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 ?

It seems to be a switching regulator. Usually the output voltage is defined by the external components. Replace it with the same part number and also check the rectifier diode for shorts. This section probably outputs 5V to the front panel microcontroller and vco. No warranty it will work, but who knows.

Probably an LM2596 but don't take my word for it - read the numbers on the top.

I also agree that it's very likely a LM2596, or more modern equivalent, but as thewisepranker said, can't be sure. I assume, since you're asking, that there is nothing visible stamped on the case. It's definitely a switching regulator, the VCc is in the right place for a LM2596 (pin 1), and the device output looks to be too (Pin 2). If it's a LM2596, the other side of the inductor should connect to pin 4 (inductor connected between pin 2 and pin 4, with the final, regulated output from pin 4), pin 3 should be internally connected to the tab (therefore also connected to ground), and pin 5 is likely to be grounded too. Screenshot is for LM2596 typical application.

I put a wire on pin 2 and turned the voltage up slowly. It was working fine at 12v so I bought a lm2596 and placed it in. Switched it on and the output voltage went up to the same as the input voltage, so now it’s blown the mosfet gate. Not happy. What has caused this ? It’s an lm2596t-12 p+

That's bad news. What I don't understamd. If you were able to wind the voltage up to 12v, and it was working fine; before you changed the regulator, why did you think the regulator was blown (unless you meant that's what it did before the reversal) ? I think the LM2596t-12 is designed as a fixed 12v regulator, whereas the straight LM2596 is variable. Inadvertently, the schematic I copied and pasted, is visibly for a fixed 5v circuit, but I hadn't realised there were fixed voltage versions of the LM2596 itself, but the pinouts appear to be the same, and I would "think" that the variable version in a fixed circuit (whereby the feedback voltage is set by a zener diode), would likely give the same fixed voltage out. If the output voltage went up to the input voltage though, maybe a solder bridge between pins 1 & 2 of the regulator, a faulty regulator, or just incompatibility between the LM2596 and LM2596t-12. Otherwise, perhaps other stuff damaged by the previous reversal.

If you look good at the picture you see 2 resistors for setting the voltage of the LM2596
But maybe you blown the driver mosfet when you put your power the wrong way
What you can do is power the 12V direct from a 12Vs psu
Leave the LM2596 of and put a 12V direct to the coil next to the LM

IF is only about LM2596 i have part and can repair out of charge..

It might also be worth adding a series high power Schottky Diode to the supply input to prevent future supply reversals! The ones I use are MBR1660-E3/45, which cost around £1 from RS.

@albert I don’t make a habit of it, it was a mistake.

I had to modify the board slightly to get it in because the previous one was the smd version which was an absolute pigs ear to get out. Don’t judge the soldering too much, iv fiddled around with it so much it looks like crap. Iv drawn a line to where I had the feedback connected. As the track came off. Nothing is shorting out. But getting input voltage on the output. (21v) so if I can figure out why I might be able to get a replacement mosfet. Atm I can’t work out why 21v is coming out the output pin when it’s a 12v reg.

you need the LM2596 ADJ version not thw 12 version

for God's sake, why did you make this kind of hogwash. God save him. this is terrible.You have no excuse for digging a well like this!

If you remove the regulator and feed the output point with a regulated power supply (current limited to prevent further damage) does the rig function?

eiradioguy wrote: ↑Thu Nov 09, 2023 9:30 pm If you remove the regulator and feed the output point with a regulated power supply (current limited to prevent further damage) does the rig function?

Yes it did before I put the new regulator in. Now it does nothing. The reg which is suppose to be 12v, must have popped the gate. For some reason on the output it’s showing 21v which is the main voltage for the drain.

I should of just put a normal 3 pin regulator in there and put it back on air but I thought I’d buy the proper one and it’s bitten me in the ass.

Anyone know what the RF chip is ? I need a new one.

where is old smd part? can you use magnifier 25X and see if any trace of mark?

Nothing, it’s been totally sanded off.

You're into spending a lot of money when you get into replacing the output device. Have you tested it out of circuit on a tester, and is there drive RF at the input to the output device?

In my humble opinion right now if you have to replace the final step and after that modification with the regulator, you have to buy a new one

At least, you can try to enter the value of those 2 resistors that hang from pin 4 into the formula of the ADJ version to know what was the output voltage, then apply that voltage from an external power supply.