Tektronix 475 weird traces both channels - Page 1
Forum section: electronics / repair

author: MadModder
author_rank: Regular Contributor
timestamp: December 31, 2018, 04:32:56 pm
content: I have a Tektronix 475 that worked very well when I bought it in 2007. After a year all trigging stopped working. Resolved by reseating (and deoxit sockets+component legs) a few tranistors in the trigger circuit.After a few more years it got weirder, and I had no room on my bench to fix anyuthing. It has now been sitting on the shelf about 5-6 years non-working.Attached is a photo of the traces. Both channels, no input signal. The set V/div does nothing.But when feeding a signal in to either channel, the signal can be seen following the trace pattern in the photo. Very weird. And the V/div does alter the height of the trace. But the weird pattern stays the same.V position is working for both channels.And if I change the trig mode in any way, both traces disappear completely. Can then only be seen when pushing beam finder. If I turn the oscilloscope off and wait half a minute and turn it on, the traces are back.Here is a film clip when I change sweep time and V/div back and forth.I'll go through the tips and tricks I found in the other 475-threads of course, starting with cleaning contacts and then check the voltages.I'm mostly wondering if someone else has had this exact problem.

author: mzacharias
author_rank: Frequent Contributor
timestamp: December 31, 2018, 05:36:01 pm
content: I would definitely look at power supply issues (open filter cap or caps mainly) first.

author: pbarton
author_rank: Frequent Contributor
timestamp: December 31, 2018, 06:13:43 pm
content: Well, I find that very unusual. When scopes fail its normally the timebase, OR, the Y amplifiers. However, your video shows that the fault affects both timebase and the Y axis, at the same time. A very rare fault!So we have to think of something that is common to both. At first I thought a power rail, common to both timebase and Y amplifiers. But then I thought what if your CRT X and Y plates had an internal short?That may explain the combined timebase/Y amplifier symptoms, as seen in the video. If true you would need a new CRT to fix the problem! Can you meter between the two pairs of CRT X and Y plates to eliminate this possibility?You could try disconnecting the two CRT X plates and re-run the tests. Then re-connect these and disconnect the two CRT Y plates, and re-run the tests. Do you get what you would expect to see?Any sign of a ±45 degree sloping line, would indicate an internal short between an odd pairing of the X and Y plates.Re-running your video, I can already see lots of waveforms with a tendancy to be inclined at ±45 degrees!Just look at the video start screen (you don't need to click the video run button) ±45 degree sloping lines everywhere. Symptoms also visible in the initial attached .jpgThat's what you get when the timebase ramp waveform (X), inadvertently gets applied to (one of) the Y plates.

author: Dacke
author_rank: Regular Contributor
timestamp: December 31, 2018, 08:28:32 pm
content: The few times I've seen strange traces like that on the 465/475 it's been bad caps in the power supply.

author: Armadillo
author_rank: Super Contributor
timestamp: December 31, 2018, 08:42:15 pm
content: From the strength of the beam, the power supply appears OK.The problem is quite obvious I thought?,it's a joy to troubleshoot.

author: PaulAm
author_rank: Frequent Contributor
timestamp: December 31, 2018, 09:30:19 pm
content: Definitely check the ripple on the low voltage power supplies. You'll probably find at least one of them with 5-10v of ripple on it.This is a very common failure on the 4xx series. The filter caps dry out and fail open. The good news is this rarely causes other components to fail.The bridges are another thing to check.Rules 1-5 for troubleshooting Tek 4xx scopes: CHECK THE POWER SUPPLIES FIRST! Repeat ad nauseum. Nothing works if the power supplies aren't right.

author: David Hess
author_rank: Super Contributor
timestamp: January 01, 2019, 02:44:46 am
content: The electrolytic input capacitors to the power supply have worn out. As long as you have it apart, change the output capacitors as well and check the rectifiers.

author: Armadillo
author_rank: Super Contributor
timestamp: January 02, 2019, 11:28:03 am
content: This is rather unique problem here, so waiting for your discovery and next publication.Worth to clear the table and make space for it.

