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The IBM5153 monitor, with modification, displays a beautiful 9.5" diagonal EGA image.

Wow, that's really cool! I would have expected that turning a single-sync monitor into a dual-sync monitor would require many more changes than that. If I acquire an EGA card before a 5154 I may have to try this myself.

One question: how does this mod affect the resilience of the monitor in the face of software causing the graphics card to generate incorrect sync timings? I'm thinking in particular of horizontal sync frequency but horizontal sync pulse width may also be of concern. Granted, I'm not sure how resilient a stock 5153 is to such things either. I've so far managed to avoid breaking mine despite playing all sorts of games with the CGA card (though I usually try things out on my composite monitor/TV first, since that's less precious). Also, the stories I've heard about breaking monitors by reprogramming the CRTC to screwy values seem to mostly concern 5151s rather than 5153s. Still, it would be good to know whether or not your modded 5153 is "fragile" like a 5151 (and whether or not a stock 5153 is "fragile" come to that).
 
I can't see there would be any significant issue. The sync H pulse timing or sync pulse width doesn't control the flyback period as that is fixed in the monitor's hardware. The monitor's own horizontal oscillator protects the functions and maintains basic scan and EHT stability, if it is sync locked, or not. Simply if the H sync pulse frequency or width is too far off, one way or another, the image simply drops out of lock with the scan, or the video information would have a phase or horizontal position error. In other words a bad H sync is no worse for the monitor than turning the H Hold control back and forth dropping it in and out of H lock. That is one great advantage of a PLL or flywheel circuit for the H oscillator and H.Hold system; random noise or garbage on the sync signal can only do so much to the basic operating frequency because the VCO that comprises the H oscillator can only respond relatively slowly over restricted range of frequencies. For other more primitive circuits where the H oscillator is triggered line by line, the effects of sync garbage can be a lot more significant in terms of a possible stress on the horizontal output stage.

There were some primitive designs of CRT monitors where they did not have their own H oscillators and used the sync signal as the time-base for that, in those the wrong input signal could have some serious damaging effects most likely.

In the case of the vertical sync for this 5153 modification, the CGA/EGA changeover is slow (heavily filtered) so if the vertical signal was very erratic the monitor will only slowly switch between the two modes.
 
Reenigne,

I just looked at the schematic for the 5151 for the first time, it appears that the horizontal output stage is driven directly from H sync, there is no local horizontal oscillator stage, which is a disaster waiting to happen in my opinion. If the horizontal output transistor is driven into conduction too long by an abnormal H sync, then more energy is stored at the end of scan time and the flyback voltage likely will punch through the H output transistor, although they have provided some protection. This design technique seems to be unique to the world of computer monitors. Even from the late 1930's all TV sets had their own independent H oscillator. Generally these gave little trouble, even with line by line sync pulses, because the sync pulse gets differentiated for its leading edge (is coupled into the oscillator with a very low value capacitance) and couldn't cause the output device to stay on too long by excessively modifying the drive waveform out of the oscillator. I don't know why IBM decided to make a monitor with no self contained horizontal oscillator. Maybe they considered the H sync was so stable and clean, being digitally generated (rather than out of a TV sync separator) it would be ok, but it saved so few parts it wasn't work the risk.
If I owned a 5151, I would put a small H oscillator board in it, and H sync it per line with a small value coupling capacitor from the H sync pulse feed. Free- running R-C oscillators are very easy to synchronize with a small pulse injected just before they are due to change state.

PS: luckily the amber monitor in the 5155 computer has its own H oscillator so when its fed a 21.85kHz EGA image and loses H & V lock and the picture "scrambles" no harm comes to it.
 
I've posted extensively about the 5151 Hsync issue on other threads. The big danger is that you'll run the horizontal frequency too high, which generally causes the flyback transformer to arc, thereby toasting it--and sometimes takes the HOT with it. The expense of a power transistor is of little concern when viewed in the light of replacing a FBT.

Yes, in my work years ago with SIMCGA, I managed to smoke a couple of monitors... :(
 
Chuck(G), that is very interesting that the 5151 fails with increasing the H frequency. In theory at least, as noted in my 5153 article in in section 5, in a system with fixed inductances L (the yoke and H out transformer) and a fixed power supply voltage V, the rate of rise of current in the H scan circuit is fixed initially at least at V/L. So that means as the scan frequency is pushed higher, the picture width decreases and there is less energy stored in the inductances at the end of each H scan, so the flyback peak voltage drops and so does the EHT. However if the drive frequency drops, the picture width increases, the peak scan current increases, the flyback peak voltage and EHT both increase which can cause destruction to the HOT and the H output transformer/insulation/EHT & EHT rectifier. So from that I would have expected the 5151 would be more likely to "smoke" with a lower frequency H scan drive than what it was designed for.

I have a Hero Jnr Robot which has the amazing vintage SC-01-A Phoneme IC in it, and it is pre-programmed with a few phrases, one of them is "I have the utmost confidence in this mission Dave" presumably a reference to 2001 Space Odyssey but another charming one: "Smoking transistors is hazardous to my health"
 
Also, the 5151 is the only monitor that I know of that plugs into the back of the PSU and is not powered up until the system power is turned on. I'm not sure why, but I seem to remember learning years ago that the 5151 would be damaged if powered up without receiving a signal shortly there after. Is this due to the same issues discussed here? or am I completely off my rocker?
 
