Velleman K7000 probe.

[johnwater]

Made the Velleman K7000 signal injector and it seems to work great, I have a very good sine wave and frequency output.

But I don't know what kind of probe/ground to use with this device. Does anyone have advice on this.

Thanks, John.

[John M0GLN]

There's been a discussion on this before if it helps,

https://www.vintage-radio.net/forum/...d.php?p=875176

John

[David G4EBT]

I later found a circuit on internet for an automatic AF/RF probe for signal tracing, which works well.

The same probe can also be use for signal injection.

I've attached some pics of the circuit and construction of the probe. I used a 15cm length of 22mm plastic conduit for the case, 2mm brass rod for the probe tip, and turned a 'bung' from nylon rod for each end of the case. I designed and etched a PCB but perf-board would do just as well, or the few components could simply be soldered together, tucked in the tube, and would work just as well.

If the probe is to be used on valved equipment, both C1 & C2 need to be at least 400WV, but in any event, if used with the Velleman K7000 it incorporates such capacitors. (In the probe, I used a 630V for C1 and 1,000V for C2 as I had them to hand). For the two germanium diodes I used 0A90s. Other germanium diodes would work just as well, such as 1N34.

Everyone has their own approach to fault-finding and there have been many threads on the forum on the topic.

I've been in the habit of using the 'signal tracer/injector approach' which has saved me countless hours of fruitless effort. By applying the probe of a signal tracer to the slider of the volume control you can check if there are tuneable signals at that point. If so, the frequency changer and IF stages are working, if only after a fashion. If not, you can work your way to the front end of the set, seeking out signals with the probe at various points to pinpoint the fault, or at least the faulty stage.

If there are signals at the slider of the volume control you can then use a signal injector to inject a signal into the AF stages at the slider, and if you hear no tone from the speaker, you've narrowed the fault down to the AF Stages and can look further into that.

Without a signal injector/tracer and a methodical approach including voltage checks, it's easy to make speculative guesses that 'maybe this or that transistor or valve is duff or maybe the detector, and so on it goes. In a transistor radio, a dead set might only be due to something as simple an open-circuit earphone socket.

For valve radio signal tracing, there's an excellent interactive tutorial at the link below. It's a circuit of a valve radio and if you hover your mouse over certain points as instructed, it tells you what signals (if any) you should expect at certain points:

http://www.nostalgiaair.org/Referenc...ost/post01.htm

Hope that's useful.

[johnwater]

Thanks John, looks like a good project.
I have some old dvmm probes laying around, gonna give it a try.
The K7000 has input and output pin, each with a ground header, I assume the input is for signal injection of sine wave?
David G4EBT has a lot of threads on this, i will review.
John

[johnwater]

Hello guys, question about my K7000.

When I connect scope to input and ground, get nice sine wave adjustable with the 4k7 pot next to it,
but when I connect scope to output and ground, lots of noise and distorted sign wave when adjusting the 47k pot.

Is this normal for this device, or do I have a problem with my connections perhaps?

Thanks, John.

[Bigears]

See what I did I earthed the pot

[Boulevardier]

A handy modification to the probe is to terminate the input/output on the case in a 4mm socket, with a pluggable tip. Being able to output to any lead terminated with a 4mm plug can help if you need your hands for other things while injecting or tracing the probe signal.

Mike

[lubosnatelnj]

@David G4EBT
I have just ordered the K7000 signal trace and I have read your posts where you publish the AF/BF probe for this circuit (the automatic one and the the one with the manual switch).
I'm a beginner in the repairing world and I'd like to know why do I need this kind of probe. I mean... the K7000 circuit give me the possibility to listen the low level AF signal and it have an output with 1KHz signal to inject. So why the probe is necessary? Is there some reason due to impedence ?
I hope for help to find the answer.
Thank you very much for your pictures and the project of the probe.
Andrea

[David G4EBT]

Originally Posted by lubosnatelnj

@David G4EBT
I have just ordered the K7000 signal tracer and I have read your posts where you publish the AF/RF probe for this circuit (the automatic one and the the one with the manual switch).

