Probes for 5 MHz Oscilloscope

[forre65413]

I have a oscilloscope noob question. I just picked up my first oscilloscope at a garage sale for $7, a Heathkit IO-12. It came with a Pomona BNC to banana adapter, but no probes. From what I can find out, this probe was made during the 1960's and is a 5 mHz machine. I want to use this to learn about oscilloscopes. Should I just bite the bullet and buy the vintage probes on Ebay, or is there a modern probe that will will work?

BTW, I did check to see that it turned on and had a display, but will be testing and replacing caps as needed, as well as a broken pot.

[StillTrying]

At 5 mHz almost any bit of wire will do, ringing usually doesn't start until about 30Mhz, you could make something useful from some screened cable that would do to start testing.

It must be a very slow timebase and very long persistence.

[ebastler]

@StillTrying, this is the Beginners forum, so the OP may or may not appreciate your joke.

@forre65413 -- of course you meant MHz (MegaHertz), not mHz (milliHertz). Even at 5 MHz, I assume that a modern probe connected to the BNC > banana plug adapter would work OK. With the low prices on no-name Chinese probes on ebay, I would just get one. The basic probes, typically specified at 20MHz, should work fine.

[forre65413]

Thanks, guys!

I am almost mortified that i used the wrong case of m for megahertz. Just this past week at work I had to tell a guy that his Fluke 289 would not measure as low as he wanted because mΩ and MΩ are a billion times different.

I will look at cheap probes on Ebay. If nothing else, I can hack something together with RG-59, a BNC connector, and some alligator clips.

[StillTrying]

I am almost mortified that i used the wrong case of m for megahertz. Just this past week at work I had to tell a guy that his Fluke 289 would not measure as low as he wanted because mΩ and MΩ are a billion times different.

I'm sure you'll get me back one day! You can 'modify' the subject of your first post as well as the text if you want.

I will look at cheap probes on Ebay. If nothing else, I can hack something together with RG-59, a BNC connector, and some alligator clips.

I would, I quite like these quite small test clips I used for a HiTechDiyProbe.
https://www.eevblog.com/forum/beginners/diy-x1-high-bw-passive-scope-probe/msg1169643/#msg1169643

[Ian.M]

Modern x10 and switchable probes often don't have enough compensation range for vintage CROs.   Check the Heathkit manual carefully to be certain the input resistance is 1Meg and the total input capacitance is within the probe's compensation range before buying *anything* other than x1 probes.   

RG59 will have far more capacitance per unit length than a proper probe lead.  As long as you only probe low impedance circuits, no problem, but %DEITY% help you if you want to look at high impedance stuff or CMOS logic above maybe 10KHz.

[forre65413]

Thanks, Ian! That's something I like to know.

Information on this scope is a little thin on the ground. I found a PDF of a partial condensed manual, and a Heathkit catalog from 1967. The impedance for the horizontal channel is 4.9 Meg, and the vertical channel has input impedances of 2.7 and 3.3 Meg for it's X1, X10, and X100 settings. I will have to do some more research.

If it doesn't work out, I might be able to sell the Mullard tubes for enough money to buy something newer.

I'm sure you'll get me back one day! You can 'modify' the subject of your first post as well as the text if you want.

I changed the subject, but I have to let the text stand to allow the responses to make sense.

[ebastler]

Modern x10 and switchable probes often don't have enough compensation range for vintage CROs.   Check the Heathkit manual carefully to be certain the input resistance is 1Meg and the total input capacitance is within the probe's compensation range before buying *anything* other than x1 probes.   

Right, thanks for pointing that out! I was assuming the use of 1x probes only, but neglected to mention that.

[forre65413]

So 1X probes would probably work, but 1X / 10X switchable probes won't?

[ebastler]

So 1X probes would probably work, but 1X / 10X switchable probes won't?

The switchable ones will work, as long as you leave them on the 1x setting. The 10x position may still be useful fow slow signals, up to audio range maybe.

[Gyro]

...
Information on this scope is a little thin on the ground. I found a PDF of a partial condensed manual, and a Heathkit catalog from 1967. The impedance for the horizontal channel is 4.9 Meg, and the vertical channel has input impedances of 2.7 and 3.3 Meg for it's X1, X10, and X100 settings. I will have to do some more research.
...

That is going cause you an issue with a standard x10 scope probe then. Virtually all scopes (certainly in the last 30 years anyway) universally have a 1Meg input resistance, which is common to all range settings. Scopes do vary in input capacitance, most modern high speed ones being lower than older ones.

As long as you select a switchable probe with a X1 setting then you will have the convenience of a proper scope probe, although you won't benefit from the lower input capacitance of a X10 probe - much less of an issue with a 5MHz maximum bandwidth. A probe in its X1 setting, does not have any compensation issues (the little compensation trimmer only has an effect in the probe's X10 setting).

P.S. In X1 mode, the probe input will exhibit the same input resistance as your scope ranges, ie,  4.9/2.7/3.3Meg.

