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Problem with older 31" Mits CRT

4946 Views 22 Replies 4 Participants Last post by  lcaillo

Sadly, our 1996 vintage Mitsubishi 31” TV has been showing its age of late. When you turn it on, the picture collapses to a thin white horizontal line across the middle of the screen. When the picture finally kicks in a couple of hours later, it has horizontal lines on the screen that are progressively larger and more numerous towards the bottom of the screen. I’m sure it’s just a minor repair – the picture tube is still nice and bright. Any ideas? Is it something I could do? About the only relevant tools I have are a Greenlee VOM and a soldering iron.

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The flyback transformer is most certainly NOT the problem. What Wayne is experiencing is a collapse in the vertical sweep. A horizontal line and any picture at all means that there is high voltage and there is horizontal sweep. This cannot happen with a bad flyback, which is actually very RARE in these sets. What is common that will cause loss of vertical are bad solder connections. Also, in some of the Mitsubishi sets, there will be a number of leaky (physically leaking electrolyte, not d.c. leakage) electrolytic capacitors. The electrolyte can cause corrosion that will create bad connections and the caps tend to not perform as they should when they leak. The lines are likely due to poor vertical linearity (the vertical sweep is not consistent for the entire interval) and this is often due to capacitors which are off value or have high ESR.

Post the model number, Wayne. Look for wet looking circles around electrolytic caps and black goo on the legs (mostly the negative) of the polar electrolytic caps. Also, look for black spots on the traces on the bottom of the board. This is a good indication that electolyte has been doing its work on your board and lots of cleaning is needed.

The short term fix is likely to resolder the vertical output IC, usually numbered in the 400s, often IC401 in mitsubishis of this vintage. Another way to know if you have leaky caps is if you smell rotten fish when you heat the negative legs of the caps in the area.
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The CS-31305 definitely is one that tends to have leaky caps. They do not "dry out" but actually leak electrolyte because of faulty seals, likely made worse by high ESR and temperature, bu caps even far away from heat sinks do the same thing. It usually occurs on the negative side. There are often several leaky caps in the area near the low voltage regulators (5v & 12v). Scan the board for spots below the caps, black goo on the legs, and use the trick of heating the leads and smell for the ones where you cannot see the legs. Clean the board very carefully to prevent future problems.

There is also a tantalum cap that causes problems in this set. IIRC it is orange and 2.2uF.
Remove all of the screws, including the ones that Tony described, and tilt the CRT forward a little. It should slide off, but it can be tight, especially with the weight of the CRT.
The vertical output is labelled IC401. Start by reslodering it and replacing the caps in its vicinity numbered in the 400s.
You probably bumped the screen (G2) adjustment on the back of the flyback transformer. It is the big black device with the red anode wire coming out of the top. The screen control is the bottom control on it. Switch to a blank input and adjust so that you have a dark screen and don't see the lines (retrace lines).
Look for a connector that may have been pulled loose. If it changed when you worked on it you probably just have something like that. Generally, that symptom is most common with an open resistor on the CRT board, but it is unlikely that you would coincidentally have that kind of failure with the other. Another possibility is that you missed some leaky caps and moving the board finished off a solder joint that was weakened.
Check to see if there are any little white switches on the CRT board. Some mits sets had them to control the various guns and/or g2 for testing and they would get bumped easily. Also, look for broken solder joints on the CRT board or around the flyback transformer.
Somewhere something has changed. Almost certainly a damaged or open connection.
Look for open resistors or bad solder joints on that CRT board.
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