Test equipment......

[stratrat_57]

Hi everyone!
New member, first post......
I'm looking for suggestions on the purchase of audio test equipment for guitar amps. In particular, An O-scope and audio signal generator. There are thousands to choose from, but I don't have thousands of dollars! How about a used older "old-school" dual trace O-scope (CRT),and the same for the generator. The kind I visualize is the bench unit with the large rotary freq selection dial. I'm seeking brand names and places to shop. The new units are very expensive. I've heard that old CRT O-scopes can be had for $50! I can't seem to find any of those.....
Recommendations, suggestions, advice? What's everyone else using?
Thanks, Frank

[olddawg]

Techtronics, B&K, Leader are some of the names that come to mind for used scopes. You probably want a dual trace scope less than 25 years old. There are a lot of them out there. Make sure it triggers. You might find yourself paying the same amount for new probes as you did the old scope. Try to get the probes with it if you can. A signal generator must just work. I have an old tube HP that has enough output to sweep HF driver voicecoils. I just like using it. It has a huge aluminum dial. You can also use your computer for a signal generator. There are lots of free programs out there.

[gbono]

Is it a HP3301-3310 function generator or better yet an old 100A or 202A??

I would try and find an old Tek scope like a 5103N - I have one of these with the differential amp plug-in (it's now blowing fuses)

Also have a HP 1741A (100MHz) and a 54603B (60MHz). The 1741A was free and the 54603 was $300 on Epay. Both are over kill for analog audio work.

If you get a signal generator make sure it has decent spectral purity (it shouldn't be noisey or add harmonics to your measurement).

[Enzo]

Decide what you are going to do with it. if you are putzing around in the basement tweaking guitar amps, then you don;t realy care about the .000000001% accuracy of a signal generator, and most any scope is more than enough for the job. I don;t think you need to resolve the leading edge of some digital clock signal very often. And unless you want to scope the output of a 179MHz wireless, you don;t need the bandwidth.

On the other hand if you are trying to do exacting work, chasing down vanishingly small amounts of distortion, trying to characterize frequency responses to the half decibel, then you might want something more fancy.

Just about any scope will work for you - as long as it works. SOme old beater with controls that work only half the time will be more of a pain in the ass than a shop aid. My trusty old B&K finally got untrusty, so I recently bought a new cheap scope from MCM in their house brand. it works fine. You can get a scope new in the box with a warranty for under $300. Nothing wrong with an old scope, as long as it works.

You don;t need 100MHz bandwidth. And for audio, comparing a 60MHz scope to a 100MHz scope is pointless. The 100MHz one won;t be "better" than the 60MHz one for audio. DOn;t pay extra for it then. SOme ancient scopes only had a 500kHz bandwidth, end even that is more than enough.

You DO want a "triggered sweep" scope. Older and cheaper scopes might be recurrent sweep. They wil work, but it is harder to get a stable picture on your screen. ALmost all scopes are triggered anyway these days, and have been for a long time.

You don;t HAVE to have dual channel scopes. Having two inputs is often handy - I can connect one to the speaker leads to monitor the output, while using the other with my hand probe to hunt around with. But a single channel will do 99% of what you need. I could pretty easily take my hand probe and touch it to the output jack. Most scopes you see are dual channel anyway. But don;t pass up a deal on a working cheap old single channel scope just because it lacks a second channel.

Learning to use the scope itself is FAR more important than having lots of features.


One place to look for old scopes cheap is your local "ham fest." A gathering for amateur radio geeks. COntact the ARRL web site for information on such activities in your area. That is one place you really can find a $10 scope.

And watch Craigs list too. I am not an ebay person, but that is the sort of thing you can find by the truckload on ebay.

You can also find little boxes for your computer that make it into a scope. That is all well and good, except on my bench the computer screen is usually the schematic display and parts lookup. Having to toggle between sccope display and schematic would not make my day a happy one.

