New Flyback-Too Much Width

[cbenham]

I installed a Thord. FLY-1 in a Philco 50T-1400 12 inch TV and it works well. TOO WELL.
I have more width than the width coil can reduce. I'm losing an inch or so on each side beyond the edge of the CRT, which is a 12LP4, and there is 10.8 KV high voltage.

The horizontal linearity is good, set by adjusting the lin. coil for the lowest current drawn by the H. stage, 129 mA.

The vertical height is normal with good linearity and the set makes an otherwise good picture.

I have not tried any 'fixes' yet, and before I do I thought it best ask people with more expertise than I have on these older sets.

The tubes are all good, the voltages are right on the mark, all the paper caps were replaced with orange drops and the electrolytics were replaced with single section axial lead versions.

Does anyone know if this flyback produces a little more yoke drive than the original Philco 32-8421 flyback?

If so, what might I do about it?

I know Zenith used a very thin piece of sheet brass shimmed between the yoke and the CRT neck on their B&W sets to adjust width and I'm wondering if that is something I might try?

What about wiring a resistor or another width coil in series with the H. yoke?

Am I overlooking something? Any help and suggestions will be greatly appreciated!

Regards,
Cliff

[Einar72]

Not knowing whether the old flyback fried itself, or any other components, I have one important question: Did you check each resistor in the circuit for changes in value? I'm curious about the cathode resistor of the H. output tube, I assume you are measuring the cathode current with a meter in series? Is there a drive or coupling adjustment between the oscillator and the output-tube grid? Are there alternate taps for the yoke on the FLY-1? Lastly, what is the output tube? Hopefully, this info will help some others to take a stab at this.

[cbenham]

Originally Posted by Einar72

Not knowing whether the old flyback fried itself, or any other components, I have one important question: Did you check each resistor in the circuit for changes in value? I'm curious about the cathode resistor of the H. output tube, I assume you are measuring the cathode current with a meter in series? Is there a drive or coupling adjustment between the oscillator and the output-tube grid? Are there alternate taps for the yoke on the FLY-1? Lastly, what is the output tube? Hopefully, this info will help some others to take a stab at this.

The original yoke H windings shorted to ground, causing the flyback to short.

The replacement yoke is another original Philco of the same number from a similar 12 inch Philco set. I checked all the resistors. The cathode of the 6BG6 is grounded and develops -19 volts contact bias on the control grid.

Measured stage current at 129 mA by pulling the 1/2 amp Damper fuse and connecting ammeter across the fuse holder, as per Philco instructions. The Drive control has minimal effect on the width. Followed Philco instructions in setting it to eliminate drive line in picture.

There are no other output taps on the FLY-1, and the 10.8 KV is about 20% higher than the 9.0 KV Philco specification. This seems to correspond with the amount of increased width.

The 6BG6 screen has 290 volts, as per specs, through a 4700 ohm resistor from B+.

Would I endanger any critical components if I increased the size of the 6BG6 screen resistor to lower the screen voltage? Might doing this reduce both the width and the high voltage?

As stated previously, I have not tried any 'fixes' yet.

Thanks for all the comments and suggestions.

[old_tv_nut]

Is your AC line voltage high? If so, you might try reducing it a bit with a variac or tapped line transformer. I wouldn't go too far though, since it may affect bias conditions in a detrimental way - seems I recall some sets would cook themselves under brownout conditions.

[old_tv_nut]

I take it that 10.8 kV is the correct high voltage? If it is low, that would expand the scan.

[Steve McVoy]

Replacement flybacks were not exact replacements for the originals, and often circuit changes were required to get them to work, by trial and error. I'd try increasing the resistance of the screen grid resistor to see if the width goes down without significanly affecting the high voltage.

[cbenham]

Originally Posted by Steve McVoy

Replacement flybacks were not exact replacements for the originals, and often circuit changes were required to get them to work, by trial and error. I'd try increasing the resistance of the screen grid resistor to see if the width goes down without significanly affecting the high voltage.

Well, I've tried doing that and all the other 'fixes' I can think of.

I can make it 'better', but not right. Everything I tried sort of works, but causes a great reduction in the HV.

I really think now that the flyback is a 'close but no cigar' replacement for the original regardless of what the parts crossover iindex says.

I tried increasing the size of the screen resistor, even doubling it, and
got a little reduction in width, but lost HV, down to 8.3 KV. So I put the original value, 4700 ohms, back in and got the lost HV back, 10.7 KV.

I tried placing a spare width coil in series with the return lead of the H yoke winding and this helped reduce the width, and did not cause any noticeable linearity problem, like putting a resistor in series did. This alone did not reduce the width nearly enough, however. The circle in a test pattern still looks like a football.

I tried soldering several different size inductors across the set's width coil. A 20 microhenry inductor worked as far as reducing the width to normal, [along with the width coil placed in series with the yoke] but with an accompanying loss of HV, down to 7.9 KV, so this is not the answer either.

I guess this added inductor overloads the flyback so I took the width coil and the 20 uHy inductor back out.

In all these experiments, I tried varying the width and lin. coil settings, tried a smaller lin coil tuning cap [.047 down to .027] and tried various settings of the drive cap and the results were always that there was a significant loss of HV that accompanied the reduction in width.

I compared several different 50T Philco schematics and found several different
connection schemes for the flyback, so I tried them all and they made no noticable difference in the excess width and gave no increased ability to reduce it.

In all this work, I found two significant sources of information which are well worth the time to study and understand.

The first is the 'Damper Tube History' article on Steve McVoy's Early Television Foundation website. This piece gets down to the nitty gritty of the development of the Horizontal Deflection system like none other I have ever read.

The second is a Sams book, 'Servicing TV Sweep Systems' by Jesse Dines,
publication # SSD-1. Another source of hard to get information about how
things work. It even has a section on reducing width!

The only thing I have not tried is the 'Zenith' method of setting sweep width.
I also don't know if this will work with a magnetically focused CRT or not.

I will have to make a non-ferrous metal shim out of very thin sheet brass stock to wrap around the neck of the CRT and slide between it and the yoke.

Positioning it properly looks like a lot of work, with many trial and error tests.
This involves disassembling the CRT mount, the focus coil, the yoke, and finding a way to keep the shim in place if it works, and insulate it from the yoke coils, not to mention the difficulty of getting everything lined up each time I want to move the shim.

Woof!

Film @ Eleven.

Cliff

[Steve McVoy]

You would probably be better off finding a junker chassis and salvaging the flyback and yoke. Those sets are fairly common.

[cbenham]

Originally Posted by Steve McVoy

You would probably be better off finding a junker chassis and salvaging the flyback and yoke. Those sets are fairly common.

Originally Posted by ROUNDSCREEN

Did you try putting a resistor across the width coil? Start with a 50 k 5 watt and work your way down in value. I did do that to a rca set but I don't recall what chassis it was. Just a thought.

I tried a resistor across the width coil but when the width was right, the HV went down to about 7.6 KV due to the overload the resistor caused.

So, I have decided to remove the FLY-1 and use it in an RCA 630 that needs
a 211T3. The Thord. cross reference says the FLY-1 is equal to a 211T3.
We will see. ;^) ;^) ;^)

I got a a 'junker' 50T set and I'm going to install the flyback and yoke from it
in the set I've done all the repair work on.

Thanks for the suggestions and help, all very much appreciated.

When I finally make the Philco work, I'll take a picture of it displaying the Indian Head Test Pattern and post it.
Regards,
Cliff

[roundscreen]

Did you try putting a resistor across the width coil? Start with a 50 k 5 watt and work your way down in value. I did do that to a rca set but I don't recall what chassis it was. Just a thought.