Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Radio Equipment => Topic started by: dodgy geezer on August 10, 2025, 11:10:41 am
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I picked up what I thought was an old Micron combo the other day, and found that it was a part-finished kit. The RF board and the encoder board look fairly complete, but I would like to check them before I put power through.
The problem is that they look to be very early examples. The encoder board, for instance, does not have a 4017 IC chip. I have circuit diagrams for later Microns, but nothing that early. If anyone has a circuit diagram for a Micron encoder board without a 4017 chip, or an RF board with no Toko inductors, I would appreciate a copy....
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Some descent photos will suffice. I'm familiar enough with that era of radio.
All the emitters of npn transistors are tied to 0volt and you don't have to worry so much about exceeding a voltage limit.
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OK - if you want to have a go!
What turned up was a Micron receiver (looks like 27mHz AM, though no crystals), a Tx box assembled with sticks and unsoldered, an aerial which looks like a commercial radio receiver one (no Micron centre load), two PC boards populated and soldered apart from a few components which were threaded through and the wires bent back, and a box of oddments including a transformer, several resistors and a diode (parts for a charger?)
The RF board has +, - and signal wires attached, also an aerial-out lead (unscreened - later Microns were screened) pictures below. It has one PCB hole unpopulated. There are two copper sections on the board which look as if they are intended to take a wire connection.
The encoder board has no wires attached. It is populated with 4 'half-shot' transistors for four channels (no 4017 chip), with space for a further 3 channels. There is some electronics at one end that I do not recognise. I can work out the + and - lans, and the norcim site has a similar example encoder which I might be able to infer the stick pot connections from (though there are no obvious points on the lans to fix a wire...). There is one unpopulated hole in the electronics at one end, covered by a link.
I was planning to power up the RF board, connect to an aerial and see if I get any radiation. Then I was going to wireup/power up the encoder and poke around with a scope until I found a stream of pulses. And then connect the two... But I could really do with a circuit diagram to see if there are any missing/incorrect components...
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The extent of what I hoped to point out is what you seem to already understand.
As well as the half-shot transistors there is an astable timer. A recent thread about restoring a Digifleet transmitter showed one using inverting logic gates.
The ring circuit is completed with a fixed or variable frame, and the output to the rf board will be obvious if you sketch one stage. Without referring to an old Tx or diagram, I think that will be npn collector 4 to 7.
I'll look out an old set & search for the relevant thread about NiCds.
I could further add, which you may have guessed, that the polyester capacitors are the stable timing caps.
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https://www.modelboatmayhem.co.uk/forum/index.php/topic,70532.msg764983.html#msg764983 (https://www.modelboatmayhem.co.uk/forum/index.php/topic,70532.msg764983.html#msg764983)
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The extent of what I hoped to point out is what you seem to already understand.
As well as the half-shot transistors there is an astable timer. A recent thread about restoring a Digifleet transmitter showed one using inverting logic gates.
The ring circuit is completed with a fixed or variable frame, and the output to the rf board will be obvious if you sketch one stage. Without referring to an old Tx or diagram, I think that will be npn collector 4 to 7.
I'll look out an old set & search for the relevant thread about NiCds.
I could further add, which you may have guessed, that the polyester capacitors are the stable timing caps.
Alas, I suspect that you think I understand more than I do! I can see that there is a multivibrator feeding pulses into a series of transistors which will switch at variable times depending on the relevant pot setting. But what happens to the signal after that I don't know - I suspect that the electronics I don't understand have something to do with that... But, in the absence of any circuit diagram, does my proposed way ahead sound sensible to you? I suspect that the original circuit worked off 9.6v nominal, and was proposing to use 7.2v NiAmh, since I don't have a bench power supply...
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Disregard my earlier description as I was working from memory.
Fig 76 in scimitarjohn's post shows the collector outputs fed into 4001 logic gates but your much simpler circuit will probably use diode logic.
The function is the same as Fig 76. Each one-shot triggers the next one and also triggers the r.f.
In your board it is possibly done via a diode. So r.f. input should be a copper rail with diodes connecting to each channel's transistor collector.
You'll begin to see a similarity with Fig76 where your colour coded polyester caps equate to Ca and C is a cheap ceramic disc cap.
* I just had a closer look at the photo and can see the 4148 diodes.
