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wgoodf
August 19th, 2009, 11:06 AM
howdy guys - i am thinking about buying an oscilloscope.

now then, i know nothing about such things (apart from the fact that they are pretty! - especially some of the old ones) but what i do know is that as i move further away from the standard 8bit home machines and that i find myself buying machines that are very much less common, it is more and more likely that they need repairs.
which i am going to have to try on my own.

i need tools. i dont know how to use the tools, but that comes with practice. Anyway i have it in my head that an oscilloscope would be a good buy if i could find one cheap. (ebay seems to range from 20-ish to 100-ish for a machine that may do - GBP)

there are a range of prices on ebay for vintage-ish machines, the thing is, as i am sure you know, the price for a 100khz machines is more than for a 20khz machine.

if it was for old kit would i need a 100khz scope or would 50 or 20 do?
am i daft for considering buying one? (ok i think i know that answer)

but anyway - advice for the newbie that is finding it more and more common to be looking at a board that isnt working properly and wondering where the problem lies.

cheers!

hexsane
August 19th, 2009, 03:55 PM
What would be easier to learn with would be a logic probe. Will be cheaper and will let you know if things are working (logic state changes will be indicated by a pulse LED on the probe). I would suggest finding some books or online tutorials for basic electronics and digital electronics before delving into the world of test equipment. A cheap scope is fun to play with but if you do get into it you may find the cheap scope is far to limited in its capabilities later on.

-Matt

Lou - N2MIY
August 19th, 2009, 04:16 PM
I think you meant 100MHz and not 100kHz when describing the bandwidth of a scope you might be interested in. For vintage computer repair, a two channel 100Mhz scope is a good tool. The logic probe has its place, but I think you should be able to get a good scope cheap enough. On this side of the pond, a Tektronix 465 can be found for ~$100USD. Certainly not daft at all...

If I were where you are, I might check if the University of Edinburgh or Heriot-Watt have surplus equipment sales.

Lou

wgoodf
August 20th, 2009, 04:47 AM
thanks for the considered responses guys.

its interesting that in those two posts above the argument about what equipment is needed for retro repairs pretty much mirrors the larger thread covering test equipment owned / used - ie, the need or not for a scope.

i think as you say some reading in required. logic probes - cheap ones are very cheap indeed. scopes less so.

however those dials are so very nice in that lab equipment chic kind of way.

but one last thing - considering its retro use, is 100mhz the minimum i should be looking at - 50mhz simple no good?
just trying to get the cost numbers in my head.

cheers

RetroHacker_
August 20th, 2009, 05:13 AM
Before you spend a lot of money on an oscilloscope, you should start learning electronics, and learning how to use the simple tools. A scope can be a very useful tool, but you rarely find yourself needing it. You can go very far with only basic tools - provided you know how to use them. You should get a decent multimeter, and learn how to use it. Similarly, get a logic probe, and learn how to use it.

I have a couple scopes, but I rarely use them for repairs. Most of the time, my trusty meter and the logic probe will tell me what I need to know. I think the most valuable skill in repairing this stuff isn't the ability to use a 'scope, but the ability to logically break the circuit down, and work through it to find the fault. Look at the board, the chips it uses, the 'sections', and try to figure out what it's supposed to be doing, and what could cause the failure you are seeing.

Start with the basics, then work yourself up to the 'scope. In the meantime, keep your eyes open for older models turning up cheap. My first 'scope was free - an ancient Tektronix tube-type model. But, by playing with that, I learned how to use the device, so later when I got a solid state scope, I understood how to use it. Even still, my "new" scope is still a good 30 years old. It's limited, but in the rare instances I've needed a scope, it's been good enough.

