K7000 Velleman Signal Tracer Active Probe

[@davidv1289]

Great video, Chris! Lots of good information even for those who have worked on electronics for many years. Your demonstrations of load resistance and impedance were excellent. Thank you! Regards, David.

[@oldblokeh]

Chris, you could eliminate the Miller capacitance and extend the usable frequency range of the probe by using a cascode arrangement instead of a single FET. If I get a chance I'll try it out...

[@allthegearnoidea6752]

Hi Steve if you look at link in the show notes there is some information and suggestions on that. Be interesting if you try it out etc.

[@oldblokeh]

@@allthegearnoidea6752 Sorry, Chris, missed that. I'll have to give it a go.

[@NagoyaT99]

Hi Chris. This is a wonderful video and it has inspired me to build an active probe for my Velleman using Manuel Caldeira's PCBWay board. I'll let you know how it turns out! I haven't seen you post recently. Hopefully you are just taking a small vacation. Please keep making your videos! They'll are educational and entertaining. Best to you from the USA.

[@TheDefpom]

I had a funny occurrence today, I was repairing something and the uP wasn't running, until I put my finger NEAR the uP or the crystal... (not actually touching anything!) and then it would startup and run fine... just the presence of my 50Hz finger was enough to start the oscillator... once I swapped the crystal it was OK after that, thought that was a similar situation to your demo of the 50Hz being picked up from your hand.

[@allthegearnoidea6752]

It may of been the added capacitance from your hand was just enough to maintain the oscillation I’m just guessing. Fault like that could have you running around so sounds like a good fix. Thanks for watching regards Chris

[@kkteutsch6416]

I worked some 40~45 years ago with these Avometer on my work, before handheld lcd multimeters became common, are excellent laboratory quality instrumental...

[@allthegearnoidea6752]

I agree they are excellent meter with an incredible movement and also well protected with cut outs. Thanks for watching vane comments regards chris

[@EI6DP]

Brilliant - Extremely interesting, informative and educational. Thank you for taking the time to make and upload such a great video.

[@chefbink61]

You must have been reading my mind! I have the K7000 to and have been wanting to come up with an active probe of some type. So thanks for doing the hard work!!! Got to run to the lab see ya later!!

[@allthegearnoidea6752]

See how you go. If you don’t need the additional gain the follower circuit has higher impedance band better dynamic range. Be interesting to know how it turns out if you build one. I’m sure there is room for improvement but it was just a quick easy build etc. Thanks for watching and great comments my very best regards Chris.

[@marka1986]

Yay, you still have the vtvm :) That's excellent 100 Megohm input resistance. Most service bench meters are 10 to 11 Meg input.

[@allthegearnoidea6752]

VTVM has actually been more useful than i gave it credit for. Although i would probably like something more modern. I think kiethley do some meters with gig ohm input impedance. I test a few of my DVM meters they were all around 10 to 10.5 but more variance than I expected. Thanks for watch and really great comments. My best regards Chris

[@paulc9139]

I'll be building one , I have the velleman single tracer amp ,but my home made probes need to go into retirement, so this fit's the bill all in one for my use's thank Chris

[@allthegearnoidea6752]

Yeh let me know how you get on. The follower probe has higher impedance but not so much gain so maybe try that version first. Love to know how it works out. Thanks for watching and great comments regards chris

[@paulc9139]

Will do, I'm thinking of putting it in an old disused broken solder sucker.

[@RandallCrook]

I was thinking about making a sig tracer. You’ve convinced me I need one even though I doubt I would ever use it. I am a digital guy🤣

[@allthegearnoidea6752]

Back in the day designing various processor driven systems I used an old MW radio to listen to the activity on the data / address lines every system had its own tune and you could tell a lot by listening

[@fibberscloset498]

Enjoyed the loading demonstration.

[@oldblokeh]

I tried a simple 2-JFET cascode circuit, but I only had time to try it on a breadboard, so a well-built circuit would work much better. Anyway, it seemed to give a unity voltage gain at 30 MHz (much higher lower down), which would make it useful for HF. One thing I noticed is that it's better to avoid electrolytics as bypass caps. I think this is probably down to their high parasitic inductance. I don't know about tantalums, but ceramics worked well.

