Audio Repair Oscilloscope

[fuster]

Hello:

I want to buy an oscilloscope for audio repair. I have worked on electronics and I have a friend who owned an electronics repair company and he will give me some help along the way. He recommended getting a used Tektronix oscilloscope that is designed for use on audio equipment. Can anyone here give me some help on which ones to look for on eBay or elsewhere that would work for testing home and car audio equipment?

Thanks. :thmbsp:

 

[ferninando]

1 Mhz dual channel is all you need for audio. actually 100Khz but 1 meg more common.

 

[andy]

A Tektronix analog scope is probably the best for audio. A good digital will be way out of your price range, and cheap digital scopes aren't very nice to use.

Classics like the 465, 475, 453, and 454 are good as long as they are in working order.
Any newer Tek models should also be good, and have the advantage of not being 30+ years old.

While you don't need high bandwidth for audio, it's nice to be able to see that there's no RF, or other high frequency garbage in the signal. Also, most good scopes are at least 50 MHz, and a crappy scope is harder to use than a good one.

 

[Jon_Logan]

I agree that all you need for baseband audio is 1MHz, or less and those should be available for cheap. If you want the ability to to CD players, you'll need something with higher bandwidth. You might be able to scrape by with 20MHz, but 100MHz, is much better.

 

[jstang]

I have 3 scopes in the basement.....

1. Hand held Velleman
2. A Heath Kit
3. A Tektronixs


Guess which one is used the most? Thats right the Hand Held by a factor of 1000 to 1.... It does not have the most features or bandwidth..... But I use it for everything from trouble shooting small signal circuits (pre-amps) to testing amps power output to looking at the quality of the AC supply.... and much more....

The other scopes are critical for more challenging measurements that a beginner is not likely to do for years.....

The hand held has three major advantages....

1. Its hand held..... it goes anywhere......
2. Its battery powered which means you never have to worry about creating a ground loop short (sparks and smoke)..... You can monitor the output of bridged AMPs without an isolation transformer..... You can look at 240v Mains with less safety concerns.
3. AND ITS GREAT FOR THE CAR.... I from time to time modify the code and sensors in my hotrod car's fuel injection computer..... THE HAND HELD IS KING for the car..... looking at injector pulse widths and many other functions EVEN WHEN THE CAR IS BEING DRIVEN....

For a beginner or even a pro.... I recommend having a Hand Held on the bench too......but IMHO a beginner should start with the hand held.


johnk

 

[Old1625]

I prefer a dual-beam scope over a dual trace, as it provides a more readily accurate representation of what is going on in two places at once.

I used a 502 Tek dual-beam in audio repair work for many years, and it served perfectly well. Tektronix is the way to go, IME.

But the problem with these good old Tektronix scopes is that they are now all coming of age (time flies in our electronics fun, donut...!?), and the plethora of bumblebeez within these old scopes are buzzing for renewal. The chances are that the Tek you get at a flea market is going to have issues.

Best to obtain a service manual before servicing such equipment, as the circuitry is dense, and interconnecting wires are all tightly laced, making tracing in troubleshooting nearly impossible. This is not your grandma's Magnavox console amplifier.

Also you need a good grade of silver solder when servicing those ceramic terminal strips in a Tek.

The other common problem with most Tektronix scopes is a failure of isolation of the filament winding in the power transformer that supplies the CRT itself. If you are dealing with a stray Tek scope you need to prepare to make up a separate filament transformer to supply the 6.3V~ at negative potentials of a few thousand volts. (Ratty-Old-Shack was once a good supplier of filament transformers with good isolation on large plastic bobbins with primary on one side and secondary on the other. But if I know these b*st*rds they are studying what we old-schoolers want, and have long ago DQ'd these handy parts. RS is now a toy store, IMHO.)

My 502 has this filament transformer already, but it needs capacitor attention one of these days....

 

[GeneralE-guy]

Don't drop consideration of a digital scope...

