Making Oscope test leads Qs

[Chuck H]

OK..My (new to me) scope came with probes, but to be "cool guy" with the right test leads I'm finding I have to make some stuff. I'm making :

2 ea. BNC to alligator clips (dual trace scope)
1 ea. BNC to banana jacks (in case I want to reference a sine wave while testing)
1 ea. BNC to 1/4" phone (to connect the amps output to the scope)

Plus:

1 ea. banana jacks to alligator clips (to inject signal at any point in an amp)
1 ea. banana jacks to 1/4" phone (to connect my signal gen. to an amp input)

It seems to me making these will save me $$$ on adaptors and other crap and avoid having long cables all over the place.

Some Qs... Should I make the BNC leads 10X or should I have 1X and 10X of each? And/or, does anyone know of a good way to put a switch on a homemade test lead?

And, What else am I likely to find useful?

Also, I'm planning on using regular RG58 cable for these, pros/ cons?

Any input appreciated.

Thanks

Chuck

[juice]

why not just make a BNC attenuation pad

[Enzo]

I use the regular old scope probe for most things, even the output if the chassis is open. If you look at the scope probe, the ground lead has an alligator on it, and the probe tip hook will grab things about as well as an alligator. If the chassis is open, I can usually clip the probe onto the jack or jack leads at the output. I mostly use just one input channel to the scope - the B channel, because it is the invertable one.

I reserve the other channel for speaker monitoring for the most part. And there I do use a cord with a dual banana plug on the end. I just made up a BNC to BNC cord and stick one of these on the end:



The nice Pomona one is about $7 and MCM sells a house branded one for under $5.

May I suggest a simple patch panel for your bench. Mine is very effective and quite simple. I made it from a slab of 1/4" plexi, but a metal panel would work too, or a box. There is no right or wrong, but here is my scheme. The panel is 12x3" and is mounted just under the bench riser - just under my scope and signal generator. COuld just as easily be off to the side or even configured as a snake head as if it were a PA snake. Whatever is convenient for you. On mine are six connector sets. Each is a pair of dual banana female binding posts - the common 5-way things - and a 1/4" jack beneath. The binding posts are wired parallel to the jacks. Six of these littel combinations. Two of them are wired in the back down to my bench speakers - I have a pair of 2x12 cabs under the bench. The next two are wired to my bench loads - 250 watt Dale aluminum resistors mounted under the bench. And the final two are wired to the output of my bench stereo receiver. It doesn't put out a lot, but it is useful for simple testing of speakers. Actually I use it more for working on speaker crossovers.

That's it. Speakers, loads, signal source at speaker level. I have some patch cables hanging nearby. A couple regular speaker cords to reach across the bench to the cart where I set combo amps. And then a number of 3 foot cords for on the bench. SOme are dual banana male at one end and 1/4" male on the other. If I have a guitar amp, the 1/4 plugs into the amp, and the bananas into the patch panel for one of the bench speakers. If I have a power amp with banana posts, I plug the bananas into the amp and the 1/4' end into the panel. The same cord and patch panel does double duty that way. The banana plug on the cable is stackable, so either way, ther is a female dual banana facing me at the panel. I can then take that scope cord with the banana adaptor on it and plug it into the panel - stacking is great.

When I am not scoping the output of an amp, I park that scope cord in one of the stereo's bananas, the music on the stereo then shows up on the scope screen, much to the fascination of visitors. I could just hang it somewhere I guess.

To complete the panel, I have several dual banana to banana patch jumpers that are maybe a foot long. I can parallel two speakers or loads, or connect any two things together. Nothing is grounded by the way, so I can series things if I want with clip leads.

I do have banana to clip lead speaker cords so I can clip onto OT wires or whatever and send them to a speaker or load. I used to have some BNC to clip leads cords, but when they wore out, I never replaced them. I could use something like this with the BNC/BNC cord I now use to the patch panel to make it more versatile



But anything they can do, I can do with the conventional scope probe, so I don';t bother.

I do have a dual banana to 1/4M signal cord plugged into my signal genny, if I want to mionitor it, I can move that speaker banana cord up to that and stack it.

Can I assume when you say banana jacks you really mean banana plugs?


Other than the regular scope probe itself, the only coax I have is that BNC/BNC running to the adaptor at the patch panel, that and the one from the signal genny. My other cords are speaker wire. Certainly nothing wrong with doing things a different way, of course.

Another tool that I guess goes with the patch panel is a short speaker wire with a 1/4 female to alligator clips. I can slap that on the end of any 1/4 male to make it clips.

