tektronix 475 restoration - Page 1
Forum section: electronics / repair

author: raymond008
author_rank: Contributor
timestamp: December 14, 2022, 12:09:51 am
content: I am trying to restore a tektronix 475 oscilloscope.First turned on ended up with smoke coming out from the bottom side of the scope ...My gut feel is a lot of issues will be related to bad cap somewhere.The first one is the -15v got shorted down to -5v. It's the tantalum cap (C1478) on the output that shorted. That also killed the drive transistor (Q1478).However, that is for external probe supply and doesn't interfere with the normal operation. Replaced the transistor and cap.Now I got a trace but it look fuzzy ... It's likely on the vertical amp somewhere ...Checked all the main supplies and they seems to be ok.Anyone seen it and have some pointers for me to look at?

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: December 14, 2022, 02:01:30 am
content: What happens when you adjust the intensity, focus, or astig controls? I'm not an expert, but that doesn't look like a vertical amp problem to me.

author: raymond008
author_rank: Contributor
timestamp: December 14, 2022, 02:35:33 am
content: It turns out half of the 'noise' are because of several factors:1. I was using 10x probe for the calibration which is 300mV. So I have to increase the vertical sensitive to 10mV2. I have the case open with a bunch of equipment and LED lights around itOnce I turned on the 20MHz BW limiter, the noise level become more acceptable.I then compared that with tek 2465 (300MHz) scope and I see similar noise level as well.I then close the case, connect a signal generator with coax with 50ohm termination, the noise level drop back to normal (sort of).However, I still can see some fuzziness and noticeable noisier than the 2465.So, I think there is still room for improvement (i.e. bad cap somewhere ...).It look similar to another tek464 I have which also went through some repair ... I would expect tek465 will be closer to 2465 behavior.Attached is what it looks like now with 20MHz BW limiter.

author: raymond008
author_rank: Contributor
timestamp: December 14, 2022, 02:49:15 am
content: BTW, fine tune astig further improve the trace a bit.Thanks for the tips.

author: raymond008
author_rank: Contributor
timestamp: December 14, 2022, 03:58:05 am
content: While the quality/performance of the scope is still under evaluation, I have another problem to solve.The connection 'nut' between the vertical sensitivity variable control and the pot behind it broke.It doesn't seems listed in the service manual.Anyone happen to know the part number of this thing or suggestion how to fix the crack in the aluminum ring?

author: med6753
author_rank: Super Contributor
timestamp: December 14, 2022, 04:10:47 am
content: JB Weld and clamp it tight for at least 24 hours.

author: jonpaul
author_rank: Super Contributor
timestamp: December 14, 2022, 07:21:56 am
content: Bonjour cher MonsieurBravo, 465/475/B are One of my first portable Tektronix scopes, Wonderful classic instruments, well worth restoring! Get the service manual for your serial number.Typically these classic 1970s scopes need recapping of the PSU, and fan attention.Heir worst issue is degraded HV transformers and HV multipliers over the decades, eventually causing sudden or intermittent HV failure. See the many threads on 465,475,B HV transformers and multipliers.Besides this forum, highly reccomend groups.io tektronixscope and Tektronixscopes 2 forums, many top Tektronix CRT veterans and afficiaidos.For recapping tips see Condor Audio in Israel.Parts may be still available from Qservice in Greece.Bon courageJon

author: raymond008
author_rank: Contributor
timestamp: December 14, 2022, 08:28:47 am
content: Thanks Jon for the info.Part of my goal in this restoration effort is to learn the process and methodology in fixing tektronix scopes.Blindly recap will take away the fun of debugging and diagnosis process and I won't be learning much out of it.So, I'll rather fix/learn it the 'hard way' :-) i.e. find the exact problem and fix what's needed.Back to the last problem : broken shaft connector.Before I can find the replacement part, I managed to fix it up with a spring and hot-shrink tube.It seems does the tricks for now.

author: raymond008
author_rank: Contributor
timestamp: December 14, 2022, 09:27:05 am
content: The next step is heat issue ...The scope is consuming 90W and the transformer gets pretty hot.The +50v (Q1426) and +15v (Q1448) driver transistor also seems hot.I am afraid it's going to burn if I leave it on for a prolong period of time.Is that normal?On the other hand, tek464 uses the same power but doesn't seems to produce that much heat.

author: jonpaul
author_rank: Super Contributor
timestamp: December 14, 2022, 11:01:36 am
content: Dear Raymond bravo pour l'effort0/ first get service manual, see w140, Artech had ( costless) printed reprod.Now check every power supply V, ripple, and troubleshooting or adjust if out of specs.Then exercise and clean every switch and control, specifically attention to the beam finder, and HV ( focus intensity astigmatism)1/ shaft coupler is available as spare part,2/ power 60..90W is about normal"seems hot"....use thermocouple or IR temp gun ( not accurate) for transistors températureFinger touche can be misleading3/ The old Fans often fail, or slow down, check fan and air filter for good airflow.4/ recapping...agree, easy to spot visually inspection of all lytics and tant in PSU.symptoms of dead lytics..Bus V out of spec, .leaking corrosion, Cap has High has high ESR, or low capacity, high 120 Hz ripple on bus.enjoyJon

author: raymond008
author_rank: Contributor
timestamp: December 14, 2022, 01:43:39 pm
content: Hi Jon,0/ I downloaded the service manual.I’ve checked all the supply and that’s how I found out the -15v were off. I didn’t check the ripple in detail though.A glance with a scope don’t see major issue. But will revisit.I spray contact cleaner for all the switches/pot as much I could reach without dissemble PCBs.1/ Do you know the part number of the shaft coupler?2/ I am waiting for my thermogun to arrive to recheck heat source.The obvious ones are the transformer and power transistors.Seems abnormally hot. The whole case warm up after 10 minutes or so.I’ll do a detail temperature and heat source inspection later on.3/ Is there a spec for the fan speed?4/ I am pretty sure there were smoke coming out when I first turn it on.But I can’t find burning marks anywhere (yet). The titanium cap I replaced don’t have any burn mark either.When titanium cap burn, what does it normally look like? e.g. bad lyric cap usually bulge, leak etc …

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: December 14, 2022, 04:32:17 pm
content: I'd be very careful around any titanium caps, especially if they are burning!

author: raymond008
author_rank: Contributor
timestamp: December 15, 2022, 04:47:22 pm
content: Found the part number for the shaft coupler: 376-0051-00 Flexible couplerFor the power/heat side, I changed the power selector to be 120v (Hi), (that's what we have here in the US). It was on 115v (M).I didn't expect it will change much but somehow the heat level seems to drop down to a more acceptable level. Still higher than I expected.The current consumption is 82w now instead of 90w.Next step is to verify the delay sweep function which seems wacky ...Likely need some cleaning on the pot or time/div knob.The scope is now functional for the most part.