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Here is an RC circuit link you have possibly found. http://sm0vpo.altervista.org/use/rc_enc_02.htm (http://sm0vpo.altervista.org/use/rc_enc_02.htm)
The 2 and 6 channel diagrams show the 'wired-OR logic' by common-cathode connected diodes. The output is buffered by a BC557 to make a positive pulse.
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Disregard my earlier description as I was working from memory.
Fig 76 in scimitarjohn's post shows the collector outputs fed into 4001 logic gates but your much simpler circuit will probably use diode logic.
The function is the same as Fig 76. Each one-shot triggers the next one and also triggers the r.f.
In your board it is possibly done via a diode. So r.f. input should be a copper rail with diodes connecting to each channel's transistor collector.
You'll begin to see a similarity with Fig76 where your colour coded polyester caps equate to Ca and C is a cheap ceramic disc cap.
* I just had a closer look at the photo and can see the 4148 diodes.
I can see the diodes along the output rail - that rail leads into the transistor pair in the mystery electronics. A bit of smplification?
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Hi, I made up the transmitter and receiver way back. But although 27Mhtz my set is FM not AM. I also have a Futaba 27Mhtz FM Tx but use the Micron rx as it is far more stable and reliable.
Roy
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I can see the diodes along the output rail - that rail leads into the transistor pair in the mystery electronics. A bit of smplification?
Three copper lands for each pot are adjacent to the holes.
The next task is to identify the output. I use a phone camera to read semiconductor numbers, but you can assume you have two normal signal transistors then determine npn or pnp polarity from connection or by diode testing.
If you place the pcb over a diffuse light source you should be able to sketch the encoder circuit and it should closely match the 6-channel one hosted on altervista.
I'd happily power up the encoder board but not the rf board. I'd look at the Rx components to see what band it was operating in.
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Three copper lands for each pot are adjacent to the holes.
The next task is to identify the output. I use a phone camera to read semiconductor numbers, but you can assume you have two normal signal transistors then determine npn or pnp polarity from connection or by diode testing.
If you place the pcb over a diffuse light source you should be able to sketch the encoder circuit and it should closely match the 6-channel one hosted on altervista.
I'd happily power up the encoder board but not the rf board. I'd look at the Rx components to see what band it was operating in.
To hear is to obey!
I use a head-mounted loupe to read components - by now I need it for resistors as well as semiconductors! The encoder section looks in reasonable nick, though I cannot tell whether the component values are correct.
I enclose a schematic of the 'mystery electronics'. It looks like what I think they call a 'darlington pair' to give the signal a bit more 'oomf', but you can see that the items in red do not make sense, and may be mistakes, or partial assemblies for some added circuitry... I do not know whether the suporting resistors/caps make sense, or why there are two parallel caps on the right hand side, one an electrolytic. The rectangles are points on the copper lands which look as if they could take a connected wire..
Why not power up the RK board with a 27MHz Futaba crystal? The Rx looks like a Micron 27Mhz, but it has no crystal, and I can't tell the band from the components. I assume that the RX board has not been tuned to oscillate, but there is only one adjustable coil, and a little fiddling should get it to radiate something which can be detected using a diode/multimeter...
P.S. Just noticed that the 3906s are PNP, so the arrows on the emitters should be going the other way.... :embarrassed: :embarrassed: :embarrassed:
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That diagram makes perfect sense with the arrows changed to reflect 2N3906.
An Internet search of "capacitor frequency response" will turn up many diagram hits of impedance Vs frequency graphs. The resonace point of electrolytic caps is possibly lower than you think. At high frequency a parallel connected ceramic capacitor has lower impedance and takes over decoupling.
Testing the Rx with a working 27MHz crystal is a logical task. I didn't dabble in rf circuit design enough to recognise what I'm looking at at a finger click. All I can say is the rf board has the wrong copper layout for 459MHz vhf, ferrite & crystal socket.
I'm just overly methodical. You'll have a set that doesn't conform to the revised regulation for 27AM but you won't come to any harm!
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That diagram makes perfect sense with the arrows changed to reflect 2N3906.
An Internet search of "capacitor frequency response" will turn up many diagram hits of impedance Vs frequency graphs. The resonace point of electrolytic caps is possibly lower than you think. At high frequency a parallel connected ceramic capacitor has lower impedance and takes over decoupling.
Testing the Rx with a working 27MHz crystal is a logical task. I didn't dabble in rf circuit design enough to recognise what I'm looking at at a finger click. All I can say is the rf board has the wrong copper layout for 459MHz vhf, ferrite & crystal socket.
