Author Topic: Learning CRT repair.. black screen, no raster, but sound?  (Read 7383 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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Learning CRT repair.. black screen, no raster, but sound?
« on: January 16, 2018,01:14:33 »

downunder

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #1 on: January 17, 2018,06:05:09 »

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

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #2 on: January 17, 2018,08:35:39 »
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
« Last Edit: January 17, 2018,08:38:57 by flgliderpilot »

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #2 on: January 17, 2018,08:35:39 »

downunder

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #3 on: January 18, 2018,04:20:31 »

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

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #4 on: January 24, 2018,00:55:40 »
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.



« Last Edit: January 24, 2018,01:02:48 by flgliderpilot »

downunder

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #5 on: January 24, 2018,04:07:55 »

Yep, seems like you're on the right track. Keep at it.                   Bruce

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #5 on: January 24, 2018,04:07:55 »

astracat

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #6 on: January 28, 2018,14:45:29 »
hi,notice this is a monitor vid signal from computer ,if 120 voltage correct it may be poor earth connections on flyback tx suggest you resolder these first or make sure you dont have a frame collapse as that will blank the screen
good luck
astracats
ps final anode cap is between 18/20 kv so keep hands away from that
« Last Edit: January 28, 2018,15:01:59 by astracat »

Keith

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #7 on: February 01, 2018,04:22:35 »
Hi,

Working from my fading memory! Some of these monitors had 2 inputs TTL and analogue and I think there was a switch to select the input, make sure you are using the correct input.

Keith

And some only had TTL inputs!!
« Last Edit: February 01, 2018,10:46:50 by Keith »

flgliderpilot

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #8 on: February 05, 2018,11:26:52 »
Hey guys thanks for the advice.  Ok, so I got ripped off on my isolation transformer, seller was a scammer and it never arrived : |

I decided I'd work without it and omit the oscilloscope.  Grounded one lead with a clip and only held one probe when powered up and stayed clear of power supply and HV section.

So I first checked all of the transistors and they seemed good in diode test mode. I checked the power supply outputs and they are good.  I applied video to both inputs on the monitor (composite and digital RGB) and neither worked so this let me bypass a huge section of the schematic until those signals meet at the TDA3505 IC. 

VCC was supposed to be 12V at the resistor before pin 6, which I saw. After the resistor I saw 0V.  I pulled and replaced the cap in parallel with this resistor, and now I had .9V so I am at least going in the right direction.

I checked every cap surrounding this chip and connected in any way to 12V and all seemed ok.

I checked the resistance from pin 6 VCC to pin 24 GND and it did not seem to be shorted.  I decided an output pin must be shorted to ground further along, so I desoldered all output pins with no improvement.

I reconnected outputs, and desoldered the ground pin of TDA3505. 

With ground pin disconnected I have 5V at VCC.   So it was actually VCC shorting to ground.

After this I was able to press the green/monochrome button on the front of the monitor and the display would change from grey to green, but still no display (obviously since TDA7305 is off), and I can see retrace lines because screen control is turned up.  So the section AFTER this chip appears to be working.

My conclusion is that it must be a bad TDA3505, the chip is shorted out.

I'll order one today and see what happens next!

Not as scary as I expected as long as you discharge everything before hand. 

In the meantime I think I'll desolder all pins of TDA3505 except VCC and GND just to be sure since I have to desolder them when the new chip arrives anyway.



« Last Edit: February 05, 2018,21:48:20 by flgliderpilot »

Keith

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #9 on: February 06, 2018,05:18:04 »
An isolation transformer will only protect you from the live AC mains. You still have to work with caution as you will still get a shock from anything after the rectifier.

Keith

flgliderpilot

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #10 on: February 10, 2018,00:12:02 »
Ok, I was wrong on the 3505  :(  Replaced it but same symptoms.

The monitor has a high pitched whine. Flyback transformer failing?

Symptom is horizontal diagonal lines when screen control turned way up, but no picture.  Brightness control has no effect.

I can feel and hear static at the screen when it powers up.

So would a failing flyback transformer exibit these symptoms?

I'm not understanding how there can be diagonal lines and screen brightness when I crank up the screen control, and still no picture if it is really a bad flyback transformer. Could someone please explain.  My monitor repair book never describes a failure like this.

The whine had me convinced it was the flyback transformer so I've removed it and am searching for a (probably impossible to find) replacement.

Thanks
« Last Edit: February 10, 2018,00:13:35 by flgliderpilot »

jordan

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #11 on: February 10, 2018,01:46:45 »
Have you checked the transistors and other elements on CRT board on the tube ?
Greeting from Slovenia
Jordan Čebron

downunder

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #12 on: February 10, 2018,05:25:57 »

To my way of thinking, what you are seeing on the screen is normal. With the screen pot turned up, you are overdriving the picture tube. The retrace lines you see are normally blanked out in a normal picture so they are not seen (but they are still there).

The screen is lighting up so you can discount a faulty flyback transformer. It's been a long time since I worked on CRT technology, but I seem to recall a feature known as x-ray protection, which would mute the screen in the event of too much radiation. Might be an avenue worth researching.

Might be worth checking pin 7 of the fbt as well as R330 and R476 just to the right of pin 7 in the schematic. These are part of the ABL
[auto. beam. limiting] circuit and could KO the picture.                                    Bruce

astracat

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #13 on: February 10, 2018,08:51:12 »
hi have known the 12v supply to be bad for this problem, try hot air on this transistor to see if voltage fuctuates
regards
astracat

flgliderpilot

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #14 on: February 10, 2018,10:06:42 »
Have you checked the transistors and other elements on CRT board on the tube ?

Yes, I've checked the transistors on the neck board, and all the transistors from the TDA3505 to the neck board. Also check the HOT.

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Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #14 on: February 10, 2018,10:06:42 »

 

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