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LCD retrofit old monochrome tube - questions

Updog molding

Plastic
Joined
Nov 16, 2017
Hello all,
I've done a fair amount of digging to try and solve this one myself, but I'm not an expert on video signals and could use a hand.

New to the forum, but long time lurker. A few years ago I started my own injection molding shop. I do all custom runs, mostly my own products. I have 3 machines, and they aren't new 😀.

So, my early 1990s arburg multronica decided to poop a tube. Replacement crts are $800 used, LCD kits are over $1500, and I think I've got it figured out how to diy....

The original tube was a Philips m24, monochrome green screen. 8.4" and 800x600 resolution. I pulled the board out, and the video chip is a tda2595. Looked up the datasheet and noted pin 11 is the composite signal in... I grabbed that pin and hooked up my o scope. Looks like the signal is a 20khz frequency. None of my monitors laying around can display it.

Can anyone point me in the right direction? I can post o scope images, modern LCD displays when plugged in, and anything else helpful.

Looking into frequency doublers and all that, but I don't know the first thing...
Thanks a ton,
Jason
 
Have you tried any troubleshooting on the CRT? It's usually pretty easy to fix them. Composite video is usually 15kHz. You might have a TTL signal.

Verify you have high voltage by arcing the high voltage lead to the chassis (the one with the weird looking suction cup on it). Be careful, it's like 12,000 volts. It'll leave you hurting, like a coil on a gas engine.

If you have high voltage, the flyback transformer is probably good. You can then check the grid voltages. There's not much to them.
 
Thanks all to the quick reply.

Yes, I did check the tube. Bad fly back for sure! I tried to source a replacement, but it is impossible to find. At2140/160 I believe was the part number.

So, TTL signal vs composite, how do I tell? The chip claims it is composite. I'll look at some of those monitors in previous posts now...

Thanks!!
 
Typical monochrome composite just has 2 wires. One composite video signal and a ground. There may be another ground and a 12V power connection.

TTL will have horizontal sync, vertical sync, and intensity, and ground. Again, also will have a 12V and ground to power the thing.

Google "XVGA converter". It's kind of a universal converter that can take about any input and make VGA. Seems to be about $150 to $200 and all of them seem to be Chinese.


Have you checked the output of the horizontal output transistor (HOT)? Typically it is labeled Q2 and will probably have a heat sink on it. The HOT creates the pulsed DC that power the flyback transformer.
 
Ewsley, thanks again for the response.

I have tested the transistor with a meter, and it appears to be functioning correctly. Meaning, not shorted or otherwise bad that I can tell. I've tried to attach an image of the board... my signal appears to have h and v sync, not sure about intensity, and it does have 12v. Looking at the tracks, it is not too difficult to identify where they go..

I guess, I would much rather spend the time and effort replacing it with the LCD than repairing this old tube that is likely to fail again soon . In either case, composite or ttl, it looks like the xvga converters will do the job.. just need to identify the wires?
 
Here are the images... 10 pin connector, overall board..
fc0927c59d999f0e7e9255f14718148a.jpg
4389d290cbf5cf70d4a4c6e7f69bdcee.jpg
13fffda8455df6a71a86d5b24482ad89.jpg


Sent from my SM-G930V using Tapatalk
 
In the last picture, the trace at the bottom that goes to the left from the 10 pin connector is probably the video signal. It goes to a pot, that's probably the contrast pot.

The trace at the bottom that goes to the right could be the horizontal or vertical depending on the function of the IC it heads to.

I'd stick my scope on the HOT and see if you have any horizontal output. I know LCDs are the way of the future, but you might be able to fix this thing with $10 in capacitors and a new horizontal driver IC.
 
R22? near the middle of the board would be my first choice to test for open circuit with a multimeter. It looks like a high value resistor running hot and a prime candidate to fail.
 
Okay, I will have to reconnect it all tomorrow, as the board is not with me in the shop. I can provide additional pictures of my testing peocedure. I have experience fixing many things, but not tubes!

I sincerely appreciate your help.
Jason

Sent from my SM-G930V using Tapatalk
 
OK. Normally, no high voltage output from the flyback means a problem in the horizontal section of the board. The actual flyback transformers are pretty reliable.

