Who makes these?

[Shedbuilt]

There's been a lot of talk about the SRF261, which seem to have appeared cheaply, from a number of sources. The package, power, and voltage look similar. A bit of a stab in the dark, but could be....

[Shedbuilt]

Agreed on the comments about gate voltage. 1) Yes, the original (assuming it was one of the LM2596s), would have been the adjustable (ADJ). I'd only ever dealt with the adjustable ones before - didn't know there were fixed voltage versions. Easy to say with hindsight, but in cases like this, especially if you're changing for an unknown regulator, try to find a way to disconnect the output before turning it on, then switch it on, and make sure the voltage is sane, before connecting to anything sensitive. Also agree that using a variable supply to test (after changing the FET if it's definitely blown), and starting off at 0, is a reasonable alternative approach.

[mixfm]

bin it, sprog box

[XXL]

bin it, sprog box

Did you buy one of these ? Yes they do sprog but not much. Can only hear it right next to the tx site. Luckily there was nothing on the frequencies it was sprogging on.

[King Croccy]

bin it, sprog box

Did you buy one of these ? Yes they do sprog but not much. Can only hear it right next to the tx site. Luckily there was nothing on the frequencies it was sprogging on.

I've seen them on Aliexpress, is that the 120w or 200w?

I've only ever seen the 60w and 150w Chinese transmitters up close. Harmonically they were fairly clean however if you dropped the power (both were variable), all these spurs started coming out of the noise floor (in band) till it was a dirty piece of crap!

Is it not possible to mod the board to take a different mosfet? I doubt the Chinese will tell you the part they are using. The design is very similar to those "180w kits" that you used to be able to get on Ebay. Although the mosfets they used were not great (mrf186 inefficient, mrf9180 unreliable), it might get you out of jail and back on air.

[BlackBeard]

Hi I have one of these china 200w fm transmitters but the regulator is popped from accidentally connecting the polarity the wrong way round. Can anyone confirm what voltage it could be ?

30E93493-4C08-4623-AA46-CEA047676C79.jpeg

Saw those on Aliexpress as well. Just out of curiosity, is it powered by one of those all-in-one BH/BA/Quintic-chips? Or does it have a real VCO/PLL or DDS? Does it have a MPX input?

Did you check the harmonics, XXL?

[XXL]

I haven’t looked into the circuit much, and it’s under a metal screen soldered to the board. I didn’t check the harmonics but at the transmission site I can get it on several other frequencies around the broadcast frequency, but they don’t go far, maybe 200m and they’re gone.

[BlackBeard]

I haven’t looked into the circuit much, and it’s under a metal screen soldered to the board. I didn’t check the harmonics but at the transmission site I can get it on several other frequencies around the broadcast frequency, but they don’t go far, maybe 200m and they’re gone.

XXL, I have good news for you. I asked the seller "TZT Direct Official Store" the following question about the transmitter you're fixing:

Friend, how is the RF-signal generated? Does it use an IC-chip like BH1415F or QN8066? Or does it use a DDS chip? Does it have a MPX-input? Can I use RDS on this transmitter?

He replied: 1418 chip, no MPX input, can be modified

I suppose the tx is using one of the BH1418xx-chips. Or is there another chip called "1418"? Dunno how clean it is and how exactly the RDS "can be modified". Maybe the IC is only used for stereo encoding (which would be better) but I suppose that it's used for everything - at least it would make sense concerning the spurs.

[XXL]

Really ? When I asked them they wouldn't give me any info at all. And I have successfully got RDS to work with this. I had to remove a few caps or resistors (can’t remember) from the audio input path.

[XXL]

Also you need a seriously beefy power supply to run these. 600w wasn’t enough. I had to use 1000w supply.

[BlackBeard]

Really ? When I asked them they wouldn't give me any info at all. And I have successfully got RDS to work with this. I had to remove a few caps or resistors (can’t remember) from the audio input path.

Interesting. Can you send a photo of the RDS-mod?

It's a pity that those modules are still quite expensive - 405€ on Ali/385€ on Ebay incl. shipping - otherwise I'd get one to play around with. If you add the cost for a PSU + case, it'd be about 500€ in total. For that money you can almost build a solid 300w rig with pre-made components from Dutchrfshop, Amateurradioshop and an exciter from JPL995 - without the need to solder anything critical. The chinese module's price compared to it's benefit kinda makes it obsolete.

You said that you got it on your table for a repair job, right?

[XXL]

yeh but if you purchase a built rig from anyone its always going to be at a labour cost and inflated price.

[BlackBeard]

I ordered the 120w version and examined it using my spectrum analyzer. I also used my Tef6686 to check for possible in-band spurs & the general noise level as well as the reception of other, licensed stations. I did so in the room next to my living room as the antenna's on a 10m mast on my balcony (next to my living room) - so I was about 14m away from the antenna with brick walls and my roof in between.

