Tektronix 475 in need of repair - Page 1
Forum section: electronics / repair

author: guenthert
author_rank: Frequent Contributor
timestamp: October 22, 2019, 05:29:21 pm
content: A buddy of mine acquired a non-working Tektronix 475 and handed it to me to repair. I cautioned already, that such a project exceeds my level of experience and skill, but he couldn't be discouragedFirst it did greet me with the magic smoke. At least that was an easy fix: a blown Tantalum capacitor in the +15V PSU sektion. There was another bad one (which at first glance looked ok) and two blown Transistors in the -15V PSU section.It works now in as much as there is a trace, albeit it is very wavy/fuzzy and I can't adjust the Y offset sufficiently to center it (the 'beamfinder' has some effect, but not exactly the desired one). At least HV and timing seems to be working.The DC values of the various PSU subunits look Ok, there is some excess ripple, but I'm not sure how much that'll be expected from a 40+ year old instrument and how much of an impact that'll have (might explain the wavy/fuzzy trace, but hardly the offset).Fortunately I found a service manual online (else I wouldn't have attempted the repair). There are however test points on the PCB which don't seem to be described in the manual. Is there another document out there which does? I was even tempted to buy a working unit (recently there were *two* Tek 465 on craigslist, alas on seller didn't respond at all and I was apparently too slow for the other) to compare them.I also am a bit stumbled by some quite high frequency noise (with components as fast as ~140MHz), seemingly (I used a small loop of wire as make-shift EMF probe) emanating from the area where U1454B and Q1446 sit. I'm pretty sure, they're not meant to do that (and don't think U1454B actually can oscillate that fast). What could that be (I *think* it comes from the area and not the nearest FM transmitter ;-} ? Is that normal? If not, how is that meant to be suppressed?

author: m3vuv
author_rank: Super Contributor
timestamp: October 22, 2019, 06:05:28 pm
content: dont know if it helps,a few weeks ago i repaired a 453a with no trace,found a couple of bad transistors,that got a trace of sorts,it was fat and fuzzy and wouldnt focus,asked on a few forums with no joy,noticed i was testing if with the hv aluminium covers off,as soon as i put them back on,it had a clean nice trace,no one seemed to spot it here!,just a thought m3vuv,73

author: guenthert
author_rank: Frequent Contributor
timestamp: October 23, 2019, 05:04:58 pm
content: Quote from: m3vuv on October 22, 2019, 06:05:28 pmThanks, but I didn't have a need to remove the HV covers (and am not equipped to test/repair anything in that section anyhow).

author: james_s
author_rank: Super Contributor
timestamp: October 23, 2019, 06:42:58 pm
content: The excess ripple is absolutely going to be an issue. The very first thing I would do is check/replace all of the electrolytic capacitors in the power supply, the 475 is old enough that some of these are likely to be bad. Get all of the power supply outputs in spec before you even start to troubleshoot other areas.

author: guenthert
author_rank: Frequent Contributor
timestamp: October 25, 2019, 09:12:35 pm
content: Earlier today I noticed that the fuzzy/wavy waveform displayed changes a bit when I tap the PCB at various places with a pen (around the middle of the PCB on the button it seems to be most sensitive). Not sure, if that's to be expected. Forgot to mention, but I checked most, if not all, transistors for seating.The fuzzy/wavy waveform (seemingly independent of input signal) is only displayed when the vertical offset is adjusted to one of the maxima (and even then it's off-center). Earlier I had it display 'GND' signal, triggered on line. I tested with the buitl-in (in spec) square wave test signal and noticed that the trigger lamp is lit only when the vertical offset is adjusted sufficient to also display the waveform.More interesting perhaps, the aforementioned high frequency 'ripple' (that might be a misnomer, as others might think of ripple as a line frequency component) or rather noise varies with the setting of the vertical offset! On the +50V line, there is this noise with a pk-to-pk level of up to 350mV, but with the vertical offset, it can be adjusted to as little as about 20mVpp. Unfortunately, then the wavy/fuzzy signal is displayed off screen. Baffling.