[@allthegearnoidea6752]

Yes I’m sure your right on the bypass capacitor

[@erikdenhouter]

RepairOldRadios gave a tip for shielding. He shielded the tip apart with a little copper pipe, and connected it through a 470K resistor to ground to keep the capacitance to the tip as low as possible. Maybe that can be adopted for the whole outer shielding, making it a bit safer. Also possible, aluminium or copper foil glued on the outside with a long wire under it, and cover it up with duct tape. As an addition, to make the amp peaking, you could temporary adjust the values of the drain- and source resistors with two pots, and find a sweet spot. B.t.w., I wonder why Fairchild's datasheet tells us that the 2N5457 is an audio amp JFET: "This device is a low level audio amplifier and switching transistors, and can be used for analog switching applications. Sourced from Process 55".

[@aduedc]

To minimize miller capacitance use cascade structure for your FET ( that is two FETs on top each other) or better yet use common Darin stage followed by your current common source. Good Video.

[@fullwaverecked]

Great video Chris! Thanks!

[@allthegearnoidea6752]

Thanks for watching and nice comments

[@fullwaverecked]

@@allthegearnoidea6752 Very detailed and informative.

[@danishnative9555]

Measuring high impedance circuits in the giga-ohm range can also be problematic. Ion detection chamber with Radium or Americium source + special FET. Those are common household devices used in smoke detectors. Thanks Chris.

[@radio-ged4626]

Another very informative video. Thanks again. I think maybe the only way to improve the probe would be circuit layout and maybe lookjng at a few similar FETs to see if they have a lower Miller effect i.e. faster switching FETs?

[@allthegearnoidea6752]

I did also build it with a MPF102 which is a VHF FET it didn’t really work any better but it did have similar value of capacitance. It’s always interesting to improve things but you know sometimes good enough is good enough. But it’s a interesting project. Thanks for watching and nice comments regards Chris

[@wa7215]

To get rid of the stray hum problem, just use an earthed 20mm diameter stainless, or aluminium metal outer tube instead of that plastic one, and then to avoid any other, possible, ‘Shocking’ issues, when using it to test HV Valve circuits, just cover the outside of that metal tube, with a suitable sized heat-shrink plastic tubing, to insulate it. 😉👍

[@robtitheridge9708]

Well done Chris i have been going on fo a few years now about the short comings of the vellemen for radio work (which it was not desighned for) same goes for the injector part great fo audio but for radio work a simple multivibrator with loads of harmonics would be more usefull.

[@allthegearnoidea6752]

Yes the signal tracer is meant to be harmonic rich but I think having to connect it via long leads suppresses it somewhat. I am thinking of building an injection probe next

[@fullwaverecked]

Chris, this video hit me in the depths of my personal places that I don't usually show others... As getting arrested for indecent exposure is, I've found, nonhabit forming... But yeah, so I've been noodling around with an Eico 147A Signal Tracer for a while now, and I replaced the original 4" 1.5 watt 8 ohm speaker for a 4" Eminence 30 watt 8 ohm. "Watt were you thinking?", you might ask? Well, I plug my Tele into the RF input, and it screams like a vintage Fender 3 watt amp, and the evil green cat's eye valve winks at me while I play the guitar badly. Up untill Tuesday, March 17th, 2020 that is. The Great Quake of Salt Lake. The poor bastard waddled off the bench and landed on the floor, busting the the cat's eye tube, basically blinding the whole project, not to mention me nearly crapping my chaps... But this video has renewed my interest in said protect, as I've always wanted a effective signal tracer that i could jam with badly. Thank you for motivating my motivation! Cheers!

