Author Topic: New Guy seeking Oscilloscope advice  (Read 3856 times)

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Offline The_Boots

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New Guy seeking Oscilloscope advice
« on: March 14, 2018, 02:07:37 am »
I'm new to the site and am still quite ignorant about electronics in general at the moment (haven't really done anything with it in at least a decade), so be gentle!
I got bitten by the electronics bug and wanted to get back into it, so I dusted off my copy of The Art of Electronics and got reading. Only a few pages in, I was frustrated by having to guess the answers to questions and whatnot, and when I checked for an answer guide online, I read what might be the most profound, yet obvious thing I've read in ages: why look for an answer key when you can just build the circuit yourself and see if you've got the right answer? To help see how the circuits were behaving, I wanted to get an oscilloscope.

Having watched several videos and read a bunch online, the general consensus seemed to be that (for people on a budget) it was better to get an old scope that was a proper piece of equipment than a cheap diy kit or USB scope. With that in mind, I found a working Tek 475 on craigslist, and bought it. I brought it home and immediately began fiddling with all the pretty switches and dials, and learning the ins and outs. It's amazing the stuff they crammed into such an old scope!  It was tons of fun-- right up until the point where it stopped working.  :-BROKE The horizontal sweep quit completely, even though I still have a dot that moves vertically when I feed it a signal. Just when it was getting good!

I started looking through the repair and troubleshooting guides (signs point to the horizontal amp...?) and even tried to post on the TekScopes group site, but I think I lack the basic electronics knowledge to troubleshoot the problem myself. Honestly, that makes sense, since the whole reason I got the scope was so that I could learn these kinds of skills! Making it worse, my TekScopes post looks to be caught in new-post-needs-mod-approval purgatory. I've already spent enough money that I'm not comfortable shoveling more money into such a nascent hobby to buy a more expensive scope, and I don't think She Who Must Be Obeyed would tolerate it, anyways. The most I think I could spend is $100 or so.
As I see it, I have a few options:
1) Give up on this whole electronics thing before it's too late (Boo!)
2) Bang my head against trying to repair the Tek when I have no hope of understanding what I'm doing while trying to find a local person who can save me from my ignorance.
3) See if lightning strikes twice and try to get another second-hand scope and hope it lasts more than 30 minutes.
4) Buy a super cheap scope just to learn on until I feel I have a chance at understanding the repair advice people give me when I ask for it.
For cheap scopes, I mean one of the $50-$100 USB ones, or the $20-$30 diy kits (which also have the benefit of being another project to practice and learn on). I'm leaning towards this option, since it's the second cheapest after just giving up, and my requirements are extremely modest. To be clear, my end goal would still be to get the Tek working again, but this way I'll have something to play with in the mean time. If I DO go this route, what is the recommended scope? I think the dirt cheap electronics lab EEV video suggested the isolated Owon USB scope, but that's pushing the limit of what I want to spend...

So that's my very first post here. Any advice would be hugely appreciated!
 

Offline tautech

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Re: New Guy seeking Oscilloscope advice
« Reply #1 on: March 14, 2018, 02:27:53 am »
Welcome to the forum.

Fault is more likely in the sweep generator unless you have the scope in XY mode and don't realize it.

Have you got a analog meter ?
With sloooow timebase settings it's possible to see the needle rise in value as the triangular wave sweep progresses.
Check the service manual for the sweep test points and the typical amplitude values.
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Offline EddyK59

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Re: New Guy seeking Oscilloscope advice
« Reply #2 on: March 14, 2018, 03:46:01 am »
What Tautech said.

Usually it's true that to fix a scope you need a scope, but exceptions can be found.

The manuals for those Tek scopes are great reading, a real goldmine of electronics knowledge and information about how the circuits function.

Best of luck.
 

Online Shock

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Re: New Guy seeking Oscilloscope advice
« Reply #3 on: March 14, 2018, 05:53:31 am »
You're expecting a lot, have you repaired any electronics before?
Hopefully you're aware of the high voltage risks.

I mean it can be done but if you have no idea how to read a schematic or follow the service manual you are asking a lot for your first outing. Have you ever soldered before or made measurements on a PCB with the multimeter?

The first thing I'd be doing is going over all the connections and make sure everything is plugged in. Especially if the oscilloscopes been in transit recently. Again high voltage risk. Don't be messing with it plugged in and turned on and around the high voltage circuit, CRT, and mains voltage caps.
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Offline palpurul

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Re: New Guy seeking Oscilloscope advice
« Reply #4 on: March 14, 2018, 06:28:19 am »
I'm going o back Shock here about the high voltage risk. It's especially important in old analog scopes because those old cathode ray screens require really high voltages to operate.
 
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Offline Ian.M

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Re: New Guy seeking Oscilloscope advice
« Reply #5 on: March 14, 2018, 06:46:33 am »
To eliminate user error,  start by posting a photo of the whole front panel of your scope, looking down at a slight angle so we can see whether buttons etc. are pushed in or not.

If you've got it in some odd mode where it doesn't trigger the sweep, I'd bet we have a Tek 475 user who can spot what's wrong and tell you how to set it back to normal.

