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Old 13th Aug 2017, 12:00 pm   #1
David G4EBT
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Default Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

I've always been of the view that a signal injector/tracer is one of the speediest ways of pinpointing where faults lie in malfunctioning radio receivers. I'd rank an injector/tracer as the second most useful item of test gear after the multi-meter. In a dead set, having done all the usual checks such as voltages as compared to the circuit, continuity tests of switches, inductors, valve heaters, checking for out of spec resistors, and so on, I usually start by injecting an AF signal from the injector/tracer at the slider of the volume control to see if the audio stages are working. If so, a loud tone will be heard in the speaker. If not, then the faulty audio stage needs investigation.

If the injector/tracer is then switched to 'trace' and the probe applied to the slider of the volume control, if the RF & IF stages are working, tunable stations should be heard from the speaker in the tracer. If stations are only heard on one waveband, or one mode (eg, AM but not on FM), then that indicates a fault in the front end of the set - the 'switchery' for example, or if signals are fine on AM bands but silent on FM, that may point to a duff ECC85 twin triode valve in the FM stage - not noted for longevity. If no signals are heard by the tracer at the slider of the volume control, then work back to see if the fault is in the IF stages, or earlier in the set - the RF stage. No signals at the output of the IF stage, but good signals at the input to the IF from the front end of the set points to a fault in the IF stage(s).

This thread isn't intended to be a comprehensive guide to how use an injector tracer, it's about how to build an injector/tracer - notably the Velleman K7000 kit - and to outline its usefulness as an aid to methodical fault finding aimed at newcomers to the hobby. There are lots of videos on youtube which expand on this, some better than others. For example, if you click on this link below and go to the circuit of a valve receiver, you'll see various points on the circuit marked with capital letters in brackets EG ('A') etc. If you put your cursor over those points in turn, it will tell you what to expect, and what should be happening. For example, it explains that you should not expect to hear any signals at points C, F, K & V - the Screen Grids.

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

Building an injector/tracer is a nice introduction to vintage radio restoration, and there are countless circuits on internet on how to go about that. An injector is simply an audio oscillator usually at a frequency of between 800 and 1200 HZ, and a tracer, is a small amplifier. An LM386 amplifier module can typically be bought on e-bay post free from China and little more than £1.00, but the quickest and surest way for a novice to build an injector/tracer is from a kit, and one that is widely favoured in the Velleman 'K7000' available for under £10.00 from several suppliers, EG: ESR Electronics & Quasar Electronics:

https://www.esr.co.uk/frame_terms.htm

https://www.quasarelectronics.co.uk/...r-injector-kit

The kit consists of a nicely made printed circuit board with two potentiometers mounted directly onto the board - one for the injector level - the other for the tracer volume. In addition to that, some additional items are needed - a small speaker (say 5 - 6cms diameter), a project box a switch, a PP3 battery connector, a couple of RCA ('phono') sockets, and if desired, a high brightness/low consumption LED so you don't forget to switch the injector/tracer off after use. It can be powered from a small transformer or external 9v 'wall wart' but uses such a small amount of current, that's hardly worthwhile.

In addition to the tracer, an AF & RF probe is required - whether separate or combined.

The circuit of the Velleman K7000 is attached and as can be seen, there are three facets to it: namely, the power supply, the tracer, and the injector - all on the one PCB. D1 & D2 on the power supply are reverse polarity protection diodes in the event of inadvertent reversal of polarity. (A nice touch - a pity Roberts didn't do the same on the Roberts Revival!). ZD1 is the stabilise the voltage. As to the injector circuit, T1 & T2 form a simple sine wave oscillator, and RV1 controls the signal injection level. I've attached a picture of the trace on one that I built - the sine wave at full volume was 4.5 Volts. Nominally, the frequency is stated as 1 kHz, but doesn't need to be precise and will depend on the actual value of the components in the kit. That particular one turned out to be 1.253 kHz, which is fine.

The tracer amplifier section is a simple LM386 amplifier, with RV2 to control the volume.

C4 & C5 are high voltage DC isolating capacitors to protect the injector/tracer circuits whilst allowing AC signals to pass.

To house the kit, a project box - either homemade, or say an ABS or Aluminium enclosure is needed, at least 11.5cm x 15cm x 7cm (4" x 6" x 2.75"). I built mine in a home-made comb-jointed wooden box as I do most of my homebrew test gear, and have attached the dimensions of the front panel, for which I used scrap single-sided SRBP PCB laminate, sprayed gloss white on the copper side.

Having built the kit, an AF & RF probe will be needed for injecting and tracing signals. In the next post I'll outline how to build a simple combined probe.

