Build an A.F./R.F. Autosensing Probe
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- Опубликовано: 7 авг 2024
- Build this simple, but effective Audio Frequency/Radio Frequency Demodulating Probe that requires no switching between input sources. The Probe feeds an external high gain amplifier w/ 60dB of gain (x1000) to detect those weak R.F. signals.
As-built drawings:
drive.google.com/drive/folder...
Thanks for watching and subscribing to my channel on vintage radio repair. 
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Wow, Don, another great circuit, love the no switching feature. I really like this overall design, a great idea to use the prebuilt modules too!
Thanks Tim. The no switch feature adds value and using the MAX9814 microphone amplifier circuit provides excellent results; surprised me. Thanks again and stay well. Best, Don
Thanks for sharing your designs. I've been wanting a signal tracer for some time and this looks very nice.
Andy, you’re welcome. Have fun building it. All the best. Don
You are a great help to us out in utube land who enjoy old radio repair. Thanks.
Much appreciated John. Thanks again for watching. Best, Don
Wow the front end capacitance looks awesome with your design Don. So small an effect on the signal tested. I will be build this great design very soon for testing my 557 radio, thanks buddy!
Tony, enjoy and I hope the build goes as planned and your 557 is singing again soon. Best, Don
Ingenious design you've got there. Thanks for sharing.
Ovi - thank you; glad you found it added value. Best, Don
Well Don,,, you did it again good buddy. Sensitive and clean with minimal components, ( the most modern being a FET transistor)
Simple with ingenuity in the background makes this another success. WTG pal.
Thank you Carl. It performs well and I like the fact no switching is required between input sources. Stay well my friend. Best, Don
Looks like a handy tool! I like the fact there is no loading! Thanks for sharing...Take Care-Larry
Thanks Larry. Let me know if you build one and your feedback afterwards. Best, Don
Don, that looks like fun. Thanks for putting it up.
Josh, thank you. Glad you found it interesting. Best, Don
Hello Don, Very nice job as always! It may look simple enough, but, a lot of great engineering of the circuit and the selection of the component values and the probe design went into it. You always do such clean, nice looking builds. Great to "see" you again, I'm glad your doing well. 98 degrees today, the 26th, to hot for this kid, who turned 60 June 10th. A download of this to go with the other designs you have shared, Thank you Don. Day's getting shorter fast. Until next time my friend, take care & all the best, Cass.
Thank you Cass. It’s been hot here too, low 90s for a week or more. Fall will be here before we know it. Stay well and thanks again for your kind words. Best, Don
Great build there Don. Nice high impedance there.
Thank you George. It seems to work well based on my limited usage. Best, Don
Nice easy circuit. Definitely on my to build list.
Thank you Bob; have fun building it. Best, Don
Good work Don. I take my hat off to you. Have a nice day.
Ketk (From España).
Thank you very much! I appreciate you watching Ketk. Best, Don
That's a signal tracer on steroids, very sensitive and won't load down the circuit. That's a great signal tracer Don. The auto selection circuit works well.
Thank you Jim. The design is very simple, but seems to be effective passing the signal along to the amplifier regardless of the source. I appreciate you watching and reaching out. Best, Don
Great job !!!
Much appreciated Greg. Best, Don
Thank you for sharing! I wish I would have seen this last week's as I built one that works, but it is a mess compared to your design. I think I will start over with your when I have the chance.
Hope it’s helpful and all the best on your build down the road. Don
Finding caps like the yellow one at the probe (visible at 9:09): Cornell Dubilier (nee Mallory) Type 150 Series Axial 630 VDC Film Capacitors at Mouser Electronics site
Thanks for sharing. Best, Don
Excellent!
Thank you. I’ve built a few now and they seem to work well. Thanks for watching. Best, Don
Hello Don. Great to see you back buddy. Umm Did you get lost in your warehouse Again ? : ) Thank you for the build video. Liked,shared. All my best.
LOL. Bobby, between this little project and supporting my family, I’ve been busy. Hope you are doing well my friend. Best, Don
Looks easier than using a scope. Especially for low level signals. I've been looking for a good signal tracer for tube audio and some RF use. This might fit the bill. Thanks!
Bill - it works great in my limited testing. Hope you enjoy it. Best, Don
Hi Don,
How are you feeling?
This is a great project.
Thanks,
Ian
Ian - I’m doing great. I still have some pain at times, but nothing to complain about. Glad you enjoyed the new project and I hope you are doing well yourself. Best, Don
Hi Don, thanks for sharing, only one question . How do you get the 3/32" tube centred inside the 1/4" tube and how is it fixed inside, did you use some form of packing?