author: guenthert
author_rank: Frequent Contributor
timestamp: October 25, 2019, 11:30:14 pm
content: The Tek 475 has a signal output, reflecting channel 2 input. The signal there matches the input (here the calibration square wave) closely when set for 20mV/gratitude, as it should. The output signal's DC offset can be minimized ("centered") by adjusting the vertical offset to some value around the middle of its range.I understand this to mean that the vertical amplifier is working just fine, but the signal gets somehow severely distorted (squashed, made fuzzy expanded, with offset) before being displayed.

author: guenthert
author_rank: Frequent Contributor
timestamp: October 26, 2019, 02:20:23 am
content: The position and form of the displayed signal changes when I tilt the chassis (in no angle there's a proper signal though). Attempting to bend the PCB doesn't change the signal. Seems somethings is lose, rather than a fissure in the PCB ...

author: guenthert
author_rank: Frequent Contributor
timestamp: October 26, 2019, 10:09:33 pm
content: There seem to be quite a number of revisions of the Tek 475A. I had trouble finding R488 ("vertical output bias adjustment") given the picture in the service manual. Turns out that mine has a protection plate mounted over the CRT which also covers the vertical output amplifier. Uncovering that, I found that the range of that potentiometer is insufficient to center the signal, a good indication that something else is wrong.Incidentally, I was able to find the source of the dependency of the displayed signal from the angle of the chassis: the vertical output amplifier PCB was mounted only with one screw remaining ...While there, I also probe'd the output of the delay line and found that the signal there matches the displayed signal, i.e. fuzzy/wavy, offset and in odd ways depended on the vertical offset adjustment (the signal expands and gets quite large in certain positions). This makes me thing, the problem is not with the vertical output amplifier, but before (and after the signal is taken to the external output).I don't think anyone is interested in watching me performing my first repair of an analogue oscilloscope, but this is just too convenient to keep notes to pass.

author: guenthert
author_rank: Frequent Contributor
timestamp: October 27, 2019, 10:12:18 pm
content: I just noticed that the delay line carries a differential signal (huh? Everything else is GND referenced). With the vertical offset adjusted to near center, the difference of the signals on the delay line does indeed become zero. Then I'd think there ought to be a signal displayed in the center of the screen, but the "vertical output bias adjustment" cannot be sufficiently adjusted.I had a look at the voltages on the plates. There is a (small) signal on top of a (large) DC offset: about 11V on one and >30V on the other, which changes very little when I vary aforementioned "vertical output bias adjustment".I wanted to have a close look at the vertical output amplifier and then noticed that U470 is meant to be screwed to the chassis and wasn't. I wonder now how badly it missed its heat-sink.

author: bd139
author_rank: Super Contributor
timestamp: October 27, 2019, 11:46:50 pm
content: The whole scope is differential past the initial input amplifier.Sounds like half of the final amplifier is gone. I would however screw it back to the chassis and check the DC voltages around the IC while the trace should be centred. It may be an ancillary component ie resistor that has gone bad and someone got half way through fixing it and gave up. U470 is a 152-0082-00 if I remember so probably will require obtaining a parts mule 475 or 475A. I think that has distributed deflection so it’s got two sets of horizontal drivers as well.This is why I’ve given up fixing tek scopesEdit: it’s important that you trace the differential signal from the front end if you can. I tend to stick a 1khz waveform on an input then trace it through with another scope until I get to the bit that doesn’t look right.

