Author Topic: Fix vertical hold on a monochrome monitor  (Read 1096 times)

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Offline rea5245

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Fix vertical hold on a monochrome monitor
« on: June 10, 2021, 07:12:57 pm »
Hi,

I have a monochrome monitor with a beautiful amber display, but I can't get the vertical hold to stay put.

It's a Magnavox 7BM623 074T. I can carefully find a sweet spot for the V Hold to stop the screen from rolling, but if I turn the computer and monitor off then on, it loses the hold and starts rolling slowly.

I've replaced all the electrolytic caps and the V Hold potentiometer. The problem remains.

Anything else I can try?

Thank you,
   Bob
 

Offline bob91343

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Re: Fix vertical hold on a monochrome monitor
« Reply #1 on: June 11, 2021, 05:48:00 am »
You would get better results if you had a schematic.  The synchronizing pulse for the vertical oscillator is apparently weak or nonexistent.  You need to find that circuit and troubleshoot it.
 

Offline PKTKS

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Re: Fix vertical hold on a monochrome monitor
« Reply #2 on: June 11, 2021, 11:15:54 am »
You would get better results if you had a schematic.  The synchronizing pulse for the vertical oscillator is apparently weak or nonexistent.  You need to find that circuit and troubleshoot it.

That is true..

When you *CAN*  pause the rolling even for a second...

it  means the local oscillator is perfect

but you have problem w/the sync integrator..
Just try to find hard that part of the circuit...

The fault should be at the integrator.

Paul
 

Offline rea5245

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Re: Fix vertical hold on a monochrome monitor
« Reply #3 on: June 11, 2021, 12:00:43 pm »
You would get better results if you had a schematic.

Yeah. Alas, I do not, and Google can't find one either.
 

Offline themadhippy

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Re: Fix vertical hold on a monochrome monitor
« Reply #4 on: June 11, 2021, 12:13:24 pm »
I thought the correct fix for a rolling crt display was a sharp thump on case,or does that only apply to televisions?
 

Offline rea5245

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Re: Fix vertical hold on a monochrome monitor
« Reply #5 on: June 11, 2021, 04:32:46 pm »
but you have problem w/the sync integrator..

Pardon my ignorance, but what is the integrator? Is it related to phase-locking with the input video signal? Is it likely to be an integrated circuit, or just some discrete components? (This monitor was manufactured in 1990. I only see one IC in it.)

- Bob
 

Offline PKTKS

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Re: Fix vertical hold on a monochrome monitor
« Reply #6 on: June 11, 2021, 04:36:43 pm »
but you have problem w/the sync integrator..

Pardon my ignorance, but what is the integrator? Is it related to phase-locking with the input video signal? Is it likely to be an integrated circuit, or just some discrete components? (This monitor was manufactured in 1990. I only see one IC in it.)

- Bob

the integrator is the modern term used to a LPF (low pass filter)

The synch pulse train is generally combined and the vertical
sync is mixed with the horizontal.. so you need to LPF the vertical
and HPF the horizontal pulses..

When they are combined in a usual stream..
Which is the likely in more old stuff CRT..

Digitally generated pulses are not like that

The integrator is just a LPF which will separate
the V synch in the SYNCH train.. (always H based freq,)

Paul
 

Offline rea5245

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Re: Fix vertical hold on a monochrome monitor
« Reply #7 on: June 11, 2021, 04:43:58 pm »
This monochrome computer monitor is driven from IBM's Monochrome Display Adapter standard, which has separate Horizontal Sync and Vertical Sync pins. So wouldn't that mean there's no need for a LPF?
 

Offline PKTKS

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Re: Fix vertical hold on a monochrome monitor
« Reply #8 on: June 11, 2021, 04:53:40 pm »
This monochrome computer monitor is driven from IBM's Monochrome Display Adapter standard, which has separate Horizontal Sync and Vertical Sync pins. So wouldn't that mean there's no need for a LPF?