Good question! I used heat shrink tubing over the most inner before inserting it into the larger… Hope this helps and I apologize for the belated response. Best, Don
Hi Don, Sorry for so many questions. Do you recommend a pot with a plastic shaft for lower noise? And does audio taper work better? Thanks again.
I did another breadboard build sometime back with a 2M linear and audio taper with metal shafts and the results were equal to my published design and they are definitely more common/plentiful. Hope this helps. Best, Don
I finally got your tracer finished and I really like it. I set the pot gain on the 386 board to the maximum position without too much hum out the speaker. Can pick up 1st IF signals no problem. I got the same aluminum case you used from ebay and it was a great idea for a probe. Grounding to the case was a little tricky but I tried the motor oil trick to get a solder ball to stick. However, don't use a new soldering tip as it gunks it up pretty good. One of the hardest parts to find was a 5M pot that would fit in the case. I happened to have some reclaimed TV pots that I bent the leads to fit. I also tried a LM386 amp with volume and gain control setup that I like from (www.circuitbasics.com/build-a-great-sounding-audio-amplifier-with-bass-boost-from-the-lm386/). Great probe and project and look forward to more great stuff!
Tom - glad it’s working good for you and I appreciate the motor oil tip trick in addition to the URL on... I’m working on a new project now that I hope to share soon if it actually works. Thanks again for following my channel and your feedback on the probe. Best, Don
Looks like another great tool to build, thanks Don! Would a J310 N-JFET be OK in this circuit? I built your last signal tracer so would be interesting to compare the two - with that one I made a separate "probe" containing the 1N34A diode and connected it to the main circuit box via a BNC cable and connectors.
Chris - I would think the J310 will work equally as well. Let me know after you build it; would love to know how it stacks up. All the best. Don
Hi Don, I'm wondering which type and value of (C1) capacitor currently works best for the probe? Polypropylene, Polyester? Do you use .1uf or smaller? Yours looks like a Mallory 150M?
Thanks for watching and reaching out. C1 value and type other than the voltage rating is not critical. When I built the circuit, any value from .01 uF to 0.1 uF worked equally well. Again, just ensure the voltage rating is adequate for your application. Hope this helps. All the best. Don
Thanks for the video. What are you using to make those schematic drawings?
Andrew - an Alvin template like: Alvin, TD1279, Professional Graphic Symbols Ruling Template, For Electrical and Electronic Diagrams - 5 x 7 x .03 inches www.amazon.com/dp/B000KNLR4A/ref=cm_sw_r_cp_tai_zkIhFb1KPDEZG Best, Don
Hi Don, I still haven't had the time to build this yet. However, I'm wondering if you've made any improvements to the design? Thanks.
Nothing major. You can make R1 a 2 megohm linear pot, which is more common than the 5 megohm I originally used and will not change the performance. I’ve built two of these, one as-built using the aluminum enclosure for the probe and another one using a plastic enclosure (all in one) and both work equally well. The MAX9814 does a great job amplifying those low level signals; removing the electret microphone soldered on can be a pain. Please let me know if you have any additional questions. Best, Don
Nice Probe man! Is this your favorite version?
Hi Blade - it’s my favorite. Thanks for reaching out. Best, Don
@@RestoreOldRadios Ive built my 1st probe to find a no audio fault in my Stryker 955. I found a dead TDA2003v audio amplifier so far.
A just barely shorted out speaker by one strand of wire killed my audio.
I want to build this probe you have here as well.
Hi, built your first probe and loved it and look forward to building this second one. Did you make R4 on the probe tip removable as you note on the schematic? If so, why? Great videos!
Tom, I did make it removable. Check out my latest video: Signal Tracing Demo - Low & High Impedance Test Point Locations
ruclips.net/video/SP6uJRjpCzY/видео.html It demonstrates how using a resistor and capacitor can improve the ST performance. Best, Don
@@RestoreOldRadios Ok great, thank you. One other quick question. Does your grounding clip go direct to ground or through the 470k resistor? I assume its through the resistor. Great videos!
Tom - the 470k attaches to the outer probe that is isolated from the metal enclosure. The ground wire is attached directly to the circuit ground, which also connects to the metal enclosure. Keep in mind the metal enclosure is at the same potential as the receiver chassis ground, so an isolation transformer is always required. I updated the as-built file for the probe; added another 1N34A to improve RF sensitivity. Hope this answers your question. Best, Don
@@RestoreOldRadios Ok good. The schematic shows it as you describe it but in your other video, it looked like you had the ground clip directly attached to the outer shield on the probe. Thanks again.
