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Tektronix 475(a?) I broke my oscilloscope, just when the fun had started...
Posted by kevingnet on 29 Jul, 2021 09:30 -
Not to seem overly dramatic, but I'm quite sad at my stupidity. I'm actually a software engineer, and recently I started a hobby of learning electronics. I've been buying vintage audio stuff and restore, fix some, clean up, etc... for sort of fun...as the case usually ends up being, eventually my nature demands that I have ALL that's necessary in terms of tools and equipment, talk about going overboard.
Long story short, I had been doing fine without an oscilloscope, but by hook or crook, I must have it. I bought a 475, the lady that sold it said she got it at a estate sell and didn't know what to do with it, got it for $80 bucks taking a risk that it may not work, oh well, so be it. Got it home, started messing with it, read the manual a bit watched some videos, bought a signal generator and was able to see the long awaited sine wave.
So far so good. Then, since I had been doing clean up of audio equipment, the genius in me decides to 'clean it up,' turns out that it had several broken knobs, I think the shipping agency likes to play soccer with the items, to kill the boredom. So... I decide to buy a 'broken' one of the same model for $50 and then it turned out to be newer a 475A, I contacted the guy and he said he had no idea what to do with it or even the state it is in, he actually sold it as non working. Eventually found out that it does display a sweep line, but did not test it otherwise, since it was, the 'bad one.'
I then proceed to cannibalize from the bad one and put the good controls, knobs, including some connecting rods from the knobs to the pots that were broken on the 475 one, so far so good. Then, totally ignoring at my peril coming first to this site and reading, had the great idea to.... deoxit the oscilloscope, since I've become used to it and now I wanna put deoxit on everthing...not content enough with this, I also reach for the can of pressurized air and have a good go at it on the scope. Now I was happy, to have a nice oscilloscope that was wholesome and clean. I put it back together, and end up with a voluminous paper weight. Ok! no worries, I just gotta wait 'til the good ol' deoxit dries off, sure, that must be it, at this point it is *not* conceivable that my hard earned oscilloscope could be damaged, I mean these things are built like tanks. Right!?
Several hours go by, couple of days later, after more reading of the manual, trying this, doing that...BIG WTF!!! Ok, come on, why did this happen to ME.
So, I'm at a cross roads, here is some data that I can use to reason with on how to proceed:
1) The bad scope is newer, I can see the circuit boards, etc... it's the A model, it *did* appear to work
2) The good scope (now a big paper weight) is toast, what do I gotta do to get this beast to wake up? It turns on, but no matter how I fuss with the knobs, no scan line, nothing at all on the CRT.
3) I've watched these youtube vids where this guy is doing a restoration on a dumped 475 and apparently it had been damaged by getting hit in the knob that you can pull out a bit and it has like 3 axis and it goes into a big rectangular box, he found a micro switch that came off, and the back pot was broken inside and he said it was for a timing something something and he decided to remove the broken pot and disable that functionality and bridge it through a wire, ok, to each his own, I guess, then he was actually able to make the thing work and it started slinging sine waves like the best of them... ok that was awesome. However! in my scope, no such microswitch was ever found, could it be it fell off when I stupidly took out the knob, I was just trying to remove the face plate and polish it, but abandoned the idea due to the damn complexity of it, just to get it clean. So... I don't know if there's something to it about a micro switch which because if there was and it's not activated, apparently the guy said that it would have the wonderful effect of no signal on the CRT.... Anybody has ideas about this, is it supposed to have this switch, for my model
4) At this point, I'm thinking, well, the 'bad one' *is* newer, and it *does* appear to work... why not just use the bad one and put the good knobs on it, and... BE DONE WITH IT?
In any case, been reading the manual, but it is huge, and I'd rather not spent too much time on it, since I'd really like to get back to the audio equipment...
Please share any advise on what to do, what would you do? Would you fix the 'good' scope or just give up? I mean the NOT giving up demon in me already cost me about a week and I've done jack about the vintage audio equipment.
Thanks for reading this sad sad story... -
- Get the manual out and read up on the functional description. The 475 is old enough that the manual is assumed to be read by adults and thus is less insulting than most modern equivalents.
- Check voltages at the PSU.
- Follow the signal. Where it stops, there's trouble.
- Yes, you need a scope to fix a scope. Even my humble collection includes 3.
PS: Capacitors are Murphy's foot soldiers. In something as old as a 475, they're often overdue for replacement. -
Please share any advise on what to do, what would you do? Would you fix the 'good' scope or just give up? I mean the NOT giving up demon in me already cost me about a week and I've done jack about the vintage audio equipment.
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I would fix one of them but I'm a EE who does software as opposed to a Software Engineer who does hardware.
Your first decision is to either attempt a repair or sell both and use the money towards a modern oscilliscope. Since you are a software engineer, you might prefer a hackable scope that runs linux.
The old Tek manuals are very very good. It can be done, but you've got a lot to learn. Sort of like debugging a large multi-thread appliction running in Docker when you just got your first Hello World to compile and run.
You need to learn the subsystems and what they do. Use that along with the manual to begin systematic troubleshooting. Completely dead with no beam? Power Supply and blanking are two areas to look at. After that start looking at X-axis and time base.
One more hint -- don't mess with any of the calibration settings until you get it working. Don't mess with them after you get it working either. Turning them wont' fix a dead 'scope and you dont' have what is needed to adjust the calibration.
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I have a beautiful old 475, bought in 2005 when I knew nada about scopes. I turn it on a couple times a year and hold my breath, waiting for the inevitable cap to fail. If you would really rather work on your vintage audio than fiddle with a scope, pickup a Rigol 1054Z for $350 new. I prefer the Siglent 1104x-e but that will set you back $500. You will have a reliable piece of equipment instead of something else to fix. Make sure you have a couple of decent DMM’s first, far more important that a scope.
