Re: 577 Bad CRT?

Tom Lee

There could be several causes to the flashing. Sorry if I am repeating some advice you've gotten, as I haven't the patience to read the entire thread.

Grid-cathode shorts are not an uncommon problem, and intermittent ones can cause intermittent brightening. These shorts can be external as well as internal. Check the area around the crt pins on the little board that holds the socket (if there is one), and along the entire path that G1 takes, from CRT back to the intensity control circuitry. Clean up any carbon tracks you might find, and touch up any solder joints that look suspicious.

If, after extensive troubleshooting, you become absolutely certain that the crt itself is to blame, you have nothing to lose, so you can attempt a couple of fixes. The first is purely mechanical. Orient the unit so that the crt faces downward. Then gently, with a well insulated tool, power up the unit and tap on the rear of the crt. With luck you may be able to dislodge the fleck of cathode material that may be the cause of the problem. The reason for the face-down position is to make sure that the fleck, if there is one, falls out of the way. The grid-to-cathode spacing is extremely small, perhaps a mm or two, I would guess.

If tapping has no effect, then you go to Danger, Will Robinson mode. Details are here:

In my TV service days, I was the designated DWR tech (kids were expendable; boss was not). I rescued many CRTs this way, both back then, and afterward (most recently an ancient Mac Plus). There's a risk of killing the CRT, so you don't do this until you've ruled out everything else.

Good luck. That curve tracer is a beauty, and well worth repairing!


Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 1/17/2021 20:50, DW wrote:

I am crossing my fingers that hopefully the CRT is okay but in the case that it isn't I have another broken 577 which I think I can salvage a CRT from but in any case your offer is appreciated.

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