author: guenthert
author_rank: Frequent Contributor
timestamp: October 28, 2019, 04:37:32 pm
content: Darn, I keep writing 475A, but in fact it's the older, slightly slower, but more sensitive, 475.Quote from: bd139 on October 27, 2019, 11:46:50 pmIndeed, I just didn't notice before. I was a bit unsettled by that, as more than once, I was frustrated by the single-endedness of oscilloscope inputs. Why aren't they differential all the way?Quote from: bd139 on October 27, 2019, 11:46:50 pmYou mean, when the + and - components of the signal (as on the delay line) add to zero (and no trace is visible on the CRT)? I can probe the DC voltages around the IC, but without list of expected values, I'm not sure how much that's going to tell me. With the help of the schematic, I might come up with some approximated expected values for some, but as U470 is a black box, I'd have to assume high impedance inputs (which clearly will be wrong for the outputs, if I can identify those). But yes, that'll be the next step.I have to screw it back in anyhow as, if there is a trace displayed (with the vertical offset sufficiently adjusted), that varies greatly whether U470's brass(?) screw touches the chassis or not. I was even wondering, whether that's its sole GND connection, but at least the thick cable soldered (!) to its head and connected to the protection tube around the CRT is on GND as well. So first I have to find such a nut ...Quote from: bd139 on October 27, 2019, 11:46:50 pmThe service manual I'm looking at lists it as 155-0082-00, but I have difficulties remembering the four digit PIN of my checking card ...I'm afraid, due to its weight, purchasing a defect Tek 475 of fleebay is financially not a viable option (need to clarify with the owner, whether there's interest beyond that though). Sadly the "vertical output amplifier sub-assembly" doesn't seem to have a product number or catchy name either, so the chances of acquiring such separately seem slim.Quote from: bd139 on October 27, 2019, 11:46:50 pmI didn't intend to pick that up.Quote from: bd139 on October 27, 2019, 11:46:50 pmYup, that's the approach I attempted (might have subconsciously remembered what I read about folks repairing HiFi amplifiers). I'm using the 'calibration' signal of my (thankfully working) 'scope.

author: guenthert
author_rank: Frequent Contributor
timestamp: October 28, 2019, 05:08:38 pm
content: Hmmh, Fleebay has a 155-0082-00, so that might be an option, once I determine that the one installed is defect (I'm not quite there).

author: guenthert
author_rank: Frequent Contributor
timestamp: October 29, 2019, 05:17:43 am
content: Funny, the nameless little PCB holding LR496/7, R497, R491 and C491 is mounted using just the right nut. So yeah, this is Peter/Paul, but I'm still only determining, whether this can be repaired with reasonable effort.With U470 screwed on tight (but not too tight -- the brass screw doesn't look all that strong), I _think_ the trace is now displaced at a lesser offset, but still, quite off and the displayed trace doesn't really resemble the signal (that might be due to the excessive noise).If I'm very quiet, I can barely make out that the fan noise depends on the vertical offset adjustment! So something is still seriously wrong.

author: bd139
author_rank: Super Contributor
timestamp: October 29, 2019, 08:06:35 am
content: Interesting. I’d check the power supplies again if that’s happening. Worth checking it at the fan boardIf it’s definitely doing that. It’s possible a tantalum has gone short somewhere and is pulling a stage down as well. Infernal things!

author: guenthert
author_rank: Frequent Contributor
timestamp: November 02, 2019, 10:03:38 pm
content: Quote from: bd139 on October 29, 2019, 08:06:35 amWell, I did check the voltage across R1691, essentially measuring the current through the fan motor's coils and while the effect is small, it is significant.I then disconnected the vertical output amplifier completely and then the fan speed (and current through the fan motor) remains more or less constant (it varies a little, as the temperature varies).I then removed the vertical output amplifier sub assembly and when I attempted to verify the conductance of the connectors to the plate, I noticed that one connector was lose and sadly, the wire on the CRT badly bent. When I attempted to reconnect it, I bend it only worse. It's difficult to get there -- I'd think one would have to rotate the CRT to access those plate connectors. For now, I just used a mini-grabber (won't perform as specified, but should at least get me in a trace on the screen), but sadly the offset remains.

author: Andrey_Ak
author_rank: Regular Contributor
timestamp: September 16, 2023, 08:01:16 am
content: Hello!I am engaged in the Tektronix 475 oscilloscope,When connecting the input to the test calibrator, time/frequency division does not match,The output of the test calibrator is 1kHz, the time position is at 200 µs/delay,then the test meander should be 5 cells on the screen, but in fact it is more.Which potentiometer needs to be adjusted to correct?I can't find what I need in the Service Manual,there are a lot of other calibrations, I can't figureout which one I need, because I don't speak the language.https://tis.kz/temp/Tek-475_070133200web.pdf---It turns out that the calibrator itself is not accurate...

author: bd139
author_rank: Super Contributor
timestamp: September 16, 2023, 08:09:50 pm
content: The calibrator is approximately 1KHz. It cannot be adjusted. It is designed only for compensating probes rather than checking the oscilloscope calibration.It's fine

author: p.larner
author_rank: Frequent Contributor
timestamp: September 23, 2023, 06:19:56 pm
content: what happened with this?,seems to me the power rails should of been checked and possibly sorted firstly.