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I have a beautiful old 475, bought in 2005 when I knew nada about scopes. I turn it on a couple times a year and hold my breath, waiting for an unobtanium hybrid
capto fail
Fixed that for you
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If you would really rather work on your vintage audio than fiddle with a scope, pickup a Rigol 1054Z for $350 new. I prefer the Siglent 1104x-e but that will set you back $500.
If you're in the USA you can get a GW-Instek for under $300 (find the EEVBLOG discount)
https://www.tequipment.net/Instek/GDS-1054B/Digital-Oscilloscopes/?search=true
It's a complete bargain at that price, and better FFT than the Rigol.
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If you're in the USA you can get a GW-Instek for under $300 (find the EEVBLOG discount)
https://www.tequipment.net/Instek/GDS-1054B/Digital-Oscilloscopes/?search=true
It's a complete bargain at that price, and better FFT than the Rigol.
Seconded. It's also upgradeable for free, except with respect to bandwidth (from what I've seen in this forum, you can't hit 100 MHz with its "upgrade"). For audio work you won't need more bandwidth than this has, and that's likely true for fixing the scopes the OP has as well.
It would be nice if the "spectrum analyzer" option were available for it, but its FFT is still worlds better than the Rigol's.
At this time, this Instek scope has to be the best bang for the buck scope you can buy new. I didn't need one myself, but bought one anyway because it's so cheap, I wanted to see just how responsive its UI is (it's very responsive until you have it doing long captures with multiple channels), and it's always useful to have a cheap scope you can use for those situations where you might not want to risk your more expensive scope (I have an SDS2104X+ as well).
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4) At this point, I'm thinking, well, the 'bad one' *is* newer, and it *does* appear to work... why not just use the bad one and put the good knobs on it, and... BE DONE WITH IT?
I think that decision is a no-brainer if you think the newer is working. Change the knobs! And use the newer to debug the older as others have advised.
IF restoring the scope is a PITA compared to the vintage audio equipment, then dump the whole package in a recycle bin and move on. Nonetheless, it's still repairing old equipment so in essence it's about the same.
All in all, "looking on the bright side of life", i find that it was a learning lesson and I've paid much more for lessons where I've learned much less.
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I have a beautiful old 475, bought in 2005 when I knew nada about scopes. I turn it on a couple times a year and hold my breath, waiting for an unobtanium hybrid
capto fail
Fixed that for you
IIRC there are no hybrids in the 475. Only basic components. But that's hearsay. -
I seem to recall that using any kind of contact cleaner in the Tek rotary switches destroys them, I don't remember where I heard that but I have left mine alone. Those old Tek analog scopes are amazing instruments, there is a lot of electronic voodoo going on inside one. If nothing else, at least there is a probably good CRT, that is one of the most unobtanium parts, the only source for a CRT is from another Tek scope.
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I have a beautiful old 475, bought in 2005 when I knew nada about scopes. I turn it on a couple times a year and hold my breath, waiting for an unobtanium hybrid
capto fail
Fixed that for you
IIRC there are no hybrids in the 475. Only basic components. But that's hearsay.
Correct, no hybrids but there are custom Tek IC's which may be difficult to source. -
I seem to recall that using any kind of contact cleaner in the Tek rotary switches destroys them, I don't remember where I heard that but I have left mine alone. Those old Tek analog scopes are amazing instruments, there is a lot of electronic voodoo going on inside one. If nothing else, at least there is a probably good CRT, that is one of the most unobtanium parts, the only source for a CRT is from another Tek scope.
If you are referring to the attenuator switch decks yes, the recommended cleaning agent for those leaf contacts is 100% IPA. However, in tough cleaning cases I have successfully used deoxit D5 then followed up with a rinse of 100% IPA then a blow dry. No damage. -
Other options if what you really want to do is work on audio equipment instead of fixing old scopes.
1. Software oscilloscope using the sound input on your PC. Free, but comes with some serious limitations. You can trace signals, see the magic sine wave, and even track down noise and bandwidth issues, but no way to see DC offsets or know actual voltages.
2. Various pocket scopes on Ebay and elsewhere, usually $80 to $100 bucks. The scope guys will all sneer because they have limited features and bandwidth, but they are all you really need for audio work.
3. The analog discovery kit. About the same cost as the Rigol but is a great way to learn electronics while providing all of the scope functionality needed for audio equipment.
I will caveat some of my statements here. Most audio work requires little bandwidth or precision, but if you are trying to track down a tiny fraction of a percent of harmonic distortion or do some of the other work on the high end of audio you need some quality gear. -
So, I'm at a cross roads, here is some data that I can use to reason with on how to proceed:
Welcome to the forum.
Bad scope, good scope, well who's to judge this and it seems quite obvious neither of your suppliers had a clue and without trying to be rude nor do you.
However simple checks are easy to do with just the probe compensation output and a scope probe.
This is always your sanity check until you become familiar and confident of how scopes should work.
Down the road some.... the service manual for any scope lists performance verification measurements and while this looks like a deep rabbit hole just reading and understanding simple checks can help you down the path to gain confidence in equipment.
Enjoy and learn your journey.
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I have a beautiful old 475, bought in 2005 when I knew nada about scopes. I turn it on a couple times a year and hold my breath, waiting for an unobtanium hybrid
capto fail
Fixed that for you
I believe you are thinking of the 2400 series, like the 2465. No hybrids in the old 475’s, but plenty of other stuff to keep you busy, like power supply caps. Easy enough to source replacement caps, but still a bit of work to replace. IMO they make a great second scope once you have a decent digital as your everyday driver. There is good reason the industry has moved on to digital.