Tom B You are correct. The all in one I used in the Demo video is a different design. The probe is grounded and R3 was not used. Sorry about the confusion. Best, Don
Hi Don, I have another question. I'm wondering how you changed the gain of the LM386 module? Did you just cut a trace on the PCB between pins# 1 & 8 to use the LM386 default gain or did you change the resistor and capacitors values? Thanks
Correct, I cut the trace to one (or both) side(s) of the 10uF capacitor between pins 1 & 8. Hope this helps. Best, Don
@@RestoreOldRadios Thanks!
Hi Don, I'm collecting parts to build this probe. Wondering if you can recommend a good source for authentic1N34 diodes? There seems to be alot of counterfeits out there. I'm also wondering which currently available JFETs work best? I've already received the two needed modules. They seem quite a bit smaller than I they appear in the pictures. Thanks for all your great ideas!
The supplier I’ve used in the past is: www.mikeselectronicparts.com/product/1n34a/ and 2N5457 JFETs is what I’ve used for many of my projects. Hope the build goes well. Best, Don
Jfet that cross reference to the 2n5457. All of these Jfet Transistor should work.
2N5458N, 2N5459, 2N5639, 2N5653N 2N5654N J105N J106N J107N J309N, PF5102N, PF5103N 1NJFET0, PN4092N, PN4093N, PN4302N, PN4303N, PN4861, TIS74N, U1898N
Hi Don, I've got nearly all the parts to make this RF/AF Demodulating Probe. The only thing I haven't gotten is the case. Is it necessary to use a metal case as you have in your example?
The metal enclosure (for the Probe) being at the same potential of the chassis helps mitigate hum and an isolation transformer is required to safe guard against shock. My first probe was in a plastic pill bottle and it performs well aside from the hum factor, but provides isolation. You’ll notice in another video I built an all in one in a plastic case and included a piece of copper tape (tied to the circuit ground) on the side that my fingers touch when powering up the unit when pressing the momentary switch; this worked well for me. All the best moving forward. Don
Mr. Don, my friend, I cannot find the 5M pot for the probe anywhere. I found big ol' wire wound clunkers that won't fit in the case, but simply cannot find where you sourced that part from. Can you let me know where you found it? It's the only remaining piece between me and rebuilding a stack of old Zeniths. Many thanks in advance!
I think you’ll get good results with a 1 Meg pot as well, which are more common. I got the 5 Meg from Mouser Electronics. I hope things go well on your upcoming Zenith projects. All the best. Don
I'm wondering what type those yellow capacitors are? And voltage rating. Thanks.
Thanks for watching and reaching out. They are 630V Metalized Polypropylene Film like: www.justradios.com/PPTcapacitors.html
Hope this helps.
Best,
Don
@@RestoreOldRadios Hi Don, I understand that a film capacitor is used between the probe and the FET. I was wondering about the other little caps like the 680pf and .1uf caps in the remainder of the circuit? Are those just ceramic caps? Thanks.
Sorry about that, yes I used ceramic capacitors for the others...
This looks like the best signal tracer I've seen online. Problem is, I can't find a 50 MOhm pot. Does anyone know where I can find one?
My mistake, its a 5 MOhm pot. No problem
Thank you. It performs well. I got the 5 meg ohm at www.mouser.com. I have since used a more common 1 to 2 Meg pot with very good results. Hope this helps. All the best. Don
bonjour possible un schema de la sonde avec liste des composants merci
Check the video description for a link to the schematic and BOM. Best, Don
Hi Don, I was about to flip the switch on my breadboard of the Autosensing Probe when I noticed in the schematic that the LM386 is wired directly to 9 volts. Is that correct? The reason I'm asking is my LM386 is in a Seeed Studio Speaker module that uses 5.5 Volts max. Thanks.
The 386 modules I used are rated from 5 to 12 volts. With your configuration being different, I wouldn’t exceed the 5.5 volts. Hope this helps. Best, Don
Don, thanks for confirming. I will connect this module to the 5V supply in parallel with the MAX9814 to be safe.