Sorry Andy, I have to disagree somewhat with <A good digital will be way out of your price range, and cheap digital scopes aren't very nice to use.>

I'm a huge fan of digital scopes, with a long background in analogs. A quick check on ebay revealed a seller with a fixed price of $275 for Tek 2430A's, the first digital I spent much time with -- a real workhorse, totally adequate for audio work. As to usage, it's true there's a little bit more of a learning curve because digital has so much more to offer.

The two biggest advantages to digitals are 1) the ability to see pre-trigger information (wonderful for troubleshooting) and 2) the ability to store waveform information on your computer (either graphically or numerically).

If you can afford it, go with jstang's advice to go battery powered, coupled with digital. IMO, Tektronix offers the best quality, having worked with Lecroy and a few consumer brands....

 

[Troutsqueezr]

This may go one step beyond repair but I'd steer towards something closer to 10 or 15MHz (Tektronics T922 for example) and one that has differential inputs.

The higher bandwidth is nice because there may be an occasion when you want to look at a square wave output on some of the better amplifiers. A square wave can give you a good picture of both frequency response and stability of the amp. For example, if you input a 1 KHz square wave you can tell how well the amp handles (or doesn't handle) frequencies up to five octaves above the 1 KHz. On better amps we'll typically input a 20KHz square wave and run it into a capacitor to judge how well the amp handles reactive loads. You also measure slew rate this way. Square waves have fast rise times and to reproduce those accurately requires a bit more bandwidth than a 100KHz scope offers.

I would also steer towards differential input capability for the simple reason that at some point you may want to examine the waveform between two points neither of which is connected to ground, across an emitter resistor for example. Using only one probe, you must use a ground for reference so that limits what you can measure.

 

[d3imlay]

I bought a hitachi V222 25 years ago and it still works great, never had any trouble. Don't rule out an HP or B&K. USB scopes are available.

 

[dshoaf]

I'm an old HP'er and have always had a preference for HP test gear for that reason. Its nice stuff but, like the Tek scopes, they have their own vintage status themselves, unless you spend the $$ for a new digitized scope.

I agree that the Tek T9xx series of scopes, typically used in college labs, IIRC, would be a good low-cost option. The biggest problem with a lot of the scopes is getting them shipped. The old ones are big and pretty heavy with lots of knobs and connectors on the faceplate to break due to poor packing. There are fewer places doing alignments on them, too, and that can be expensive.

In some ways, its almost worth buying a new, digizited scope, but all depends on what you're willing to spend.

Cheers,

David

 

[fuster]

Thank you all for your help. Definitely helps me.

What about for a good used sound generator to input a signal to the amp? What would I look for?

Also, does anyone know what a "Variac" is? Is this something I can find used on the web or elsewhere?

 

[fuster]

I forgot to ask about whether I need to buy a "dual trace" oscilloscope?

 

[Jon_Logan]

I'd recommend dual (or more) trace. Then you can make relative timing measurements

 

[avionic]

Also, does anyone know what a "Variac" is? Is this something I can find used on the web or elsewhere?

Variable transformer, Basically and adjustable AC transformer. I picked up a 20amp unit on EBAY for $18 "no meters though" ajustable from 0 -140vac . You can easily spend over $100 on a new one. http://www.newark.com/spc-technolog...P_N_C_TransformersAndAccessories_PrivateLabel

 

[nashvillebill]

Plenty of good responses here. So, of course, I'll throw in my viewpoint.

I've got an ancient HP 1205 scope, made in the late 60's. Dual trace and a whopping 500 khz. Paid around $50 for it 8 or so years ago. I had to de-oxit the controls and revive a flaky channel, but it works now. For basic audio work, it's passable. But I can't fix a CD player with it (need 5 Mhz), much less work on an FM tuner (need 100 Mhz). And it doesn't have the "smartcursors" that the Tektronix 2200 series has, for example, so if I want to know voltage or frequency of a waveform on the display, it's time to start counting divisions manually. Nevertheless, it gets me by for basic troubleshooting, and it survived the UPS shipping, despite being merely thrown in a box without a single bit of packing.

If cash is tight, a similar scope can be had cheap; even a single channel, 500 khz scope is better than no scope, but you may/probably wind up working on an ancient used scope scope before actually using it.

