Oscilloscope and Signal Generator

[EETStudent]

Im ready to take my troubleshoot skills, as they relate to guitar amps, to another level. I would like to add to my tools an oscilloscope and a signal generator. Any recommendations for models that would make a good starting point?

[Chuck H]

Cheap ones... Seriously.

You don't need mhz. Even most any oscillation issues with guitar amps will be visible with a 20mhz scope. Just find one that is reported to be in good working condition. Get one with a 10X probe if you can. An owners manual is nice. Otherwise, research if an owners manual is available for the model your looking at before you buy.

Same with the signal generator. Pass on the "function" generators. Too much $$$. A simple sine/square wave audio signal with an adjustable output is fine. I bought a scope and signal generator on Ebay. I think I spent US $230.00 for both after shipping. They've been valuable tools without any need to upgrade for about three years now.

[olddawg]

See if there is an electronics swap meet near you and check Craigs List. I got my old tube HP sign wave generator for $20. It has enough output to sweep voice coils in HF drivers. A scope you want to be able to trigger and want a dual trace if possible. You will want to test the scope. Most have a calibration test point on the front. I would suggest buying new probes. They may cost you more than the scope. You can buy a decent scope for audio for $50 if you look around. But funky probes can drive you crazy.

[Chuck H]

A scope you want to be able to trigger and want a dual trace if possible.

Sure. Mine is dual trace and triggers. I haven't used the dual trace yet but I can imagine a few cases where I'd want it. I haven't used the trigger either. But I do more design/build than repair. The trigger could be a help in detecting bugs for repairs.

[nashvillebill]

Parts Express has a kit function generator for less than $50 if my memory is right. I built it maybe a year ago, square wave, triangle,a nd sine wave. Square wave was perfect, sine wave not quite perfect but plenty good enough for audio work. Good value for the money IMHO.

My ancient Eico tube RF sig gen has a sine wave generator, but it's not very pretty, but then it's not intended for audio troubleshooting.

For clean looking sine waves my old HP200CD is my fave.

I used an old HP scope (only 500 kcps) for many years, now I've got a BK 1477 (15 MHz, dual channel) that I got off of CL for $40 which works great. Probe set from Parts Express for around $25.

All that merely to point out that it doesn't have to cost a lot of cash. Of course, I'm no expert, I'm just a hack.

[Ronsonic]

For scopes and sig gens I use targets of opportunity. If I find a cool old scope cheap I snag it.

I've got an HP200CD too and love the thing. I've also got a major battleship boat anchor that's probably 60 years old and will drive speakers for testing if needed.

I also use the cheap little handheld sig gen from MCM, at the -20dB setting it about matches a strong guitar pickup, convenient.

Check ebay, flea markets, yard sales, take advantage of people with drug habits, whatever. There's a lot of good usable gear out there.

[Chuck H]

When I needed a scope and sig gen I went on Ebay and bought the first clean working examples I could get and though my expense was still greater than some tales here what I spent wasn't even missed the following week.

I think the general theme of the thread is that you absolutely don't need to spend a lot of money. I didn't get a scope and sig gen for years because I just figured it was expensive gear I couldn't afford. Once I looked into it I was surprised at how much used gear is on the market. These are really common tools as it happens. It's sort of the same as getting a Crescent wrench at a garage sale for 50 cents. Sure, a good one costs thirty times that at Sears, there's just sooo many used ones that no one knows what to do with that they're of almost no value.

[doctor]

When I needed a scope and sig gen I went on Ebay and bought the first clean working examples I could get and though my expense was still greater than some tales here what I spent wasn't even missed the following week.

I think the general theme of the thread is that you absolutely don't need to spend a lot of money. I didn't get a scope and sig gen for years because I just figured it was expensive gear I couldn't afford. Once I looked into it I was surprised at how much used gear is on the market. These are really common tools as it happens. It's sort of the same as getting a Crescent wrench at a garage sale for 50 cents. Sure, a good one costs thirty times that at Sears, there's just sooo many used ones that no one knows what to do with that they're of almost no value.

I find it hard to understand how one can perform any repair without
measurement equipment (besides not glowing tube replacement),
but… better late than never.

If you still remain in the analog audio area, 20MHz analog scope and
20KHz signal generator will be sufficient.

If chosen area is digital audio (digital effects, keyboards, synthesizers
and so on), you need digital scope about 100MHz and signal generator
up to 20KHz.

My common recommendations are:

1. Scope must be dual channel.
2. Do not purchase Chinese scope that do not perform properly even
basic modes.
One second hand Tektronix is thousand times better than two first
hand Chinese scopes.
3. If you need relatively low bandwidth analog scope, I would recommend
dual channel scope with built-in component tester (Hameg, Metrix and so on).
4. Probes must correspond to the highest scope frequency.
5. Signal generator with digital (at least 4 digits) display is preferable
(for multi-channel EQ testing and so on).

If chosen area is analog RF, you need analog scope and signal generator
not less than 1GHz.

If chosen area is common digital, you need digital scope and signal generator
about 500MHz.

