Reverse engineering of the
The VR299 Decstation monitor

The VR299 RGB monitor was manufactured by the Digital Corporation. It was a very large nineteen inch monitor for "professional use" (whatever that means) meant to be used with Digitals' range of unix workstations like the Decstation 3100.  I had acquired on of these  monitors in 1996 when 19 inch monitors were still "very cool" and only heads-of-dept were important enough to have one. I was given mine because it was faulty and out of  warranty and the repair quote was just short of $1000.

The monitor was beautifully made, solid ,fully EMC screened, pressed steel chassis, a magnificent Mitshubishi CRT. It has a high bandwidth RBG video chain and a ruggedised power supply.   As they came they were not suitable for personal computer displays. The RGB video signal came on 3 seperate 75ohm coaxial cable with a composite XORed sync mixed on the Green signal.  This was a fixed scan rate monitor and contained circuits to detect if the the horizontal pulse rate was outside spec and if so, to protectively shut down the monitor. It was also insanely heavy and bulky!

The acquisition of this monitor launched a successfull project to convert the video and the deflection on the monitor so it would be suitable for PC use.  IT was not possible to get service manuals or schematics from DEC at the time (now or ever!) so I began the heavy task of reverse engineering it so I could discover the horizontal and verticle timing components and substitute suitable  values that would permit operation on PC SVGA  signal timings.

The process of deriving the circuit consumed about two weeks.  It required totally methodical persistence and a large supply of pencils and erasers!  I have not redrawn the circuit as "enginnering grade" schematics because it wasnt necessary for me and I had run out of time!  Even though it was a long time ago I still remember the thrill of running it in scattered pieces on the floor connected up with jumper wires and paperclips  and getting the first sync locked stable display of 1024 x 768 16bit colour. This was big cheese back then. With this I could impress my friends and awe my enemies!

My reverse engineering drawings are presented here, as well as a short paper I wrote in 1996 that documented the use of this monitor for PCs and the required modifications.

I really have no idea why these drawing might be usefull, but I guess I am sort of attached to them as they took me about two months of my scarce time to create.  Maybe you are repairing large CRT monitors and cannot get any schematics ?  This design appears to use the chip manufacturers application note reference designs. This design would be representative of fixed scan professional monitors from this era, more recent monitors make heavier uses of VLSI for dynamic scan correction and variable scan rates.

Sadly, the display tube in my monitor became gassy with the symptom being that the bottom third of the display became darker and darker, with the zone of darkness increasing in size when finally in 2001 I bade a sad farewell to it and it continued the journey to landfill that I had interupted. For the few years that I had it running it played games, ran windoze, impressed my friends and awed my enemies.  I have no pictures from this era, it was before cheap digital cameras.  These reverse engineering notes were used successfully by other radio amatuers; we finally discarded the last lab of these  some time around year 2000.

The Modification of the Digital VR229 monitor for PC use
or "my monitor is bigger than your monitor"

The CRT cathode drivers used MRF548 microwave power transistors and probably the complete video chain had a video bandwidth of over 400Mhz. Video amp also used BRF95 BRF96 microwave transistors.  

This diagram shows how the differant PCB modules were linked together with their cable harnesses.  

The vertical deflection system was on the same pcb as was most of the deflection system. It was easier at the time to derive to seperate circuits.  As with the horizontal deflection system there are elaborate function generators to correct the scan for linearity, dynamic brightness control and dynamic focus.  The deflection system used a generic consumer TDA2653A chip but with significant elaboration.

The above circuit is my sync generator and automatic monitor switcher.  The VR299 was a fixed scan rate monitor and for pc use, with its varying scan rate changes as it first booted DOS and then fired up Windoze made it hard to use without a standard scan vga monitor around. The standard vga monitor would be activated when the pc was in dos, or windoze had crashed out to a DOS screen. In 32bit mode, the SVGA drivers and scan rates would become active, this change being detected and latched with the monostable and D flip flop.  I reused the VR299 composite sync seperator as this was easier. I derived a composite sync from the PCs seperate H and V sync with the XOR gates.  I used CMOS logic here because there was a conveinient spot in the monitor that had 12 volts availalbe.  The annoying propensity of the pc to switch scan rates was the motivation for the incredible added complexity of the multiscan monitor.

This is the horizontal deflection system. It includes dynamic coarse and fine focus and scan intensity correction,pin cushion correction  and elaborate linearity controls.  An auxilliary winding on the scan/EHT transformer is monitored for a sustained collapse of flux which could indicate the H sync frequency being too low/high  or a failure, this would initiate an automatic shutdown of the horizontal deflection system.

This is the video processing card. This card was mounted on the back of the monitor and had 75ohm BNC connectors right on the card. The video gain/offset is managed with MC1496 multipliers and then amplified up to CRT drive level through microwave transistors. There were three video boards, this board which had low level video signals, a smaller board with heatsink directly mounted on the CRT which held the CRT cathode drivers and yet another board mounted directly on the CRT terminal pins.

But time has moved on. In the ten years since I went to this trouble, my converted monitor continued its interupted journey to landfill. Nineteen inch monitors became the standard, and now corporate dumpsters and council hard rubbish collections consist of functional nineteen inch monitors.  They have all been replaced by various LCD flatscreen devices that are mostly horrible to look at. A real CRT trully has an "inifinite" colorspace, whereas the LCD monitor at best has a 16bit colorspace. At least they dont flicker, eat deskspace and cost the earth to run.  There are many dead nineteen inch LCD monitors now appearing in hard rubbish. They are there because the cheesy power supplies  all suffer from bulging electrolytic capacitors.  This is a very easily repairable condition.  Thats a pity because there is allmost never anything wrong with the display.  Really old ones may have dim  illuminators