Sanwa PM1723C, loses V-hold when warm

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frsj8112
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Sanwa PM1723C, loses V-hold when warm

Post by frsj8112 »

Hi, I'm working on a Sanwa PM1723C which loses V-hold when it has been running about 20 minutes.

It all starts out ok, but after 5-10 minutes the image start to twitch a bit. And after 15 minutes the image starts to roll vertically.
After 20 minutes it rolls so fast that I cant distinguish any graphics on screen.

I've checked all solder joints around the HA11235 sync IC.
I've replaced the V-hold pot
All caps have been replaced.
I've also socketed the HA11235 sync IC and replaced it with a known working one.
No difference at all.

Any pointers at what to look for?
I'm going to buy one of those air-in-a-can so that I can flip it upside down and shoot some ice cold air onto some areas of the chassis handling vertical sync.

I attached the schematics for this chassis as well.
Thanks
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grantspain
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Re: Sanwa PM1723C, loses V-hold when warm

Post by grantspain »

could be a diode or transistor breaking down, is it in the sync or hold circuit is the question
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frsj8112
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Re: Sanwa PM1723C, loses V-hold when warm

Post by frsj8112 »

Update!

I've found the problem.
This chassis has been put aside, I've taken a look at it and put aside again.
But now I had some spare time again and started to go over it again with the freeze spray.

Found that in the area around the H-size coil I could get the vertical rolling to almost go away, see video



I then sprayed this capacitor, C320, which some previous owner had replaced with a tantalum capacitor instead of an electrolytic. I had found the issue. When spraying this the rolling effect went away.
The tantalum had the correct specification, 47uF 20V (original was a 47uF 16V electrolytic) and I measured both ESR and capacitance and it checked out OK.

I replaced it anyways with an electrolytic capacitor, 47uF 50V and heres the result after having the monitor running for about 2 hours:


And here's that tantalum that desoldered:
forum_attach.jpg
Happy days :)
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ChrisBEANS
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Re: Sanwa PM1723C, loses V-hold when warm

Post by ChrisBEANS »

I have no idea what half of that meant, but congrats dude, glad it’s sorted :D
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grantspain
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Re: Sanwa PM1723C, loses V-hold when warm

Post by grantspain »

yeah good work, again someones previous repair causing problems- i see this sort of thing all the time
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