Tektronix 475 Oscilloscope - Help With Repair - Page 1
Forum section: electronics / repair

author: Ookpik
author_rank: Contributor
timestamp: October 22, 2019, 01:04:24 am
content: I recently acquired a Tektronix 475 oscilloscope.The scope powers up but does not get a spot or trace.I checked all the voltages at the test points with the scope powered ON . The readings were as follows:+5 - 4.99-8 - 7.99+15 - 15.0-15 - 14.99+50 - 49.99+110 - 111.5Everything appears to be within tolerance.What do I check for next? I have repaired monitors and PCB's but this is my first attempt at scope rebuild.As an aside - with the unit OFF the +5 and -8 test points have continuity to ground (shorted).Any help would be greatly appreciated.

author: tautech
author_rank: Super Contributor
timestamp: October 22, 2019, 01:59:53 am
content: Quote from: Ookpik on October 22, 2019, 01:04:24 amDo each of the rails meet the ripple spec ?Pop a link to the SM here too so others can play along at home.This might be useful to study:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2019, 03:39:19 am
content: Is the CRT heater glowing? Can you check the voltages on the control grids?

author: lordium
author_rank: Regular Contributor
timestamp: October 22, 2019, 06:47:05 am
content: Check all internal connectors. I had something similar with my 475A. Turned out to be a connector to the crt got loose during shipping.

author: andy3055
author_rank: Super Contributor
timestamp: October 22, 2019, 07:09:41 am
content: Looks close to the 465 M I am trying to fix. Check fuse 1318 (1.5 A) to start with.

author: med6753
author_rank: Super Contributor
timestamp: October 22, 2019, 08:30:20 am
content: Agreed, check the HV fuse on the primary side of the HV transformer. Known fail point is shorted tantalum capacitors just before the transformer. Will be either two 22uf's or one 47uf.

author: bd139
author_rank: Super Contributor
timestamp: October 22, 2019, 08:33:17 am
content: Definitely those tants are a weak point. Lost count of the amount of them I’ve fixed with that failure.

author: Ookpik
author_rank: Contributor
timestamp: October 22, 2019, 02:28:31 pm
content: Thanks for all the responses!I did check the 1.5amp fuse in and out of the mount and it tested fine. I can’t verify heater glow because the tube has a shroud around it and I’m unfamiliar with how to remove it to see the CRT neck. Any advice to confirm heater glow appreciated! Also if there are specific CRT connections you can point out as I’m not sure where to look to verify looseness, cracked solder, etc. I’m beginning to believe this might be the issue.I know the scope is close to working as I stood it on its back end after removing the CRT cover (to try and see the CRT neck glow)..I got this!But the trace seemed to fade out or lose connection when I laid it back down…..then no trace.Any guidance appreciated as the high voltage section makes me very cautious to not poke around until I’m completely familiar with this unit.

author: andy3055
author_rank: Super Contributor
timestamp: October 22, 2019, 04:58:38 pm
content: Great! Now that you have a trace, you need not worry about the HV supplies or the tube etc. My guess is that you have a poor connection between the back panel mounted parts and the main board. Hook up a probe to the Calibrator output and see if you get the square wave. After that shut down the power and after giving it some time to discharge various voltages, try removing/re-seating all the connectors on the back panel. I am pretty sure you have a perfectly working scope other than a loose connection which is very common in this vintage machines.Let us know when/what you find.

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2019, 05:05:13 pm
content: You can also pull the cover off, power it up and start poking at things with a non-conductive stick, see if you can find a ticklish spot.

author: Ookpik
author_rank: Contributor
timestamp: October 23, 2019, 02:08:24 am
content: Any particular back panel mounted parts I should be concerned with?I did quite a bit of poking around with a non-conductive stick but I was unable to produce any desirable results.I shook the scope out but unfortunately there was no change. The trace comes and goes based on the orientation of the unit. There is a very slight CRT crackle that corresponds to the screen and trace going in and out. I poked around in the high voltage section and nothing changed. When the trace was on I did get the calibration square wave on both channels. I can't seem to find a loose connection.I have worked on vector tubes/chassis before and this issue is usually caused by cracked solder on the deflection/HV PCB.Can the tube be easily discharged, connected and reconnected or would a complete tear down be necessary to accomplish this?The Channel 2 inverter switch appears to work when the trace is on the screen but the trace will then fade out and reappear in the same position. The problem doesn’t appear to be vertical or horizontal collapse but more like the tube isn’t being continually energized. Is there a way I can see if the CRT has neck glow or is this something that is not easily accomplished on this particular scope?Any ideas would be greatly appreciated!

