For about a year I've had these Monster Power inverters and they've been sitting around waiting for me to fix them. They're cheap 150 watt inverters, part number MCPI-150. One of them had it's overload LED on and it was squawking. The other one was squawking without any warning lights. Both of them were working fine otherwise.
So I tore them apart and started swapping parts on them. I swapped the output boards and then one was squawking really annoyingly with the overload light on. The other was working silently still putting out power. So I figured there was a problem with the output board so I go poking around pulling parts and finally I find out that the LM339 is bad on it. I dug around in my junk pile and found a 400 watt Powmax power supply that has bad caps in it, so I took a gamble that the LM339 comparator was still good in it.
I unsoldered both of them and checked with a meter. Pin 1 measured 13.3 ohms to ground on the bad one and it should have read open. So after I replaced it, being careful not to get the chip too hot, it worked without making noise and no more overload light.
On the next one I looked around on the schematic that I made and I couldn't quite figure out how it worked, so I used my meter to test components on both inverters. I found that P1, which is a KSP2907A, was reading differently on both. So I unsolder the transistor and it turns out that from base to collector I read 92 ohms. Measuring from base to emitter was open. Maybe heat killed it.
I couldn't find a direct replacement in my parts pile, so after looking around for a while I found that I had some spare SMD transistors for some UPS's with exactly the same specs just the wrong package, part number MMBT2907A. I figured I could make it work with a little ingenuity. The pictures show the result.
Now both of them are happily working. Just goes to show you, with a little bit of determination and time, almost anything can be fixed.
Hope this is helpful to someone else.
So I tore them apart and started swapping parts on them. I swapped the output boards and then one was squawking really annoyingly with the overload light on. The other was working silently still putting out power. So I figured there was a problem with the output board so I go poking around pulling parts and finally I find out that the LM339 is bad on it. I dug around in my junk pile and found a 400 watt Powmax power supply that has bad caps in it, so I took a gamble that the LM339 comparator was still good in it.
I unsoldered both of them and checked with a meter. Pin 1 measured 13.3 ohms to ground on the bad one and it should have read open. So after I replaced it, being careful not to get the chip too hot, it worked without making noise and no more overload light.
On the next one I looked around on the schematic that I made and I couldn't quite figure out how it worked, so I used my meter to test components on both inverters. I found that P1, which is a KSP2907A, was reading differently on both. So I unsolder the transistor and it turns out that from base to collector I read 92 ohms. Measuring from base to emitter was open. Maybe heat killed it.
I couldn't find a direct replacement in my parts pile, so after looking around for a while I found that I had some spare SMD transistors for some UPS's with exactly the same specs just the wrong package, part number MMBT2907A. I figured I could make it work with a little ingenuity. The pictures show the result.
Now both of them are happily working. Just goes to show you, with a little bit of determination and time, almost anything can be fixed.
Hope this is helpful to someone else.
Attached Files
Comment