Flyback replacement for a Mitsubishi AM-1401R

[StarquestMan]

I thought I would post a little update on my TV in case it helps someone else out with a similar set. I ended up getting what basically is the HR Diemen 7718 although its probably a different brand since it goes under the part number 334P-12602. I went this route to avoid having to import one from Ireland. the same seller on ebay also had a 334P-12603 that looked identical except the wire coming from the base near the screen control was grey instead of orange like the original one in the TV which had a grey wire, though im betting this is simply a manufacturing variance, although I could be wrong. I could not find a cross reference for the 603 so I went with the 602 since I could at least verify the pinout.

when it arrived, the 602 matched all the DC resistances of the original flyback although the primary between pins 1-2 had a lower resistance of about .8-.9 ohms vs the original of about 1.3 ohms. I understand that DC resistances on a transformer are just a guess but i'm unsure how to properly test inductive components, i'm thinking I need an LCR meter for this sort of thing but that's besides the point.

The new flyback is a bit larger than the old one but it still fit snugly once I loosened up the Input board and bracket on the back of the main PCB. once installed I powered it up half expecting something to explode and short out but was greeted with the sound of static from the speaker and the very faint high pitch whine of the deflection/high voltage working just fine. I then turned up the screen control to light the screen and adjusted the focus and everything was normal.

One thing I forgot to mention is that I had tried again to repair the original flyback once more with epoxy, and while it worked fair, it would start ticking and popping after a few minutes of operation and just gave me the feeling that it was trying to arc out again. The new part does not do any of this and is much much quieter overall than the original part I had tried to fix.

Two things I have noticed though is a slight tilt in the picture and a slight gap on the right side of the screen. The screen tilt or rotation i'm sure is easy to fix by simply rotating the yoke slightly but the horizontal deflection does not appear to have any sort of adjustment other than a horizontal hold pot buried under the crt at the front of the chassis with a glob of epoxy holding it in place, the service manual basically says its set at the factory and to leave it alone. I checked the B+ voltage and while it goes a bit low on a snowy screen, it stays right at 123v when a picture is displayed through the inputs regardless of brightness. the screen does show a little blooming too that was not there before (if thats the right term for a growing and shrinking screen depending on brightness although it could be picture bending since I can see the linearity change as an object moves up or down along the screen. I am wondering if their may be some ripple effect on the B+ due to the lower impedance of the new flyback primary winding that isn't showing up on my meter. there is one unpopulated electrolytic capacitor spot on the chassis between the B+ and ground although there is another 47uf 200v cap between the same two traces but the service manual does not give me a value for it.

sorry if I bored anyone with my little novel

[JohnCT]

You can tweak the width and HV by changing the value of the retrace capacitor. The cap is across the horiz output and is a low value high voltage plastic or dipped cap.

If you lower the value, the HV goes up but the width comes in. Raise the value and the HV drops and the width will stretch.

If the picture is off center, you may be able to add some DC voltage to the horiz winding. Sony used to do that to offset the horiz on projection TVs. I don't recall how they did that but any Sony PTV schematic from the 1990s will show that.

John

[StarquestMan]

Thanks for the reply. I will have to try that. I started doing some comparisons with the pictures I had posted of the set working when I had first repaired it and noticed that not only has the horizontal gotten narrower but but the vertical is narrower as well. both the horizontal and vertical seem to be at the right ratio to one another but the overall picture is slightly smaller although centered pretty close on composite input but shifted to the left when the tuner is on a blank station where as it took up the full width before, it centers pretty well when tuned to a signal. I also noticed that the snow on the tuner starts out at normal brightness when first turned on but continues to darken until barely visible after a while and does not brighten when i switch to an input and back again. I am starting to wonder what the voltages coming off of the secondarys look like. I suppose they could be high or low. I suppose that changing that capacitor in the horizontal may correct these voltages if they are off?

On a side note are there any recommended places I could scan and upload the service manual I have? I know there are places that take service manuals for HiFi equipment but I am unsure about TVs.

[JohnCT]

Originally Posted by StarquestMan

I suppose that changing that capacitor in the horizontal may correct these voltages if they are off?

Yes, or make them worse. The low voltages off the flyback are proportional to the HV winding, so changing anything on the primary side affects them.

There might be a way to get a bit more width without sacrificing HV. On tube TVs we used to add a capacitor to the damper circuit. I'll ask my older brother - he's a lot brighter than I am on this stuff. There are a bunch of guys here who know a lot more about this than I do, hopefully they'll see it and chime in.

We used to do stuff like this back when TVs were a lot more money and worth the time, but when done as a hobby, the time is often diversionary and fun.

If you can get the sweep close enough and the HV is within reason, the lower secondary voltages can be fudged a lot easier or even subbed from another area, particularly if it's a low current source.

