TM 808 White's

[Skippy]

I've looked into what frequencies these ceramic resonators have as "standard" values, in the 420-430 kHz range. They are:
420 kHz : not common, obsolete, used in remote controls, divide by 12 -> 35 kHz
421 kHz : very rare
425 kHz : seems unavailable
426 kHz : rare, but available
429 kHz : common, remote controls, divide by 12 -> 35.75 kHz, a near-enough match to 36 kHz
432 kHz : common, remote controls, divide by 12 -> 36 kHz exactly

I'm thinking that 429 kHz would be a sensible choice as a "primary" frequency, and the choice of 432 kHz or 426 kHz as secondary options. These two alternatives are 0.7% above and below the 429k frequency. This would make the detector operate at 6703 Hz ( and +/- 0.7% ).
Regarding availability:
429k and 432k are pretty easy to find.
A UK seller, Continental Electronics, has 429k, 432k,( and two different 420 kHz types ). His shop :

https://www.ebay.co.uk/itm/255143325489

https://www.continentelec.co.uk/

The 426k part is available from Cricklewood Electronics in the UK:

https://www.cricklewoodelectronics.com/D426.html

There's this German seller, who also lists 420k, 429k :

https://www.williges-elektronik.de/426-khz-keramik-resonator.html

Running the detector at 6703 Hz instead of 6590 Hz means you will need 3.5% less tuning capacitance on the coil ... or 3.5% less inductance in the coil windings.

[berreizeta]

Received xtal 25.6Khz

Aliexpress, I have found a seller who has offered me 420Khz ceramics for sale:

https://es.aliexpress.com/item/10050....0.0.7c4a7a9dn 3dXuO&mp=1&gatewayAdapt=glo2esp

[Skippy]

For info, here is a technical guide to using ceramic resonators. It includes suggested component values for a simple 4000-series inverter-based oscillator. https://ecsxtal.com/127-ceramic-resonator-principles/

[Skippy]

If you built your own oscillator / divider using the 3.2768 MHz crystal, it would be powered from the 15V supply, so it would not need additional voltage-shifting transistors etc to connect to the existing circuit ( which your module would need ).
It would also work with either the NAND gate oscillator circuit, plus a '4024 divider ....
.... or with the '4060 oscillator / divider chip.

An additional idea:
If you layout your PCB so you can select between two different Q outputs from a divider IC ( for example with a wire link ), you would have the option of using a 3.2768 MHz crystal ( to make 25600 Hz and then -> 6400 Hz ) or .. use a 6.7458 MHz crystal ( to make 26351 Hz and then -> 6588 Hz ).

[berreizeta]

3.2768 Mhz easy to find :

[Skippy]

"The '4060 I'm seeing is less stable than a 7555 timer" The '4060 just has an inverter at the heart of it's oscillator. It's never going to be any good used as an R-C oscillator. But ICL7555 CMOS timers are pretty rubbish, too, don't use them. By far the best of the "555" devices are the LinCMOS parts, like the TLC555. They are fast, low power, don't get warm and drift about. They even have reasonable output current capability ( pin 3 ). The maximum supply voltage is lower than the bipolar ( NE555 ) version, worth remembering.
By the way, the 25600 Hz crystal module is likely to be a 3.276800 Mhz crystal, divided down by 128. This crystal freq is widely used in timing circuits, if you divide it down more, you get 100 Hz, useful for decimal timers, like stopwatches.

[berreizeta]

At the moment I don't have anything else to "play",

I can't get the 420, 429 is easy to get and is another viable option

The 4060 I'm seeing is less stable than a 7555, is ruled out

I am waiting for the 25600 crystals, which for me is still the best option 25600/4 = 6400 Hz

I keep doing tests with the coils but I have doubts about the values: 175mH for RX and 645uH for TX ?

[Skippy]

I've just discovered these ceramic resonators are also made for 426 kHz, intended for TV remote control use, again. So that's another alternative option.

[Skippy]

As an additional thought: you could make the oscillator selectable between 420 kHz and 429 kHz, for example by using a 3-pin header, and a jumper link that you place in one of two positions. Use either 2mm pitch or 0.1 inch pitch, depending on space available.
This could be useful if you are having interference problems - sometimes a small change in detector frequency is enough to quieten it down.

