Author Topic: Learning CRT repair.. black screen, no raster, but sound?  (Read 12599 times)

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flgliderpilot

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Learning CRT repair.. black screen, no raster, but sound?
« on: January 16, 2018,01:14:33 »
Hi, it's nice to find this forum .. it's very hard to find information about CRT repair these days.

I am learning to repair CRT's and a Commodore 1902A / 1080 / Magnavox CM8505 is my first patient.

I obtained both schematics and service manual, and I have several CRT / TV repair books, but none specifically mention the issue I am seeing.

https://gona.mactar.hu/Commodore/monitor/schematics/Commodore_1084S-P_Monitor.pdf

There is no display, just a black screen. 

Brightness all the way up, still black.  No lines, no dots.

I can feel static electricity on the screen when I rub my hand across it.

When it turn the monitor off, there is a click and a short white flash on the screen as it goes off.

Audio works fine.

I see three orange glows (gun heaters?) in the neck.

I checked the voltage at the output transistors and I see around 100V on the collector, and 11V on the base.  Emitter to Collector is not shorted.

While checking these transistors my probe bumped the emitter one time, and I saw some red activity on the screen... which made me feel like the CRT is good.

I examined the circuit board and replaced some capacitors. I see no damage or bulging capacitors. I do see some work has been done to the flyback traces.

At this point I have some guesses:

1) Voltage from Flyback is too high or too low.
2) B+ from power supply is dead.. but if this were true there would be no static electricity feeling on the screen correct?

Based on your experience where should I check first?

I'm not going any further until my isolation transformer arrives.

Should I purchase a high voltage probe?  I don't think the tube voltage is adjustable.. I dont see a sub brightness or anything in this monitor.

Thanks!!

« Last Edit: January 16, 2018,01:16:45 by flgliderpilot »

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