Oscope ?

[gtrplayr1976]

What would be considered the minimum standard for an Oscope for troubleshooting amps ? Any recommendations that wouldn't be outrageously expensive ?

Thanks !

[km6xz]

The best would be older lab type scopes or portable lab scopes such as the very popular Tektronix 465b which can be had for as little as $150. They will last longer than most techs. There are lots of low cost Chinese made digital scopes which give some advantages in screen displayed calculated values but they are essentially throw away items if the die. They are not intended for tube circuits, even with a 10:1 or 100:1 attenuator probe because the cheap probes they include can't handle the occasional parasitic HF arcing that attaching or disengaging a probe to a live plate can generate.
You can find tv repair level scopes from B&K or other brands for $50 or so but they will not be as reliable or accurate as the Tektronix scopes which cost new 5 times as much.
The best way to find a good scope is locally, not eBay which has driven up the prices of even junker scopes or any test gear. Search for your local amateur radio club and ask members if anyone has one for sale, you might even get one for free.
What specs to look for? If you want to work on any circuits besides old tube amps, you will want at least a 20Mhz vertical bandwidth but 100Mhz is a lot better and is useful in digital circuits. You will want at least 5mv per division sensitivity, if you are wanting to be correcting noise problems in front ends of high gain amps. An analog scope is much better for viewing the character of noise at low levels because it shows noise signals without distortion while the digital scopes have high percentages of quantizing error with very low level signals, resulting in you having a harder time seeing what is actual noise and what is artifact of the A/D conversion with the signal defines by only 8 bit words.
If getting a used scope, hope that good probes come with it. Good probes are expensive, a pair of new 100Mhz 10:1/1:1 switchable probes will cost more than the scope. The probes that come with new Chinese digital scopes are junk but can be replaced for less than $35 each. Your readings and your scope are only as good as your probes.

If setting up a good amp workbench, you will also need a Variac which luckily, are avaiable for a modest amount if current Chinese manufacturer. Used quality metered variacs are worth more than new Chinese but the new variacs do not have current meters. You need current as well as voltage metering on a variac. That can be solved by getting a cheap clamp on AC ammeter. I have one that works really well, that is also a multimeter and came with thermocouples for accurate temperature measurements for $40.
You will need a signal generator. There are inexpensive function generators but they have a lot of distortion and normally do not have calibrated level output. One of the most effective signal generators is an old HeathKit IG-5218. It has accurate frequency selection, output level metering and low distortion, 3 traits not found all in one unit from many other sources. I used to buy any I could find for my shop even though I already had every HP and Sound Technology generator you could imagine.
Low cost DMM's are a bargain, pretty accurate and tossable. But do not cheap out too much, be sure to get one that has "True RMS" reading, it will allow accurate power measurements with complex waveforms, such as a sinewave being distorted by clipping in a tube amp. A tube amp repair bench should have an Analog meter also. Analog is very useful for spotting trends or adjusting for a peak or null value. Older VTVMs, which were cheap as kits in the 50s and 60s can be had for very little money but they are very good for working on high voltage and are very forgiving of abuses such as drawing an arc from a high z plate circuit which will digital meters.
Heath and Eico are two common types of old VTVMs but HP also made lab quality meters that can be found for a reasonable price. A HP410B is bullet proof and goes for about $75 and the more modern, smaller HP410C with a good AC probe is about $250. If you find a used one, make sure it has the AC probe, if not, it is questionable to buy it, most on eBay do not have them.
That is a good beginning....

[gtrplayr1976]

Thank you !. You are correct. The ones on Ebay are mostly over $300. I'll look around ,and see what I can find. I have a decent Fluke DMM ,and that's about it. Looks like I need more

[Enzo]

Our audio stuff is about the least demanding of a scope. Just about any scope that makes a picture on its screen is useful. In all my years I have never seen a scope with insufficient range for an amplifier.

AN old beater B&K for $10 at a ham fest can be all you need.

[kolaking]

Thanks for saving me from about to ask the same question. Great info!

[gtrplayr1976]

Our audio stuff is about the least demanding of a scope. Just about any scope that makes a picture on its screen is useful. In all my years I have never seen a scope with insufficient range for an amplifier.

AN old beater B&K for $10 at a ham fest can be all you need.

Finding a ham fest might be the hard part

[Leo_Gnardo]

My old Hewlett Packard 122R (made for the US Air Force @ 1960) has been serving me with hardly a hassle over the last 27 years. Yes I did have to replace a resistor, a tube and a fuse, also spray the pots & switches a couple times but that's it. It's big and draws 2 amps of 120V power. Only has 200 KHz bandwidth too but that's fine for audio. The 122 model is a "upright" not rack mount build but otherwise identical. Got it from Fair Radio in Ohio. They're still in business selling all sorts of surplus gear & parts. Google 'em up & see what they got. There won't be any $10 beater bargains but you might find something that fits your budget.

I see your location is Virginia. If you're anywhere near the naval base, you might snoop surplus & pawn shops in that area.

Good luck!

[salvarsan]

For audio, you need two channels, triggered sweep, no more than 250kHz bandwidth on the scope input.
A refurbished Tektronix 221, a tiny old portable, gets to the point very quickly.

