Repairing the mamouth WG 33K3301 33" Med Res Color Monitor
By, William Stephens
(clicking any picture in this article will bring up an enlarged version)
I preface this article to say that my work area is on the second floor. I say this because my shop guy had to carry this behemoth monitor up and down the stairs... a couple times!!! I apologize for not fixing all the problems the first time.
This one starts off by not working at all. In examining this thing I notice that things have been in and out several times before, it may be a composite of a couple monitors. None the less it all seems to be there and it all seems to be the correct versions. These models of wells-gardner have a switching power supply (SMPS) instead of the old linear ones of models like the 7000 series and the like. After foolng around with unconnected ground wires and stuff like that I realized that things can get unplugged, possibly shorted or tugged a lot due to the great weight and difficult grip problems a monitor of this size has. I saw that the feedback IC on the SMPS was in a socket, so seeing that I had some of those chips, UC3842N or KC3842N I popped a new one in and voilla!!! it came to life. I did this because when you turned it on you could see the old familiar SMPS faint with a volt meter on b+.
Wow!!! that was an easy fix. I ran it for a couple hours and proclaimed it FIXED! Back to the shop floor.
2 days later I got a call and my shop guy told me that the monitor looks good but when he moves the cabinet the image jiggles. I told him and he agreed that there was a loose wire somewhere. He decided to track it down on the floor first, so good, hope he finds it easily.
A day later I got a call and now this time the image was collapsed horizontally and the thing still jiggles, he's at the limits of his test equipment and tools so, unfortunately it's back to the benck, up all those stairs.
I got the thing back on the bench and after a great while of poking a prodding I found nothing wrong with it... I love it when this happens. I could make it jiggle but could not see anything loose and the image was shrunken and hour-glassed just like my shop guy said and no amount of tuning and tweaking got rid of the display anomoly.
I checked the pincushion circuit on the horizontal, seems logical for hour-glassing and once again everything looked good... readers note: remember this for later on.
I decided that the jiggle had to be a bad solder connection on the PWB, so out the chassis came and I broke out the solder sucker and good iron. I started with the big things and worked my way down but not all the way... there are a lot of solder connection on the board. I reflowed the connectors, then the flyback and while doing the flyback as I sucked off the old solder from pin 10, the pad came along for the ride. It's cracked right where the pad meets the trace... this was the jiggle, my shop guy would have never found this one. Fixed it up re-installed the chassis and the jiggle was gone but the image was still screwed up.
I looked and tested and looked some more but to no avail, I could not find a problem anywhere. This went on for about an hour a day for the next 4 days... this was really bugging me, and anyway, I needed the bench space back for other work. I remembered how my shop guy told me about the problems removing and re-installing the chassis and how the floppy wiring often got caught on stuff. I even experience this the several times I had the chassis in and out doing my checks and tests.
Finally and for a reason I don't know, I decided to take a few voltage readings in the pincushion circuit I had previously given a clean bill of health. Surprisingly, I found that the emitter of Q705 (chassis mounted transistor with it's own connector) was floating several volts above ground and according to the schematic it should be grounded. Other voltages were off from the schematic in this area as well. In checking out the Q705 connectors emitter wire was open circuited. With the monitor on, I took a test lead and grounded one side then touched the other end to the emitter of Q705 and amazingly, the shrunken hour glass anomoly immediately disappeared. When I removed the lead it returned, when I grounded it, everything looked perfect. What had happened is that this connector would get snagged on the chassis when you removed it usually and from this abuse over time the wire in the connector broke. I disassembled the connector, fixed the connection and put it back together. The monitor worked perfectly thereafter.
This job would have taken an hour or two instead of the 5 days it sat there frustrating me if only I had followed comprehensive testing procedures. I should have found this in the initial first few minutes of the troubleshooting when I immediately suspected the pincushion circuit. I guess I did a less than cursory test sequence and totally missed the error. Once I blessed this circuit I never re-visited it until some sort of inspiration made me question my original findings... If only Einstein had done this, he could have advanced physics nearly 50 years, at least for me I got this behemoth off my bench!