If it is faulty, then repair really needs someone with at least some service trade experience or equivalent. (e.g. fixing CRT TVs etc. or high power Valve equipment)  There's thousands of volts on some of the CRT connections, and old CRT scopes with transformer based EHT supplies can be rather dangerous to work on.   IMHO a CRT scope is *NOT* a good repair project for a novice to learn on without hands-on mentoring.
« Last Edit: March 14, 2018, 06:53:09 am by Ian.M »
 
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Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #6 on: March 14, 2018, 01:21:10 pm »
First off, thanks for the replies! I should add that I'm *pretty* sure that it's a hardware problem, since it happened when I wasn't actually touching the scope, and there was an audible pop, and then the trace went bananas. Power cycled it, and it was as it is. Still, being a newbie, I'd appreciate the double-check!
To give people a better idea where I'm at:
I know the basic components of a circuit and am comfortable reading simple diagrams. I know what capacitors/resistors/transistors look like on a diagram and their respective units. I know the difference between Volts, Watts, and Amps. I know that when something says "Danger! High Voltage" to leave the damn covers on. I know what the CRT looks like and that it works in kilovolts. I know enough to be scared of poking metal bits into a mains-powered electronics device and I know that capacitors can hold a charge for quite a while, even when the device is unplugged. I think I can safely identify and probe parts in the low-voltage sections of the scope without killing myself (famous last words).
The place where my knowledge falls flat is when these individual components combine. I know there IS such a thing as a voltage divider and that it involves resistors, but I don't have the background to recognize it if it isn't in a diagram by itself with the words "VOLTAGE DIVIDER" below it.
The videos I've watched of other people repairing the scopes had their success hinge on their understanding of how the larger circuits behaved-- if a voltage was off, the problem was probably in part a, but not part b. Without understanding or experience, I'll just be flailing about at random, and that's when mistakes happen.
In other words: I agree that this is not what I'd pick for my first project. That was supposed to be the cute lil' power supply kit I have in the mail.

Which is why the advice I'm looking for isn't really "how do I fix it?" so much as "what should I do as a stopgap while I build up the skills necessary to make a serious attempt?"
I know I can be wordy, so it got lost in the noise. Sorry!
« Last Edit: March 14, 2018, 01:25:21 pm by The_Boots »
 

Offline alsetalokin4017

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Re: New Guy seeking Oscilloscope advice
« Reply #7 on: March 14, 2018, 02:26:10 pm »
Well, the good news is that you have HV, or you wouldn't have a beam at all. So you can leave the covers on the HV section and don't mess with it. Since you heard a "pop" my first guess will be a blown tantalum capacitor, which should be visible by inspection. Use a magnifying glass and look at all the components on the circuit boards to see if anything looks wonky.

You will learn a lot by wading through the Tek Service Manual. Find the troubleshooting flowchart in the SM. This should help you isolate the problem to specific sub-circuits, for closer inspection. Generally one begins (after the careful visual inspection) with checking the various low-voltage Power Supply voltages at the testpoints you will find indicated in the SM.

Something you might try is to put the Time/Div knob(s) in the X-Y position, send a signal into CH1 (x) and see if the trace makes a horizontal line.
« Last Edit: March 14, 2018, 02:31:57 pm by alsetalokin4017 »
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Offline Cliff Matthews

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Re: New Guy seeking Oscilloscope advice
« Reply #8 on: March 14, 2018, 02:30:55 pm »
She's an American thing of beauty..  :-+  Have you checked the timebase? *(that looks like a compressed square wave..)
 
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Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #9 on: March 14, 2018, 02:44:01 pm »
Been there, done that. 

Study the horizontal section schematic.  Assuming you don't spot the damaged part, start at the sweep oscillator and work towards the output.

If the spot moves vertically when you connect the probe to the calibrator signal you can use the scope itself  to find the fault.  Something I did not realize when my ex-Air Force 60 MHz Dumont suffered the same malady a few days after my 30 day warranty expired, though in my case it was  bad solder joints.  So it just didn't work the next time I turned it on.  I didn't fix it until I picked  up a wonky 465 which worked just enough to fix the Dumont which I then used to fix the Tek. 

I still have both, but when I decided to get back into electronics I bought a Rigol DS1102E rather than face the effects of another 15 years.  I'll get them going again, but I want to do things besides just repair my gear so I can repair my gear.

 
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Offline Old Printer

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Re: New Guy seeking Oscilloscope advice
« Reply #10 on: March 14, 2018, 02:45:00 pm »
That's a great scope, I have one, but not the experience to fix it besides something really obvious. If you really are serious about electronics you will eventually get another scope, most likely a digital one. Unplug the 475, take the cover off and have a look around. Keep your fingers and metal objects out of there for a while. There is an amazing amount of stuff crammed in that scope, pretty daunting. Unless you see something easily, I would put the cover back on and get a new scope until you get the skills to fix this one. It is too nice of a scope to mangle, and it wouldn't be hard to do. At the price new Rigol's are going for now it is hard to recommend anything else. You can probably find a "new" demo at the $300 point. If that is too much, buy a $50 multimeter and start your learning there, you can get very far with just that and some basic components. I bought an Analog Discovery USB scope because I couldn't make up my mind which DSO to get right now, but that requires a computer on your bench. What it does give you is a load of basic electronic tools for just under $300, instead of just a scope. It's range is limited, but that has not stopped me from doing all the learning I have time for. We are in the golden age of electronics, there is more info for beginners at our finger tips that ever. Between this site and YouTube you could spend years so enjoy, and welcome.
 
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Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #11 on: March 14, 2018, 03:05:14 pm »
I have a few multimeters (although one was a Harbor Freight freebie).
I know that scopes are available at the $300 (or even $250) level, but as I mentioned, I don't really want to spend that much on one until I'm a little farther along. I want to keep it at $100 or below, and that rules out most DSO's, especially ones from the last 15 years or so.
Even with educational pricing (I'm a High School Math/CS teacher and I coach our Robotics team) the Analog Discovery is still more than I want to pay. I have computers I can use at the bench, though. That's not a huge issue.

She's an American thing of beauty..  :-+  Have you checked the timebase? *(that looks like a compressed square wave..)
Yep. It's the signal from the calibrator.
 

Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #12 on: March 14, 2018, 03:10:14 pm »
Firstly, get the service manual, either free online at the usual places or purchase one from Artek.

Secondly, look at each component to see if it is damaged. In this case it may be that a decoupling capacitor or resistor has ceased to exist. Reseat connectors.

Thirdly, with any fault, check the power supplies for voltage and ripple. Having said that, I doubt that's your problem since you have a trace.

