Transient Voltage Suppressor replacement

[Richmurphy]

Hi,

I'm trying to repair my split system air conditioner.  After a quick visual inspection, I found what I thought was a suspect looking zener diode and a couple of rather burnt looking resistors.  After consulting the service manual, the zener turned out to be a TVS, specifically, this guy:  http://www.shindengen.co.jp/product_e/semi/list_detail_NEW.php?category_id=06&sub_id=02&product_id=ST02D-200

I haven't been able to find anyone with that exact component in stock.  My local electronics retailer doesn't seem to have anything even close:  https://www.jaycar.com.au/products/passive-components/circuit-protection/transient-voltage-suppressors/c/202C

I'd be appreciative if someone could guide me towards a suitable alternative.  I've attached a circuit diagram with the damaged components highlighted in red if that helps.

Thanks,
Richard.

[Armadillo]

https://www.cdiweb.com/ProductDetail/ST02D2007070--shindengen/560981/

[wraper]

You can also replace it with a zener and a fast diode connected in series.

[Ian.M]

I *ASS*U*ME* the PSU is dead with no great signs of distress other than the parts you mentioned?  If not, please tell us what is actually wrong with the unit.

If the TVS diode ZD11 in that circuit has blown, I would also expect damage to the chopper chip IC01 and probably other active devices and low value resistors on the primary side of the PSU.   If ZD11 has failed open circuit, you *MIGHT* be lucky, but if if failed shorted, you'll need to rebuild the circuit round IC01 with new parts.   If F702 has blown its almost certain you'll need to rebuild the primary side of the PSU.

Unless ZD11 is physically damaged, or reads shorted or leaky, I'd simply replace the two cooked startup resistors and see if you get lucky.

[Richmurphy]

I *ASS*U*ME* the PSU is dead with no great signs of distress other than the parts you mentioned?  If not, please tell us what is actually wrong with the unit.

If the TVS diode ZD11 in that circuit has blown, I would also expect damage to the chopper chip IC01 and probably other active devices and low value resistors on the primary side of the PSU.   If ZD11 has failed open circuit, you *MIGHT* be lucky, but if if failed shorted, you'll need to rebuild the circuit round IC01 with new parts.   If F702 has blown its almost certain you'll need to rebuild the primary side of the PSU.

Unless ZD11 is physically damaged, or reads shorted or leaky, I'd simply replace the two cooked startup resistors and see if you get lucky.

The unit isn't switching power to the outside fan/condenser but the internal unit seems to work perfectly.  The two thermistors and relays checked out okay initially so from there I disassembled the cover to access the rest of the PCB and I immediately spotted the blackened resistors.  ZD11 reads 2 ohms and the wires appear slightly blackened.  I haven't removed ZD11 from the circuit so that's the extent of my testing so far but I didn't think the low resistance or colour seemed healthy.  I was going to replace these 3 and hope for the best but you're probably right regarding IC01.  All other resistors and caps have tested okay.

EDIT: The stand-by feature is working okay and I gather this is the purpose of IC01...?

[Ian.M]

You are reading the primary resistance of the chopper transformer shunting ZD11. 2 ohms would be normal.   Check the 12V rail on the secondary side of the PSU - if its there, close to nominal voltage and without grossly excessive ripple, the PSU is fine.  Depending on how badly cooked the two resistors are they may or may not need replacement.  If the colour codes are readable, isolate one end of one and check their value.  If OK leave them, otherwise if unrecognisable or way out of tolerance, replace.

If I were you, I'd start checking the relay control circuit round IC06 . . ..

[Richmurphy]

You are reading the primary resistance of the chopper transformer shunting ZD11. 2vohms would be normal.   Check the 12V rail on the secondary side of the PSU - if its there, close to nominal voltage and without grossly excessive ripple, the PSU is fine.  Depending on how badly cooked the two resistors are they may or may not need replacement.  If the colour codes are readable, isolate one end of one and check their value.  If OK leave them, otherwise if unrecognisable or way out of tolerance, replace.

If I were you, I'd start checking the relay control circuit round IC06 . . ..

Thanks Ian. I've confirmed those 2 resistors are completely open so they'll need replacing. After looking at the circuit diagram my initial suspicions were around IC06 but then I saw the fried components so I'm focusing on the obvious problems first.  I'll replace the two resistors tomorrow (its 2am here) and see how it all goes. Thanks for your advice.

[Armadillo]

I've confirmed those 2 resistors are completely open so they'll need replacing. After looking at the circuit diagram my initial suspicions were around IC06 but then I saw the fried components so I'm focusing on the obvious problems first.  I'll replace the two resistors tomorrow (its 2am here) and see how it all goes.

Even if ZD12 is shorted or IC01 is shorted, how would R102 + R103 forming 660K across ~170Vdc, charred it to cause it to open circuited? It's puzzling? This could be merely just heat discoloration but its puzzling why both are confirmed open circuited at the same time.
And if anyone of these resistors is open circuited, there will be no enable signal and hence no 12v or VCC 5v and the circuit will appears dead.
Before speculate away too far, I suggest to revive or confirm the 12v and VCC 5v prior.
Interesting.   

[Ian.M]

The 0.P.s local parts supplier is Jaycar (Australia) so the odds are he's got 325V +/-10% feeding those resistors.  However, I agree that at less than 0.1W each they shouldn't have cooked.  Possibly an undocumented factory mod to a lower value to give the opto's phototransistor more current?