Too much voltage from isolation transformer

srarcade80

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So i've been trying to figure out this strange problem on this one cab. The screen fades out to black and stays dark. It randomly brightens a little bit and goes dark again. I checked out the signal cable, it was good, swapped out the chassis, same problem on another chassis. Figured the tube was shot, so I swapped the tube out, worked great for a day then problem came back. I put in another jamma harness and power supply, same problem. So then i tested the voltage going into the chassis from the isolation transformer. Its 134V! Tested the voltage going in, its 122V. So I ran the power to the monitor from another cab, confirming it was getting 120V from the line out of the isolation transformer. Powered up both cabs with signal from one, voltage for monitor from the other. Pic is still dark.

So with the 134V coming out of the isolation transformer, is that causing the tubes to go bad? I chucked the other tube I switched thinking it was dead since the new one fixed it right away but then had the problem again later. I would suspect it would wreck the chassis first. I have never seen this problem in 15 years.
 
sounds like a cold solder joint to me.
it could be your fly is failing as well.

what chassis is it??

Peace
Buffett
 
Thanks for the links, definitely exactly what I was experiencing.

Not a williams transformer. This was in an Atari drawer cabinet on the factory mounting plate, didn't note the brand on the iso. I have a few of these cabs/transformers and they have all been fine, just this one went bad suddenly. Now I have to determine if the damage was done on the chassis or tube, I'll be checking later.
 
Monitors are rated at plus or minus 10%.....some 20% on the input voltage. 134 volts shouldn't hurt them.

Edward
 
Just wanted to post the resolution to this mystery. After swapping chassis and tubes and still getting the same fade out problem, I knew it had to be something consistent, and I was out of ideas. I actually discovered by banging on the front of the tube with your fist caused the color to come back temporarily which said to me there's a poor connection somewhere. I had this same problem with my VGA crt back in the late 90s and I would beat the crap out of it to play everquest. I was digging on some TV repair logs from long ago and saw a similar problem and a suggestion that the high voltage was not persistent. So i discharged the tube again and took a good look at the anode and cap. The cap was a bit loose and I really didn't like the look of the solder joint on the wire to the clip so I re-soldered it very generously and also spaced out the gap on the clip so it would be tight in the anode. Sure enough this fixed the problem! Which means I trashed a perfectly good tube in the process of discovering this as well as replacing the isolation transformer. I went back and did the same to the other chassis I tested as well to be sure it won't have the same fate again too. Just another thing to add to my pre-charge check list when installing a chassis.
 
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