That could be the case. Ideally, I'd guess that the 5151 is better to have a stable horizontal sync drive before or at least when it is powered up.

Thinking more about how a higher than normal H sync frequency could damage it, since the most of the horizontal output transistor's dissipation occurs when its in an intermediate state of conduction (as it is in the process of switching off or on) then the heat generated in it would be roughly proportional to the switching frequency. So if driving the monitor at some much higher H frequency the transistor could heat up and melt its junction, that would put DC across the output transformer and smoke it that way. So probably both low and high H sync frequencies away from the design center frequency will damage the 5151 monitor.
 
One thing I did notice before the smoke started was that the image became much brighter than usual. I suspect that the flyback circuit is designed with some compromises involving linearity and output. Time to get out my old "RCA Radiotron Designer's Handbook" which had a very good section on deflection circuits.
 
The brightness could have increased due to alterations in the crt's grid to cathode voltage, if it is just that and the image brightens up, there is increased EHT current, the EHT voltage drops a tad and the picture size slightly increases. On the other hand the picture will brighten with increased EHT alone, and if that is the cause, the picture size shrinks a little. (The amount of beam deflection in electromagnetically deflected tubes is inversely proportional to the square root of the EHT voltage, but beam deflection in electrostatically deflected tubes like scope tubes and early TV tubes like RCA's 7JP4 is inversely proportional to the EHT directly. This mathematical relation is one reason why electromagnetically deflected tubes won out for TV work, because for electrostatic ones they ended up requiring very high deflection plate voltages, even for tubes of 7 to 12" size as the required EHT got higher for brighter images and aluminised screens. Therefore they needed deflection amplifiers running very high plate voltages to get the required linear sawtooth voltages)
 
Very interesting modification!

Hugo Holden said:
There were some primitive designs of CRT monitors where they did not have their own H oscillators and used the sync signal as the time-base for that, in those the wrong input signal could have some serious damaging effects most likely.
Click to expand...

This maens the 5151 apparently.
Since I am currently playing around a bit with MDA/Hercules CRTC programming, do you have any idea whether this just applies to the 5151, or if most other MDA/Hercules monitors are just as easy to damage?
Because in my 'collection', I have an original IBM 5151, but also an amber Philips monitor and two Commodore paperwhite ones.
I am currently using the Philips because I trust the 5151 the least, but who knows, the Philips may be just as easy to blow up :)
 
ibmapc said:
Also, the 5151 is the only monitor that I know of that plugs into the back of the PSU and is not powered up until the system power is turned on. I'm not sure why, but I seem to remember learning years ago that the 5151 would be damaged if powered up without receiving a signal shortly there after. Is this due to the same issues discussed here? or am I completely off my rocker?
Click to expand...

My Philips monitor also plugs into the back of the PSU, but it has its own power button.
I've never understood why IBM didn't bother to put in a power button. Especially with monochrome, prone to burn-in, you'd want to turn off the monitor sometimes, while the system continues to run.
I think it's another case of poor decisions in cutting cost.
 
Hi Scali,

I think the way to tell for sure for any monitor type is to look at the schematics to see what happens to the incoming H sync, if it is used to synchronize an oscillator, or goes directly to the H output transistor's driver stage. Or simply if the monitor has a H hold control anywhere it is obviously fine.

I think the fix for this dilemma is simply a small add on pcb that has a two transistor or two gate multivibrator arranged with two different capacitor values to create the same frequency and on off ratio of whatever the original H sync has and one of the resistors partially variable for a H hold control, then an R-C differentiator to select the leading edge of the incoming H sync and inject that at one of the gates (or transistor bases) to synchronize it. That way when the H sync disappears or is random signals the monitor basically scans normally and doesn't shake off its mortal coil.
 
Hugo Holden said:
I think the way to tell for sure for any monitor type is to look at the schematics to see what happens to the incoming H sync, if it is used to synchronize an oscillator, or goes directly to the H output transistor's driver stage. Or simply if the monitor has a H hold control anywhere it is obviously fine.
Click to expand...

Ah, I see.
Having a H hold control implies that it has internal synchronization I suppose.
Well, at least on the Commodore 1402s I have, there are adjustments on the back:
monitor_1402_04.jpg


So that one might be 'safe' :)
Edit: here is the circuit, perhaps you can tell something from it, I'm not that good with electronics: http://www.manualslib.com/manual/1002615/Commodore-1402.html?page=17#manual

I'll have to check the Philips when I get home, but pretty sure it has those controls as well, at the back.
Then I need to see if any of them are H-hold.
 
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Unfortunately it appears that circuit you attached is another one without a local H oscillator inside the monitor.
 
Hugo Holden said:
Unfortunately it appears that circuit you attached is another one without a local H oscillator inside the monitor.
Click to expand...

Yup, I looked at the controls, and three of them were marked for vertical adjustments, and the last one was 'sub brightness'.

However, we seem to have a winner in the Philips. It's a BM7523 model. It has 4 controls on the back, for vertical size, vertical position, but also horizontal size and horizontal position.
Edit: This seems to be the schematic for that model: http://gona.mactar.hu/Commodore/mon...523_(likely_76BM13_too)_schematics_side_A.jpg
 
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