author: MadModder
author_rank: Regular Contributor
timestamp: January 02, 2019, 04:02:58 pm
content: I did clear my lab bench yesterday.Had some time to spare today so I opened the case and jotted down the voltages:RailVoltageRipple+105/160135.85V40mV but rising slowly+110110.4V50mV+50 unreg64.9V2VPPugly triangle wave ripple+5049.79V50mV+1514.97V30mV+54.1V3VPPouch...-8-7.97V1mV-15-14.96V1mVBut not as bad as the suggested 10V ripple!I noticed that CR1462 (bridge rectifier for -8V) and C1472 (big cap for -15V) have been replaced. Broken traces around that cap is fixed with wires.I guess next step is to dismantle the whole shabang and take the board out. *sigh*

author: Armadillo
author_rank: Super Contributor
timestamp: January 02, 2019, 04:25:53 pm
content: I think you need bigger space for this unique problem.Watch at 40 sec, the trace moving to the left, unblanked, timebase looks ok too.Obviously you are on alternate mode, its tracing alternatecurves retrace lines... heheheheheCan't take all the joy out of your hunt....Happy Hunting..

author: MadModder
author_rank: Regular Contributor
timestamp: January 02, 2019, 04:54:50 pm
content: You devilish you...No I'm in chop mode there, as can be seen by the depressed CHOP button. That's why there are a bunch of "squares" at the bottom.I tried the alternating mode too later, and single channel, but it makes exactly the same pattern, but without the cut up line.No that is not why there are chopped up lines. It's still chopped up in single channel mode.Damn it, why is it retracing in that weird pattern while it is drawing the curve?

author: Armadillo
author_rank: Super Contributor
timestamp: January 02, 2019, 04:58:22 pm
content: No, serious, waiting for your discovery and finished video of this not easy unique problem. heheheheheheh

author: MadModder
author_rank: Regular Contributor
timestamp: January 02, 2019, 05:53:15 pm
content: More fun weirdness. This time with channel one only. No alternating, no chopping. But it's chopping it up in its own way anyway...time/div 1ms, 1Hztime/div 0.5ms, 1Hz. Why is the pattern mostly gone here?time/div 1ms, some arbitrary frequency around 1.3kHzOh well. I'll buy a handfull new caps some time to start with, after I've gotten the board out for examination. Later. Some time.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: January 02, 2019, 06:01:52 pm
content: Is that a waveform from Treez latest LED driver?

author: MadModder
author_rank: Regular Contributor
timestamp: January 02, 2019, 06:05:56 pm
content: That would be a quite bad driver. A really annoying 1.3kHz buzz

author: PaulAm
author_rank: Frequent Contributor
timestamp: January 02, 2019, 07:43:10 pm
content: You can buy or make little adapter boards so a standard snap cap can fit the weird 4 pin pcb footprint of the oem caps. That makes for a nice repair. Tektronix sometimes used the can as a jumper so you need to connect the 3 ground pins together.

author: tggzzz
author_rank: Super Contributor
timestamp: January 02, 2019, 07:58:16 pm
content: Fix the 5V line first.If there are no short circuits (5V is 4,1V no 0V), then a dirty technique is to solder tack a suitable value good capacitor across the relevant PSU lines, in parallel with the suspect caps with high ESR. If the problem behaviour is diminished, indentify and replace the faulty caps.

author: james_s
author_rank: Super Contributor
timestamp: January 02, 2019, 10:14:58 pm
content: Wow that is a *really* weird symptom. I do agree that fixing the 5V supply is the obvious place to start. No point in troubleshooting further until that is solid.

author: MadModder
author_rank: Regular Contributor
timestamp: January 03, 2019, 10:40:56 am
content: @PaulAm, yes, Iv'e been looking for thetriangle type adapterbut the ones I find are bloody expensive.Guess I'll just roll my own.And the Harwin pins look neat. I'll have to hunt those down.