I hope these notes will prove useful, especially to novices, at whom they're aimed.
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Last edited by David G4EBT; 13th Aug 2017 at 12:07 pm.
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Old 13th Aug 2017, 12:36 pm   #2
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Following on from my earlier post, firstly, someone asked how to fit an LED. All that's needed is a high brightness/low consumption LED with a series resistor. The value of the resistor will be dependent on the current that the LED requires to enable it to light at the desired level. The easiest way to find the value of resistor needed is by substitution. In my case, a 2K2 resistor was fine and at good brightness drew only 3mA - a modest amount to warn the user not to inadvertently leave the injector/tracer on when not in use. See pic 1 below.

For an AF probe which can be used to trace AF signals, and to also inject a signal, all that's needed is a probe tip in say a disused felt tip pen case, but a combined AF/RF probe extends the usefulness of the tracer. I've attached a circuit and tagboard layout of one that I've made, which can be switched from RF to AF as desired. It only requires four components - two capacitors, a resistor and a germanium diode, plus a switch, tagboard and two RCA ('phono') sockets. I built it into a little ABS project box, 8cms x 5cms x 2.5cms.

A useful accessory to the Velleman 'K7000' injector/tracer, or and other make come to that.

Happy fault tracing!
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Old 13th Aug 2017, 12:44 pm   #3
David G4EBT
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Pic two in my second post, showing the circuit, tagboard and front panel of the probe box all on one page, were rather small. To make them a bit clearer, I've attached three separate pictures. (Incidentally, no originality is claimed for the circuit - such circuits and many more are widely available on internet. The tag-board/box are all my own work though!).

Hope that's helpful.
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Old 13th Aug 2017, 1:01 pm   #4
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Hi David, has anyone tried adding a squarer circuit to the output of sinewave for harmonic rf injection. Cheers Billy G7LER
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Old 13th Aug 2017, 1:15 pm   #5
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

The RF detector would work better in the probe head, otherwise depending how long the cable is from the probe to the detector box, the cables capacitance could seriously kill the input at, and the output from, an IF transformer...

Lawrence.
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Old 13th Aug 2017, 1:31 pm   #6
David G4EBT
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Thanks for reading Lawrence, and for your helpful comments - quite right of course - the shorter the lead, the better. I'm just waiting for a metal RCA male plug on which I can mount a probe to plug directly into the into to the probe box. I'll update the thread when I've done that. Also, a short (15cm) phono 'patch lead' to connect the box to the K7000, but the practicalities of convenience of use make lead lengths a bit of a compromise.
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Old 13th Aug 2017, 1:40 pm   #7
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Back in the day we used to use a discarded aluminium pickup cartridge container (cylindrical) to build the detector in or sometimes a cigar tube, the injector was a simple two transistor multivib dirty square wave job full of harmonics job so it could be used to inject at the IF and RF circuits

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Old 13th Aug 2017, 1:52 pm   #8
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

You can certainly build combined injector/tracers, but I've always thought it makes more sense to keep them separate. I built a two transistor injector last year to remind myself how easy it was - just a couple of junk box silicon transistors and a few other bits birdsnested together in an old remote control case.
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Old 13th Aug 2017, 2:36 pm   #9
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

The old Multicore Solder came in an aluminium tube, ideal as it could
accommodate the old half-width PP3. Later we fitted silicon transistors,
and cause annoying patterns on channel B1 on the telly.
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Old 13th Aug 2017, 3:26 pm   #10
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Beautifully finished project as per your usual standard David. I find that the tone generator and amplifier one of the most useful bits of test gear at work for fault finding on telephone lines. Plug it in at the subscriber end, and you can instantly find the pair you want from the hundreds in a typical cabinet by the presence of the tone. When you find the pair, short them together. If the tone disappears, you have a good loop. If you can still hear the tone, one leg is open circuit. Our tracer amps only have one input connection, no "earth" as such. The tone genny has two connections, output and earth. The very important thing...it fits in your pocket so no lugging large items around with you.
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Old 13th Aug 2017, 4:06 pm   #11
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Like many I guess, I've made any number of injectors over the years - the one in the yellow container is a Woolworth's icing syringe which I made as a teenager in 1957 - the other is in a felt tip case with a push button at the end and a darning needle for a probe. One is closer to a square wave than sine, the other can't make its mind up what it wants to be! Really, whilst not of course seeking to inhibit discussion, I hadn't meant to spark off a debate about a range of homebrew options - simply to say that for those with limited skills and experience, the 'K7000' is a quick and easy to build well-proven kit with all the PCB mounted components to hand. (Hence, the title of the thread).

The main point I wanted to get across is that so many inexperienced enthusiasts seem to flounder about making subjective judgements about what might be stopping a set from working and for example, start replacing all the paper caps willy nilly thinking that might make the set burst into life. They'd stand a much better chance of success by adopting a methodical objective approach to fault diagnosis, disturbing things as little as possible to trace the fault(s). In that regard, in my view, signal injecting/tracing can play an important part in that and I'm surprised that it seems not to be the norm.

The best example of jumping to erroneous conclusions that I can think of is with Bush TR82s with AF117 transistors.