I'm a beginner in the repairing world and I'd like to know why do I need this kind of probe. I mean... the K7000 circuit gives me the possibility to listen the low level AF signal and it has an output with 1KHz signal to inject. So why the probe is necessary? Is there some reason due to impedance ?
I hope for help to find the answer.
Thank you very much for your pictures and the project of the probe.
Andrea

To inject the AF signal all you need is a simple probe as shown in the picture below of two probes - the upper one for AF tracing/injecting, the lower one for RF tracing. The capacitor is to protect the K7000 circuitry if you put the probe on a high voltage point in a valve ('tube') radio by mistake, but the K7000 does itself have protection capacitors for that purpose, (C5 on the K7000 circuit to protect the injector output from damage, C4 on the tracer input), so it's just an extra precaution. You can also use that AF probe to trace audio signals after the detector stage of a radio with the probe in the 'tracer' socket of the K7000.

But you also need to be able to trace RF/IF signals before the detector stage of radios (whether tube or transistor radios). In those stages, the audio part of the RF/IF signals needs to be detected ('demodulated') for you to be able to hear them, so a detector diode needs to be included in the probe for that to happen. For that, a simple RF probe can be made. There are lots of circuits on internet but the lower diagram in the first picture below will work fine.

However, it's more convenient to use a combined automatic AF RF probe which can be used to either inject an AF signal, or to trace an RF/IF/AF signal throughout the radio. Some years ago I found the circuit in the second picture below on internet. The link on the diagram in the second picture below which explained how the probe works is now dead, but my understanding is that when use for AF injecting or tracing, it's like any other AF probe - the AF signal flows via R1/C1/R2, whether injecting or tracing. But if the probe is applied to an RF/IF signal point in the radio, as C1 is quite a high value capacitor, the RF/IF signal will instead flow via C2 (100pF) to the two detector diodes, which will pick off the audio component from the signal to make it audible on the tracer.

I'm sure that other forum members who may be reading this will put me right if my understanding is mistaken!

I just used that circuit to design a practical probe which will fit into a piece of plastic tube. I doesn't need to be built on a PCB - I just happened to be etching several PCBs at the time, so it was no trouble to add another one, but a piece of stripboard would be fine. There are other circuits for AF/RF probes such as this YouTube video:

https://www.youtube.com/watch?v=OGrkIC-Mov0

The best way to learn how to use your K7000 is to try it on a working radio at various points to discover what signals you can hear. For example, if you put the tracer probe on the slider of the volume control in a working radio, you will hear signals as you rotate the radio tuning knob. Then if you inject an audio tone from your K7000 into the slider of the volume control, you will hear a loud tone in the radio speaker. So if you did the same on a dead radio using your tracer and heard signals at the volume control you'd know that the RF/IF stages are working. But if you inject a signal from the K7000 at the volume control and hear nothing from the speaker, that would suggest there's a fault in the audio stage.

There are lots of videos on YouTube which explain signal tracing. Here are two:

https://www.youtube.com/watch?v=Vf7hopZ7TUs

https://www.youtube.com/watch?v=maIJC4J_jZw

When building the K7000, if you follow the instructions carefully, it will work right away.

The components are on strips for you to place in the right order on the printed circuit board and the board is clearly marked 'R1, C1 etc, but for some reason, there have been several people on the forum who have put the resistors in the wrong places so their K7000s have failed to work. (Maybe lots of people not on the forum have done that too?). Why they did that is a mystery, but in case of difficulty, at Pic 3 below, I've given some tips on the K7000 circuit diagram which might help. There are three stages in the K7000- the power supply, the injector, and the tracer amplifier.

I hope these note help you to build and get the best from your K7000 Andrea.

Every success with it.

[Radio Wrangler]

The RF probe circuit in the first attached picture has a diode entirely in series with a capacitor. There is no DC path for the rectified component. I don't think it's going to work...

David

[ms660]

Reverse leakage? :

https://www.vintage-radio.net/forum/...ad.php?t=51438

https://www.vintage-radio.net/forum/...8&d=1267781394

Lawrence.

[lubosnatelnj]

David.

Thank you very much for your clear, detailed and exhaustive explanation! And also for your wishes!