author: Armadillo
author_rank: Super Contributor
timestamp: January 03, 2019, 10:54:13 am
content: Is it difficult to salvage the BASE from the old cap?

author: MadModder
author_rank: Regular Contributor
timestamp: January 03, 2019, 11:10:55 am
content: That has crossed my mind. I'll try to loot the old caps first. But mounting a new cap on that base isn't going to be pretty.The replaced cap though... it is hanging in the air with a zip tie around the other caps, and wires going down the holes.

author: PaulAm
author_rank: Frequent Contributor
timestamp: January 03, 2019, 12:46:29 pm
content: Search on ebay for "Tektronix 7000 capacitor" and you'll see a set for $12. There are also gerbers in a 7000 repair thread on this board. I have a postscript file for printing to pnp blue if you want to etch your own.It's easy to damage the pcb when pulling the caps, so I wouldn't recommend reusing the base.You had zip ties? I'm impressed; I've seen them just hanging in the airEdit: That 7000 kit only has 1 triangle adapter , try item 273254508468, that's a kit of 5 and it's only $10.

author: MadModder
author_rank: Regular Contributor
timestamp: January 03, 2019, 01:28:21 pm
content: Ya, I saw the $10 one, but I need more than 5 if I want this properly done, and $14 shipping here, and our postal service adds VAT to all that, plus $8.27 for VAT handling fee for non-EU packages. That makes it $50.77

author: Armadillo
author_rank: Super Contributor
timestamp: January 03, 2019, 02:57:25 pm
content: Like many others, you don't only own one scope.That amount $50.77 for just the bases, can already buy a new used scope.Furthermore, its so heavy, so boxy, takes so much space, and after repairing it, more than likely you are going to put it aside to collect dust, example it don't meet modern days measurement needs.Just float it or drill holes on the salvage base, nobody is going to look inside.Well, that's just my opinion.

author: MadModder
author_rank: Regular Contributor
timestamp: January 03, 2019, 04:53:55 pm
content: Yes, that is what I’m going with to start with. The bodged on cap though... Alright I ziptie it back when done.I got the 5V cap out now. Only one of my instruments showed something, once. 255µF 0.14kΩ.Now all instruments say theres nothing there. It’s shot alright.

author: bd139
author_rank: Super Contributor
timestamp: January 03, 2019, 04:58:41 pm
content: Was that the axial one on the main board right in the middle? Sprague 150D. Those buggers die all the time.

author: MadModder
author_rank: Regular Contributor
timestamp: January 03, 2019, 06:18:37 pm
content: Yes on sprague, but not 150D. 68D, big alu can with transparent shrinkwrap around. Bone dry inside.I cut the base off above the waist, gutted the butyl rubber and trimmed the edge down and crimped it around the fenolic board.I think that'll work well. Then drill two holes for the new cap, shove it down and fasten it with hot glue and then connect the pins to the old ones.One of the leads from the A TRIG HOLDOFF pot was broken off beside the solder. Easy fix.Both mains leads were hangning by a thread on the switch, and came loose when I took the switch assembly off the board. A really shoddy solder job. Strands spreading out on the sides, half of them cut off. Maybe to fit in the solder lugs. But after cleaning the lugs from old solder, all strands went in just fine. Also easy fix.I'm not sure how to remove the big bottom PCB from the frame though. All screws taken out and all offboard soldered leads detached, but I still can't take it out. The board is all loose and can be rocked back and forth some. But something is holding it back from further movement. The exploded view over the bottom board assembly says nothing about more screws or something.I can replace the big caps anyway, but every small tantalum ones idk.

author: Armadillo
author_rank: Super Contributor
timestamp: January 03, 2019, 07:00:14 pm
content: show the way to saving $$$$$$$$$$.Also saving the earth from unnecessary chemical etching.