John

[StarquestMan]

I went and checked the voltages coming off of the secondary, other than the heater, there are two sources. One is for the neck board and is listed as 195.5 volts. I read about 192-193 with a picture. The other was a 14 volt supply for the ICs on the main board, I read a fairly stable 14.1-14.3 volts.

I came across some interesting info here
https://www.repairfaq.org/REPAIR/F_m...tml#MONFAQ_018

It mentions that beam deflection in a CRT gets "stiffer" at higher voltages resulting in a smaller picture both horizontally and vertically. since the service manual states that the HV is non adjustable and is specified as 22.2 kv for the original flyback, I wonder if the output of my new flyback is ok for the secondaries but a little too high for the HV part?

I had always assumed that the screen pot on a flyback fine tuned the high voltage output, but now I realize that this isn't the way it works, unless i'm mistaken. looking at the data for an HR7718 flyback shows an output of 24.5 kv. However I don't necessarily trust their specs because I do remember watching a video of someone repairing a 1990 Samsung TV with a failed flyback. He showed the Sams Photofact specs for the OE flyback output and the part number for the replacement. I went and crossed this number on HR Diemen's site and it listed a voltage that was about 7kv lower if I remember correctly. Im thinking that a high voltage probe would be a good tool for me to buy next.

Im not so sure how I would correct this if it does turn out to be the issue? turning down the B+ would lower HV but throw off all the other voltages.

I feel like I am learning quite a bit from all this though

[JohnCT]

Originally Posted by StarquestMan

I went and checked the voltages coming off of the secondary, other than the heater, there are two sources. One is for the neck board and is listed as 195.5 volts. I read about 192-193 with a picture. The other was a 14 volt supply for the ICs on the main board, I read a fairly stable 14.1-14.3 volts.

Those are certainly in the ballpark. I would double check the heater voltage though using a TRMS meter. It should read 6.3VAC (or whatever the CRT uses for heater) and not a tenth over. A tenth or so under is fine, but if you have to tweak things to get the heater on the dot, do so for tube longevity.

I came across some interesting info here
https://www.repairfaq.org/REPAIR/F_m...tml#MONFAQ_018

It mentions that beam deflection in a CRT gets "stiffer" at higher voltages resulting in a smaller picture both horizontally and vertically. since the service manual states that the HV is non adjustable and is specified as 22.2 kv for the original flyback, I wonder if the output of my new flyback is ok for the secondaries but a little too high for the HV part?

Possibly. In any case, a couple of KV high shouldn't make a dramatic difference in picture size, at least not enough that the circuit adjustments wouldn't compensate for.

I had always assumed that the screen pot on a flyback fine tuned the high voltage output, but now I realize that this isn't the way it works, unless i'm mistaken.

No, the G2 or screen adjusts the DC to the second grid of the gun, and the F adjusts the voltage to the focus element. There are no other adjustments on the fly.

looking at the data for an HR7718 flyback shows an output of 24.5 kv. However I don't necessarily trust their specs because I do remember watching a video of someone repairing a 1990 Samsung TV with a failed flyback. He showed the Sams Photofact specs for the OE flyback output and the part number for the replacement. I went and crossed this number on HR Diemen's site and it listed a voltage that was about 7kv lower if I remember correctly. Im thinking that a high voltage probe would be a good tool for me to buy next.

Im not so sure how I would correct this if it does turn out to be the issue? turning down the B+ would lower HV but throw off all the other voltages.

I feel like I am learning quite a bit from all this though

The turns ratio of the secondaries set up the voltages. If your replacement fly has more turns on the HV winding, it will have more HV than the original, all other things being equal.

At this point, you can raise the value of the retrace cap across the horiz output transistor to drop the HV and this should also give you a bit more width. The vertical should be easily adjustable with no circuit changes. You will lose a bit on the other flyback secondaries, but those will have little to no effect generally unless the HV drop required is significant.

A HV probe in this case would be nice. Ordinarily, you can test HV by measuring the heater voltage. If the flyback is original and working properly, you can tell by an accurate reading of the heater what the rest of your voltages are. The CRT being very particular about heater voltage, the designers are very good about setting the heater to 6.3VAC. In this case of course, we have a different fly and can't trust the heater or other secondary readings to verify HV.

John

[StarquestMan]

Just now getting back to this neat little TV after being busy for a while with other work. I checked out the heater voltage for the CRT, it is sitting at 6.19 volts. I used the TV for a little while just to check it out. After I turned it off I noticed a small splotch of colors on the screen that was sort of rectangular shaped and had a shimmering effect like a candle burning or a flag waving in the wind. I was very curios to what the HV was running at so I bought an old HV probe on ebay and when I got it I checked it for accuracy on another working TV that I knew the specification and it read right on. So I checked the Mitsubishi and it checked spot on 22.5 Kv as per the service manual. what would cause this strange effect on the screen after the set is powered off and is this harmful? I still need to correct the width deficiency but I'm guessing this isn't related although I have never seen this before on a TV so I don't know what to think.