That depends a lot on the circuitry

Do you have the CHIP number?  if it is a chip ..

All discrete ?

You need to carve that detail .. if you have the CHIP
controller number .. search for a usable datasheet..

old stuff is likely to be scanned in PDFs these days..

If it is really OLD  (amber stuff is old) it may be all discrete
and likely to have a mixed tech of analog and digital..

hard to say from here by guessing..

but just ONE CERTAIN:  the is a problem w/SYNCH PULSE somewhere
not tripping the local vertical oscillator properly

Paul
« Last Edit: June 11, 2021, 04:56:16 pm by PKTKS »
 

Offline rea5245

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Re: Fix vertical hold on a monochrome monitor
« Reply #9 on: June 11, 2021, 04:59:26 pm »
The only IC is a 74LS86 - a quad XOR gate. Everything else is discrete.
 

Offline PKTKS

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Re: Fix vertical hold on a monochrome monitor
« Reply #10 on: June 11, 2021, 05:04:27 pm »
The only IC is a 74LS86 - a quad XOR gate. Everything else is discrete.

For that shit  you need a 4 chan scope and a lot of patience.

Paul
 

Offline rea5245

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Re: Fix vertical hold on a monochrome monitor
« Reply #11 on: June 11, 2021, 08:06:47 pm »
For that shit  you need a 4 chan scope and a lot of patience.

I'm more worried that I need more knowledge of analog electronics than I have.

- Bob
 

Online BrokenYugo

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Re: Fix vertical hold on a monochrome monitor
« Reply #12 on: June 12, 2021, 07:21:41 pm »
Without a schematic for the forum to look at and guess the trouble point you're pretty much reduced to tracing the V sync signal and sketching out that partial schematic, and figuring out where it gets weak to pinpoint the faulty area.

Another thought, are you certain it's the monitor and not the equally old display adapter driving it? A scope of any sort will be useful here.
 

Offline rea5245

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Re: Fix vertical hold on a monochrome monitor
« Reply #13 on: June 12, 2021, 07:31:50 pm »
Another thought, are you certain it's the monitor and not the equally old display adapter driving it?

Well... no... now that you mention it, I guess I'm not certain. I don't have another monochrome monitor to test with the PC.

I have a scope though. What would you suggest I look at? The vertical signal, and whether it has a rock-solid frequency?
 

Online BrokenYugo

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Re: Fix vertical hold on a monochrome monitor
« Reply #14 on: June 12, 2021, 08:07:17 pm »
Without looking up the standard my assumption (a dangerous word) would be there should be a 5V TTL signal on the port and it shouldn't be distorted much when the monitor is plugged in.
 

Offline Ground_Loop

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Re: Fix vertical hold on a monochrome monitor
« Reply #15 on: June 13, 2021, 03:20:36 am »
I had the same problem with an old computer and monitor. It turned out to be the sync signal in the computer. I had a bad CMOS logic chip. Have you tried connecting a different device to the monitor?
There's no point getting old if you don't have stories.
 

Offline james_s

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Re: Fix vertical hold on a monochrome monitor
« Reply #16 on: June 13, 2021, 04:42:27 am »
I'm not sure why you replaced the pot, if adjusting the pot affects the rolling then the pot is obviously not the problem. I do not advocate shotgun replacement of electrolytic capacitors either, much too often I've seen people replace good quality parts with the cheapest no-name Chinese junk around and then mess something else up in the process.

Are you sure the video card is producing a valid sync signal? Is the signal getting all the way to the board in the monitor? The symptom you describe is exactly what you will see if there's a bad connection somewhere and the sync signal is not getting through.
 

Online vk6zgo

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Re: Fix vertical hold on a monochrome monitor
« Reply #17 on: June 13, 2021, 05:16:01 am »
Without looking up the standard my assumption (a dangerous word) would be there should be a 5V TTL signal on the port and it shouldn't be distorted much when the monitor is plugged in.

