to C1071 to the gate of Q920.The thing is that I can't follow the trace because it goes underneath the switch assembly and then it gets lost.My board is newer or maybe older than the one shown in the SM so I can't follow these traces.I really need to find a photo of that board (gd-3980-02) with the switch cover removed so I can see where this leads.The other issue is that I can't find the connection of Q920's base to the timing board connector either.I really hope it's not a connection problem since this gigantic switch is scary and I don't want to take the cover off.Capacitors measure fine so it must be this thing that's keeping it from working.

author: ondraN
author_rank: Regular Contributor
timestamp: August 25, 2025, 01:38:07 pm
content: Don't worry about the switch. The carousel can be removed quite easily and the contacts are soldered to the PCB. I have the time base board at home, so I'll try to take a picture tonight or tomorrow.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 25, 2025, 02:53:20 pm
content: Thanks so much for offering to take photos!Luckily it won't be needed since it's now fixedI disassembled the switch and found corrosion underneath specifically in this pin!Took the time to do the IPA and paper strip thing and this restored the connection from the capacitor to the PCB header.The other connection maybe needed a reseat of the whole board or maybe was just a glitch but I swear I couldn't measure any resistance from the pin 6 of the connector to either transistor (Q904 and Q920).I'm glad I didn't give up on this although I admit it took much longer than I anticipated.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 25, 2025, 06:35:46 pm
content: One thing that I think is not quite right is that I have trace flicker up to about 20uS/div which is pretty high up in time/div.The trace flickering on my 2235 stops at .5mS/div.Another thing is that at 'XY' mode the dot should stop but it keeps moving with the same speed as the previous setting.

author: ondraN
author_rank: Regular Contributor
timestamp: August 25, 2025, 08:32:15 pm
content: It is very difficult to give specific advice. I would start by checking all the contacts. Remove the carousel (just the screws from below) and see if they are in the correct position and shape, if any are broken off, and verify by measuring that each of them has good contact when pressed. I would also measure all the capacitors and resistors in the timing circuits. And also check if the carousel axes are not rotated relative to the control double knob. If everything is OK, exciting detective work begins with the help of a second oscilloscope

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 25, 2025, 09:24:11 pm
content: What is very weird to me is that the traces are now correct regarding to the timebase settings.The flickering I'm describing is persistent like I'm in a lower timebase setting.From what I understand if the screen corresponds to the frequency of the applied signal the switch contacts and capacitor/resistor combinations have to be okay otherwise wouldn't I get wrong results?I don't know about the XY mode but I don't care for this as much right now but the flickering is kind of annoying and certainly shouldn't be there as I checked a 475 on a youtube video and it doesn't have it!Would really like to troubleshoot this further but I wouldn't know where to begin since I don't know what would cause this error.Ι do have other oscilloscopes but I guess I'd need a second working 475 to troubleshoot this one..

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 26, 2025, 06:24:48 pm
content: Played with this again today and noticed that it doesn't really stop with higher timebase settings it's that the trace is faster and masks it somehow.I guess that's good as I wouldn't like to be timebase switch related since I cleaned those hard to reach contacts yesterday!I'm stuck right now as to where this flickering/pulsing issue might be coming from..So far the only thing from the front panel controls that affects this issue is the trigger holdoff potentiometer, with this pot I can make the "pulsing" of the signal slower.Would this indicate that my problem may be in the triggering logic somewhere?

author: ondraN
author_rank: Regular Contributor
timestamp: August 26, 2025, 07:58:15 pm
content: Dismount switch is easy. A Torx T5 can be used for the potentiometer coupling screw.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 26, 2025, 08:04:38 pm
content: Yes I have already did that to fix my timebase settings that were erratic, thanks for the pictures though!The timebase settings are okay(minus the XY mode which I will probably investigate last) and I don't see any glitches even by moving the time/div.My issue now is the trace flickering/pulsing on and off like the holdoff setting is not fast enough to keep it stable.I already dismantled and cleaned the holdoff pot/switch but nothing changed.Do you think that the current problem has something to do with the time/div switch?

author: ondraN
author_rank: Regular Contributor
timestamp: August 27, 2025, 07:45:19 am
content: These problems may or may not be related. I would first solve the X-Y mode problem. There is one contact (section 1, P5-2) that switches the 5V power supply to three other boards. I would check if this voltage reaches all the connected circuits and that they work.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 27, 2025, 12:16:53 pm
content: When in X-Y mode I don't see any continuity from 5v to P5-2 (anodes of CR1111, CR1104 etc) which is a huge lead to go on!I took the timing board out again (for 3rd time) and re-cleaned all the contacts, reassembled it and then looked underneath when it's at the CCW position.The switch contact that passes 5V to the P5-2 pin is the very first one counting from the left.Both traces of the switch contact are correct (5V and P5-2) but I don't see the contact pressed down when I'm in X-Y position.Is there any possibility to assemble the switch assembly in a wrong way?Because if it is I may have done so?! Which is also weird because how all the other timebase settings are correct and only this one is false?Wouldn't it be all over the place??If it's impossible to assemble this then how is it possible for this switch contact only to not press right since this done by the weight of the carousel thing.update: As it turns out you can actually put this thing the wrong way..I placed it with the switch turned CCW and I measured continuity between 5v and that P5-2 pin, I put it back together and now it's somehow stuck in something like X-Y mode, dot stuck in middle but I get no horizontal movement when I play with CH1 I get diagonal movement(?!).Also no matter how I turn the switch,I'm stuck in this weird mode.I really hope I didn't break it entirely..Is there a certain way that I should put this back togehter??I'm really lost here..

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 27, 2025, 02:10:30 pm
content: Update #2:It turns out that half of the carousel was rotated probably by not setting the knob set screw correctly and the concentric part of the assembly was in a wrong position.Probably I did this by mistake or whatever.Setting both parts of the switch CCW and screwing both knob parts tight restored the functionality of the scope.I also have stable signal and X-Y mode which is nice!I don't know what fixed it because I did the contacts twice.It must've been that the plastic part of the switch was somehow rotated.

author: ondraN
author_rank: Regular Contributor
timestamp: August 27, 2025, 02:41:09 pm
content: Both carousel shafts move independently and have only a stop on both sides. Their movement is synchronized by a lock between the TBase A and TBase B knobs. When you insert the knobs, it is necessary to have both sections on the stop counterclockwise and secure the knobs in the X-Y position. Otherwise, there will be mutual displacement and problems. In the first post I asked if the knobs were in the correct position, so I meant this.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: August 27, 2025, 02:59:53 pm
content: I didn't quite understand what you meant by position so I had to learn it the hard way I guess.One thing is for sure though, the contacts really did need cleaning so the first pass was required.I think thats's where I messed up the position of those two sections and created the flickering issue which was related to the X-Y mode as it seemed.I got distracted because the timebase settings were correct so I thought the issue was elsewhere and lost quite bit of time in random parts of the board reseating and checking transistors etc.Anyway I'm glad this is over.I really appreciate your help ondraN

author: ondraN
author_rank: Regular Contributor
timestamp: August 27, 2025, 06:44:29 pm
content: Yes, it's quite tricky. I've renovated many of these oscilloscopes, so it didn't occur to me that these things are unknown to anyone. The main thing is that it turned out well and the oscilloscope is ready again. They are beautiful devices, even after all these years.