[Chuck H]

Great ideas. I guess I only thought to use individual cables because I don't have a "bench" per se. When I want to build or fix an amp I drag out all my crap and make a mess out of my guest room for awhile. I am planning a bench though. As soon as I can find a good place for it. Until then I'll just use the cables. But once I have a "bench" area I'll go with a patch bay. Makes perfect sense. Also, I see no reason why the 1x/10x switches for the scope channels shouldn't be built into it.

Thanks

Chuck

[Steve Conner]

I wouldn't bother with a bunch of BNC-to-whatever leads. You need x10 probes anyway to protect your scope from tube amp voltages, and protect your amp from the loading of the scope's 1M input resistance, and more importantly the capacitance of 6 feet of RG58 at 30pF/ft. Both of which will alter the waveforms and tone when you clip it onto the guts. The x10 probe improves this loading situation by a factor of, oddly enough, 10.

For scoping the speaker output, these concerns probably aren't valid, but I always just use a x10 for everything, except for the likes of power tube plate voltages, where you need a x100.

[Chuck H]

You need x10 probes anyway to protect your scope from tube amp voltages, and protect your amp from the loading of the scope's 1M input resistance

Thats what I thought too. But never having used a scope, I thought I'd ask about it in case there was something I didn't consider.

and more importantly the capacitance of 6 feet of RG58 at 30pF/ft. Both of which will alter the waveforms and tone when you clip it onto the guts. The x10 probe improves this loading situation by a factor of, oddly enough, 10.

Again, thats what I thought too. Which is why I asked about it. So why is this stuff commonly used for scope probes that need to measure in MHZ? My test leads will only be three feet long. So at 60pf/10 that'll only be 6pf.

For scoping the speaker output, these concerns probably aren't valid, but I always just use a x10 for everything, except for the likes of power tube plate voltages, where you need a x100.

This I don't get. Even if your measuring 1000 volts a X10 probe would bring that to a nice safe 100 volts. Most scopes will take a few hundred volts at the input. Mine is rated at max 400 volts in.

Not being antagonistic. And I do want practical advice on how to avoid blowing up my test gear. So if I'm missing something please let me know.

Thanks

Chuck

[loudthud]

Many scope probes can only take 600V, some are only rated at 400V. I clipped a cheap probe on the plate of a 6L6 once. It died instantly when a signal was applied. Getting a scope repaired is expensive. Getting it repaired right usually means returning it to where it was made or replacing a circuit board. It's worth it to protect those scope inputs. A 100X probe is usually rated for 1500-2000V.

A 10X or 100X probe will have an adjustment for compensation. This adjusts the probe for the input capacitance of your scope. Usually a 1KHz square wave is used, you adjust it for a nice flat top. This can affect the accuracy of your scope greatly, don't ignore it.

[Steve Conner]

+1 on what Loudthud said As he's hinted, the X10 is not just a potential divider, it has capacitors for frequency compensation. The simplest X10 probe you could possibly make would be a 9 meg resistor with a trimmer cap in parallel, of approximately 1/9th of the sum of the scope's input capacitance and the cable capacitance, and then adjust the cap with a square wave calibration signal.

Also, RG58 isn't used for scope probes because of its high capacitance. They use a custom coax with a foamed dielectric and hair-thin stainless steel center conductor. A pig to repair if it ever breaks, and another reason to just buy probes. You can get special tips for them that stick into a BNC socket, if you ever want to integrate one permanently into something.

Most scopes I've used only go up to 5V/div, so you need a X10 at least to fit tube amp voltage swings onto the screen. The old tube scopes went up to 50V/div, I think, but they were before my time.

[Chuck H]

Excellent, Thanks guys.

I had no idea the scope probes were made with a specialized lead because in the "test leads" section of my catalog all the BNC fitted cables are made with RG58. But at audio is the 6pf for a 3 foot x10 RG58 test lead really going to matter? I mean, we're not building cell phone transmitters.

Chuck

[Enzo]

It won't affect the "tone" much, but it will affect sharp waveforms

On your commercial probe, is there a little hole in the BNC connector with a little screw slot adjustment? Does your scope have a little "CAL" test point? The CAL should be a square wave, typical is 1kHz, so clip the probe to it and set the screen up for a nice easy to see square wave.

Are the corners perfectly square? Take a tiny screwdriver and twiddle the probe adjuster back and forth. See the leading edge corners of the square wave curl over or peak? Set the probe for perfect square, then contemplate what cable capacitance can do to simple signals.

RG58 is fine for RF transmission lines, which other than scope leads is a common use for BNC conectors.

[Chuck H]

Thank you guys. I really appreciate the tips and guidance (and patience).

Chuck