If money's no object, it sure would be nice to have a brand-new 1 Ghz, four-channel, color display digital storage scope. A colleague at work has a newer portable 4 channel color Tek that probably runs around $2400 now, maybe less. The black-and-white handhelds have their place too.

I can't decide what other people need; for me, money was/is tight, so buying old used stuff is a necessity. Newer would be nice. There was a guy on the 'bay recently that had new-old-stock Tek 2246's (I think), 4 channel analog 100 Mhz with smartcursors for $400 BIN. (Tempting but my property taxes are due)

So there are lots of choices, boils down to "how much do you want to spend" IMHO.

 

[fuster]

Thank you again for all your opinions. This really helps. Any other opinions are appreciated.

The argument made for me to get a hand held unit first seems very strong. I think I will likely buy one of those first, then get a bench unit. I assume all the features discussed would apply to the hand held unit. I saw a Vellman 10mhz hand held oscilloscope for $195.00. I think it said the model was HPS-5. I saw a Tektronix that printed a graph in the display, THS710, 60 mhz, they wanted $955 for it. I would like to keep it under $500 for the thing. Any suggestions?

The frequency ranges suggested seem to point to at least 10-20,000 mhz, right? And it sounds like I need maybe four inputs or channels on it, and a dual trace.

Is there a practical difference between a Tektronix 465b and a 475?

Am I looking for some language to tell me if the unit I am looking at will give me the data I need for audio diagnostics? Or are all of them the same?

Several of you have chimed in on possible repair issues with a used oscilloscope. It sounds like that might be a risk I have to assume, or look at buying a new one, maybe a digital unit that is not priced sky high, if there is such a thing.

The Variac: do they make those in a DC power supply model? And what about the sound wave generator needed for signal input?

Thanks.

 

[dshoaf]

Fuster, yes, DC power supplies are made with adjustable outputs. And yes, a signal generator is a good thing to have around for chasing signals through a circuit.

I'd want to back up and ask what, exactly, you want to be able to do with this gear?

Frankly, when I worked on a bench at a shop in the 70s, the majority of my troubleshooting was with another small system (with patch cables between a tuner/phono source, preamp and power amp with volume controls and a set of stout speakers which were fused), a voltmeter, and a scope. I did have other signal generators and test gear but those were the items I got 80% of my work done with.

This gear was intended for troubleshooting and failed part identification - not confirmation of all the gear's specs according to the spec sheet. That took a major investment in gear that I could not justify the return on investment.

Hope that helps.

Cheers,

David

 

[fuster]

I want to be able to test units that they work properly on all functions. That includes at least a rough idea of power output of a power amplifier and an idea of signal strength/quality from an audio source including a tape, CD or other deck, a preamplifier, etc.

Main thing I am trying to set up right now would be for purpose of trouble shooting problems with a PC board within a receiver or amplifier and isolating the problem to a specific circuit. I need to be able to run tests on the bench to make sure everything is working properly first, or if it isn't to be able to know where to look for a repair. I need to be able to safely test on a bench top. I realize that a volt ohmmeter is going to be more useful for isolating circuits and trying to do basic tests of parts on the board. But I know there are tests done with a power supply, a signal generator and an oscilloscope which are useful for finding out if the thing works OK.

 

[Troutsqueezr]

Generally you would only need a power supply per se if you were repairing car amps, in which case you'd need a high current 12 volt supply. What you might want is a Variac with an ammeter hooked to it so you can slowly raise the line voltage while monitoring the current and looking for smoke. Some receivers many not lend themselves to a slow rise in line voltage but most do.

 

[Troutsqueezr]

There are two things that go wrong with a receiver more often than anything else.

The first is easy to fix and that is the potentiometers or panel switches will oxidize and will stop a channel from working. Spray some tuner cleaner in there, give the pot or switch a few tweaks and you're on your way. It's best to use a tuner cleaner that contains a lubricant.

The second thing is blown outputs. Not so hard to fix but takes some soldering skill, some basic transistor knowledge and a voltmeter.