If you can spring for a digital scope + signal generator, do so. Note that for phase information, the bandwidth is samples/sec/10, i.e., a 20 Msample/sec scope has a useful 2MHz bandwidth, so read the HanTec specs carefully when you peruse eBay.

I use the SysCompDesign CGR-101 which is complete with probes for ~$230 shipped. Software runs on any PC, Mac, Linux that has a USB port. Open source, too.

[pontiacpete]

Hamfests and Conventions Calendar

[Mick Bailey]

You need to check that any scope is working properly before buying, as replacement components can be expensive or difficult to find and faults can sometimes be tricky to diagnose. If in doubt walk away. Also, make sure that the display isn't burned - some lab scopes get left on all day long with the brightness turned up way too high. I'd agree that the 465b is a really good scope (I'm saying that because its been my main scope for many years), but they're getting on a bit and now have a few failures;

PSU electrolytics
Tantalum cap shorts
Leaking HT caps
Transistor failures

Make sure the trace brightness can be turned down fully - the 465b can fail to give a bright trace that can't be turned down.

If you can tolerate a slightly bigger scope the modular Tektronix mainframe scopes are really nice and the spare modules are inexpensive. There's a lot of documentation and schematics around for Tektronix scopes and I wouldn't be put off buying an older working scope, so long as it didn't show signs of abuse.

At the end of the day any scope is better than nothing; a 10Mhz single-channel scope will still beat an empty space on a workbench.

[Steve Conner]

I have a couple of old Tektronix 7603s, one to use and one to cannibalise for spare parts. I paid $150 for one and the other was a dumpster dive. The power supply in that one was dead, emitting puffs of smoke, but I fixed it in an afternoon. The service manuals and schematics are all available online.

I've never owned a Tek 465, but it is something of a classic. We have a 2465B at work. Sometimes an analog scope can make sense of things that a DSO can't, and vice versa. For most audio work I would prefer an old analog scope, and a PC with a 24-bit soundcard for spectrum analysis, to a low-end DSO that would make an equally poor job of time and frequency domains.

[km6xz]

I suppose those who recommend a low bandwidth scope never had to work on anything other than non-oscillating, well behaved low voltage analog circuits.
Very few products today have circuits limited to the traits, without clocks, high voltage, digital, SMPS, and non-ground references power supplies. Two channels with summing and channel inverting is essential for switching supplies. Is you are not working on modern circuits, at least occasionally, you will find yourself boxing yourself into a smaller and smaller corner, even as a pure hobbyist.
The 740x and 760x series are great bargains but need a scope cart usually and are better suited to specialty work such as using true diff-amp vertical plug-in, spectrum analyzer plug-in, etc. Since they are too big most hobby benches, they are sold cheaply between hams but expensive from used test equipment dealers.
I can't remember a day where wide bandwidth was not used in some fashion.
The sound card software combination works well for generating and analyzing signals but in a limited range of levels. I have a couple different systems but usually just use my HP3580a spectrum analyzer since calibration and input durability is not an issue. I also have an AP but those are best available for performance testing and not good for general troubleshooting.

Regarding an old 465. I would wager one kept in decent condition will still be serviceable far longer than any Chinese digital scope one has today. I have never had to do much of anything to one, and I have had dozens of 465b's and 465m's. Never had to replace a filter or transistor in any of them. There are reasons it was the most successful scope in history, because it was not made like audio gear which has no expectation of high reliability or longevity, parts were not stressed. Every part was tested before used in assembly. That is why good test gear costs more than cars, and always have. As mere mortals we generally can't afford the quality that precision test gear had in any other part of our lives. Over the years I collected and rebuilt more test gear than audio equipment by far, and one reason, besides its usefulness is that it is a joy to work on well designed equipment.

[salvarsan]

I suppose those who recommend a low bandwidth scope never had to work on anything other than non-oscillating, well behaved low voltage analog circuits.

The person who asked probably hasn't had advanced technical training but did, after all, use the word MINIMUM.

He's not checking clock skew in a data stream or looking at phase margin in pricey op amps. He said audio.

I hazard a guess that the original poster wants a low-risk entry into oscilloscopic testing without complication, i.e., an inexpensive entry-level tool and not a way of life.

An old 100+MHz scope takes up plenty of space, too.

Space? I liked my Tek 2236, 23" CRT monitor, and HP Laserjet 4 printer but it crowded the component collection and PCB rework station. I still have the last two and a USB oscilloscope+sig.gen pod. If I need more, I'll get more.

[gtrplayr1976]

Lots of good info here, but now I'm thinking I may be better off without one. ... This can be confusing for a newbie. Do I really need one ? Someone could sell me a used one ,and I would have little idea if it actually worked correctly. I could learn to use one I would just need to know where to hook it up , what to set it too, and what the wave looks like. I used one years ago at work ,but I had all that info available. I guess if I can find one reasonable that I trust I'll pick it up. Signal generator sounds like it may be more useful ?

[salvarsan]

Lots of good info here, but now I'm thinking I may be better off without one.

What are you most likely to try fixing?