There's 2 ICs on the board. One is for the vertical, one is for the horizontal. You should be able to figure out from a data sheet which is what. Find the horizontal and start poking around.

In the horizontal section, you have a horizontal input and an oscillator. It does some magic and creates a clean output pulse to a transistor and steps that up to a bigger transistor and that creates a DC pulse to run the flyback trasformer.

If you can't find a schematic for your board, just look for something close. You will find that they all basically work the same way. Many will have almost identical components.
 
Noted.

I'll do some digging and report back on my findings.

Looking forward to getting this machine back to life!

Sent from my SM-G930V using Tapatalk
 
Many-a tv repairman met their maker over the years fiddling with tube tv's. Be careful! The high voltages can hang around a while, don't take much at 12,000 volts to stop the old ticker...

ToolCat
 
Hello all,
I've done a fair amount of digging to try and solve this one myself, but I'm not an expert on video signals and could use a hand.

I grabbed that pin and hooked up my o scope. Looks like the signal is a 20khz frequency.

The signal is more complex than just 20khz.
 
Recognizing the various types of video signal is not overly difficult. If you had posted some waveform photos I and others here could probably do it in an instant. Here are some examples:

NTSC composit video signal:
This signal combines horizontal sync pulses, vertical sync pulses, and a color subcarrier reference, with the actual video signal. The video signal itself consists of the B&W information which is represented by a simple Voltage level and the color information which is encoded on a 3.579 Mhz subcarrier. In short, an NTSC composit video signal is a very complicated affair. It is easy to recognize on a scope display but you must keep in mind that it can be displayed at either a horizontal or a vertical rate and the two will look somewhat different. Here is one explanation of how it is put together. Normally, but not always, it will be a ONE Volt, peak to peak signal. This level will almost always be used for outside connections but inside a device (TV or monitor) it can be different. Also, it will normally be connected to a device with either a BNC or an RCA phone connector. But it can be passed through almost any type of connector and the exceptions are all over the place. It is a signal that is passed along on a single wire and through a single connector pin, in addition to a ground wire and pin.

http://www.broadcaststore.com/pdf/model/793698/TT148 - 4053.pdf

TTL Video:
TTL video is just that, a video signal that is converted to TTL logic levels. So it will not have all the features of an NTSC signal. No color information. No gray scale information because a logic signal can have only two states and they wind up representing black and white (or green, yellow, or whatever your tube displays). Thus, a TTL signal is used for displaying characters only because they can be generated with just a black and a white signal, NO shades of gray.

I believe TTL video was generated with three signals, the actual video, a horizontal sync, and a vertical sync signal. So there would be three pins, in addition to ground, involved.

On a scope a TTL signal would look like any other logic signal: lots of pulses with highs and lows but no intermediate values. On the video TTL line you could see longer low levels (areas with no pulses) representing the horizontal and vertical sync rates.

I would look at the pins that are used on your ten pin connector that is near the board's edge. (There is another ten pin connector in the middle which probably connects to the CRT.) If you see logic type signals on one or three of them, then you probably have a TTL connection. If one of them has something that resembles the NTSC composit video signal (without the color subcarrier) then you probably have composit video but not necessarily the NTSC variety. Higher definition versions of the composit video signal have been used.

In either case, you will need to measure the horizontal and vertical sync rates with some precision. This is necessary because monitors are built with any number of spec ranges and if you want to purchase one that can be used in place of the CRT monitor you presently have, you will need to match the specs. If you have TTL signals this should be simple, just measure the frequency of the horizontal and the vertical signals. If you have a composit video signal then you will need to measure the time interval between the leading edge of the horizontal and of the vertical sync pulses that are incorporated in it. To do this you will need to use two different horizontal sweep settings on your scope. Since the horizontal sync rate is going to be 15,750 Hz or better, you can set the sweep to display about 100 us horizontally across your scope display will be a good starting point. The vertical rate will be 50 Hz or higher so you can start with a sweep rate that displays about 50 ms.

Triggering a scope on a composit video signal can be difficult if it does not have the H and V settings which provide filters for those rates. But, if your video is not strict NTSC composit video, then even those settings may not be a lot of help. Choices of DC and positive edge can help. Low Pass for vertical sync and High Pass on none for the horizontal sync.