The subjective assumption tends to be: 'dead set must = tin whiskers'. A signal injected at the volume control slider will show if the audio stage (reliable Mullard OCxx transistors) is working. If so, the tracer can be applied to pin 6 of IFT3 - the input to the OA90 detector diode and if signals are heard there, but not at pin 5 of IFT3 (the output side of the diode) this will show that the diode is duff - a common fault. The diode is tucked away inside IFT3 can and a new diode across the IFT pins may effect a repair in minutes with the duff one left inside the can. If there are no signals at the input to the diode, the tracer/injector can be used on each stage to discover just where the fault does lie, and yes, it could be AF117 transistors, but discovering that will be based on an objective approach not on the subjective ''SWAG' technique' ('Stupid Wild Ass Guess').
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Old 13th Aug 2017, 5:58 pm   #12
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Since the input to the RF probe circuit should be short/low capacitance and the 'mode' switch is bulky and hard to fit into a probe, I wonder if it would be better to make separate RF and AF probes, the former having the diode always in-circut.
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Old 13th Aug 2017, 6:59 pm   #13
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

In practice you can just leave the diode in circuit permanently.
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Old 13th Aug 2017, 7:33 pm   #14
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Thanks for reading the thread and taking the trouble to comment Tony. I've made and used separate AF/RF probes, but thought it might be a bit fiddly for newcomers to fit the components of an RF probe into a felt tip pen case, film canister etc. I thought that a combined probe might be easier to construct. For anyone who might wish to make separate probes, I've attached some circuits.

I also designed a PCB to make a compact automatic AF/RF probe which doesn't need a switch but I haven't got round to building it yet as I have a lifelong trait of flitting from one project to another, rather than single-mindedly focusing on one project before I start another (and another, and...!). It drives everyone nuts including me.

An explanation of how the automatic probe works can be found at this link:

http://circuitswiring.com/automatic-af-rf-probe/

However, the circuit seems to have originated here:

http://www.lh-electric.net/

(An interesting site by the way).

The first pic below shows separate AF & RF probes. The AF one to be used with the injector as well as for AF tracing, simply consists of a DC blocking cap, (albeit one is built into the K7000 Kit which is the subject of this thread).

The second pic is the circuit of the auto AF/RF probe and the third pic is the PCB that I designed for it, which I'd probably house in a short length of 22mm plastic conduit and would turn plastic bungs for the probe end and the output end of the tube.

The last two pics are of RF probes I've built and use - one in a felt tip case, the other in a pill container.

Hope that's of interest.
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Old 13th Aug 2017, 9:56 pm   #15
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

The one and only time I needed an injector for radio work, I used a CMOS oscillator , set to oscilate at audio, and looked for the harmonics at RF .I'd bought two AM/FM tuner boards from the junk box at a shop ( reasoning I could make one out of two for my music centre project). For a HR job it worked reasonably well . As for tracers- I've made a useful tone tracer for telecomms from a 386 .on full gain it will detect an audio tone in telecomms cables from a short distance. ( Source -oscillator made from two 555 to give a modulated tone)
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Old 14th Aug 2017, 1:01 pm   #16
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

Quote:
Originally Posted by Restoration73 View Post
The old Multicore Solder came in an aluminium tube, ideal as it could
accommodate the old half-width PP3. Later we fitted silicon transistors,
and cause annoying patterns on channel B1 on the telly.

Bit naughty using 9V on a silicon multivibrator without emitter diodes.....


MAXIMUM RATINGS
VEBO Emitter-Base Voltage :
BC546/547 6V
BC548/549/550 5V

But probably not enough power to do any damage.
VV
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Old 7th Sep 2017, 6:18 pm   #17
David G4EBT
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Default Re: Velleman 'K7000' Signal Injector/Tracer & RF/AF Probe

In pic 2 of post 14 above I showed a circuit for an automatic AF/RF probe which doesn't need a switch, and in pic 3, a component layout and PCB that I'd designed for it. I went on to say "I haven't got round to building it yet as I have a lifelong trait of flitting from one project to another, rather than single-mindedly focusing on one project before I start another (and another, and...!). It drives everyone nuts including me".

In a modest attempt to redeem myself I have now etched a PCB and built & tested the probe, which I've just tested on a working Bush TR82 using a Velleman 'K7000'. Happy to say that the probe works fine in all stages, so that's another stalled project - if only a modest one - seen through to a successful conclusion!

I've attached some pics of the construction of the probe. The first pic is the etched and drilled PCB, the second is the completed PCB, third is the probe end, forth the flex end, and the last is the completed 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. Really, a PCB is quite unnecessary - the few components could simply be soldered together, tucked in the tube, and would work just as well, but I'm a bit borderline 'OCD ish' about neatness and not into 'dead bug', ugly style' or whatever, even when stuff is hidden away.

If the probe is to be used on valved equipment, both C1 & C2 need to be at least 400WV. (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.

Hope it's of interest and maybe some use.
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