Finally, sit down with the service manual, read the theory of operation, and most importantly, think. Take it slowly. What can you see works, what doesn't? Which parts of the circuit are involved only in the parts that don't work? Start there.

There will be a signal flow from, in this case, the sweep generator to the CRT horizontal deflection plates. Either start at the beginning and follow through until the signal is no longer apparent, or start at the end and find where the signal is first apparent. Use a multimeter and the sweep set to a low speed so that you can see the voltages change. Sometimes, with understanding and imagination, it is possible to use a faulty scope to debug itself; in this case the vertical deflection is effectively a fast acting voltmeter. (Do not connect a probe's shield to anything other than 0V, or there will be mechanical and biological noises!)

With 475s, be aware that there are EHTs (2kV and higher) around the CRT's neck end, the CRT's frontplate, and under a cover marked high voltage - these are fairly well protected from prying fingers. However, there are HT voltages (100-400V) that are less well protected and could deliver dangerous currents. Know where the relevant diodes/capacitors (and their terminals and PCB tracks) are on the circuit board, and make sure you don't touch them.

Of course, if you can't find the problem, then I'll buy it for a pittance :)

BTW apart from repairing the scope, IMNSHO there's a balance to be struck between:
  • understanding how a circuit should work, and checking that practically using test equipment
  • using test equipment to find out how it works
Worst case is randomly probing without understanding.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #13 on: March 14, 2018, 04:32:11 pm »
Wow, this is an amazing response. There's a lot of good stuff in there. I can certainly look for bits that have let the smoke out, but I'm guessing that really checking for ripple needs a scope, right? I guess I can hook the scope to itself to see that, though.

Finally, sit down with the service manual, read the theory of operation, and most importantly, think. Take it slowly. What can you see works, what doesn't? Which parts of the circuit are involved only in the parts that don't work? Start there.

There will be a signal flow from, in this case, the sweep generator to the CRT horizontal deflection plates. Either start at the beginning and follow through until the signal is no longer apparent, or start at the end and find where the signal is first apparent. Use a multimeter and the sweep set to a low speed so that you can see the voltages change. Sometimes, with understanding and imagination, it is possible to use a faulty scope to debug itself; in this case the vertical deflection is effectively a fast acting voltmeter. (Do not connect a probe's shield to anything other than 0V, or there will be mechanical and biological noises!)

So I'm not at home right now, so I can't poke at things yet, but I thought I'd check some thoughts I had about this. An earlier comment suggested that the problem was more likely to be in the Sweep generator, but reading the service manual, it says that there are three independent sweep generators (A, Delay, B, maybe), but I don't get a sweep in any of those modes. Wouldn't that suggest that the immediate problem would likely be somewhere common to all three (e.g. Horizontal Amp)?

Of course, if you can't find the problem, then I'll buy it for a pittance :)
No way! Whether today, tomorrow or ten years from now, I DO want to get it working again!
 

Offline Peabody

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Re: New Guy seeking Oscilloscope advice
« Reply #14 on: March 14, 2018, 04:35:41 pm »
I think it may be worth the trouble to unplug it, slide the case off, get a bright flashlight, and carefully look over everything you can get to, looking for something that might have gone pop - something that's charred.  You could get lucky.  Of course if you find something like a tantalum or electrolytic capacitor, or maybe a diode, that has lost its smoke, that doesn't necessarily mean that replacing it will fix it.  Something else could be wrong that caused a perfectly good cap to blow, and will do so again.

But you could get lucky.  And even if it only isolates the part of the circuit that's faulty, that's some help.

What you really need is a fellow hobbyist in your area who is more knowledgeable, and willing to spend some of his time to fix it.  Maybe a Craigslist ad would turn one up.

Also, if there is a Tek forum that you could post to, you might find help there from someone who has had the same problem.

Otherwise, it would be a daunting task.  If you have the documentation, you could perhaps test voltages at hundreds of test points on the boards, looking for anything that isn't right.  But these things are awfully complicated.

Banggood has the DSO150 for sale at $22 from the China warehouse.  And presumably you could use one of the Tek probes with it.  But there is risk even there because you have to build the kit.  But at least price-wise that's low risk, and it would at least get you a basic slow digital scope to learn with.  That's what I got, and it actually has been quite useful. But I do miss having a second channel.  If you do get that, ask me about the Li-Po battery mod.  The next step up would be the Hantek 6022be, which is a USB scope for around $55.  I have no experience with that.

 

Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #15 on: March 14, 2018, 04:52:26 pm »
This thread really belongs in Repair with the title  changed to "Dead horizontal on Tek 475".  This is not a big deal to fix.  It will take time.  I spent a week tracing my way through the Dumont, but the schematic was a copy of a copy and barely readable.  It also was not as detailed a service manual as Tek put out.  It was 1/4". The Tek is almost an inch.

So long as you are patient and careful around the HV circuits you'll be able to fix it.  There are plenty of forum member who can and will help with detailed advice.
 

Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #16 on: March 14, 2018, 05:26:42 pm »
This thread really belongs in Repair with the title  changed to "Dead horizontal on Tek 475".

It's kind of turned into that, hasn't it? My original intent was to ask for advice on what was the best alternative to fixing it that still left me with a scope.


Banggood has the DSO150 for sale at $22 from the China warehouse.  And presumably you could use one of the Tek probes with it.  But there is risk even there because you have to build the kit.  But at least price-wise that's low risk, and it would at least get you a basic slow digital scope to learn with.  That's what I got, and it actually has been quite useful. But I do miss having a second channel.  If you do get that, ask me about the Li-Po battery mod.  The next step up would be the Hantek 6022be, which is a USB scope for around $55.  I have no experience with that.

Yeah, I was kinda looking at the DSO150, Hantek 6022be, and the Owon VDS1022/1022i. They range from $20-100.
 

Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #17 on: March 14, 2018, 05:49:32 pm »
Wow, this is an amazing response. There's a lot of good stuff in there. I can certainly look for bits that have let the smoke out, but I'm guessing that really checking for ripple needs a scope, right? I guess I can hook the scope to itself to see that, though.

You could, but a multimeter set to AC volts is sufficient.

Quote
Finally, sit down with the service manual, read the theory of operation, and most importantly, think. Take it slowly. What can you see works, what doesn't? Which parts of the circuit are involved only in the parts that don't work? Start there.

There will be a signal flow from, in this case, the sweep generator to the CRT horizontal deflection plates. Either start at the beginning and follow through until the signal is no longer apparent, or start at the end and find where the signal is first apparent. Use a multimeter and the sweep set to a low speed so that you can see the voltages change. Sometimes, with understanding and imagination, it is possible to use a faulty scope to debug itself; in this case the vertical deflection is effectively a fast acting voltmeter. (Do not connect a probe's shield to anything other than 0V, or there will be mechanical and biological noises!)

So I'm not at home right now, so I can't poke at things yet, but I thought I'd check some thoughts I had about this. An earlier comment suggested that the problem was more likely to be in the Sweep generator, but reading the service manual, it says that there are three independent sweep generators (A, Delay, B, maybe), but I don't get a sweep in any of those modes. Wouldn't that suggest that the immediate problem would likely be somewhere common to all three (e.g. Horizontal Amp)?

I'd have to think to figure out if you could use the B sweep if the A sweep was broken; you probably could but it isn't the way to go (yet).

Obvious test: eliminate the sweep circuits from the equation...

First verify that both input channel position controls do move the trace up/down.

If you set it to XY mode (sweep XY, and ch2) then the sweep is disconnected. The position is determined by the "position" of the two input channels; ch2=Y and ch1=X. If twiddling the position controls doesn't move it horizontally, then the problem is between the sweep output and the CRT plates.

But do look for damaged components, wires dropped off etc.

Quote
Of course, if you can't find the problem, then I'll buy it for a pittance :)
No way! Whether today, tomorrow or ten years from now, I DO want to get it working again!

I'm here if you need me ::)
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #18 on: March 14, 2018, 05:57:05 pm »
I think it may be worth the trouble to unplug it, slide the case off, get a bright flashlight, and carefully look over everything you can get to, looking for something that might have gone pop - something that's charred.  You could get lucky.  Of course if you find something like a tantalum or electrolytic capacitor, or maybe a diode, that has lost its smoke, that doesn't necessarily mean that replacing it will fix it.  Something else could be wrong that caused a perfectly good cap to blow, and will do so again.

A faulty cap might cause something else (diode, resistor) to blow, not vice versa.

Quote
But you could get lucky.  And even if it only isolates the part of the circuit that's faulty, that's some help.

Otherwise, it would be a daunting task.  If you have the documentation, you could perhaps test voltages at hundreds of test points on the boards, looking for anything that isn't right.  But these things are awfully complicated.

Your statement appears to be based on experience with modern scopes. In contrast 475s aren't all that complicated; the signal flow is relatively straightforward and all the voltages are accessible - unlike modern scopes.

If you RTFM theory section and faultfinding section, then think, you certainly won't have to probe hundreds of points to locate the problem area.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #19 on: March 14, 2018, 05:59:42 pm »

Banggood has the DSO150 for sale at $22 from the China warehouse.  And presumably you could use one of the Tek probes with it.  But there is risk even there because you have to build the kit.  But at least price-wise that's low risk, and it would at least get you a basic slow digital scope to learn with.  That's what I got, and it actually has been quite useful. But I do miss having a second channel.  If you do get that, ask me about the Li-Po battery mod.  The next step up would be the Hantek 6022be, which is a USB scope for around $55.  I have no experience with that.

Yeah, I was kinda looking at the DSO150, Hantek 6022be, and the Owon VDS1022/1022i. They range from $20-100.

You can do a hell of a lot with a multimeter. I managed to design and build my first computer from individual components (TTL and a 6800 with 128bytes RAM) without a scope.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline Old Printer

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Re: New Guy seeking Oscilloscope advice
« Reply #20 on: March 14, 2018, 07:02:41 pm »
You will pay more for a Tek because of the name, but the other side is that there is a lot of info, manuals and used parts available for them. Certainly worth the bit extra they command. Keep an eye out for one of the popular 2000 series Teks. I picked up a 2225 in good shape for $100 on ebay. I bought it just because Dave did two lengthy videos/teardowns on it, including a rough calibration. I will often us it rather than my 475 just because it is simpler, that and I am waiting for the inevitable cap to blow in the 475, not that usiong it has that much effect on that likely-hood. I might be inclined to look for a $100 Tek 2225/35 and use a soundcard scope for free in the meantime vs a $50 china scope kit. That $99 Educational Analog Discovery was an amazing deal while it was running. There was a time when they were easy to find cheap used, but that seems to have dried up since the price jump. I still keep an eye on craigslist for some ex-student selling one that does not know the price skyrocketed.
 

Offline tautech

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Re: New Guy seeking Oscilloscope advice
« Reply #21 on: March 14, 2018, 07:35:02 pm »
All what tggzzz said and to add;
Good scope schematics list DC voltages that are only relevant when the scope is in a certain configuration.
RTFM again and again.
These DC voltages can be a quick way to narrow in on a fault.

Read, digest, read more.
The A sweep is not that complicated, they're only a RC network driving stages until the horizontal output amp.
No sweep at the plates results in equal voltages on both plates for the trace to be central.

Can you shift the trace right and left with the Horizontal position control ?
To do so implies the preamp and output stages are OK.

