Tape Project Forum
Tape Machines => Tape Tech => Topic started by: ironbut on January 08, 2009, 05:39:51 PM
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I know there's several of us who use software scopes to do our alignment. While these voltages are low enough so a purpose built probe isn't really needed, if you wish to use it for troubleshooting or building other circuits, this might be a good idea. This probe is meant to add some protection for your computer sound card just in case you make a mistake and attach it to a high voltage source ( in the case that you're using a laptop, this would be a total disaster!). I stumbled across a seller on eBay that sells these (just do a search for sound card probes) but you can easily make one yourself. Here's one of those open source diy sites that is very cool. If you follow the links within it, you'll find projects that range from this cable all the way to a diy laptop.
http://www.ladyada.net/library/equipt/diyaudioprobe.html
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I had a look at that site.
First thoughts -
The capacitor might provide some protection but it is an electrolytic with a low 16 volt rating.
It might not work if you get on the wrong side of B+ in a tube circuit.
I think I would use a .1 mfd. 600V mylar type cap of some sort.
Large enough to not roll off low frequencies.
Also, instead of a potentiometer you might consider various precision decade resistors like 1% at 100 ohms 1000 ohms 10K, 100K and 1 meg. (one at a time) in series with the cap on the positive side of the probe. If you build a network with a terminating resistor to ground, then you will know more precisely what you are measuring, like 10V in = 1V measured on the scope etc.
I know that decade (X10, X100) attenuation of signals and a switchable HV DC decoupling cap were features of "real" scopes and their probes.
I will have to see if I can find a schematic of a probe.
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Good thinking Steve! I haven't made one yet ( I've been going through a bunch of "read it latter" bookmarks) and I'm glad I haven't.
Thanks a bunch.