Let me know how things work out. Best, Don
Don, it looks like the module manufacturer was too low on the operating voltage. Texas Instrument stated a very wide operating voltage of 4-12V or 5-18v. So I was able to operate this module on 5V and 9V with no problem. Now comes the interesting task of shoe horning everything including the battery into a 1"x1.75"x5" plastic box. I plan to line the inside with aluminum duct tape. Just the other day, I learned that it's possible to solder directly to aluminum on RUclips. So I'll be able to ground the the shield. I'll let you know how this works out. Thanks, Lyman
Lyman - hope the build goes as planned. Thanks for keep me posted. All the best. Don
good circuit can i use this for audio tracing
Ian - so far it performs equally well on audio or RF sources. Thanks for watching. Best, Don
Don, great Signal Tracer but I'm confused. The input cap is a 0.1uF I thought that can pass audio but RF would need a much lower cap value? Can you help me out with this.
Thanks for reaching out. The design works well in the AM Broadcast band. Regarding the capacitance value, check out this video:
Signal Tracing Demo - Low & High Impedance Test Point Locations
ruclips.net/video/SP6uJRjpCzY/видео.html
Using a 20 to 50 pF capacitor in series with the .01 uF integrated into the probe when measuring RF at the plate of a RF tube or in the I.F. section helps mitigate low frequency signals (hum) and allows the demodulation to occur with less impairments.
Hope this answers your question.
Best,
Don
@@RestoreOldRadios Thank you Don, I'll poke around in the radio.
The radio is a AAF I get one station loud and clear at the low end of the dial. Nothing else going to the high end. I have a signal tracer with a J-fet input but the design is not that good.
When I place the probe tip on pin 7 of the 12BE6 I get nothing, working back toward the detector I start to pickup the station. The radio vol. control is at it's lowest setting with sound only coming from the signal tracers small speaker. I sure appreciate you sharing your knowledge. It's a God send. Hope to see more test instruments from you - as I consider you the master builder.
-Tom
Tom, what antenna is being used on the radio? Since pin 7 (grid) is being fed from the tank circuit, a strong signal should be able to be detected at that point if the gain of your signal tracer is ~40 to 50 dB. Do you have a signal generator that is modulated? Best, Don
@@RestoreOldRadios Don, here's where I may have messed up.
The radio is a Packard Bell 5R1 it's original antenna was an older loop in poor shape, I replaced it with a ferrite rod antenna from a later same model radio.
No alignment was ever done.
Yes, I have one that is modulated.
What do you suggest?
-Tom
Tom, only if you have an isolation transformer you can hook your SG to pin 7 through a .01uf capacitor or lower pF value and turn on the modulation and tune through the band and see if the audio is detected from the loudspeaker. Start at 600kC then 1000kC, & 1500kC. Make sure your tuning condenser tracks the frequency of your SG. Again, you shouldn’t proceed if you don’t have an isolation transformer or the knowledge to safely do so. Best, Don
Regarding the driven guard/shield, this is from "The Art of Electronics" by Paul Horowitz and Winfield Hill: drive.google.com/file/d/1fmr2eRkTZJwJ1XNLzhs7b2R1QUdxyvSb/view?usp=sharing
Thank you. Best, Don
Isn't the LM386 module gain +26 dB and not +20dB (as drawn)? At least you said 20?
Not that it would matter that much... :)
Harri - my bad. I should have said 20 x voltage gain, which is equal to 26 dB. A variation in gain of a few dB won’t change the performance. Thanks for reaching out and watching. Best, Don
@@RestoreOldRadios The design itself is very good and functional. The parallel A.F. and R.F. paths are a well-working old-school style solution.
@@RestoreOldRadios You could feed the probe shield with the input signal from a unity gain low output impedance source, to reduce the effects of input capacitance even more. Even directly from the JFET source could work: maybe this would be worth testing?
As an example, see the "driven guard" in this: www.analog.com/media/en/technical-documentation/application-notes/41727248AN_347.pdf
Hi there! How are you? I wish you to be fine and all the best.
Have a nice day.
Ketk (Spain).
Ketk - Very good! Still some soreness at times in my back, but doing much better. I appreciate you asking and I hope all is well with you. Best, Don
@@RestoreOldRadios thanks for answer. Here we are fine also. Taking precautions with each other. And also following doing new things and new fixes. All is fine here for me till now. Have a nice day and take care from those soreness.
Ketk (Spain)
Good day, how can your subscribers send you a donation. Do you accept paypal by chance? You worked on a project similar to what I've been tinkering with and would like to send a donation for saving me lots of time.
Thanks for watching and I'm glad this helped with your... No donation is necessary as sharing my thoughts and ideas is just one way of giving back to others that enjoy the hobby as much as me. Regarding the Probe design, I do have an enhancement to share that improves the R.F. performance. I'll let you know when the the documentation is updated to reflect the add on. Thanks again for tuning in and all the best to you. Don