[Skippy]

Gotta say, I'm not sure what you're doing with the '4060.
The idea of the 4060 ( or another 4024 ) was to divide down the signal from a STABLE high-frequency source, such as a quartz crystal ( ideal ) , or a ceramic resonator ( OK, but not so accurate or stable ). These all run at 100 kHz to 10 MHz, so need plenty of dividing down to get 26370 Hz ( = 4 x 6.59 kHz ). The 26370 Hz is fed into the 4024 divider, to make 6.59kHz, and other lower multiples for audio tone generation etc.
I suggested two possibilities: a 6.7458 MHz quartz crystal, divided down by 256 gives 26351 Hz, and a correct coil frequency of 6.588 kHz;
or a ceramic resonator of 420 kHz, divided by 16 to give 26250 Hz, and a detector frequency of 6.563 kHz. A 429 kHz resonator would also be close enough.
Both of these options should work with the '4060 chip, when operated at 15 Volts supply, the same as all the other logic IC's in the detector. The resonator will definitely work, the 6.7458 MHz crystal is getting towards the limit for this IC. ( IC's vary; different manufacturers, different age of the part, lots of unknowns ).
Other circuit options include:
Using the existing two NAND gates to make an oscillator with the 420 kHz resonator or 6.7458 MHz crystal. Then use a seperate divider chip, either the '4060, or a '4024, to create the 26370 Hz frequency. This would be OK for the 420k resonator, but the 6.74.. MHz crystal may be too high for a NAND based oscillator.
What you should NOT be doing is messing around with crappy R-C oscillators. There are ways to make decent R-C oscillators, but a 4060 inverter based one is not a decent one.

My honest suggestion is to use the existing two NAND gates to make a 420 kHz resonator oscillator / buffer. Feed the output of that into a CD4024 divider IC. Use the Q4 output ( divide by 16 ), and feed that into Input ( pin 1 ) of the '4024 in the existing circuit.
Sure, the 420 kHz resonator is a bit tricky to get ( it's obsolete, but was very common ), so you may have to settle on the 429 KHz version, which is much more easily found.

[berreizeta]

I have finally managed to make a 4060 that work. (10 units in the trash, bad or fake?...)

With 10k and 220p it gives me 6600 Hz

[Skippy]

That's quite funny, and shows how troublesome sound cards are ... with few exceptions, unfortunately. Your scope trace clearly shows the full cycle of 3.125k, as quarter-cycle; half-cycle; quarter-cycle. Then inbeween is absolutely nothing, where in fact there are 8 cycles of 25k. If I recall, it's a quarter-cycle, then 15 half-cycles, then the final quarter-cycle. Your trace just shows "ringing". It's not ringing, of course. If you take a square wave, and remove the 7th, 9th etc harmonics, you are left with a waveform that only has fundamental, 3rd harmonic and 5th harmonic. This looks at first glance like terrible ringing .... but notice it starts "ringing" before the big 'edge' , not just after one.
Many PC's today have "super" sound cards, that go to higher frequencies. Usually called High Definition Sound Cards, or HD Audio, they can sample audio up to 48 kHz, (I think).
What is super rare is audio inputs that go down to DC. They normally have very sharp cut-off digital high-pass filters at about 16 Hz, that can't be 'turned off', plus electrolytic caps to AC couple signals, for example from the pre-amp to the main ADC input.
I have an external USB sound box that apparently goes to DC with some minor modifications .. I just haven't tried it out, yet.

[berreizeta]

Yes !, BBS Excalibur Spectrum, from 1500Hz, 3000, 4500 ... 21Khz (Sound card no give more..) :

[Skippy]

Excal's work with standard BBS waveforms, so nominally 25 kHz and 3.125 kHz, a ratio of 8 to 1. There are 8 cycles of 25 kHz, followed by one cycle of 3.125 kHz, then repeat. The frequency shift options vary across the BBS machines, I think +/- 8% is possible for some of them, I don't know what shift the Excal is capable of.
If you're considering a change of frequency ( lower ) for the 808, I suggest halving the freq to 3.325 kHz (nominal ). This can be done by using a different divided output from the crystal / divider circuit, but leave the audio tone generation part alone. ( Obviously, many other aspects of the circuit will need changing, too ... )

[berreizeta]

I am using the Spectrum Analyzer directly without the attenuation since the coils work with millivolts.
With the Analyzer I have verified that the Cscope Metadec works at a frequency of 16,480 kHz and the Minelab Excalibur at 3,050 Hz !!
For me, it's one of the best detectors I've ever had.
I was thinking how the TM808 would work at about 3,000Khz....