I found an old HP200 sine wave generator - with the big silver knob thanks - at the MSU surplus outlet. $5. It didn;t work. One plate on the tuning cap was bent. Straightened it with my needle nose, and voila! Sine waves. I have several audio generators, again you don;t need anything fancy. I recommend something continuously variable - as opposed to a step switch for frequency adjust. if you are sweeping to find a resonant spot in a speaker cab or whatever, it is so much easier with continuous.

Don't worry about the difference between a sine generator, an audio generator, and a function generator. As long as they all make sine waves in the audio spectrum, you are covered. Sine generators or oscillators tend to make only sine waves, which is useful enough and all you need 95% of the time. Audio generators tend to have sine waves plus square waves, and maybe sawtooth or triangle waves. That's fine too, square waves cab be useful now and then. A function generator has those and more. it usually has adjustable DC offset, and sweep. The function generators tend to be less "accurate" in their waveforms, than the others, but not so you;d notice. Again, we are down to that fraction of a percent thing.

You can spend large bucks on a generator, but you can also find them cheap used. And a brand new with warranty unit can be had for maybe $200.

[bob p]

here is a helpful thread:

http://music-electronics-forum.com/t3186/

[stratrat_57]

Thanks everyone, very helpfull info/advice. I have no problem with "low budget gear". This stuff is just for use on my amps. I just want a way to trace signal through my gear, if needed. I'm troubleshooting my jcm-900 with a Fluke and using my guitar belching out an "A" chord as my signal generator! Tracing signal with this set-up, you have to be an octopus! Enzo, when you say you use "music", you mean a CD player? My main problem is knowing what a normal signal level looks like at each stage of amplification. I feel my Fluke should be just fine for this, so the main thing would be to find something to supply a steady consistant signal into the amp at the same level as the guitar.

[defaced]

Figure about 0.1v at the input, then trace it from there. You can also just measure the output of your guitar with your meter and use that. I've uploaded a 1kHz sine wave mp3 here before, that will help you tremendously, it's a few posts down:

http://music-electronics-forum.com/t12945/

There's some other stuff in that thread that may help you.

[Enzo]

I don;t see where I mentioned music, you must be referring to one of the older threads.

Yes, I mean a CD player or something. I have small cheap stereo on my bench to listen to, but I also have a cable trailing out from its line out in the back. I can play a CD or more likly just tune in an FM music station. As a rule I don;t care if it is at guitar level. it will be louder than a guitar, but to just see if an amp works it is fine. And if I have a super weak channel or a dead channel, I use it. At those times I am just looking for where the signal goes away, I can concern myself with little level issues later.

Ther is a difference between tracing down a subtle problem and a gross problem.

COntinually banging on the guitar while trying to work is a nightmare. Frankly, in a go or no go situation - like a dead channel - even just hum signal is sufficient.

You know those little personal keyboards like Yamaha and Casio make? I mean the small ones about the size of your computer keyboard, and they have the smaller keys. A lot of them have an ouput jack. You could even use one of those. Pick a nice sounding patch, wedge a matchbook or something between keys to hold one down, and set the volume to suit - VOILA!! instant tone generator.

DOn't ask me the signal level stage by stage through a Marshall, I sure don;t know. What I do know is that when a signal goes into a tube it should come out amplified, or if a cathode follower at about the same level as it went in. I follow the function of the amp, I don;t have a chart of signal levels.

[olddawg]

Is it a HP3301-3310 function generator or better yet an old 100A or 202A??

I would try and find an old Tek scope like a 5103N - I have one of these with the differential amp plug-in (it's now blowing fuses)

Also have a HP 1741A (100MHz) and a 54603B (60MHz). The 1741A was free and the 54603 was $300 on Epay. Both are over kill for analog audio work.

If you get a signal generator make sure it has decent spectral purity (it shouldn't be noisey or add harmonics to your measurement).

It's an old HP 200 CD. Big aluminum dialed, leather handled workhorse. I paid $15 for it years ago at a street sale.