Well Gardner P447 K7000. Getting desperate; need help.

[Polish-Jack]

I am working on a K7000 marked with a P447 which, form what I have gleaned from the Internet, mean is it intended for use with 19 inch tubes. I am trying to Frankenstein a cabinet together from parts that a local arcade was dumping because they were "modernizing". I am having width issues I just can't seem to solve and I'm hoping someone here knows their way around this board and can give me some pointers. Here's my story so far.

This board was attached to a tube that was necked. It was also a board that looked most promising. The voltage regulator and the horizontal transistor were shorted. The flyback had a crack. Some of the adjustment pots were cracked. I downloaded the service manual and bought all of the replacement parts and also made a cap kit while I was at it. I installed them all and checked all of the traces. Repaired a lot of broken traces and ring cracks. "

First test: I plugged it into a TV that I salvaged and crossed my fingers that the flyback did not arc. I didn't! YES! but it would not focus. So I ordered and replaced it with one form Arcade Parts and Repair.

Second test: It lives! Focus works. Brightness works! I buy the CraftyMech Test Pattern Generator to test screens so that I can choose the best looking one.

The TPG arrives and I start playing with monitors. I go through three of them and they all have width problems. I guess I just have bad luck. I find a fourth 19 inch tube on Facebook. Strip it down and plug it in. Same problem. Can't get the raster to the edges, much less to over scan. This can't just be four incompatible tubes in a row with the exact same problem.

I start to dive deep into the k7000 schematics and find that the 25" k7000 has a cap that is parallel to D18. This cap does not exist in the 19 inch schematics but it does look like it was installed as a modification in other 19 inch boards that I've seen pictures on when Google'ing. I try a few different values and find that 0.0020uF gives me the best increase in width with the least amount of distortion. It helps, a lot, but does not solve the problem. There is still half and inch of gap between the edge of the screen and the limit of the raster.

I made a gallery of pictures with and without the cap installed parallel to D18.

https://imgur.com/a/ewcyQtU


I have gone though the entire width circuit from the pins that connect to the yoke, all the way back to IC2, and tested every component out of circuit. Everything tests as expected. Caps are capacitating, diodes are dioding, resistors are resistoring, and transistors are transistoring.

I did find something interesting and don't know if this is normal or not, but when I turn off the TPG with the chassis still on the raster expands dramatically. When I turn the TPG back on the picture takes a moment to sync and the raster shrinks back down to being much too small.

Here's a video.
https://youtu.be/SMB6NSYDrHk

I'd appreciate any advice because I'm stuck at this point. Thanks for reading.

[MKL]

Increase the capacitance of C38 (and remove the cap you put in parallel with D18).

[Polish-Jack]

Thanks for trying to help, MLK. I tried what you recommended and increasing that width cap makes the raster smaller. Decreasing the value makes raster wider, but only by a little bit, and ends up crushing the picture on the right side.

Original 0.39uF cap in location C38


Result of C38 cap value increased to 0.56uF


Result of C38 cap value decreased to 0.15uF


The green painter tape is there for reference.

[MKL]

And I assume the width coil is adjusted for maximum width.

I would try different values for C36 (which now should be 6.1 or 6.2nF).

[Polish-Jack]

C36 is the "critical safety capacitor", right? The one in parallel with the horizontal transistor. I thought that was just for picture stability and nothing to do with picture size.

I'll try that today, thanks.

[Polish-Jack]

I tried different values as suggested and it worked to expand the width of the raster, but lowered voltages. I had so max out the flyback brightness to get a dim picture. The original cap was a 0.0061uF and I managed a proper width with a 0.0110uF cap.

I'm starting to think I might just be that unlucky with the tubes in getting.

[MKL]

Yes, the cap varies the flyback or retrace time of the horizontal scan which has an effect on the secondary voltages of the transformer. Increasing the capacitance does in fact lower the voltages while decreasing it has the opposite effect and when the cap opens they're so high that the xray protection stops the operation of the horizontal oscillator so the changes in capacitance should be small and the anode voltage monitored. Maybe the horizontal yoke doesn't have an optimal inductance. Typically it's 2.0-2.5mH.

[Polish-Jack]

The yoke on this tube is 3.5mH, so that makes sense. I guess there is nothing left to fix with this chassis and I just keep hunting for a better donor tube.

Nuts.

I wonder if I can do some rewinding of the yoke...