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« on: July 23, 2016, 08:44:04 AM »
Hi.
I am restoring a pacman cocktail (low serial 0275 !). Already fixed almost everything (there was a lot of work to do), and i am now struggling to get a perfect picture on the G07.
I made several fixes to get the G07 working. First, I changed all the caps and the flyback. The new flyback was making a lot of high freq noise (I couldn't hear it, but my son complained about it, wondering how I could stay in the basement which such noise). Went back to the old flyback, noise gone. Couldn't get the image to sync... after googling, found out that a 390ohm resistor in the negative sync section was populated by a 4k7... swapped that, and got a picture. Could tune it, and it was looking pretty good (for a monitor of that age, and a burnt tube). That was on my bench.
The transformer of the pacman was toasted (the secondary was, the primary was still ok, and is used as a 220->110V auto-transformer. The 110V going to the isolation transformer). I replaced the big transformer with a switching power supply that I had around, and ordered a new isolation transformer to do 220->110V, but could only find a 230->120V. Tested that on the bench, still looked ok. Adjusted B+ to 120V.
Yesterday, re-assembled the cocktail, since everything is rewired, and working on the bench. Problem: the G07 has now a picture which shakes a little bit. I checked B+, and it was over 120V (around 125V). Turning the pot couldn't lower it below 123V. (OK to go higher). Measured other side of R1, and was getting around 180-190V ! I thought that my 120V transformer was too high, so... I reconnected the old isolation transformer, connecting the output of the first transformer to 125V tap, and the monitor to the secondary 0-115V. R1 better now, and I could adjust B+ again... but.. the issue is that B+ seems to drift. It climbs slowly, then seems to stabilise. Adjust B+ again to 120V, closed the cocktail. Issue still there... reopened the cocktail, B+ was down to 103V. Image still shaking... While at it, I looked at the B+ with a scope. In AC-coupling, there is a 60Hz ripple (around 1V p-p). I replaced the rectifier diodes (with 1N5404), replaced the 600uF/200V cap, and replaced the B+ pot (which looked pretty bad). Problem is still there, so I am a bit lost. I forgot to mention that the solders are fine, I did reflow most of them and inspected all.
The image is also a bit small on the horizontal side, but the width coil seems stuck, and I don't want to risk ruining it since I have no spare.
Any advice ? There must be something obvious that I miss.
-Fred
PS: also though that the switching supply could induce noise, so I run wires from my bench supply to the board, but it didn't make a diff.
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