Re: Star Wars 25 inch Amplifone issue

And Rodger guessed correctly. Sad face for me. :-(

After probing around with a scope, and checking signals, it became clear
something was going on in the tube. So we swapped the blue and green
drive wires in the connector, and powered it back up. If the issue was
with the electronics driving the tube, we expected the overall gree hue
would become blue. And if the problem was in the tube, it would stay
green. And it stayed green.

After some research, we found this:

   http://www.repairfaq.org/REPAIR/F_monfaqa.html#MONFAQA_002

And it looks like our three choices are:
1. Tap the neck (gently) with the crt face down and try to dislodge the short.

2. Try blowing the short out (with a current pulse), but this risks blowing out
the heater filament too. I have a Sencore CR-70 which I recently acquired, but
I've not yet powered it on. And the manual says it doesn't clear heater-cathode
shorts because it can blow out the filament too.

3. Remove the ground reference from the filament and float it.

The last option sounds intriguing. Does anyone think this is a bad idea?
Do I need to worry about improving insulation on the heater winding or wires to
the CRT? IIRC, the cathodes are only about 180 volts, so I think the insulation
on the wires will be sufficient. Will the floated voltage on the wires affect
other coils in the flyback? If so, we could create a separate power supply for
the heater.

Comments? Suggestions?

___
Ken

Rodger Boots wrote:
> Heater-cathode short in CRT green gun? Don't suppose you have a CRT tester?
>
> On Sep 11, 2013 6:10 PM, "Ken Sumrall" <k_lists@scrapheap.net
> <mailto:k_lists@scrapheap.net>> wrote:
>
> The monitor on our Star Wars sit down machine at work is having
> issues. It started with an intermittent issue where the screen
> would suddenly turn green, and it was very bright and all the
> retrace lines were showing, and all the lines were out of focus.
> Smacking the cabinet would sometimes bring back a good picture.
> Then we also lost vertical deflection, so we opened it up to take a
> look.
>
> The deflection issue was easy to fix. We replaced a few
> transistors, and a fuse, and it was working again. The other issue
> we're still struggling with, in fact it's much worse, and is mostly
> in the bad state, with occasional flickers of it working correctly.
>
> We initially thought it was a bad connection in the CRT socket, as
> it seemed to change as I was poking the socket with a wooden stick.
> So I replaced the CRT socket harness with a spare one I had. That
> didn't work. So poked some more, and started to think there was a
> loose connection in the focus block. But then poking further didn't
> do anything.
>
> So, we started to suspect that something is just randomly working
> for short moments, and whatever I was poking when that happened made
> me think that was the issue.
>
> We are now wondering if maybe the green drive transistor is shorted
> on? That would explain everything looking green, and the retrace
> lines, and the brightness. Would that also explain the poor focus?
>
> Any other suggestions?
>
> Thanks!
>
> ___
> Ken
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Received on Fri Sep 27 14:42:53 2013