Even without access to another scope it's entirely possible to repair this.
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Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #22 on: March 14, 2018, 08:12:27 pm »
I had a look at the 465 manual.  Not the same scope, but obviously very similar.  Repairing the scope should be fairly easy.  There are 16 waveforms supplied for test points in the sweep section and 9 in the horizontal amplifier section.  The operation of every element of the circuits is described in great detail.  You'll learn quite a lot reading it.  Best of all, fixing it will be a big ego boost. 
 

Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #23 on: March 14, 2018, 08:58:45 pm »
So I made it home and took off the cover. Most of the center (where the CRT is) is covered by the safety shield, which I didn't touch. I took a look at the parts I could see (sides and bottom) while the manual was printing, and the only thing I saw that was immediately obvious was corrosion on some of the IC legs. Looking at the manual, this is the Vertical Channel Switching section of the vertical preamp board, and  two of those are Quad NAND-gates and the other is a dual Flip-flop. No idea what they do, though. Other than that, the coating on a ceramic cap had a tiny crack, but it's right at the leg.

I followed tggzzz's advice and put the scope in X-Y mode, and lo and behold, with the right settings, the trace rotated 90 degrees and was horizontal(pic attached). With a little improvisation (I only have one probe!) I set the calibrator signal to both pins at once and got an actual 2-D shape, as well.

Gee, if this is really going to be a repair effort, should I just make a new thread in the correct place, or I guess, could it be moved?
 

Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #24 on: March 14, 2018, 09:27:46 pm »
I suggested moving it and renaming it to the moderators, but a request from you would not hurt.

You have no sweep, so if you probe the test points for which the scope waveforms are given you should isolate the problem fairly quickly just using the vertical response as an indicator.  Then a bit more testing and study of the schematic and theory of operation should get it working.  As you have observed, there are maintenance issues and also calibration tasks to be performed.

Leo Bodnar sells a very nice little 10 MHz pulser with a 40 pS rise time.  There's a long thread about it.  The BNC version is about $80 US IIRC.  I have one and love it.  Aside from checking the rise time, it also provides a timebase check.  Reading the Tek manual makes me want a 485 I was offered and passed on.  But reading the 465 manual makes it seem like a pleasant project.

Have Fun!
Reg
 

Offline alsetalokin4017

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Re: New Guy seeking Oscilloscope advice
« Reply #25 on: March 14, 2018, 10:23:48 pm »
Looks like the cat pissed in there somehow. I've never seen corrosion like that unless something bad spilled on the parts. It looks like there may be some inter-leg conductivity happening.

I think I'd replace all three of those ICs (and inspect their sockets and maybe replace sockets as well) and also that cracked capacitor, before going much further.  Even though those parts are in the vertical circuitry... shorted pins could cause problems in other subcircuits too.
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Offline tautech

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Re: New Guy seeking Oscilloscope advice
« Reply #26 on: March 14, 2018, 10:25:03 pm »
Gee, if this is really going to be a repair effort, should I just make a new thread in the correct place, or I guess, could it be moved?
As the OP you can move it to Repair yourself.
Very bottom of this page, bottom left: Move Topic.
Click that and follow your nose.
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Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #27 on: March 14, 2018, 10:29:42 pm »
So I made it home and took off the cover. Most of the center (where the CRT is) is covered by the safety shield, which I didn't touch. I took a look at the parts I could see (sides and bottom) while the manual was printing, and the only thing I saw that was immediately obvious was corrosion on some of the IC legs. Looking at the manual, this is the Vertical Channel Switching section of the vertical preamp board, and  two of those are Quad NAND-gates and the other is a dual Flip-flop. No idea what they do, though. Other than that, the coating on a ceramic cap had a tiny crack, but it's right at the leg.

That corrosion looks unpleasant - but contained to the IC itself. It doesn't appear to have spread to the PCB or even the socket. If that's the case, at worst the ICs would need to be replaced, but I don't know whether that is necessary. Cleaning them with IPA might reveal the actual extent of the corrosion. Be careful not to allow any corrosion products to fall into any of the switches!

An interestion observation is that those ICs are in the ch1/ch2 trigger select circuit. As you will know from reading the manual (hint hint :) ), the trigger is the thing that kicks off a sweep. If you look at the manual, you will see that the ext trig input avoids those ICs and kicks off the sweep directly. So, try connecting a signal to the "ext trig" input, selecting "ext trig", and fiddling with the slope/level to see if you can kick off a sweep and/or get the "triggered" light to illuminate.

Replacing a socketed IC is trivial, provided you have a replacement. Now 74xx devices are rare, but, without checking the timing, I would try using a 74LSxx or 74xCTxx device. Be aware that 74xx devices have a 5V power supply - but in this case the +ve (pin 14 (or 16)) is not at +5V and the -ve (pin 7 (or 8)) is not at 0V. So do not connect a scopes probe's shield to the -ve ! Either use a multimeter, or do some mental arithmetic!

But before jumping to the conclusion that the ICs are faulty, it would be worth isolating the fault to that part of the circuit.


Quote
I followed tggzzz's advice and put the scope in X-Y mode, and lo and behold, with the right settings, the trace rotated 90 degrees and was horizontal(pic attached). With a little improvisation (I only have one probe!) I set the calibrator signal to both pins at once and got an actual 2-D shape, as well.

If the deflection is roughly equal in both directions, then the horizontal amp is OK and the problem is in the sweep circuits.

There are a lot of switches and ribbon cable connectors around the sweep circuit, and they might or might not be the cause of the problem. Some of the switches are easily accessible and others are not. Most have exposed contacts and can have grit or oxidation problems. If you really think the switches are the cause of the problem, then it is best to carefully ascertain what is still working so that you can avoid fiddling with working components.

Having said that, your audible "pop" doesn't indicate a switch problem; a capacitor is still a likely cause. Look for tantalum or electrolytic caps which are damaged.
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Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #28 on: March 14, 2018, 10:36:38 pm »
Looks like the cat pissed in there somehow. I've never seen corrosion like that unless something bad spilled on the parts. It looks like there may be some inter-leg conductivity happening.