author: MadModder
author_rank: Regular Contributor
timestamp: January 04, 2019, 06:27:43 pm
content: The replaced cap is for -15V. The schematic says 3000µF, but the new one is 6800µF. I've decided to replace that too since it's a Sprague. I don't trust Sprague even though it's a snap-in with blue shrinkwrap and they were acquired by Vishay in 1992. Maybe if it said Vishay, not Sprague.But why is it a 6800, and not a 3300 for example? Maybe they just took something they had at hand?

author: Armadillo
author_rank: Super Contributor
timestamp: January 04, 2019, 06:44:51 pm
content: This is in a "Super" high end amplifier. If it still exist today, it's going to cost a bomb each.It lasted for decades and still going strong.

author: MadModder
author_rank: Regular Contributor
timestamp: January 04, 2019, 07:01:41 pm
content: This one is an 80D, and its ESR is 0.04 Ohm. Some random new capacitors of the same size in my bins has an ESR around 0.01 Ohm.

author: Armadillo
author_rank: Super Contributor
timestamp: January 04, 2019, 07:20:21 pm
content: My point is, Sprague was a good brand. Sprague cap was a prestige to have in a set. Built to last. A "Tanky".But of-course, You can change to a dollar or less Caps that will easily out-beat it anytime now.

author: bd139
author_rank: Super Contributor
timestamp: January 04, 2019, 07:29:13 pm
content: Still is a good brand. Vishay sell them now and they still say sprague.40 years is pretty good for an electrolytic cap.

author: james_s
author_rank: Super Contributor
timestamp: January 04, 2019, 07:35:07 pm
content: Sprague is top-notch stuff, they have always been very high quality parts. Even high quality parts don't last forever though.

author: Armadillo
author_rank: Super Contributor
timestamp: January 04, 2019, 07:43:33 pm
content: 0.04 ohms vs 0.01 ohms over -15 rails at max 1.5A with an auto output voltage regulation circuit;The next effect is 0.0675 watts loss, output voltage still stay at 15volt well within 1.5% tolerance.And you take branded stuff away...... goodness. hahahahhahahahBut your choice of-course.

author: MadModder
author_rank: Regular Contributor
timestamp: January 04, 2019, 07:48:14 pm
content: I don't buy unbranded chinesium for my gear.And the "new" cap was put in there in the early 90's, so around 25 years old.But what about the value. Should I put in a 3300µF (3000 isn't a standard value) as per the manual?

author: james_s
author_rank: Super Contributor
timestamp: January 04, 2019, 07:54:20 pm
content: I can't imagine you will have any trouble with a 3300uF filter cap when 3000uF is specified, electrolytic capacitors are not high precision components in the first place.

author: MadModder
author_rank: Regular Contributor
timestamp: January 04, 2019, 08:00:18 pm
content: No it's usually +-20%, but the "new" one is 6800µF. I thought maybe it was some "updated" info that person had on the capacitance.

author: Armadillo
author_rank: Super Contributor
timestamp: January 04, 2019, 08:08:07 pm
content: Maybe he has the same thinking, lower ESR is Best.Maybe select 10,000 uF better. sure lower with your non chinesum easy pick in your Bin.

author: MadModder
author_rank: Regular Contributor
timestamp: January 04, 2019, 08:29:19 pm
content: Haha, a too high capacitance isn't good either. Don't want to pop the poor rectifier when turning it on.

author: Armadillo
author_rank: Super Contributor
timestamp: January 04, 2019, 09:39:02 pm
content: Quote from: MadModder on January 04, 2019, 08:29:19 pmMy Goodness? Hahahahahahah;At the secondary, this inrush current is limited by inductive reactance of the secondary 15v coil; which I calculated to be around 0.020524466732 Henry = 6.24354278 ohms + Dc resistance (0.044563384066 ohms) = 6.288106164 ohms;Hence excluding the ESR of the capacitor, the inrush current is ~ 2.385455908 Ampere which is lower than the published IFSM of the diode bridge of 50 Ampere.Or you imagine is 15v / 0.01 ohms = 1500 Ampere.Don't know what to say, just hope that you get more challenging troubleshooting to do than changing just the capacitors. Hehehehehehe

author: MadModder
author_rank: Regular Contributor
timestamp: January 04, 2019, 10:09:57 pm
content: No why would I imagine 1.5kA. That's preposterous.Do you know the VA ratings for the transformer's windings or what?