[GerryBags]

I received a couple of probes, an Iwatsu 50 MHz probe and a Tek P6011, with a 'scope I got recently. I'm not likely to need them, and you could experiment with them to your heart's content, as disposable. Perhaps even use the parts to make your own. PM me your address and I'll send them over to you.... won't be express delivery, given the distance, but they'll get there eventually! 

[Ian.M]

Well, from the sound of that, you wont be probing any CMOS logic unless you build it an active probe, or can mod it to 1Meg || 30pF, or the logic's got a very low clock speed.  Not that its great for slow logic either - its purely AC coupled so unless the signal's got frequent enough edges and a duty cycle in the 20% to 80% range, all you'll get is a confusing display of the aggregate of its coupling caps' RC time constants or a straight line that occasionally twitches.

Yep.  I've just found the schematic: http://www.rsp-italy.it/Electronics/Kits/_contents/Heathkit/Kits/Heathkit%20IO-12%20oscilloscope.pdf

It looks like 1.375Meg is needed across the Y input to get it down to 1 Meg and 1 Meg is needed across the Ext X input as that's got no DC path (as *very* high input impedances were preferred back in the days of valves).   However, that's pointless unless the capacitance seen at the Y  input socket with the adapter fitted (on a capmeter that can read down to pF)  is under 40pF  (probes with over 50pF compensation range are rare like hen's teeth and you need a bit of margin).  Check it in each attenuator setting.   You *MAY* be lucky enough to find its within the possible compensation range for a modern probe.

If the specs are to be believed: http://www.heathkit.nu/IO-12_1968.jpg
I calculate that  should be around 40pF on x1 and 20pF on x10 and x100.
I'd be reluctant to mod a classic Heathkit other than to bring it back into working condition, or for electrical safety, but making up your own banana to BNC adaptor with the shunt resistor and a switch and a 30pF trimmer so you can pad the x10 and x100 ranges to match a probe compensated for the x1 range would be a possibility.   It looks like its sensitivity is 10mV/cm on x1, so on X10 with an x10 probe you'd get 1V/cm which is in the ballpark for comfortably looking at logic signals.

However if you do that and want help decoding a serial protocol, I think *ALL* the experts here will say "*HELL* *NO* !" when they see your photo of the trace!

[forre65413]

I bought this to learn about using a scope. It's not the best compared to modern units, but it was only $7! If it's more trouble than it's worth than it can sit on the shelf looking cool while I check out Ebay for something more modern.

I don't plan on modding this any more than replacing the power cord, a broken pot and changing out some caps (any caps that aren't ceramic are wildly out of spec). My multimeter should measure down to 10's of picoFarads, but I don't won't know the good readings until I replace the bad caps.

I don't mess with that many electronics that are this old (especially when they aren't solid-state). Even though this is a factory-built model, the point-to-point wiring is pretty higgledy-piggledy.

[Ian.M]

I wouldn't mod it either, but a PCB carrying a BNC socket, a miniature slide switch for the extra x10/x100 range compensation padding cap, said trimcap and a 1.2 M resistor in series with a subminiture 500K trimpot could easily be made to clamp directly under the input terminals so you can get the full benefits of reduced circuit loading of a modern x10 probe.   

As is, you'll have to use active probing techniques  if you want to look at high impedance signals at the top end of its usable frequency range.    A DIY JFET source follower probe could be rather useful if you dont want an extra 100pF load on whaqt you are probing.  At 1MHz 100pF has an impedance slightly under 1.6K  which is plenty low enough to be an excessive load on high impedance circuit nodes.   A crude JFET probe could easily get that below 10pF, an order of magnitude improvement.    While you are building accessories, another useful probe would be a RF envelope detector.   Such DIY probes were traditionally built in aluminum cigar tubes, reinforced with a roll of stiff card stuck in with a dab of varnish, as it provided screening in a convenient form factor and the 'guts' could be accessed simply by unscrewing the end cap that the cable entered through and carefully withdrawing them.  If you know anyone who smokes cigars get them to start saving the tubes for you.   There was an official Heatkit high impedance x10 probe that was compatible with it  - if you can find the instructions for that it would be nice to build a clone of it as well, though we can probably work it out from first principles - it needs a string of resistors totalling 33Meg, with 1/9 of the total input capacitance includimg the coax shunting them. If you want to use it for HV work, use 10 3.3Meg 1% resistors in series. Design the probe for about 90pF total input capacitance  and pad the  cable end with a trimcap to compensate it.  Allowing for strays, 82pF ceramic across each resistor should get you in the right ballpark and you'll get about 12Pf and 36Meg at DC at the tip.  With a 10 resistor chain that should be good for probing HT nodes in valve gear up to about 1KV DC and 100V Pk-Pk AC, assuming of course that the scope input coupling cap can still take 100V DC across it.

[David Hess]

So 1X probes would probably work, but 1X / 10X switchable probes won't?

The Heathkit IO-12 does not have 1 megohm inputs and its input impedance changes with attenuator setting so attenuating probes like x10 and x100 are not going to work.  It was intended to just use wires however 1x and 1x/10x probes in 1x mode will work fine and are what I would recommend.