little max 2870 1w transmitter

[radionortheast]

Thought i’d share something I made, a little tiny 1w tx using the 2870

Photo on 07-11-2024 at 09.22.jpg

Photo on 07-11-2024 at 09.20 #3.jpg

[shuffy]

Is that an FM rig Northeast? How are you modulating it? And what's on the veroboard on left hand side?

[FMEnjoyer]

Clever stuff, must know a few things to put that together.

[jvok]

When will we be hearing it on air then?

[radionortheast]

Is that an FM rig Northeast? How are you modulating it? And what's on the veroboard on left hand side?

Yeah its fm, there is an audio buffer op amp the smaller board, this makes it pass the full frequency range, its like an audio amplifier, you have a direct connection at the output with nothing resistive, at the input is were you have your variable resistor, ajust the input level without effecting the audio, you can get the bass and pass the stereo too, the larger board is a stereo encoder.

Clever stuff, must know a few things to put that together.

Thanks it did take a lot of work, alot of trial and error, I liked that it was easy to get the parts, I do like pcs, you don't know if the tax remittance has been paid

When will we be hearing it on air then?

Maybe you hear it already its on daily, I mostly listen nearby, when I first made it i’d listen to it for hours as I couldn’t believe the sound, I don’t have much interest in listening far away.

[BlackBeard]

Thought i’d share something I made, a little tiny 1w tx using the 2870

Thanks for sharing! Can you maybe post a photo of the op amp circuit? Where exactly did you connect it?

[BlackBeard]

Oh yeah, and did you check the spextrum?

[marqueemoon]

Cool project! I can't claim great skills or anything, but I do enjoy fiddling around with this sort of thing.

[shuffy]

Yeah its fm, there is an audio buffer op amp the smaller board, this makes it pass the full frequency range, its like an audio amplifier, you have a direct connection at the output with nothing resistive, at the input is were you have your variable resistor, ajust the input level without effecting the audio, you can get the bass and pass the stereo too, the larger board is a stereo encoder.

Nice work northeast. Pricey way of doing it but I know that wasn't the point of your project! I read that there was no modulation input as such on these modules but as usual, I didn't look at the MAX2870 datasheet and assumed (incorrectly) that it was one of these direct synthesis things! That's why I asked how you're modulating it. I now see that it's a regular PLL synth on the one chip allowing an external loop filter and I expect you've gone in with your mod around here somewhere. I'm guessing you haven't changed the supplied loop filter components which could be one reason you need the amp to get closer to the required frequency response and no doubt that's why others are asking to see your amp circuit. Is the TX staying on frequency when you modulate?

[radionortheast]

Thanks for sharing! Can you maybe post a photo of the op amp circuit? Where exactly did you connect it?

This is the little circuit you need, I came up with this combination of capacitors it stopped the rf hum, the resistor keeps the voltage at a simliar level to the 2870, a 5v dc to dc converter can be used instead of the resistor.

littleopampcircuit.jpg

With the one with the touch screen display it was a capactitor just below r5, next to the main chip, alot of these do work in a simliar way some of the components to control the pll are on the board acround the chip.

connectionfor2870t.jpg

With the koon, one with the oled display, it is connected on the left hand side of c33.

koonconnectionpoint.jpg

I have another different one that users a touch screen display, it seems they’ve changed design, the font is different not as good. I did think it might not be possible, I was able to find a connection point, it requires taking out the 1k resistor its too low needs to be higher, the pll tries to correct the audio. Then it would be connecting up the side which is closest to the chip to get audio in high lighted in the picture. So the 2871 may work too, hasn’t been testd out yet.

connectionpointfornew2870.jpg

Its best to use the thinnest wire you can find for soldering, old usb cables are good for this, tape it down with small bits of sticky tape, put a blob of solder on the end of the iron, go in fast. I used a big magnifying glass, managed to make the connection, better using a usb electronic microscope.

The original configiration was using a 100uf capacitor without the op amp, with the amplifer been powered by 12v and the 2870 on a 5v dc converter connected to the same supply which was alot simplier, the stereo encoded using stereo tool an old laptop. It did work well enough, the audio lacked abit of bass, almost every other audio source wouldn’t work.

Abit later on it was configured with the op amp, stereo encoder and amplifier all powered by 12v supply, 2870 has to be powered by its own 5v supply, as I found that they couldn't all be powered from the same 12v supply, without undesired operation. The amplifer does seem a good one, it barely gets warm, i’ve even had it were just the signal isn’t connected at the output it wouldn’t spur. There was a situation when it wasn’t connected up I could barely hear it beyond 50 meters, I was suprized it didn’t break.

I was planning to put in a case then have the separate power suppiles inside, the amp is meant to work at 15v provide 2.5w, beable to switch the rf amp between 12 and 15v. I feel the 1w is enough

Cool project! I can't claim great skills or anything, but I do enjoy fiddling around with this sort of thing.

It was amazing that it worked, I bust the original amp, when disconnected the c1971 amp in an 8066, tried connecting it up, this amplifier is alot more robust. I tried afew boxed signal generators looked cool but put out spurs over other fm frequencies, you couldn’t connect audio. I came across this, it was abit of a fluke, it didn’t have spurs, you could connnect up the output using an sma lead to the amp it just so happened it was the perfect match for the amplifier. So you could get the 2870, amp, make a filter, you’ve got something that puts out a clean carrier, putting audio onto it was the problem.

Nice work northeast. Pricey way of doing it but I know that wasn't the point of your project! I read that there was no modulation input as such on these modules but as usual, I didn't look at the MAX2870 datasheet and assumed (incorrectly) that it was one of these direct synthesis things! That's why I asked how you're modulating it. I now see that it's a regular PLL synth on the one chip allowing an external loop filter and I expect you've gone in with your mod around here somewhere. I'm guessing you haven't changed the supplied loop filter components which could be one reason you need the amp to get closer to the required frequency response and no doubt that's why others are asking to see your amp circuit. Is the TX staying on frequency when you modulate?

Yes, its lucky some of the components that control the pll exist outside of the chip. Its usually resistor close to were you’d connect audio in were the value is too low, the pll is trying to correct the audio. I was able to do with the koon, solder the thinnest wires, put a higher value resistor, it has to be very high mega ohms, something like a 100k resistor will cause the pll to unlock at high volume. So I guess you start off with something were the pll is trying to correct the audio, end up with something that is the opposite.

[radionortheast]

Oh yeah, and did you check the spextrum?

At this power level an 8066 could be used, I wanted to extra sure, it wouldn't bother anyone, its why I made it, to try my own stereo encoder. You can hear other fm stations below the aerial, there is also visual confirmation, with the peaks of them still present when it is on.

[shuffy]

Yes, its lucky some of the components that control the pll exist outside of the chip. Its usually resistor close to were you’d connect audio in were the value is too low, the pll is trying to correct the audio. I was able to do with the koon, solder the thinnest wires, put a higher value resistor, it has to be very high mega ohms, something like a 100k resistor will cause the pll to unlock at high volume. So I guess you start off with something were the pll is trying to correct the audio, end up with something that is the opposite.

Yeah I thought it would be something like this you'd done. It looks like the time constant of the filter as shipped is pretty low so you'll have had to get round that by whatever means. If your arrangement sounds good with decent stereo, the loop is staying in lock and you're not overdeviating then you've hit the goal!