author: james_s
author_rank: Super Contributor
timestamp: January 04, 2019, 10:23:25 pm
content: What does it matter? This is not rocket science. Replace the capacitors with something close to the original value, it will be fine. In most cases you could double, triple or more the original values without problems but sometimes not, there's not really any reason to try. If someone put a much higher value capacitor in there before it's probably either because they used what they had on hand or they tried to improve the design without really knowing what they were doing, doesn't really matter now.

author: Armadillo
author_rank: Super Contributor
timestamp: January 04, 2019, 10:49:42 pm
content: FYI, the VA rating of the transformer applies to all windings of the secondaries and yes including this, so to speak, auxiliary current limited -15 volts power supply.But it's galaxies away if we will to delve into transformer design, yeah!.Quote from: MadModder on January 04, 2019, 10:09:57 pm

author: Inverted18650
author_rank: Frequent Contributor
timestamp: January 05, 2019, 07:36:16 am
content: Following. (*has donor parts).

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 05, 2019, 08:05:13 am
content: Quote from: MadModder on January 02, 2019, 05:53:15 pmYou know, if you WANTED a display to do that you would have a HELL of a time...ok, I'll let myself out...

author: MadModder
author_rank: Regular Contributor
timestamp: January 05, 2019, 12:23:36 pm
content: I did not try that, but did this in 2007

author: MadModder
author_rank: Regular Contributor
timestamp: July 12, 2020, 09:56:25 pm
content: The victim has been sitting on the healing bench for quite a while now.I've not been able to reach the bench for at least a year...But today I spent a few hours searching for all bits and pieces. They were put in a few good spots that I was sure I'd remember, while doing a proper cleaning a few months ago...Anyway. I soldered the caps, plugged the mains in and pulled the power knob.I trimmed the +50V rail, and all voltages were OK.No weird traces!I let it sit there and warm up for half an hour just to be sure.Still good.However, no trigging. At all. In any mode.So, back to troubleshooting.But at least there's good traces.One step forward.

author: tggzzz
author_rank: Super Contributor
timestamp: July 12, 2020, 10:07:37 pm
content: Quote from: MadModder on July 12, 2020, 09:56:25 pmCheck input signals are getting to the trigger circuit.If they are, check if there is a trigger output.If not, follow the setup procedure in the manual.

author: MadModder
author_rank: Regular Contributor
timestamp: July 13, 2020, 12:09:39 pm
content: I followed the trigger signal every step all the way from the vertical amp down the trig board. When I was almost at the end of the trig generator circuit, I got only garbage.Wiggled the four transistors there a bit, and everything went back on track.Removed those four trannies, applied a drop of non lubricating de-oxit on each leg and pushed them back in, and out, and in a few times.Then I wiggled every transistor on that board for good measure.Sometimes I hate those little buggers.I wonder if there exists some sort of tiny pipe cleaners so one can clean the sockets.Working like a charm

author: tggzzz
author_rank: Super Contributor
timestamp: July 13, 2020, 12:28:52 pm
content: Quote from: MadModder on July 13, 2020, 12:09:39 pmYes, there is.... The transistor legs, with a bit of IPA or DeOxit on them. But I suspect you realise thatGlad it is working; they are nice to use.

author: MadModder
author_rank: Regular Contributor
timestamp: July 13, 2020, 01:32:32 pm
content: Hehe, yes.I guess there's not much crud getting in there so it'll do.BTW, Here's the mounted new caps.They're quite a bit smaller.Worked really good with the old cap bases.

author: tggzzz
author_rank: Super Contributor
timestamp: July 13, 2020, 02:21:27 pm
content: Quote from: MadModder on July 13, 2020, 01:32:32 pmToo much hard work for me; I've used circular "adapter" PCBs to do the same job.