I'm just overly methodical. You'll have a set that doesn't conform to the revised regulation for 27AM but you won't come to any harm!
Thanks for that! I doubt that I'll be using it for real - 2.4Ghz is so much more convenient - but I would like to get it completed and functional after around 50 years, and perhaps add some more channels as an exercise. Learning all the time... {:-{
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Well, I attached 4 pots to the board and gave it 5v, which should have been enough to wake it up. And got nothing. No pulses at the output end and, as far as I could tell, no pulses around the multivibrator....
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If you have a suitable resistor, like 10 to 100 ohm you can put that in series for safety as you up the voltage.
If it doesn't burst into life there is something wrong in the circuit or the soldering. It's pretty basic stuff that I've used.
I'd be drawing ( by hand) the astable multivibrator.
Double check the function then centralise pots without desoldering. If the combined length of the pulses > the frame it goes haywire. However I think it's just component values that don't work at low voltage or an error.
I'd be working off a full schematic, double checked!
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Haywire would be nice! All I can detect on the scope is 0v, going up to 5v occasionally as I try each land. Do you think that more volts are needed? It would have been run at 9.6v originally, but the transistors ought to start at about 3v...
A full schematic would be great. Maybe I'll draw one...
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Haywire would be nice! All I can detect on the scope is 0v, going up to 5v occasionally as I try each land. Do you think that more volts are needed? It would have been run at 9.6v originally, but the transistors ought to start at about 3v...
A full schematic would be great. Maybe I'll draw one...
No, the voltage was an accidental misdirection and you should be OK at 5.
The voltages at the astable part will point you right to the problem if you use the diagram in the link below for number reference.
The astable timing capacitor colour codes, diode check of the transistors, and dmm measurements of its resistors is enough.
If there is a component missing it won't work.
https://www.electronics-tutorials.ws/waveforms/astable.html (https://www.electronics-tutorials.ws/waveforms/astable.html)
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I will need to do the schematic. I wonder why there are a pair of what look like diodes in the MV circuitry....
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The two extra diodes conduct to clamp the negative kick to -1 volt.
The clue to what is going wrong will come from the Vc, Vb state at which the oscillator transistors settled at, and if diode tests say the transistors are OK.
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'Sort' of schematic for the MV done. I thought I would copy Terry Tippett's style a bit, and 'draw' the components - but I only managed the ttransistors... I think that the PCB is very elegant. And I notice that I can't spell 'linear'. The transistors pass the diode test...Caps 2 and 3 are actually 0.01, i think - I'll update later
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'Sort' of schematic for the MV done. I thought I would copy Terry Tippett's style a bit, and 'draw' the components - but I only managed the ttransistors... I think that the PCB is very elegant. And I notice that I can't spell 'linear'. The transistors pass the diode test...Caps 2 and 3 are actually 0.01, i think - I'll update later
Notes
-The effect of C4 to C7 (1nF) caps is to soften the switching for rf suppression.
- The extra diodes are series connected to block the -Vbe kick, not anti-parallel to conduct and clamp at -Vf.
That's the alternative way to do the same job.
Remarks
I earlier suggested reading off voltages thinking it always stopped at the same condition where proper circuit function is flipping between two.
The value of C4-C7 is too uncomfortably high, to near to the timing caps for my liking.
If it was me I'd snip these out if the 10nF polyester caps and resistors fit the timing formula.
If it suddenly worked, 100pF is more than enough & there are other ways to soften switching.
So check timing component values.
Still check if it stops in one state to pinpoint a solder error or damaged component.
Snip out the extra suppression caps.
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Some success.... I found a poor ground connection (mine) and up came some pulses. Only two. I tried to upload a scope pic, but for some reason the site wouldn't take it...
Looked at the transistor where the pulses stopped and found a poor solder connection (theirs). Mended it and got three pulses. Can't see anything obviously wrong with the next transistor stage, but there must be something somewhere...
I'm unimpressed with the trimmer pots I put on to do the testing - the sliders seem unreliable. I will get a set of (slightly) better ones. That could be the problem.
I should have a lot of 101 caps somewhere, but it will be a fiddle to replace them, so I'd like to see if I can get that last pulse first... moving forward(with lots of help!!)
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If the astable is working there is no need to change it.
