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Learning CRT repair.. black screen, no raster, but sound?

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flgliderpilot:
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!!

downunder:

Turn up the screen pot (shown just above the focus pot in the schematic). It may be on the flyback transformer or the CRT base board.

The extra brightness may indicate frame collapse which will normally blank the screen to prevent phosphor burn.

You have HT, EHT and filament volts, and yes, shorting collector to emitter on each of the 3 RGB output transistors will turn the relevant gun hard on.

If the frame is collapsed (no vertical scan) don't forget to restore the screen pot to it's original position. This will give you a start point.

Bruce

flgliderpilot:
Hi Bruce, thanks for the reply.  I don't know how I missed that screen control knob, I actually spent a lot of time hunting for it duh.

Ok so I turned the screen control up, and I can see the raster line.  There is no brightness and screen setting where I can get visible brightness without the raster line. Picture width and height, pincushion, etc, adjustments on the back of the monitor do seem to work.

I have a composite video signal attached at this point but no display.

See photo attached

downunder:

It seems to me there is no video drive to the CRT - don't suppose you have an oscilloscope - no. Check the voltages on the CRT base again - I see there is a -20V derived from the power supply, and check the outputs of the power supply.                            Bruce

flgliderpilot:
Hi Bruce, I do actually have an oscilloscope and when I looked at the base of the output transistors I didn't see a video signal... just some very low (microvolt) noise.

Ok so I am on the right track. HV side looks good it's got to be the power supply or PCB.

I'm just waiting on my isolation transformer to arrive so I can start tracking backwards hunting for the video signal (just to be safe).

In the meantime I'll check the power supply outputs.

Thanks for your help I really appreciate it.  I come across lots of old collectible but broken (1980s) computer monitors so I'd love to learn to repair them and save them from the dumpster. I have an interest in Arcade games too so it's good practice.  Some day these things will be valuable again.. sort of like vacuum tubes.

CRT repair books are kinda scarce these days. I just got a new one today "Computer Monitor Troubleshooting and Repair by Joe Desposito".. it actually has a picture of and discusses this exact monitor I'm working on, except they call it a Magnavox TY15. Seems this monitor has at least 4 different names.

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