I think I'd replace all three of those ICs (and inspect their sockets and maybe replace sockets as well) and also that cracked capacitor, before going much further.  Even though those parts are in the vertical circuitry... shorted pins could cause problems in other subcircuits too.

The ICs are in the ch1/ch2 trigger select, and can be avoided by using the external trigger.

C437 is a PSU decoupler for those ICs, and thus probably isn't the cause of a complete sweep failure.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #29 on: March 14, 2018, 11:07:13 pm »
Noob moment alert: how do I use the external trigger? Do I just hook up any old device to the external trigger plug and choose external from the trigger menu? What kind of signal do I need to feed it?
 

Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #30 on: March 14, 2018, 11:16:11 pm »
Noob moment alert: how do I use the external trigger? Do I just hook up any old device to the external trigger plug and choose external from the trigger menu? What kind of signal do I need to feed it?

RTFM. It is effectively a third channel. Use the calibration output or anything else convenient.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #31 on: March 15, 2018, 12:26:46 am »
Get a Harbor Freight toothbrush size stainless steel brush and brush the IC legs.  Place a piece of wood between the legs to to support them as you brush each side. Then tin them with solder.  You *really* don't want to replace old 7400 series logic if you don't have to.   A lot of it borders on unobtanium.  Replacing the sockets is probably a good idea.  But a diamond nail file ground down to match an IC pin might be less work.  There is also Deoxit.  I've never used it, but I've heard a lot of very favorable comments.  So I'm going to try it on the transistor sockets in my Tek and  Dumont scopes.
 

Offline Peabody

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Re: New Guy seeking Oscilloscope advice
« Reply #32 on: March 15, 2018, 02:01:33 am »
So I made it home and took off the cover. Most of the center (where the CRT is) is covered by the safety shield, which I didn't touch. I took a look at the parts I could see (sides and bottom) while the manual was printing, and the only thing I saw that was immediately obvious was corrosion on some of the IC legs. Looking at the manual, this is the Vertical Channel Switching section of the vertical preamp board, and  two of those are Quad NAND-gates and the other is a dual Flip-flop. No idea what they do, though. Other than that, the coating on a ceramic cap had a tiny crack, but it's right at the leg.

I have three used 7400's  and four used 74LS74's that I'd be happy to send you.  But you know, since the chips are socketed, you could just take them out, clean up the pins and test them on your breadboard.  Truth tables from the datasheets.

My my guess is these are unlikely to be the cause of the pop, but you never know.  I would look again for blown caps and such.
 
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Offline Peabody

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Re: New Guy seeking Oscilloscope advice
« Reply #33 on: March 15, 2018, 02:32:25 am »
I found this week-old video on Youtube.  Possibly similar issues.  It's promised to be the first of a series on repairing his 475.


 
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Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #34 on: March 15, 2018, 12:33:17 pm »
So I spent an hour or so last night trying to get used to reading the diagrams and learning what boards did what, and testing voltages. Most of the voltages came back within about 5%, which I figure is close enough, right? All voltages were taken with no input signal.
The only voltage level that was way off was the unregulated 50V which read 70 Volts. That's not 50V, but since it's unregulated, I wasn't sure that was a really big deal.
There was also some substantial corrosion on the "B Trigger Generator" IC pins 5 and 6. That's an input preamp.
Still haven't found any proper smoking guns, or... Smoked guns?
I'll keep looking. Going to start reading through that Tek Troubleshooting guide, too... Seems like good stuff.
 

Offline Cliff Matthews

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Re: New Guy seeking Oscilloscope advice
« Reply #35 on: March 15, 2018, 01:47:58 pm »
I wonder if this vintage Tek stuff had any Tin-whisker issues... maybe get out the magnifying glass and see.
 

Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #36 on: March 15, 2018, 02:53:13 pm »
Corrosion, cracked solder joints and electrolytic caps are the biggest problems with vintage gear like the Tek.  The trace pitch is such that tin whiskers seem an unlikely source of problems relative to those and general dirt.  I had a step in the calibrator response when using the delay on mine that I tracked down to a delay line termination resistor.  When I put the iron to  one lead of the resistor it fell out of the board!

My 465 is all socketed transistors.  In a humid environment, corrosion at the socket-lead interface is a big headache.  Unfortunately I see no easy way to address that because of the plastic sockets used in my unit.  The OP's unit appears to be either soldered or uses individual pin sockets.

A note to The_Boots.  Be very careful about cleaning switch contacts,  especially in the vertical attenuator section.  RTFM for the section of the instrument you're working on.  Also I forgot to mention, get a 12-18" section of wooden broom handle and whack the frame of the scope while watching the CRT with the scope in free running sweep mode to test for intermittent connections.  I learned this from an old Tek service guy I took my 465 to for advice.  I was quite alarmed at first, but after fixing a slew of broken solder joints it makes complete sense.  Just make sure you hit the frame and not some other part.  Also slap the sides of the scope with both hands.  This won't find an intermittent, but it will tell you you're looking for one.  Once you narrow it down to a small area an application of liquid flux and a soldering iron to all the joints is the solution.  If one joint is bad, likely several more in that area are also.
 
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Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #37 on: March 15, 2018, 03:56:30 pm »
Checking for physical connections makes sense, even if it doesn't help for the sweep issues.

There's a separate issue that I kind of back-burnered, but I probably should mention it as well:
The channel 1 voltage seems very low. While channel 2 shows voltage like I would expect, channel 1 is always about 1/10 at the same V/Div. I can sometimes get it to jump just by brushing against the V/Div knob, so it seemed likely that it was a bad connection somewhere. Since channel 2 still had correct voltage, I didn't think it could be related to the sweep issue and I could tackle it once I got the sweep worked out. Of course, I'm nowhere near experienced enough to know for sure.
 

Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #38 on: March 15, 2018, 04:04:36 pm »
So I spent an hour or so last night trying to get used to reading the diagrams and learning what boards did what, and testing voltages. Most of the voltages came back within about 5%, which I figure is close enough, right? All voltages were taken with no input signal.

Table 5.5 gives the DC tolerances and the ripple tolerances. Having said that, the sweep won't be affected by voltages being a bit out. If you look at the circuit diagrams, all "precision" analogue electronics comes from the regulated supplies.

Be aware that once you change the DC PSU voltages, just about everything in the scope may need tweaking - so that is a "let sleeping dogs lie" area.

Quote
The only voltage level that was way off was the unregulated 50V which read 70 Volts. That's not 50V, but since it's unregulated, I wasn't sure that was a really big deal.
There was also some substantial corrosion on the "B Trigger Generator" IC pins 5 and 6. That's an input preamp.
Still haven't found any proper smoking guns, or... Smoked guns?
I'll keep looking. Going to start reading through that Tek Troubleshooting guide, too... Seems like good stuff.

Read the circuit description for the 475. Look at the block diagram to see how the signals flow - that will help you ignore much of the detailed circuit description.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #39 on: March 15, 2018, 06:19:31 pm »
You've probably got a dirty attenuator switch.  I don't recall the Tek recommended procedure for cleaning them.  But I did read, *after* doing it, that you should *not* spray tuner cleaner on the attenuator section.  When I read that I immediately washed them many times with 99% isopropyl alcohol.  That's the reason for my RTFM comment.

A safe way to clean switch contacts is to close the switch on a piece of good paper and pull the paper out. Potentially tricky to do with a wafer switch, but teasing the contacts apart with a toothpick or  bamboo fondue skewer will work if you're careful to not bend the contacts too much. You can safely push the contacts together when you pull out the paper.

Once it is working, you're going to want to check and adjust it.  I bought one of Leo Bodnar's 40 pS rise time pulsers, so I now have two Tek 106 pulsers which are surplus to my needs.
 

Offline tautech

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Re: New Guy seeking Oscilloscope advice
« Reply #40 on: March 15, 2018, 07:04:04 pm »
So I spent an hour or so last night trying to get used to reading the diagrams and learning what boards did what, and testing voltages. Most of the voltages came back within about 5%, which I figure is close enough, right? All voltages were taken with no input signal.

Table 5.5 gives the DC tolerances and the ripple tolerances. Having said that, the sweep won't be affected by voltages being a bit out. If you look at the circuit diagrams, all "precision" analogue electronics comes from the regulated supplies.

Be aware that once you change the DC PSU voltages, just about everything in the scope may need tweaking - so that is a "let sleeping dogs lie" area.
Good advice to a point.

Minor inaccuracies can often be corrected by bringing a PSU voltage/s back into spec but for now just the sweep needs be fixed.
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Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #41 on: March 15, 2018, 08:21:55 pm »
So I spent an hour or so last night trying to get used to reading the diagrams and learning what boards did what, and testing voltages. Most of the voltages came back within about 5%, which I figure is close enough, right? All voltages were taken with no input signal.

Table 5.5 gives the DC tolerances and the ripple tolerances. Having said that, the sweep won't be affected by voltages being a bit out. If you look at the circuit diagrams, all "precision" analogue electronics comes from the regulated supplies.

Be aware that once you change the DC PSU voltages, just about everything in the scope may need tweaking - so that is a "let sleeping dogs lie" area.
Good advice to a point.

Minor inaccuracies can often be corrected by bringing a PSU voltage/s back into spec but for now just the sweep needs be fixed.

Just so, as I've written in other posts, and implied in the rest of that post.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline Ray Gianelli

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Re: New Guy seeking Oscilloscope advice
« Reply #42 on: March 15, 2018, 08:40:55 pm »
I lot of folks gravitate toward the Tek 400 series scopes for various reasons, i.e. no NLA IC's for one.  OTOH, you can get a newer Tek 2000 series for around $100.  I picked up a 2445A for $105.  Yeah, it's got plenty of ASIC's in it, but for $100 it's practically disposable.  It's a much newer scope and therefore won't have as many of the age related issues you might find in an older scope.

I know you've already bought your 475 but it's a complex piece for a novice to troubleshoot.
 

Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #43 on: March 15, 2018, 10:23:57 pm »
It depends a lot upon the "novice".  The quality of the Tek manual is just breathtaking.  After 20+ years I'd forgotten until I pulled it down to look at the horizontal section.  Great care went into documenting the whys and wherefores of each stage in the instrument.  It's a fantastic education in the realities of high performance analog design.
 

Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #44 on: March 15, 2018, 11:25:09 pm »
I lot of folks gravitate toward the Tek 400 series scopes for various reasons, i.e. no NLA IC's for one.  OTOH, you can get a newer Tek 2000 series for around $100.  I picked up a 2445A for $105.  Yeah, it's got plenty of ASIC's in it, but for $100 it's practically disposable.  It's a much newer scope and therefore won't have as many of the age related issues you might find in an older scope.

I know you've already bought your 475 but it's a complex piece for a novice to troubleshoot.

In my experience, having repaired 465,475,485,2445B,2465, it is swings and roundabouts.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline JohnPen

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Re: New Guy seeking Oscilloscope advice
« Reply #45 on: March 16, 2018, 12:05:49 pm »
I am not sure if this applies to Tek 475s but in the Tek 453 they use tunnel diodes for triggering the timebase.  Also Nuvistors (Valves) were used for the timebase A and B in the 453,  these do not apply to the 475.  After some searching and probing for 'no sweep' in timebase A of my 453 I discovered that the Nuvistor had failed but more importantly the tunnel diode had also failed.  I modified timebase A to duplicate the FET circuit as used in the TEK 453A but still had 'no sweep until I replaced the tunnel diode.  It appeared that even free running the timebase would not work properly unless a working tunnel diode was present.  My timebase B circuit also did not trigger but this time it was only the tunnel diode that had failed.

