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Repairing the Hantarex Polo Color Raster Monitor
By, William Stephens
(clicking any picture in this article will bring up an enlarged version)

The procedures outlined in this article are very dangerous for the clumsy and inept among us. If you think this applies to you then read but don't do, you might kill yourself.

This article covers 2 Hantarex Polo monitors with different problems. The first one is a Polo 25" and the second on e is a Polo 33" monitor.

My first patient is the 25" Polo and it came in with a peculiar fault, It would come up and then shut off and then come up and then down and so on. By this behavior I could tell it was not the familiar LOPT(flyback)/HOT(horizontal output transistor) problem that plagued this model. It seems that Hantarex outsourced their LOPT's and they got a bad batch of them from their supplier which began to fail in the field shortly after they were released back in the early 90's.

No, this seemed to be a SMPS (switch mode power supply) problem. The heart of the SMPS is a UC3842 (IC101) SMPS Controller which regulates and provides the feedback circuitry for a SMPS. It's kind of like one stop shopping for control parts in the power supply, and they're cheap. I replaced this without any further tests, hooked it up and the monitor came up... at least for a while. After a few minutes it stopped cold! A little checking around and I found that (T101) was shorted solid. This is the switching FET for the supply and the power heart of the SMPS. I immediately checked the fuse and suprisingly it was O.K.. Normally when the FET shorts the fuse will blow, but this time it did not.

I did a further check of the main rectifier diodes (D101 and D103) finding no fault so I replace the FET and monitored TP6, the +138 volt supply with a multimeter. I turned it back on and it came up again, there was a steady +138 Volts at TP6 and the picture looked great. I let it soak for several hours while I deduced what happened.

It appears that the SMPS controller went a little funny in the head and was whipping the FET with transient and higher than normal voltages when it would surge, this weakened the FET. The FET in it's weakened condition could not cope with this steady state restored by the new IC101 and subsequently failed, weldeding it into a complete GDS short, a normal but disasterous mode of failure for an FET.

--William Stephens

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