If you have questions on this, please post some scope photos.



Ewsley, thanks again for the response.

I have tested the transistor with a meter, and it appears to be functioning correctly. Meaning, not shorted or otherwise bad that I can tell. I've tried to attach an image of the board... my signal appears to have h and v sync, not sure about intensity, and it does have 12v. Looking at the tracks, it is not too difficult to identify where they go..

I guess, I would much rather spend the time and effort replacing it with the LCD than repairing this old tube that is likely to fail again soon . In either case, composite or ttl, it looks like the xvga converters will do the job.. just need to identify the wires?
 
Recognizing the various types of video signal is not overly difficult. If you had posted some waveform photos I and others here could probably do it in an instant. Here are some examples:

NTSC composit video signal:
This signal combines horizontal sync pulses, vertical sync pulses, and a color subcarrier reference, with the actual video signal. The video signal itself consists of the B&W information which is represented by a simple Voltage level and the color information which is encoded on a 3.579 Mhz subcarrier. In short, an NTSC composit video signal is a very complicated affair. It is easy to recognize on a scope display but you must keep in mind that it can be displayed at either a horizontal or a vertical rate and the two will look somewhat different. Here is one explanation of how it is put together. Normally, but not always, it will be a ONE Volt, peak to peak signal. This level will almost always be used for outside connections but inside a device (TV or monitor) it can be different. Also, it will normally be connected to a device with either a BNC or an RCA phone connector. But it can be passed through almost any type of connector and the exceptions are all over the place. It is a signal that is passed along on a single wire and through a single connector pin, in addition to a ground wire and pin.

http://www.broadcaststore.com/pdf/model/793698/TT148 - 4053.pdf

TTL Video:
TTL video is just that, a video signal that is converted to TTL logic levels. So it will not have all the features of an NTSC signal. No color information. No gray scale information because a logic signal can have only two states and they wind up representing black and white (or green, yellow, or whatever your tube displays). Thus, a TTL signal is used for displaying characters only because they can be generated with just a black and a white signal, NO shades of gray.

I believe TTL video was generated with three signals, the actual video, a horizontal sync, and a vertical sync signal. So there would be three pins, in addition to ground, involved.

On a scope a TTL signal would look like any other logic signal: lots of pulses with highs and lows but no intermediate values. On the video TTL line you could see longer low levels (areas with no pulses) representing the horizontal and vertical sync rates.

I would look at the pins that are used on your ten pin connector that is near the board's edge. (There is another ten pin connector in the middle which probably connects to the CRT.) If you see logic type signals on one or three of them, then you probably have a TTL connection. If one of them has something that resembles the NTSC composit video signal (without the color subcarrier) then you probably have composit video but not necessarily the NTSC variety. Higher definition versions of the composit video signal have been used.

In either case, you will need to measure the horizontal and vertical sync rates with some precision. This is necessary because monitors are built with any number of spec ranges and if you want to purchase one that can be used in place of the CRT monitor you presently have, you will need to match the specs. If you have TTL signals this should be simple, just measure the frequency of the horizontal and the vertical signals. If you have a composit video signal then you will need to measure the time interval between the leading edge of the horizontal and of the vertical sync pulses that are incorporated in it. To do this you will need to use two different horizontal sweep settings on your scope. Since the horizontal sync rate is going to be 15,750 Hz or better, you can set the sweep to display about 100 us horizontally across your scope display will be a good starting point. The vertical rate will be 50 Hz or higher so you can start with a sweep rate that displays about 50 ms.

Triggering a scope on a composit video signal can be difficult if it does not have the H and V settings which provide filters for those rates. But, if your video is not strict NTSC composit video, then even those settings may not be a lot of help. Choices of DC and positive edge can help. Low Pass for vertical sync and High Pass on none for the horizontal sync.

If you have questions on this, please post some scope photos.
Very detailed description. I appreciate all those who are trying to help. I'll get some scope images tonight, hook up the old board, and probe around.

Maybe we will get this thing figured out.

Jason

Sent from my SM-G930V using Tapatalk
 








 
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