Flyback replacement for a Mitsubishi AM-1401R

[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.

[Electronic M]

Originally Posted by 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.

That's probably normal. It takes a few seconds for HV to bleed down on most sets and the same way it takes the CRT heater ~11 sec to warm up to full emission from cold it takes about as long to stop emission when turned off from warmed up. Capacitors in the sweep and gun drive circuits don't fully discharge immediately either so the set will still be semi-running off of stored charge and stored cathode heat for a few seconds after power off.

Each different circuit design will behave differently on switch off. Old monochrome sets would often have a lingering white dot in the center of the screen for the same reasons.

[StarquestMan]

I hoped it was something normal, it lasted about 30-45 seconds, maybe a full minute.

[damen]

I know someone who did RCA warranty work back in the day and he said some customers complained of this happening with solid state sets. RCA said to inform the customer that it was caused by an "above average quality picture tube". Longer the glow, the better the tube. Customers stopped complaining.