Oscilloscope features

[jackmech]

I'm thinking about getting an oscilloscope to help me diagnose problems and I was wondering what kinds of features I should look for for working on cars. I'm thinking of getting one off eBay and most of them are Techniks (?) or something like that. Any tips?

[campbellcj]

Um, it's Tektronix http://www.tektronix.com

Agilent Technolgies is another leading supplier. Also Anritsu (Japan), possibly Philips and a couple of others, but those first two have huge market share.

Sorry, I can't answer your question about automotive applications though. Actually, on our early cars at least, I can't imagine much that a scope would be good for. Perhaps diagnosing or rebuilding D-Jet ECU's?

[pbanders]

The best deal on oscilloscopes is the 7000 series from Tektronix off of Ebay. This sites lists all of the plug-ins and mainframes:

http://www.caip.rutgers.edu/~kahrs/testeq/7000.html

I would go for a 7834 mainframe if you want performance. For cheap but good, a 7623 would be a good choice. I personally like scopes with analog storage, but if you don't want storage, that opens up what you can get.

As far as plug-ins, you want two dual-channel vertical amps that match the speed of your mainframe, and a dual delay timebase. The BEST additional plug-in is the 7D15 Counter/Timer. I have one and can use it to measure injector pulse widths to a very high degree of accuracy (up to 7 digits, far better than is needed).

Total cost for all of this stuff should be $300-$400. You absolutely cannot obtain this kind of capability for this price buying new.

If you're going for absolute bare-bones, try a T922 from Tek off Ebay. Good 15 MHz dual-channel scope that you can get for as little as $50 in perfect condition.

[jackmech]

Campbellcj: I'm planning on using the oscilloscope for such things as testing the ignition system (you can see if all the spark plugs are firing well and if the points are bouncing and such) among other things that I can't think of right now because it's early and I'm braindead right now.


Thanks Herr Anders,
Those sound in my price range ($200-300) but I have a few questions, since I know next to nothing about oscilloscopes.

1. Can you define mainframe and scope? Are they two separate hardware pieces? All the oscilloscopes I've seen are all in one, single case things. I'm sure I'm misunderstanding.

2. How do the plug-ins work? I'm guessing they extend the features of the machine. If so, is there a standard plug type? How do they attach? Are there some scopes that can't be "plugged-in" to? Would I need an amp? Why two-dual channel amps? And what's a dual-delay timebase? Is this the function that will hold a waveform on the screen for closer inspection? That is a function that I would really like.

Lots of questions. I'll check at school for books about oscilloscopes and try to learn more. Thanks.

[pbanders]

Go to the webpage I referenced, there are photographs. The 7000 Tek scopes were modular. The mainframe contained the display, storage controls (if present), triggering and backplane. You buy plug-in modules to complete the scope. Mainframes had either 3 or 4 plug-in slots, usually 2 vertical and 1 horizontal, or 2 horizontal for the 4 slot units. At a minimum, you need a single vertical amplifier and a time base.

My scope is a 3 slot 7313 (65 MHz), which has split-screen analog storage. I generally use a 7A26 2-channel vertical amp and a 2-channel 7A18 vertical amp with a 7B53A dual delaying time base. This setup gives me four vertical channels with readout (you can see the vertical amp and time base settings on the screen). My 7A18 has an offset voltage capability that is nice for looking at signals with a large DC component w/o distoring the waveform by using AC coupling (e.g. looking at the output of the speed control circuit of the D-Jet ECU). The 7B53A timebase is extremely flexible, with magnification, external delay, many triggering modes, etc.

Storage allows you to look at one-shot events and time-dependent events. For example, the acceleration enrichment section of the D-Jet ECU provides immediate and delayed enrichment. The delayed enrichment causes the injector pulse width to widen by a few tenths of a millisecond for about 2 or 3 seconds after TPS pulses are detected. With storage, you can put the injection pulse waveform up on the screen, give the ECU some accel pulses, and watch the pulse widen. The storage shows exactly how much additonal time was added. See my ECU web page for photos. All of the photos of scope traces were taken using my 7313. I didn't have my 7D15 counter/timer when I did the page, it's a powerful tool for measuring pulse widths and time delays with nanosecond accuracy.