[Rhodesplyr]

It can be handy having a stand-alone signal generator, but there are computer programs that can generate all sorts of audio-frequency tones if you only need audio. I recently used a computer program to provide the calibration tones for my Peterson tuner. Fifteen years ago, Peterson would play the tones for you over the telephone.... There are also diagnostic test signal recordings.

I frequently use my Roland Jupiter-6 analog synth to sweep speakers to check for buzzes or resonances. I can dial in different waveforms and frequencies from below human hearing to signals that annoy dogs and bats.

But maybe I don't know--are there things that the old-school signal generators can do that the things I mentioned can't?

[km6xz]

A bare minimum bench would have enough quality of the instruments to have confidence in the readings. That is not as simple as it seems.
For a scope, there is no better deal than a 25-30 year old Tektronix such as the 100Mhz 465b for $200-300 or the ruggedized version 465M for $150
Buy two good probes, the best you can afford or a bunch of cheap ones and replaces them often. If you are working in tube gear, find a 100x probe.
There are lots of signal generators but few have all you need in one box. You should have accurate calibrated output, metered and step attenuator, you need low distortion, you need pretty accurate frequency calibration. One of best for meeting these criteria in one box is the old Heathkit IG-5218. I have lots of precision ultra low distortion generators but I use one of my Heathkits most because it gives what a service bench needs, with >2% level meter accuracy from -60 to +20, a very good step attenuator, 1% percent frequency calibration with decade selection switches for frequency and multipler(very useful to have resetable frequency switching instead of continuous sweep variable. And the ThD is 0.01% or better. The next step up from that model would be $1000 or more. The Heath is available at ham swap meets for $40-60.
You will also need a metered variac with voltage and current meters. That might take a little searching but they can be found. You can get new Chinese variac with voltage meter for $100 or so. Build a current meter/shunt into a socket box mounted in clear line of sight.
You will need high power dummy loads. You can find power resistors in the 100-250 power range surplus. Many of them are inductive so if there is a stability problem with a solid state amp you will see it on the bench. You can buy new non-inductive dummy loads for $35-50 each or surplus inductive power resistors for 1/3 that. You will want several so you can jumper them to match many difference impedances. Tube amps will need 4, 8 and 16 ohm loads which can be handled by 2 8ohm resistors. Solid start will need a wider range, down to 2 ohms.
You will need a good meter that can handle tube voltages. An old VTVM can be very hardy and take the high voltage in stride that would fry digital meters. A good analog meter for tube work is the old HP 410b or the newer more modern looking 410C. You will also want a cheap digital meter that is handy for general testing where trends in changing values are not needed to be seen as quickly. A $30 digital meter nowadays has very good accuracy and lots of functions, they just are not great for high voltage. There are a few that handle 1000volts ok such as the old Fluke 8050a which is really a great meter. I had a dozen in my old shop back when they were $450 each. Now you can find them in swap meets for less than $100. I still have a couple on my bench and they get used a lot more than my lab type 6.6 digit HP and Fluke lab meters. There is very little in repair that needs more than 3 significant digits of resolution.
If you are working on solid state gear you are going to deal a lot with surface mount, so a hot air rework station or some other rework solder station will be needed. Getting parts out of pc boards with several layers and soldered onto 2 layers of ground plane is not easy.
That is a good start that would be pretty much both a minimum and perfectly suitable bench for effective repair and calibration of a wide variety of equipment.

[trem]

Handheld Audio Generator

Anybody have experience, or an opinion on the above Audio Gen.?
Thanks

[nashvillebill]

Handheld Audio Generator

Anybody have experience, or an opinion on the above Audio Gen.?
Thanks

I couldn't be sure from the description, but it appears that you can't fine-tune the frequency, you're stuck with the discreet increments on the rotary selector; the only adjustment seems to be amplitude. I could be wrong though.

The kit I ordered and made was PE's 320-118 1 Hz-100 kHz Function Generator Kit for $41 (manual here http://manuals.elenco.com/manuals/fg-500k.pdf which even describes how the oscillator functions )

I suspect the two sig gens both use the same XR-2206 chip.

[bkahuna]

I haven't built one of these, yet, but it sure seems like a decent quality unit for very little outlay. The pcb is available for around $15. I'm thinking about building one to put in my field service toolbox.
Miniature Audio Oscillator

[Ronsonic]

Handheld Audio Generator

Anybody have experience, or an opinion on the above Audio Gen.?
Thanks

That's the one I was talking about. I paid a good bit less than that. It's "Tenma" brand.

It works for me. It's portable, easy to leave on and deplete the battery and you'll have to make up some test leads that work for your application.

[doctor]

That's the one I was talking about. I paid a good bit less than that. It's "Tenma" brand.

It works for me. It's portable, easy to leave on and deplete the battery and you'll have to make up some test leads that work for your application.

I completely agree with Ronsonic.
Such self-made devices are only suitable for rough estimate.

Offered self-made generator may be used only as a audio probe,
but not as even semi-professional generator (like lamp with
battery can be used for continuity checking instead of ohmmeter).
I also use a little audio generator manufactured by Tenma
(see attached Tenma 72-505 specification) for out-of-laboratory works,

The only upgrade, which I made is DC connector for external power
supply instead of 9V battery.

“Science begins where the measure start”