* It's repetition period (frame time) needs to be the maximum width of six channels plus another ~ 2.5msec for a receiver to sync.
Each one-shot triggers the next so you know where to look if your not exceeding the time limit.
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Well, I now have 4 nice negative-going pulses (after hitting the trim pots a lot), and I can vary the three gaps between them smoothly using the first three trim pots. But I do not have a fifth pulse, and the fourth pot does nothing. A pulse arrives at it from the third transistor, but the fourth 4124 does nothing. It passes the diode test, and all the soldering seems fine - I redid it anyway.
I note that the collector of each 4124 passes a pulse to the next. But the fourth 4124 collector is just hanging in the air. All the others are connected to the +ve rail via the 5k pot. Shouldn't the last 4124 also be connected this way? I note that there is a 'spare' 4.7k resistor on the darlington pair schematic which would perform this task for the last 4124 if there were 7 channels. Perhaps connecting the last 4124 collector to this resistor would wake up the fourth channel?
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I think this is the situation....
Yes, for four channels you have five negative going ~10uS pulses at the anode rail so five diodes, four 47nF caps but one of the two astable transistors kicks off the sequence.
You end up with an extra transistor or feed it back into one of the two astable transistors.
I think that makes a fixed frame (say 20mSec for 7ch ) OR a variable frame with fixed syc pulse which needs to be not less than 2.3mSec.
If that is right then you have a decision about redoing the layout and some calculations to do.
This diagram of grouped timing components might help.
I think Futaba did variable frame, Hitec maybe did the other, and so Hitec IPD receivers didn't work with Futaba.- just a thought , and it may be wrong!
One 6channel radio I was given to look at had already been tweaked from 1.3mS to 1.5mS neutrals causing it to crash so one timing resistor needed changed.
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Er... wow! Redesigning a Micron kit is rather beyond my pay grade. It OUGHT to work as specified. I see your circuit, which is rather like the situation I have got. I have put a drawing down below. Are you suggesting adding an extra 4124 - which, I suppose, could be done...
The 4124 outputs a pulse to the signal line as shown through the diode, and also along the line marked in red to the pot for the next channel. Only, for the last 4124, the components marked in red are just missing. There is, however, a spare 4.7k sitting unconnected on the board at the end of the cascaded half-shots, and it looks as if the last 4124 for channel 7 should be connected to this as a sort of termination. If less than 7 channels are populated, I suspect that a fly lead should go to that resistor from the last 4124, as shown in green on the diagram. Does that make sense?
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I connected the green link as above - got another pulse! At that stage my skill (?) with using a scope took a dive, and the trim pot sliders started failling again, making it very hard to see what was going on.
It seems that altering the pots alters the gap between the pulses rather than the pulse width, but trying to investigate that collapses into noise whenever I move the trims. I have ordered a set of standard pots and will see if it gets better when they are connected.
Meanwhile - onto the RF board!
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All this is way over my head, ( I did learn how to repair HiFi and stuff ) but good to see reals skill / knowledge is still very much alive and kicking! :-))
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The skill is all with Invisible - I probably know less about it than you do, Martin. But it's interesting to work on what looks like a new Micron kit that's 50 years old, and it looks like this is an early design that no one has a schematic for, so publishing that will be useful...
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Looks like the encoder is working. On with the RF board.
I wanted to power it up, but as soon as I twiddled the ferrite slug it collapsed in a heap of dirt. So I need to order another one - hope they are standard sizes... In the meantime, here is my schematic drawing..
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Hi DG, I think they must be interchangeable as I do not know if there is a measure of reluctance for them.
By the way I have all the plans for building the 6 ch. 27Mhtz FM Tx and Rx, which I did many years back.
Roy
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https://en.m.wikipedia.org/wiki/Permeability_(electromagnetism)#Relative (https://en.m.wikipedia.org/wiki/Permeability_(electromagnetism)#Relative)
μr
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Hi DG, I think they must be interchangeable as I do not know if there is a measure of reluctance for them.
By the way I have all the plans for building the 6 ch. 27Mhtz FM Tx and Rx, which I did many years back.
Roy
Well, we shall find out!