If you have a scope, have a look at the back of the sync input socket, or the equivalent pin of a multiway socket.
If there is a signal, & it looks correct, the connector, cable, & display adaptor is ok.
You should be able to trace either a cable, or if it directly connects to the PCB, the track which runs from that point.(often, if a cable is used, it connects to the PCB, so you will end up following a track, anyway).
There may be a dry joint, or other disconnection--------if so, the signal will either disappear, or become reduced in amplitude.

If that hasn't happened, you can then probe around the device it ends up connected to.
If this is an unidentified IC, take note of the type number, then Google for it, as these often show typical circuits using the device.

Many of these monitors are fairly generic, so it also may be possible to find the schematic of a similar one.



 

Offline rea5245

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Re: Fix vertical hold on a monochrome monitor
« Reply #18 on: June 13, 2021, 01:16:10 pm »
I don't have another monitor or another PC to try it with.

On my scope, the V Sync signal seems fairly steady, but there are two odd things, and I don't know if they're significant. First, the scope (a Rigol DS1054Z) calculates the V Sync signal alternately as 50Hz and 49.5Hz. Second, if I watch the trace for several seconds, I can notice that it's slowly crawling the left. But is that really a problem with the signal, or just with how the scope is calculating and drawing it?

At the same time the scope's trace is slowly crawling, the monitor's V Hold is stable enough that the screen is not rolling.

As I mentioned before, if I turn the monitor off and on, it loses the vertical hold. If the problem is in the signal from the PC, would you expect that I could adjust the V Hold knob and stabilize the screen, then lose it by turning the monitor off and on?
 

Online BrokenYugo

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Re: Fix vertical hold on a monochrome monitor
« Reply #19 on: June 13, 2021, 01:57:15 pm »
Did you happen to get a picture of the scope trace? Probably a monitor side problem if it looked something like this.

http://minuszerodegrees.net/mda_cga_ega/mda_sync_ver.bmp
 
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Offline rea5245

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Re: Fix vertical hold on a monochrome monitor
« Reply #20 on: June 13, 2021, 02:02:49 pm »
Here's a pic of the vertical sync.
[attachimg=1]
 

Offline james_s

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Re: Fix vertical hold on a monochrome monitor
« Reply #21 on: June 13, 2021, 07:02:03 pm »
That sync signal looks fine to me, if it's crawling it's due to the trigger settings on the scope, you'll have to adjust those to get it to display properly there. The frequency readout changing is also totally normal, scopes are not precision frequency counters. The sync at whatever point you are measuring is ok, but it must be disappearing somewhere before it gets to the vertical oscillator.
 

Offline PKTKS

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Re: Fix vertical hold on a monochrome monitor
« Reply #22 on: June 14, 2021, 10:38:37 am »
Here's a pic of the vertical sync.
[attachimg=1]

Place BOTH waves using the OSC as primary trigger...

The SYNC is failing to adjust the OSC properly..

if it is deviating due to aging components *you may* see that..

Paul
 

Online vk6zgo

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Re: Fix vertical hold on a monochrome monitor
« Reply #23 on: June 14, 2021, 01:39:26 pm »
Here's a pic of the vertical sync.
[attachimg=1]

The 1054 should have no trouble triggering, but the very low displayed signal level may be a bit problematical.
It would probably be a good idea to reduce the volts/div so the display is say, 5 divisions in amplitude, then try retriggering.
Another idea is to reduce the time/div setting, as you don't really need to look at so many cycles.
 

Offline rea5245

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Re: Fix vertical hold on a monochrome monitor
« Reply #24 on: June 14, 2021, 01:52:30 pm »
the very low displayed signal level may be a bit problematical.

According to the MDA spec, high voltage levels are 2.4V to 3.5V. See http://minuszerodegrees.net/oa/OA%20-%20IBM%20Monochrome%20Display%20and%20Printer%20Adapter.pdf, page 9.

I'm surprised. I would've guessed 5V. But apparently not.
 


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