It would seem likely that the 'pop' you heard was, as others have suggested, the failure of a capacitor but it probably disconnected itself as well as the most of the scope is still working OK.  However that sort of transient behaviour could well have taken out a sensitive semiconductor component in the triggering circuitry which may be the cause of your problem.  Best of luck.
 

Offline tggzzz

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Re: New Guy seeking Oscilloscope advice
« Reply #46 on: March 16, 2018, 03:24:40 pm »
I am not sure if this applies to Tek 475s but in the Tek 453 they use tunnel diodes for triggering the timebase.  Also Nuvistors (Valves) were used for the timebase A and B in the 453,  these do not apply to the 475.  After some searching and probing for 'no sweep' in timebase A of my 453 I discovered that the Nuvistor had failed but more importantly the tunnel diode had also failed.  I modified timebase A to duplicate the FET circuit as used in the TEK 453A but still had 'no sweep until I replaced the tunnel diode.  It appeared that even free running the timebase would not work properly unless a working tunnel diode was present.  My timebase B circuit also did not trigger but this time it was only the tunnel diode that had failed.

Except for the nuvistor, that applies to the 475.

To proceed down that path, the OP needs to see where signals are entering/leaving each of the major functional blocks shown in the block diagram.

Quote
It would seem likely that the 'pop' you heard was, as others have suggested, the failure of a capacitor but it probably disconnected itself as well as the most of the scope is still working OK.  However that sort of transient behaviour could well have taken out a sensitive semiconductor component in the triggering circuitry which may be the cause of your problem.  Best of luck.

It can do - and sometimes it takes out a resistor as well :) I've seen that in a 485 on the "hidden" 13V rail, and in a 2465 where the Rifa firework took out a resistor and a 1cm diameter patch of prepreg :)
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline JohnPen

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Re: New Guy seeking Oscilloscope advice
« Reply #47 on: March 17, 2018, 12:05:11 pm »
At the time I had nothing fast enough to see the trigger output of the tunnel diode so after considerable checking I bit the bullet and purchased some tunnel diodes from somewhere in Russia.  Replacing the tunnel diode on the A timebase cured the problem but I still couldn't see the trigger pulse. ;D
 

Online Gyro

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Re: New Guy seeking Oscilloscope advice
« Reply #48 on: March 17, 2018, 01:34:15 pm »
Be very careful messing about with the tunnel diodes - in fact I would have advised not touching them at all if it wasn't too late (Not too late for the OP). They are delicate and sensitive parts. It's unlikely that the ones from Russia would have matching characteristics.

http://w140.com/tekwiki/wiki/Tunnel_diodes
Chris

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Offline JohnPen

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Re: New Guy seeking Oscilloscope advice
« Reply #49 on: March 17, 2018, 02:21:27 pm »
I basically selected one from the attached list of Russian Tunnel diodes list.  I went for the one with the closest current rating to the TD 4.7ma as 'sort of specified' in the 453 parts list.  The ones I used were identified as 1l304A and these seemed to work fine fortunately.  Certainly triggering appeared to work properly and reliably for both A and B timebases.   I don't know if these tunnel diodes are available now as this repair was done about 4 years ago.  I still have 4 out of the 6 I purchased as spares if anyone is desperate.

Edit: Oops I forgot the attachment.

Edit 2:  Found my original receipt from 2014 so  the actual identity of the TD is Quantity     
   
GI304A Ge Amplifier Tunnel Diode military USSR Lot of 6 pcs
« Last Edit: March 17, 2018, 02:46:44 pm by JohnPen »
 
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Online Gyro

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Re: New Guy seeking Oscilloscope advice
« Reply #50 on: March 17, 2018, 02:55:21 pm »
A useful list, saved for future reference. Thanks!  :-+
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Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #51 on: March 17, 2018, 11:22:15 pm »
I don't know... I tried hooking the A gate output on the back to another oscilloscope (a cheap USB one) as was recommended in the "Troubleshooting Your Oscilloscope" manual and nothing.
I also now have no dot at all, and back when I did, the intensity knob just messed with the focus, although not as much as the actual focus knob. Beam finder still makes the dot show up just fine.
It feels like things are deteriorating... I'm thinking of trying to find a local hobbyist who might help. Any suggestions on where to look?
 

Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #52 on: March 17, 2018, 11:52:02 pm »
Knowing where you are would help.
 

Offline The_Boots

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Re: New Guy seeking Oscilloscope advice
« Reply #53 on: March 18, 2018, 12:01:19 am »
I'm in Ann Arbor, Southeast Michigan.
 

Offline james_s

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Re: New Guy seeking Oscilloscope advice
« Reply #54 on: March 18, 2018, 06:04:14 pm »
You said you heard an audible pop, that suggests that there should be some visible sign of problems somewhere. Given the age of the scope, I would suggest starting by looking at the electrolytic capacitors in the power supply and check all the voltages there. Capacitors have been known to pop their seal and that could cause a low or missing power supply voltage.

If you feel like you're getting in too far over your head I would suggest putting it all back together so you don't forget where anything goes then set it aside until you have some more experience. Heck sometimes just setting something aside for a day or two then looking at it again works wonders.
 

Offline Peabody

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Re: New Guy seeking Oscilloscope advice
« Reply #55 on: March 18, 2018, 06:16:38 pm »
I'm thinking of trying to find a local hobbyist who might help. Any suggestions on where to look?

Try to contact some of the organizers of this Meetup group in Ann Arbor, and see if they know of anyone who might be able to help:

https://www.meetup.com/AllHandsActive/
 

Offline rhb

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Re: New Guy seeking Oscilloscope advice
« Reply #56 on: March 18, 2018, 08:38:15 pm »
I second the suggestions by @james_s.  You've got a power issue if the beam  goes off the screen.
 


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