[@allthegearnoidea6752]

Erm sounds exciting LOL. Thanks for watching regards Chris

[@elsaarmstrong-zp6ng]

I have a short length of 22 mill’r copper pipe with a inner plastic sheath and a plastic sheath to fit over the outer copper! I have a piece of co-ax braiding soldered to the pipe and ground inside! No hum attall even at full volume! The outer sheath is 20mm plastic poly pipe and u simply submerge the plastic into boiling water then stretch the poly over the outer copper! When it cools it is tight over the copper and will never move! An old sharp dart covered with heatshrink serves as the probe! Fraser

[@zx8401ztv]

Well that worked way better than i thought it would, i may take a screen snap of the circuit as one day i might need one. For r.f signals i waft the antenna cable from my frg7700 receiver near the stage, handy for listerning to the i.f products in a c.b or radio, handy for checking for the 1st mixer local oscillator. Yes i cheat, and i do dodgy things to find faults lol. Just for sillyness you could make a parallel L C in parallel circuit, earth the coil and a long wire on the other end. Your impressive probe could connect across the coil, tune for radio 5 live lol, yes pointless but fun 😀 Smashing job 😀

[@allthegearnoidea6752]

There is a drop box link in the show notes with the schematic and additional information. I do like a nice frog but don’t own one at the moment. Thanks for watching and comments best regards Chris

[@Kresve]

@@allthegearnoidea6752 Hello. Thank you for sharing your knowledge, I take a lot of attention in your explanations because I am learning a lot. I have to tell you that the link to DropBox does not work. I would like to have the diagram to do the probe. Health!!!! (sorry for the inaccuracies, I'm using Google Translate)

[@hestheMaster]

Darn sensitive active probe. Quite impressed Chris. Definitely not rubbish.

[@allthegearnoidea6752]

Certainly sensitive perhaps a bit over sensitive and it maybe lacks dynamic range but it is better than the passive prove. I’m sure it could be improved with a little more effort etc.

[@ptronix]

Excellent explanation of load resistance & capacitive reactance on loading circuits, reminds me of when I learnt all this at college. Ever thought of being a college lecturer? Of course I know all this stuff but couldn't explain it as well as you did

[@allthegearnoidea6752]

Thanks Nick. To be honest I struggled a bit with the explanation so I’m really hoping it made some sense to people. Even though I’m familiar with the concept it’s not always something I think of when taking measurements. That impedance PCB with the decade resistances was really quite an eye opener. Thanks for watching and nice comments regards Chris

[@andrewskater8813]

very very interesting. and very nostalgic since i dont live in the UK since many years. But i still say three like you do :-)

[@clifffiftytwo]

Very interesting presentation and quite educational. I particularly enjoyed your Input Impedance Test fixture - quite useful! Thanks for sharing your thoughts on this -

[@allthegearnoidea6752]

Hello Cliff I’m really glad you enjoyed it and for taking the time to leave a comment. Best regards Chris

[@haraldlonn898]

Heat shrink tubing around a metal tube works very well for me.

[@surgingcircuits6955]

An awesome video for me. Excellent approach, information, etc. Answered many questions. TY so much!

[@allthegearnoidea6752]

Thanks very kind of you to leave such a nice comment. Best regards Chris

[@davidkerr8030]

Really great video Chris, thanks very much. I had already bought the Velleman probe kit and was in the process of building it. Had wondered how to make a probe so your video was very timely indeed. I will definitely have a shot at building one. Also learned quite a lot too!

[@allthegearnoidea6752]

Hello David thanks for watching and nice comments. Unless you really need the additional sensitivity of the FET just start of by building a simple passive probe using a Germanium diode. That’s quite good enough most of the time and is simpler. If you need a diode let me know and I will post you one.

[@davidkerr8030]

@@allthegearnoidea6752 Hi Chris, that sounds like a good idea. I will have a go at that first. I do have some Ge diodes knocking about, but thanks for the offer.

[@TonyAlbus]

Thank you Chris, very interesting!

[@allthegearnoidea6752]

Thanks Tony good to see you today

[@JesusIsLordofMybeing]

Awesome tutorial and thanks so much for sharing your design.