-Ian

wgoodf
August 20th, 2009, 05:15 AM
Before you spend a lot of money on an oscilloscope,

trust me - i can assure you that was the one thing that was not going to happen.
i am cheap cheap cheap!

cosam
August 20th, 2009, 05:26 AM
however those dials are so very nice in that lab equipment chic kind of way.
Absolutely. I don't need any of my old computers either, but that doesn't stop me collecting them ;-) You probably don't need a scope, but if you like twiddling knobs and are curious about what signals "look like" then why not? A logic probe will however get you a long way and is still useful even if you do get a scope later (they're a bit easier to set up if you just want to do some basic checks).


but one last thing - considering its retro use, is 100mhz the minimum i should be looking at - 50mhz simple no good?
just trying to get the cost numbers in my head.
One rule of thumb is that you want the bandwidth to be about five times the highest frequency you plan to measure. A 100MHz scope will therefore be good for up to about 20MHz, which should be plenty for vintage purposes. A 50MHz scope will cap your maximum useful range to 10MHz, which may well be enough depending on what you're doing.

wgoodf
August 20th, 2009, 06:18 AM
Absolutely. I don't need any of my old computers either, but that doesn't stop me collecting them ;-)

well yes - i guess thats the thing if we are honest, sometimes we justify the purchase long after we have decided to buy something.



You probably don't need a scope, but if you like twiddling knobs and are curious about what signals "look like" then why not?


is there someone who doesnt like twiddling with knobs?



A logic probe will however get you a long way and is still useful even if you do get a scope later (they're a bit easier to set up if you just want to do some basic checks).


i think what is clear that i could do with a wee bit of reading and a logic probe seems the sensible way to go as far as hardware- which i really knew. i had considered buying one in the past for all the reasons stated above.

but saying all that - if i find a 50mhz scope on ebay for dirt cheap as seems to be the case this way, then there is no particular reason not to buy one, even if as you say, it proves limited in use due to the nature of problem solving circuits.
because when it comes down to it - its more old kit to play with and something new to learn.

cheers and thanks again guys for the advice.
appreciated.

dave_m
August 20th, 2009, 07:44 AM
but saying all that - if i find a 50mhz scope on ebay for dirt cheap

Be careful on ebay. Make sure there is a statement that the unit is in working order. Also do not buy it if they do not have a photo of it showing both traces hopefully with a signals applied. And there should be a statement about foam-in packing, etc. Look for Tekronix models 454 and 465 for a good price and note that shipping will be costly. Manuals for these models are available on the web. Familiarize yourself with them enough so you can tell from the ebay photo that the critical knobs are not missing, broken, etc. Scopes are only fun when they work.

barythrin
August 20th, 2009, 08:04 AM
Although the opinion on these events has been subject to criticism lately (for those of us who go for "other" equipment only) that's where I did like HAM radio conventions if you find one in the area. Some cheap scopes can be found and usually they'll have them connected to something to show you they work. The Mhz game is one that folks are right you can be limited to but I'm also not an expert enough to know what the highest hz you'd end up playing with would be.

The stuff I've seen/read troubleshooting with a scope are bad circuits or electricity where you can see the wave going through and not being smooth as expected. Can detect bad processor or RAM, other chips that way. Also for troubleshooting a TV or monitor they can come in handy but I have yet to use it for this purpose (it's somewhere in my several year long to-do list).

edit: Not sure what's out there education wise that's good reading, found a few sites googling around but http://www.archive.org/details/TroubleshootingWithTheOscilloscope despite being old I'm sure is a good read and seems right on the topic of I have an oscilloscope, how could I use it to troubleshoot things.

Chuck(G)
August 20th, 2009, 08:26 AM
A very useful tool that may come in handy can probably be had by most of the forum's members for little more than beer money.

A parallel-port logic analyzer:

http://developer.berlios.de/projects/tfla-01/

To be sure, this thing has its limitations, but it can be very useful when you're wondering what is going down some lines. While an o-scope will give you lots of detail about the shape of a pulse, there are times when you don't need that information and are more interested in the data on the lines.

Yes, this super-cheap analyzer has severe limitations, but for looking at data coming down a serial line or over a keyboard port, or down an 8-bit bus, it may be all that you need.

wgoodf
August 20th, 2009, 11:05 AM
http://www.archive.org/details/TroubleshootingWithTheOscilloscope despite being old I'm sure is a good read and seems right on the topic of I have an oscilloscope, how could I use it to troubleshoot things.

bravo!
and thanks

tezza
August 20th, 2009, 11:10 AM
Hi,

One thing puzzels me, and it may be just ignorance. However I figure if you don't ask, you'll never get to know.

