What kind of oscilloscope do I need?

Discussion in 'Effects, Pedals, Strings & Things' started by Roodillon, Aug 2, 2008.

  1. Roodillon

    Roodillon Member

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    I wish to purchase an oscilloscope to help me diagnose and repair analog and digital effects pedals, and to work on and optimize microphones. I'd probably use it on amps and electric guitars too. I need to know what features I need on an oscilloscope to do the work stated above. I have been looking at oscilloscopes on eBay that were $200 or less and there are dozens of them. Mostly used, some smaller ones that are new. If I buy a used one I will make sure that is calibrated and warranteed. There is also one for the PC called Multi-Instrument 3.0 that looks interesting. Here is a link for it http://www.virtins.com/page2.html. If you scroll down the page there is a chart that shows you what the various versions do. There is a Lite version for $49, a Standard version for $99, and a Pro for $199. If I decide to go that route, which one would be best for me?

    Thank you!
     
  2. Montez

    Montez Member

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    All you need is a 10mhz scope. nothing fancy unless you are scoping digital. I use a Protek 6502 20mhz, 2 channel with great results. $195 new. Be careful of used scopes on Ebay I bought 2 junk piles before I sprung for a new scope.

    I really don't recommend PC based scopes. I didn't have much luck with them. Just my experience...

    You might also pick up a function generator. I use a vintage Heathkit IG-18, used on ebay $30. Anything that generates audio sine wave signal in the 100-10Khz range and 100mv -1v rms will work.
     
  3. Roodillon

    Roodillon Member

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    I'm with you on the used/computer based scopes. I will buy new/hardware if I can. I saw some on eBay that were new and on the small side for around $125, that would probably work fine since all I need is the basic stuff. One thing about the PC based though was that it could do digital.

    So - what about digital? I've got plenty of digital stuff around here and sometimes it breaks. And that stuff is usually much more expensive to have someone else repair or to replace. I just had to send a brand new Holiest Grail back to E-H because it failed after one use. It worked out of the box but the next time I used it it passed audio through when the effect was bypassed but went silent and did nothing when the effect was engaged (which is funny because I have a 35 year old original Memory Man which started doing the exact same thing all of the sudden too). It took E-H three weeks to get another pedal to me so I would just as soon have repaired it myself if possible. So I would like the capability to work on digital pedals too. Actually that's pretty important to me. I can get along on the analog pedals with my meters and stuff for the most part but when it goes to digital I'm in the dark right now.

    BTW - Is there any general diagnosis for pedals that suddenly stop working when the effect is engaged? For example, one day they work fine and then the next day they pass audio when plugged in and the effect is bypassed but when you engage the effect there is nothing? No sound, processed or unprocessed? I've had that twice in the past few weeks and have seen it several times before. There must be some sort of a common denominator here. What you you check first with this problem, assuming that it's not something simple like a cold joint, or bad jacks, pots, switches, or power leads etc.?
     
  4. amp_surgeon

    amp_surgeon Member

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    There's nothing particularly unique about a scope that makes it useful for analog but not for digital. The only thing that would matter for digital is the clock rate. A 20Mhz scope should be just fine for most any audio work you'd use a scope for, including digital pedals.

    Having said that, a scope is not the most useful tool for troubleshooting digital circuits, even if the bandwidth is adequate for the clock rate. All you can see is a train of pulses on a maximum of two channels at once. This is fine for verifying that there is "activity" on a wire or trace, but it isn't going to tell you exactly what that activity consists of. Fortunately, in the vast majority of cases it's enough just to know that there is activity. It will confirm that a particular chip isn't completely dead. A cheap logic probe will suffice for this.

    On the other hand, if you're actually trying to design a digital circuit, then you might very well need to know what data is moving between two chips. For that, you'd need a logic analyzer.

    I've had decent luck with inexpensive 20Mhz scopes. As long as you treat them with a little respect they'll last a long time. They aren't rugged, though, and won't stand up to being tossed around by a UPS driver. I'd be reluctant to buy a used one on eBay. They're cheap enough - just get a new one.

    I also keep an old Tektronix tube scope in my shop. It's huge, heavy, takes forever to warm up and stabilize, and uses as much current as a refrigerator. But, it has an input impedance 20 times higher than any of my solid state analog or digital scopes, so it's great for working on tube amps. I keep a vacuum tube voltmeter for the same reason.
     
  5. Montez

    Montez Member

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    I have found that 9 out of 10 repairs involve the cold joint, or bad jacks, pots, switches, or power leads etc. type of problems as you suggested. After that, get the schematic and follow the signal in and out of each component with the scope. where the signal stops, that is most likely the culprit. That is assuming all bias voltages are correct.
     
  6. amp_surgeon

    amp_surgeon Member

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    +1

    I've been telling my "protege" that 50% of troubleshooting is done with your eyes (the majority of problems can be seen), 40% with your brain (if you understand how the circuit works then you'll know where to look), and the other 10% is when you use your test equipment to confirm your suspicions.
     

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