[@acestudioscouk-Ace-G0ACE]

I loved this and learnt a lot! I do hope you do some more... pretty please. 🙏😃

[@Zonfeair]

Hello I just subscribed and have been learning electronics over the past two years. I've struggled with the concept of impedance but this video helped a lot. What I am not sure of is Impedance good or bad and is high impedance better than low impedance? or does it not matter as long as impedance is matched between input and output or DUT and the testing gear?

[@allthegearnoidea6752]

Unfortunately this isn’t a yes or no answer. In some situations it makes sense to match impedance but sometimes it’s not important or even counterintuitive. For test gear higher impedance is usually better as it doesn’t load the circuit under test but at radio frequency this is wrong as the mismatch can cause reflections and standing waves. Don’t worry about it to much, you will figure it out when you need too. Thanks best regards Chris

[@TechneMoira]

Very nice explanation. I wonder how any work was done with those AVO's considering how inacurate they are. I have one myself (an older model) but I guess it'll have to live its life out as a display object :)

[@allthegearnoidea6752]

They are useful for service sheets that specify a particular model of meter etc. PS I haven’t forgotten about the valve tube just had a lot going on recently. I have a few days off this week so hope to send it.

[@cranegantry868]

I will build one of those probes. Subbed too!

[@davidroberts965]

Great video Chris I will have a go at building the probe

[@allthegearnoidea6752]

Thanks David. Be interesting to see if it works out. If you don’t need the gain try the follower probe design first it has higher impedance and probably better dynamic range.

[@davidroberts965]

@@allthegearnoidea6752 Thanks Chris Will do just waiting for Jfets to arrive

[@GeorgeChristofi]

Well well, all the clangers were being chased by the soup dragon with all those bleeps and blarps. I bet he was coming down to give them a probing like the ufo's do to unsuspecting hillbillys

[@allthegearnoidea6752]

I’m sure I dropped enough clangers is this video. Thanks For watching regards chris

[@jamesaguirre853]

Is the schematic available somewhere else, the dropbox file is gone and I would really like to get it. I know I might be able to piece one together from video, but having the schematic to download would be nicer.

[@allthegearnoidea6752]

I’m upto my eyeballs at the moment but if I have the files i will send you new link

[@FisherWallofSound]

I’m sold. I’m a subscriber!

[@user-ur1ny8wt3c]

Well done !

[@paulperano9236]

I was wondering about the 10M resistor at the front end. If that is not there what is the current draw on the battery ? What affect does removing it have on the input impedance ?

[@allthegearnoidea6752]

Hello Paul thanks for watching and comments. The current draw is very small there is really no current flowing into the gate of the FET. The purpose of the resistor is to discharge the gate capacitance of the FET. At low frequency removing the resistor would effect the impedance but it’s the gate capacitance and Miller effect that will reduce the impedance at higher frequencies and the resistor is not significant

[@rádiosantigos1958]

Nice demonstrations. I will try to make the probe. Just a question: If instead of a R2 of 10M we install a 20M one, will that increase the final input impedance of the probe? Thanks.

[@allthegearnoidea6752]

Probably not as at higher frequency it’s governed by the gate capacitance of the FET and Miller effect. This circuit was just a quick build using parts from the junk box I’m sure you can improve my design. Thanks best regards Chris

[@Stuart_MM7SHX]

Hi there, the dropbox link seems to have expired - would it be possible to please re-post it? Thank you!

[@maciejsierzega]

Project link dont work !

[@radio-ged4626]

..or how to make a calculator sound like bagpipes.

[@allthegearnoidea6752]

Funny but most of my radio repairs sound the same way ?

[@radio-ged4626]

@@allthegearnoidea6752 🤣

[@kkteutsch6416]

I worked some 40~45 years ago with these Avometer on my work, before handheld lcd multimeters became common, are excellent laboratory quality instrumental...

[@kkteutsch6416]

Digital reading multimeters those days are really workbench type instrumental like HP and other famous - and really expensive - instruments and analog ones are so common, my 100Meg Honor refuses to operating after years of good service - and years no used too, it seems to me the galvanometer is open circuit ...