What does a logic probe tell you, that a multimeter does not? I've used a multimeter a number of time to ascertain whether a circuit is on or off simply by seeing if there is high voltage (say 5v) or low voltage. Does a logic probe tell you any more than this?

Incidently you can do a lot with a multimeter. Of all the repairs I've done (http://www.classic-computers.org.nz/blog/index.htm)keeping my vintage units going, I've only really needed a scope (well, someone else's scope and scope expertise) twice, and that was to align drives and to check for AC ripples on a suspect PSU (which turned out to be ok actually).

They are a good thing to have but I wouldn't break the bank over one.

Tez

RichCini
August 20th, 2009, 11:43 AM
I'm no expert at this kind of stuff, but I've collected a bunch of tools that I've used on-and-off over the years for my vintage work. My list probably isn't the best, and there are most certainly other ways to perform the same task, but this works for me. YMMV.

* A (or more than "a") good digital multimeter. Mine has an RS-232 port for data logging. I also have a plug-in current probe and temperature sensor. In total, I have three multimeters (this one, a Radio Shack bench multimeter, and one on the Tek 465 -- sometimes you need to watch more than one voltage at a time.)
* Dual-voltage adjustable bench power supply.
* 10a variac
* Tek 465 scope with multimeter.
* A Mastech 1.4ghz frequency counter.
* A logic probe and signal injector. These I bought when I was in high school and I still have them. I use the logic probe now and again when I need something quick.
* A HP 4952 serial protocol analyzer. Sometimes I find this invaluable in troubleshooting serial port connections. This is the one "luxury item" on my list. It was $25 on eBay and came complete.

Again, some of this might be overkill but a lot of it I got inexpensively from friends (the oscope) or on eBay. The only other thing I wish I had is a logic analyzer but even then, it's just geek-wear for the workbench.

dave_m
August 20th, 2009, 12:55 PM
What does a logic probe tell you, that a multimeter does not? I've used a multimeter a number of time to ascertain whether a circuit is on or off simply by seeing if there is high voltage (say 5v) or low voltage. Does a logic probe tell you any more than this?

You are correct for static conditions. But what about when the signal is switching? If the switching duty cycle is very low or very high, you can be tricked. The probe gives you on, off and blinking. some of them have a pulse injector that can force a high or low. This is very handy for testing gate outputs.



I've only really needed a scope (well, someone else's scope and scope expertise) twice, and that was to align drives and to check for AC ripples on a suspect PSU (which turned out to be ok actually).

They are a good thing to have but I wouldn't break the bank over one.

Tez

Quite right.

tezza
August 20th, 2009, 02:15 PM
You are correct for static conditions. But what about when the signal is switching? If the switching duty cycle is very low or very high, you can be tricked. The probe gives you on, off and blinking. some of them have a pulse injector that can force a high or low. This is very handy for testing gate outputs.


Ah, I see. Now I know. Thanks.

Tez

nige the hippy
August 20th, 2009, 03:08 PM
Radio rallies are definitely a good place for 'scopes, however I got my Tektronix 2430 150MHz DSO from the local tip for a fiver! ( it cost the same as a house when it was new) it's a beaut! and it's only gone bang once since ;-( (fixed however)

The first 20 or 30 repairs are the worst! Be prepared to spend months wondering.

I suppose the most useful tool of all is patience,
the next most is being prepared to look up (and ask for) the information,
The third most is NEVER get angry, it's a machine, it can only do what is asked, and to the best of it's ability, if you get angry with it, you will do something stupid, and make the problem worse.
Fourth... There CAN be more than one fault on vintage kit. Way back I was really good at fault finding, it was just a case of tracing back to a point common to all symptoms of the fault, checking a few signals, one was usually wrong. There's 2 or so ends to a signal. Change driver & reciever(s) & you're there. (Unless it's a power fault) Thanks to the effect of cosmic rays & time, quite often in vintage stuff there's more than one faulty component. Get the PSU perfect. strip off any peripherals, and get the basic system going, then add in the cards, one at a time. Keep notes, and keep a cool head.