I now have 3 Micron sets here - all different. I did not realise that there was so much variation - I have a 1977 AM version, a 1979 am version converted to FM, and one of the unusual FM red ones with slider trims - unknown date. All seem to have different electronics. As well as several sets of blue construction notes, I have the following blueprint drawings:
am receiver assembly jul77
charger unit assembly aug77
fet/cmos am receiver apl 78
am/fm transmitter circuit jul 78
assembly pl-7d am/fm tx oct 78
open gimbal stick wiring oct 78
I was going to make a package and send it off to SingleChannel, but I had no 1977 AM Tx circuit - which I am having to draw up at the moment... once I get it working...
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https://en.m.wikipedia.org/wiki/Permeability_(electromagnetism)#Relative (https://en.m.wikipedia.org/wiki/Permeability_(electromagnetism)#Relative)
μr
These look likely
DG
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All I can tell you is you can exclude the lowest μr materials.
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All I can tell you is you can exclude the lowest μr materials.
I'd be happy to if I knew what they were At the moment simply getting the right (or similar) size is proving tricky... :((
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I'd be happy to if I knew what they were At the moment simply getting the right (or similar) size is proving tricky... :((
I think I see see a ferrite cored transformer from your schematic and your photo.
Here is an inductance calculator for a solenoid that you can key in your six Neosid optional values of μr. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indsol.html (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indsol.html)
The coil length is that of the copper winding of N turns, not the ferrite length. The coil diameter is of the ferrite core because the magnetic flux in the ferrite predominates over the bit in the air and the former, where μr =1
The Neosid catalogue shows six grades of ferrite material so you'll get six values of specific inductance ( Lsp for one turn.
If you had a solenoid of 8 turns, and high μr permeability grade of 10k , self inductance of the 8 turn will be 64,000 Lsp.
So matching size isn't your only problem, and it isn't the biggest problem.
If I had two or three pieces to glue together I'd be in with a chance of finding material grade from an inductance measurement.
Realistically, you've got a ton of old published circuits to track down and digest unless you have a radio ham friend.
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I keyed in some values to see the ferrite grade would be low μr. Show's how much I know!
High μr came to mind because I've used ferrite material, rather than iron, for high Lsp in larger, medium frequency transformers.
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......
So matching size isn't your only problem, and it isn't the biggest problem.
If I had two or three pieces to glue together I'd be in with a chance of finding material grade from an inductance measurement.
Realistically, you've got a ton of old published circuits to track down and digest unless you have a radio ham friend.
...and tracking circuits is where I came in.... Though I suspect that there will not be a precise specification of the inductance, even on the original design. A coil former which was standard at the time would probably have been used. I have the dust if you want to compact it, but the largest piece is 3mm dia x 2mm!!
Unless the different ferrite specifications have an impact on the saturation time-to-rise and the timing is therefore impacted, I suspect that they will mainly influence the strength of the electro-magnetic response. If this is the case, a different grade slug might simply need screwing less or futher in? I have ordered a Toko 10k slug of about the right size (very slightly smaller in diameter, from my measurements) and we will see what that does.
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You still need to make a 27MHz parallel resonant oscillator with a 15pF capacitor.
F29 has a μr around 10, about the same as the 10mm dia rod inside mw/lw/sw AM radios. An F25 core in the same wound former used for F29 material results in ten x5 times the inductance.
The hyperphysics calculator uses the k constant rather than μr.
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You still need to make a 27MHz parallel resonant oscillator with a 15pF capacitor.
F29 has a μr around 10, about the same as the 10mm dia rod inside mw/lw/sw AM radios. An F25 core in the same wound former used for F29 material results in ten times the inductance.
The hyperphysics calculator uses the k constant rather than μr.
Hmm...quite a difference, then? Even though the Neosid pamphlet shows that F25 and F29 overlap in suitability for 27MHz...
Calculations are a bit moot, however, without any base data to work on. In the absence of that, I went looking for a core that would fit. I made the original slug to be 8mm length, 3.73mm diameter over the screw threads (probably nominal 3.75mm). The best I could find was a Toko 10K Ferrite Screw Core Slug L=8mm Dia=3.65mm 1g ET08. ET08 does not seem to be enough to identify the item.
I only have a simple diode/cap/multimeter to test for RF, so I'm not too sure how to determine whether it works or not. I suspect that I might get some RF reading from the aerial even if there was no slug in the coil at all, since this component just seems to be impressing the pulse signal on the carrier wave...
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Hmm...quite a difference, then? Even though the Neosid pamphlet shows that F25 and F29 overlap in suitability for 27MHz...
Outside the suggested area is not impossible, but less practical due to parasitic effects.
