Flaky inverter. Any fix?

[calzap]

The inverter is a Portawattz 600.  It’s about 20 years old and has been in one of my trucks since the inverter was new.  It converts 12 VDC to 120 VAC at 60 Hz and is rated 600 W.  Output is a modified sinewave, which is simply a square wave with a short zero volts period between each half cycle. 

It had not been used for several years.  As soon as it was flipped on, the fault light and buzzer would activate, and there was no output.  It was removed from the truck and behaved a little better on the bench, but still not good.   On the bench, input was from a fully charged marine battery.  If two 250-watt incandescent lights were plugged-in and the inverter switched on, the fault light and buzzer activated, and there was no power output.  If the inverter was switched on with no load, there was no fault indication when a single 250-watt light was plugged-in.   When the second 250-watt light was plugged-in, sometimes there was no fault and sometimes there was a fault.

A block diagram of the inverter is below.  I could not find a schematic.  I removed the top for a peek inside and could see nothing wrong.  See the pic below.  The DIP IC’s are as follows:
a.  under the ferrite cylinder is a STI NE558N, quad 555 timer,
b.  near the toroid coil is a STI HCF4013BE, dual D flip-flop,
c.  near the center of the board is a STI LM339N, quad differential voltage comparator,
d.  remaining IC is a SEC KA3525, PWM controller.

One thing that caught my eye was a large blob of heatsink compound near the large diodes.  I removed the heatsink compound.  Under the goo was a pair of 0.1 ohm, 2 W resistors mounted in parallel with a thermistor between them. See the pic.  Is the immediate function of this assembly to allow current to be measured as a resistance?  And what function does the assembly play in the inverter circuitry?

All of the tin-can caps look OK.  Nothing looks smoked.

Any suggestions?

My use of an inverter in a truck is primarily to run power tools.   Charging batteries for cordless tools and flood lights (LEDs) are other uses.  Seems likely the present inverter will be scrapped.    Is there any advantage in getting a true sine wave versus “modified” sine wave inverter for these uses?

Mike in California

[TheMG]

You'd have to measure the output waveform with an oscilloscope to really figure out what's going on. Note: operate inverter from an isolated DC source when doing such tests. Many of these type of inverters do not have an isolated output, and neutral is not at ground potential either.

Could be a bad connection or solder joint providing insufficient DC voltage (everything on the 12V side needs to carry quite a bit of current). Bad capacitors could cause the high voltage DC from the DC-DC converter stage to sag too much under load. I've also seen it where one part of the H-bridge output is not functioning, causing an incorrect waveform.

[bdunham7]

You should look at the 160V output of the DC-DC converter to see if it is noisy or sagging under load.  I assume that capacitor at the output is the 330uF 200V one, what are the other capacitors for?