Conversion of a Digital Monitor Model VR299 for use with IBM Personal Computers.


  C opyleft July 1996 Ralph Klimek





The Digital Model VR299 Monitor is designed to be used in conjunction with a Decstation Model 2100/3100. It is a fixed frequency scan rate monitor and was designed for use with the Decstation only. With the obsolescence of the Decstation, it was noted that there was still 2-3 years of life left in the monitor. The extreme cost of 20 inch Personal Computer Monitors and the question of whether the VR229 could be converted cheaply to PC use inspired this research; notwithstanding the "status symbol" attribute of a twenty inch monitor on ones' desk.



Background Information 



The monitor is supplied with video with RGB 75Ohm coaxial feeds with composite sync on green. Its designed scan rate is 54Khz line and 60Hz frame; which is not closely generated by any pc svga card. To make the project worthwhile the target scan rate should be able to accommodate 1024 by 768 pixels by 256 colours for use by Windows.  The nearest compatible set of frequencies generated by my Tseng Labs ET4000 svga card running in mode 38H was 56Khz line and 70Hz frame. The other frame rates were too low for the Vr229 vertical scan circuitry and would have needed extensive re-engineering. 70Hz was chosen as it meant as it was merely 14% higher. As a consequence only one capacitor in the Vertical Oscillator circuit need changing. The difference in the line rate was close enough to be accommodated by the pre-set controls. 1024 by 1024 pixels is not an option with this monitor because of its aspect ratio. The ET4000 could not produce 1024 x 1024 at 70Hz frame rate.


The other major concern was if different scan rates would be tolerated by line and frame flyback and EHT systems. To answer this question and not risk destruction of my VR229 would require the schematics. These ,off course, were not available so the entire circuit was traced by hand. Would there be enough leeway in the scan correction circuits? I could not determine this from my schematics and the SigneticsÔ data sheets for the TDA2595 and TDA2953A line and frame controller chips. In any case the design examples in the Signetics documentation and Digital's implementation were very different The scan correction and dynamic focus is very sophisticated and was meant to be scan rate independent 


Technical Description. 




The VR229 has composite sync fed on the Green RGB signal. This is picked up in the video amplifier board from the green coax feed, passes through some amplifiers and pulse shapers and what appears to be a MIller integrator to remove transient video signals that might confuse the line and frame sync processing.  This signal is passed to the Deflection Board which contains all line and frame signal and power circuits. It goes to the TDA2595 chip. This chip does composite sync separation, horizontal and vertical blanking generation, spot suppression, horizontal oscillator and phase locked loop. It was designed to be used for domestic television sets but still works well enough at a 400% increase of its design spec. (Well done Signetics!)


The extracted vertical sync goes to the TDA2653A chip and was meant to work in tandem with the  TDA2595 as the vertical sync input is also used as the Vertical Blanking output stage. Its quite convoluted; I decided against trying to create a vertical sync input stage here but let Signetics do it. ( on the assumption that they know more than me!) The vertical controller chip also directly drives the vertical deflection yoke and contains the rudiments of scan correction circuits. It provides a linear ramp that signals the vertical scan position which is then used by the trapezoid, pincushion and dynamic focus circuits.


The first video amplifier board contains board traces that were meant to accommodate separate line and frame sync inputs. This was also meant to work in conjunction with the TDA2653A/TDA2595 tandem. The separated vertical sync makes a detour via the video board as a fossil relic of a different Model monitor design. I could not make use of the relic circuits; another IC was meant to be installed on the video board it was not possible to determine what it was from the relic board traces. It was necessary to generate a composite sync signal from the separate H and V sync from the svga card. This was simply accomplished with some pulse amplifiers and a CMOS XOR gate which XOR the H and V Sync signals. As the TDA2595 no longer had to discriminate the Green video signal as well it has no difficulty with the XORed composite. The front and back porch signals are no longer within the original specification but the effect is not objectionable. It may be possible to obtain extra horizontal line drive to increase the width of the display by another 10% if power dissipation in the line output stage is not excessive. Set the width control to maximum 


There is a protection circuit which shuts down both vertical and horizontal output stages if insufficient flux is detected in the line output flyback coil, the threshold being set by the 10 turn pot nearest the TDA2595. This should not have to be reset at the modified scan rate. 


Some care needs to be taken with the video feed. Ideally the video feeds from the svga card should be 75 ohm coaxial cable. However, subminiature 75 ohm coax cable is not readily available and ready made cable assemblies or composite cable is not commercially available except to large OEM's I have used a controlled impedance twisted pair cable Belden 9903 which is designed for Ethernet AUI cables and has an impedance of 100 ohms. The terminating resistors on the input of the first video amplifiers must be changed; the mismatch results in objectionable ghosting. For my development purposes, my video cable was 4 meters long. Each twisted pair gave me a signal and return ground and each pair has an independent screen returned to the same ground at both svga card and monitor end. 


Modifications required. 


The following circuit changes are required. 


1.        Change C44 from 0.47uF to 0.39uF . This will alter the Vertical Oscillator centre frequency and put the the VHOLD control in the right range. The range of the VHOLD control has been deliberately restricted, I suspect, to minimise dissipation in the TDA2653A chip at higher scan rates.


2.        Change R409, R439 and R4xx from 75Ohms to 100 ohms.

Note: It should be equal to the impedance of the video cable. 


3.        Fit a small muffin fan at the base of the main heatsink on the deflection board. The altered scan rates do make it run unacceptably hot otherwise. 15 volts is available from the main PSU. Mount the fan external to the metal case so that air can move through the went. Do not mount it internally as the rotating field from the fan will reduce colour purity and cause scan irregularities.


4.        Remove one end of R529 (1k0)that connects to R439. This is the composite Sync-on-Green take off point and will become the point where your generated composite sync will be fed.


5.        Construct and fit the composite sync assembly. Circuit is provided. Its inputs are the raw V and H sync from the svga card. Its only output is the composite sync signal connected to the lifted end of R529 as above. Power for this circuit is derived from the hot end of V502. This is a 1 watt zener providing a nominal 12 volts for the first video amplifier board. The XOR gate used for the composite sync assembly is a CD4070 quad XOR gate and was chosen for its ability to make do with arbitrary power sources.


6.        Using your svga card documentation and utilities ( last one to actually locate them is a rotten egg!!!) verify that your svga card can actually produce 1024 by 768 at 56Khz line and 70Hz frame rate. If it cannot then you require an upgrade. You will need at least One meg of video ram in order to have 1024*768*256 colours Test your card and modified monitor. Beware. My ET4000 has to have the power cycled before it would actually use the modified scan rates.

You cannot destroy the VR229 monitor with the wrong sync rate. I have tried and failed, but go easy on it anyway!  


7.        Run Windoze set-up and choose 1024*768 256 colours  large fonts option. You must have your svga card set-up disk handy at this time as new drivers are required. Start your modified windows and plug in your modified VR229. The opening Windoze logo still runs at the original vga scan rate and will not display on the VR229. The first sensible thing you will see is the Hourglass and your wallpaper file. Then its business as usual with some disclaimers.


8.        Modifications are required to the way your operate. The modified VR229 is not a multi sync monitor. Windoze and the svga paradigm assume you have one. This rules out running dos in full screen mode as it switches the scan rate back to vanilla vga. You can still open a DOS window. Use the PIF editor to edit DOSPRMPT.PIF and set the "windowed option" . Many DOS applications wont run it this mode and with this display, particularly most modern games that access the otherwise rarely used "HiColor" Mode (15bit pixel descriptor). You cannot throw away your old vga screen.