There are no significant harmonics - only one at about -60 dBc at 139.5 MHz. My NanoVNA was not calibrated concerning the attenuator's used though. Furthermore, there are some in-band spurs, also at about -60 dBc. Of course the modulated signal is quite wide - that's a downside of the tx.

Concerning the reception of other, licensed stations: I was able to receive every station that should be available in my city in stereo & RDS. I only had trouble with one little station that's broadcasting with less than half of my power AND was only 400 kHz away from my signal. I still got it - but only in stereo, no RDS. You have to keep in mind though, that there was some heavy frontend overload going on, as I was basically standing next to my antenna. 80m away I was able to get that station again on my "average-joe" car radio in stereo AND with RDS. Conclusion: Unlike other chinese tx's, this one does NOT wipe out the entire band.

You can switch between mono & stereo. I noticed though, that the 50uS preemphasis is ALWAYS applied - also when transmitting in mono. This is another downside and doesn't make sense as the mono setting should be there for injecting a full MPX signal. So you have two options for stereo & RDS: Either you mix a normal L/R signal with the RDS on top (old trick from doityourselfchristmas) OR you set the tx to mono and deactivate the preemphasis in Stereo Tool or Airomate.

I added a photo of the tx's spectrum for you to judge

Edit: Worked perfectly fine with a MeanWell LRS-350-24, some ferrites (3 each on + & -), a 10nF & 100nF capacitor parallel & an EMI filter on the mains line to the psu. SMPSU & tx shielded from each other of course.

[radionortheast]

What XXL was saying about there been other transmissions does tally with it been a bh, the often use bh chips in cze, with that you do get smaller carriers in the fm band, not much range on a few watts but would start push out if amplified, not the wide band white noise covers the whole band like you get with the qn chips, kt ones.
With the 8066 transmitter 5/7w, you couldn’t hear any fm stations 15 meters, the noise was fairly typical for chips, the only ones i’ve found that don’t have it are the bh ones. The stereo degrades the range on the bh ones, theres sutff that shouldn’t be there. The kt0805l or kt0806l had the lowest of the qn/kt chips i’ve seen noise was limited to afew mhz, still annoying offering some kind of hope for chips fans of these types of things.

[Albert H]

I'll say it again - these all-in-one "stereo sender" ICs are fine for feeding your car radio from a CD player or an MP3 player. That's what they're meant to do. They are NOT designed to be amplified!

I've tried EVERY one of those ICs, and NONE of them are of any use apart from flea-power in a car (a nice Faraday Shield). Even the QN8007 (supposedly the best of them) puts out as much energy in spurious crap as it does in the actually intended carrier. In theory, you could clean one up with a VERY critically tuned filter, but it would need to be precisely tweaked for each frequency it was used on. It's simply not worth the effort.

A simple four transistor, two IC exciter can be clean and stable and cost just a few pounds to build. ½W is really easy to achieve with cheap components, but proper calibration requires use of a real spectrum analyser - NOT one of those toy VNAs!

Please don't pollute the already noisy airwaves with the crap that these Chinese efforts spew everywhere!

[radium98]

SH CTL TTL.....

[radionortheast]

I think I do agree with Albert on this, I can’t believe that an 8007 would be any better than an 8066 or any other, I don’t see evidence of it, I don't think I would get 8007 one just to see, good for on the floor with the aerial retracted if they've got one.

[yellowbeard]

One thing I never could understand - screening is really important for keeping digital artifacts out of your audio and RF, it is difficult enough keeping the PLL out of the RF when the oscillator is on the same board. Now put a stereo encoder, RDS and the PLL on the same chip and tell me its clean and good - that's voodoo, alien abduction and a perpetual motion machine at the same time. It could be true, and liars on the internet may have reasons to tell you its true, but I have good information and experience that make me doubt it.

[Krakatoa]

That's true, modern mixed-signal IC's are expensive to develop and layout just to keep the analog part clean from the digital noise. QN, BH, KT or whatever chips can't be clean because it woukd take using separate analog and digital VCC and GND connections, and the manufacturers would clearly state in the datasheets the recommended layout and decoupling guidelines. Have you seen any? So haven't I.

The only function where these DSP chips would excel, is if they could be made to multiband process audio, add pre-emphasis and generate the outputs MPX with RDS signal, but leave out the serious RF and PLL part.

[radionortheast]

Yeah i’ve never seen specs, measurements of what their doing at fm frequencies, other than the ones i’ve measured myself, i’ve had lots, hard to believe there would one that would be different. On the kit I had which had the 8066, I had to put ferrites in two of the display lines, as rf would get into and overload it, then an amplifier that spurs if not matched, together with a noisy chip puts out noise over other fm frequencies, something so cheap no considerations were ever done, they were only meant for in car transmitters.
I do have an amplifier that dosen’t sprog requires a 6db input, I do have a clean signal source to put into it, a whole lot of messing, for a novice I don’t think it is possible