Mitsubishi CS-27205 troubles

[mr_fixer]

During my semi-weekly neighborhood scavenging trips, I found a 1995 Mitsubishi CS-27205 set. When I got it home and plugged it in, it only has a thin dim horizontal line on the screen. It also has no sound or hiss when i turn up the volume. I'm thinking some sort of power supply problem, Anybody encountered these symptoms before? When you turn the set off the screen has a bright flash, which makes me think the crt is still usable. Logan

[dr.ido]

It has lost vertical deflection, probably because the vertical output IC has failed (assuming there are no dry joints on or around the vertical output IC/yoke connector/flyback).

Replace the vertical output IC (probably a LA7838/LA7841/LA7845 or similar) and associated electrolytic capacitors. The vertical IC is usually powered by a winding on the flyback transformer via a fusible resistor and diode. If the original IC failed shorted it may have taken these out as well.

Many sets will blank the CRT under these conditions to prevent burning a line into the screen. Someone may have turned the screen control up (hence the dim lime) when fiddling.

[mr_fixer]

Thanks Dr. Ido, after looking in that area I discovered a poor solder joint on the regulator transistor. It was killing the B+ to the vertical and audio portions of the TV. I have never seen a tv set that received some of its power supply voltages from the Flyback trans before. Thanks for the info, Logan

[nasadowsk]

I have never seen a tv set that received some of its power supply voltages from the Flyback trans before. Thanks for the info, Logan
Reply With Quote

Hey wait! Just about all tube TVs do that - that's the 'boosted' B+ off the damper tube.

[radiotvnut]

Most, if not all, CRT based TV's use the flyback for voltage sources. Often, the only voltages produced from the main AC supply are the 130 volt B+ and maybe a start-up voltage. These voltages operate the Horizontal sweep circuit and the rest of the voltages are taken off the flyback. In remote sets, there will be a seperate stand-by supply derived from the AC line.

[zenith2134]

He's right, the flyback transformer is the most integral part of the chassis in most sets. For example, if the set's dead, I'll often check/replace the fly as an early step. Especially in modern CRT sets (they cook so fast it seems)

[mr_fixer]

it's been a long time since i was deep in a tv chassis, i thought the flyback boost was only used for the horizontal O. Tube. Does the B+ boost affect the Audio output in other tv sets? i don't think i remember older tv sets getting the CRT filament from the flyback before. but it could just be a memory lapse. logan

[radiotvnut]

Most older TV's still used a big power transformer in the AC supply for most of it's operating voltages, including the CRT filament. Even the older solid state set's (early RCA XL-100 & Zenith Chromacolor II, for example) still used a power transformer in the AC supply. I think the sets with mostly flyback derived voltages came out in the '70's when foreign TV companies were trying to eliminate the heavy and costly power transformer.

There are some later model sets that do not have a bunch of flyback derived voltages. The RCA CTC203 chassis has a switching power supply that provides most voltages for the set. I think there's about 2 or 3 voltages besides the HV generated by the flyback. I think these voltages feed the video output stage and the vertical deflection circuit. In fact, you can pull the flyback out of a CTC203 and still get audio provided the rest of the chassis is OK.