g07 pincushion after tube swap

TheShanMan

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Did a tube swap tonight with a tv tube whose yoke was almost a dead on match for a g07 yoke (a couple tenths of an ohm on the lower impedance and 1 or 2 ohms on the higher impedance). It looks great except for some pincushion on the left and right sides (horizontal monitor orientation). Not horrible but definitely noticeable.

I did a little searching and found reference to the possibility of improving a pincushion problem with changing a particular cap. The chassis has been recapped, but I'm assuming that reference meant that if you change the cap value it will alter the pincushion.

Is that method legit? If so, which cap? If not, is there anything I can do about it? Well, obviously it's possible to correct it, but it's a matter of whether anyone knows what it would take I guess!
 
So you're saying even though the schematic calls for a 100uf for C517, stick a 220 in there? Just making sure since you didn't give explanation. C518 is a 220 already (and is so in the schematic). Are you suggesting that C517 is the key component in pincushion, meaning that if I were to pick up some uncommon values like 120, 150, 200 uf (just checked mouser so I know they exist) I should try each one until I find the one that gives the best geometry?

Is there a similar cap on 4900's? I've got a 4900 with a swapped tube and yoke that has a similar issue. I can live with it but I'd much prefer to spend a half hour experimenting with cap values if I knew which cap to alter.

Thanks Ken!
 
C517 and C518 are wired in parallel. Capacitors in parallel add their value. So the 100 and 220 uf originals would total 320 uf. That value never did give a completely straight side to the picture. I've found over the years that as these capacitors heat up from use, their values deteriorate (usually it's the 100 uf that decreases it's value).

So use both C517 and C518 at 220 uf each (that equals 440 uf) and the sides will be straight. Plus as the caps age, the picture will remain straight for a much longer time. Generally, increasing the capacitor's values above 220 uf each in those two positions will make the sides bow inward.
 
I had to go to a 470 (and left the other one at 220) and it's still slightly bowed, but I can live with it and I'm afraid that if I go too far it might have some other negative side effect (longevity or something that I might not notice until the damage is done). Seems like 2 470's would be about perfect if there was no fear of a bad consequence.

It does bow inward on the top and bottom slightly too. If anyone knows what can be done for that I'd love to hear it, though it's not as bad as the pincushion I just improved so I can live with it.

The only other issue is I can only adjust the width to about a 1" gap on both sides. Doing a little searching led me to http://www.therealbobroberts.net/width.html so when I am ready for a parts order I'll have to include width caps in the order.

I love learning about all this stuff. Thanks Ken.
 
I have also seen things like bad width coils, flybacks about to blow up, low B+ voltage, and bad C412 cause pincushion problems on the G07.
 
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