help with ultra probe amplifier

@jodonne22 Hi again, 2 things you need to keep in mind. 1. On one of the videos (I think it’s the video in which MCL shows all wiring of the base unit), he mentions at some point that the audio amp can run into oscillation (out of the blue) because of excessive amplification. To solve that, you need to go back to that video and watch it once again. He mentions one of the caps that can be left out, so the amplification will be reduced to prevent any oscillations from happening. So, considering the above, this could be one of your problems. If the base unit works (when the probe head is not connected) the base unit should be almost silent with the volume at maximum) 2. On the other hand, ensure that you did not (somehow) have some wires soldered in the wrong place (inside the base unit), the probe, and the multi-pin plug/socket. It is very easy to make mistakes like that. Further on, make sure that the required voltages are present at the probe head PCB level and that one of the 2 voltages there is adjustable via the designated potentiometer (located on the base unit). Make sure that the positive is where it should be. Also, verify your soldering for bridges on the probe head PCB  (use a proper magnifier), as you cannot always trust your bare eyes. Ensure that you have properly grounded both ends of the coaxial cables. Make sure your BNC has a proper contact with the ground (is sturdy and screwed in properly). Ensure that no SMD components are coming in contact with the interior part of the aluminium tubing (inside the probe head. And the list doesn’t end here. Make sure all 3 pots are properly grounded, too.

@ovi4 thanks for the quick response, I will check all of your points tomorrow. I think I’m close. 😀

@jodonne22 PS:  In regards to the ”probe head” PCB, please ensure that all resistors and ceramic caps have the required values and that they are not soldered (by mistake) in the wrong place. The same applies to all transistors. Pay special attention to the (special J-FET) transistor that is in contact with the ”sniffing” coaxial tip, as any static electricity can easily destroy it. A lot of people (apparently) have destroyed it and didn’t even know it happened (during the soldering process).  Instructions about it can be found (again) in another video number (that presents the Probe Head construction). Good luck and be extra careful.

hi ovi4, I just wanted to drop you a note and thank you for all your help. This was a fun project and I learned so much. Well I found the issue with the probe amp. one of my vias, looked fine but when tested, there was no continuity. works great now. I also had a problem with the probe head , as it turned out there was a crack in a trace on the board. I ended  up just making a new board. just put it all back together yesterday. it is amazing. thanks, Mr C😊😊 and works so well. here are some sanitized pictures.

https://imgur.com/a/Yf0Qgce

@jodonne22 Well, the important thing is the result and that you’ve managed to find and fix the issues. And yes, the amp looks great. Tnx for sharing. Great job and good luck with your next project, build whichever one it might be. 🙂

@jodonne22 PS: just realised that you’ve used a different kind of head unit body for it. I wonder where that tube is from? Is another de-soldering (sucker). It would be nice if you could share what the part number is, and who sells them? I would like to buy a couple for myself. Thank you. The BNC idea is cool, too. Back then (when I built my Ultra-probe I was thinking of using a BNC connector for the tip, but quickly gave up on the idea thinking that the solder sucker that I had was going to be too short and perhaps not wide enough either which is why i.m curious to see the exact part number of yours. Tnx.

@ovi4 I used a cigar travel tube, it’s a little larger than the solder sucker but I wanted something larger. I have arthritis in my hands so it’s easier to hold. it is well made a pretty low cost. here is the link

https://a.co/d/gxR0lSg

@jodonne22 Wow, that tube looks good. I might try it on another (upcoming) project of mine since I’ve already built my Ultra-probe Quite some time ago, even though I never got to put any paint on the ”Base” amp box (as of yet) :)))  Tnx. Very much for the link and the idea. Great idea.