author: andy3055
author_rank: Super Contributor
timestamp: October 23, 2019, 03:52:35 am
content: Check those big transistors on the right on that image. I can see 2 of them. You will have to check all the solder points from everything mounted on the back panel to the main board.Poking may not show the issue as it seems to come up only when you put it on its back which provides some weight.The waveform looks great.

author: tautech
author_rank: Super Contributor
timestamp: October 23, 2019, 07:01:18 am
content: Quote from: Ookpik on October 23, 2019, 02:08:24 amYep you're on the right track IMO.Cracked solder joint or component where it's subject to thermal cycling.andy3055 is pointing you in this direction too.Power devices and high wattage resistors is where I look first and often there's some carbonizing that gives the game away. Still, you might need magnification to spot it if it's just a dry joint and not a decent easy to spot crack.

author: med6753
author_rank: Super Contributor
timestamp: October 23, 2019, 04:32:17 pm
content: I would suspect an issue with the CRT socket. Either loose or corrosion. But be careful, there are voltages back there that will bite.And although it's unlikely there COULD be an open in the CRT itself that is "made" when the scope is tilted up. But hopefully that is not the case.

author: james_s
author_rank: Super Contributor
timestamp: October 23, 2019, 06:28:03 pm
content: I did have a Tek 2215 that had a loose bit of metal floating around inside the CRT, occasionally it would lodge somewhere and cause the scope to sit there chirping instead of powering up. In that case though I could hear it rattle around inside.One of the pictures here looks like the tube face is glowing as if the beam is striking the side, glancing off that and spraying the screen. You might also check the wires that connect to the deflection plates on the neck, although that wouldn't cause it to fade out.IIRC the cathode on electrostatic deflection CRTs runs at a substantial negative voltage relative to ground so as was mentioned above do be careful poking around the CRT socket.

author: Ookpik
author_rank: Contributor
timestamp: October 24, 2019, 02:34:36 am
content: I discharged the CRT. I then removed and cleaned the CRT socket. I also removed and cleaned the vertical and horizontal deflection plate connections. I can hear some debris rattling in the CRT or elsewhere when the scope is tilted. The result was the trace now seems to stay on when the scope in the proper horizontal position but the image flickers.Does the rattling sound produced by the CRT indicate that it needs to be replaced?You can see the scope's current state of functionality by clicking the link below. If someone can tell me how to embed the video I would be happy to do so.https://youtu.be/tfYGadIc9LM

author: andy3055
author_rank: Super Contributor
timestamp: October 24, 2019, 05:03:57 am
content: I am not sure if the noise is from the tube itself unless you removed the tube and tried tilting it around. It could be something else rattling under that main board. I did see the video clip. It is probably something else causing it. Look for more flaky connections. It may be a good idea to unplug the boards and (clean the contacts with an eraser) and re-seat them.

author: Ookpik
author_rank: Contributor
timestamp: October 28, 2019, 01:59:59 pm
content: I removed and tested the HV oscillator – Q1318 – and it tested fine.I did find some cracked solder around the power transistor, which I reflowed. No change.I reflowed the solder around the other power transistors as well.I reseated all the board connections. No change.With the Intensity turned all the way down you can see the angled screen like the electrons are not even.

author: james_s
author_rank: Super Contributor
timestamp: October 28, 2019, 03:42:03 pm
content: With the intensity all the way down there should not be any glow on the screen at all. I think something is not right inside the CRT, stray electrons are spraying off of something. Maybe someone has other ideas of what could be going on but that definitely doesn't look right. I think at this point I would very carefully remove the CRT and rock it around in isolation to determine if that loose piece you heard is inside it or not. The Tek 2215 I had definitely had a loose piece rattling around inside the tube that would occasionally lodge somewhere and short something out.