You see the cost, size and performance penalties and design constraints when you've had to design examples. Capacitor dielectric material is analogous to core material and easier to understand the same effect where you can have the same overlapping values in capacitance with different dielectrics.
Calculations are a bit moot, however, without any base data to work on. In the absence of that, I went looking for a core that would fit. I made the original slug to be 8mm length, 3.73mm diameter over the screw threads (probably nominal 3.75mm). The best I could find was a Toko 10K Ferrite Screw Core Slug L=8mm Dia=3.65mm 1g ET08. ET08 does not seem to be enough to identify the item.
I only have a simple diode/cap/multimeter to test for RF, so I'm not too sure how to determine whether it works or not. I suspect that I might get some RF reading from the aerial even if there was no slug in the coil at all, since this component just seems to be impressing the pulse signal on the carrier wave...
The quartz crystal is just a capacitor with a known, high-Q resonance points.
It's only one part of a unity-gain tuned circuit that manages to sustain oscillation.
Internet hits will describe the function, but you have a transitor, crystal and tuned LC.
You can find L through an improvised waveform source, dropper resistor and known-value substituton inductors.
XL= 2.Pi.f.L so, ideally, you'd want a bit higher than 3kHz from an esc.
You could rewind primary and secondary on an open bobbin cored inductor for scrap breadboard experiments. 100 microhenry with ten turns is LSP OF 1 microhenry.
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A lot of work, building a signal generator! Perhaps it would be easier to use a circuit simulator - something like LTSpice? I assume there is one where you can just alter the inductance...
Looking at the Micron set-up notes, it does not seem that the coil setting is critical. The notes say:
1 - wind the core nearly out
2 - set up an RF detector multimeter and power the Tx on - there should be no RF
3 - wind the core in until you get an RF signal
4 - wind the core in 1.5 turns more for safety
5 - turn the Tx on and off a few times to check that the circuit starts properly...
which suggests to me that so long as it's oscillating the set-up is OK?
P.S. CircuitLab looks fun...
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A lot of work, building a signal generator! Perhaps it would be easier to use a circuit simulator - something like LTSpice? I assume there is one where you can just alter the inductance...
Looking at the Micron set-up notes, it does not seem that the coil setting is critical. The notes say:
1 - wind the core nearly out
2 - set up an RF detector multimeter and power the Tx on - there should be no RF
3 - wind the core in until you get an RF signal
4 - wind the core in 1.5 turns more for safety
5 - turn the Tx on and off a few times to check that the circuit starts properly...
which suggests to me that so long as it's oscillating the set-up is OK?
A 27 MHz crystal is actually a 9MHz crystal, btw. The resonator covers the whole band but the turns they refer to means a mm of lateral movement. You'll find out how tricky it is to get any range at all.
For an improvised sig gen I used a 555 circuit to hand, but all you need is a frequency high enough (eg esc at 3kHz) for XL to be high enough compared to your dropper resistor.
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Are you saying that the resonant circuit will probably work over a wide spread of inductances, but will not provide much power unless it is optimised quite precisely? I can't see how that matches with the circuit notes, which seem to indicate that so long as the oscillator is running the precise core position is not critical.
Doing a range check would give a definitive answer, of course. If this Toko 10k produces detectable RF I am inclined to put the PCBs into the Tx and see if I can get the set to talk to a receiver...
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The ferrite core arrived this morning, so I put it in and powered up the RF board with a Futaba AM crystal. I used the diode/cap/multimeter circuit to detect RF.
The first thing that I noticed was that I seemed to detect some mV on pretty much any bit of metal, which varied along it's length, making any precise measurement difficult. Next I tried a working Micron Tx, and could definitely see a jump when I turned that on.
Then I tried the RF board, with a crocodile clip to a partly extended aerial. I was running off 6v rather than 9.6v, but I could detect a small jump on power-up, even when the slug was almost completely out. I got a bigger reading (though still much less than the working Tx) as I wound the slug in, and there seemed to be a slight peak at a point where the slug was 1.5mm out of the coil.
I'm not sure how to interpret this. The inductor seems to be working, and the low output can be explained, at least in part, by the lower voltage and non-resonant aerial length. Since this is the only slug that remotely fits the coil that I can easily source, (it's a little bit loose, but the threads do connect) I'm inclined to assemble the whole Tx and do any further mods on the complete system...