Hi, first post!

Im trying to fix a broken mini split ac and I narrowed it down to a broken pcb in the outdoor unit. Incomning AC voltage is working but I have no DC on the board. When I messure different points it seems that things that are supposed to be tied to ground hangs in the air. How is that possible? Broken ground plane i pcb? For example, I suppose the first means of DC is through divider r147, r148, r2, r149 but all these has full ac voltage like they wasnt tied to ground. Would be very helpfull for some input. Schematics are seen here https://pixady.com/image/r6m/

Thanks in advance!

 

Follow R147 up, up, up, and up. It connects to L just left of, "reactor".
If the voltage doesn't get smaller as you approach R149, that would be a bad ground, but this is an AC divider string.
The reason there are three resistors in a row is the arc-over voltage limit for small resistors, and they are connected directly to the power line which can get mighty twitchy in a lightning storm.

 

Thanks. Ok so problem is elsewere. I cannot find where DC is made in the circuit. 12vdc is needed on the power relay K1 for the high voltage DC to work and the mcu must be running to pull it to ground and I dont have any kind of DC anywhere. Where to start looking?

 

Just to the right of, "reactor".
But I haven't found all of the DC sources. Maybe the voltage regulator(s) are on the bottom of the page. You're looking for where +15V, -12V, and +5V are created.

 

Yep, that's the high voltage DC stuff. I have found the low voltage DC stuff in this picture https://pixady.com/image/r7u/ but how does it get the juice to start up?? It's a bit of a catch 22. It needs the high voltage DC stuff to work to make low voltage DC but that requires 12vdc on the power relay. Thank you for helping!

 

I haven't found it all, but this is part of the mystery. This chip makes low voltage direct from the power line. Find DCV-F

 

DCV-F (Fused high voltage DC) is dead until power relay K1 kicks in and that relay needs +12 and +5 for some logic to operate the relay.

 

This sounds like, "You can't get there from here." but you know for sure it worked until it broke...right?
I'll try to get back to you today.

 

New info! According to a member of a swedish forum R1 is a softstart for the high voltage DC and it lets AC through to the diode rectifier d100 and d101. Did a measurement and I have mains AC at the rectifier and a voltage drop above it of about 1.45 volt AC so that seems to work! DC side of the rectifiers was zero so I have two broken diode rectifiers. Hope this is the complete fix

 

That board is populated with about a dozen over-voltage diodes from +15 to ground and -12 to ground. I have a feeling you aren't done.

 

Yes I see those. So you think they got cocked in the process? They are tiny little things.

 

So you think they got cocked in the process?

I think you're going to end up lifting components or cutting copper traces to see where the short is.

 

Time for a little update. Changed out the switching IC Top243pn and now every voltage on the board works! -12v is only -9 but I don't think that is critical. But the serial communication still does not work. When checking between serial terminal 3 and AC neutral terminal 2 I get 35 Vdc and I think that is correct but I think this should fluctuate if it's communicating? If I remove signal cable from terminal 3 I get +110Vdc between 2 and 3 on the outdoor unit and -210 Vdc between 2 and 3 on the indoor terminal. I suspect error in the outdoor IC that handles the serial connection. IC has 230 VAC and 5Vdc where it should but I'm not sure it outputs anything on the serial pin. What angle of attack should I approach this from here?