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This handy test instrument is just the shot for testing PC monitors, including VGA, MGA and composite video types. It’s especially valuable for servicing and for checking whether it’s the monitor or the video card that’s at fault. Design by C. C. ROHER* Y OU STARE AT the blank screen and it stares right back, as you wonder: “Is the monitor faulty or is it the video card?” If it’s simply the video card, there might be a couple of hours of work involved in getting the system up and running again. Simply buy another circuit card, install it, and away you go. On the other hand, if the monitor is sick, you might be looking at a lot more than just a few hours of down time – not to mention, lightening your wallet by at least $200. If you are like me, then you do your own repairs regardless of how much hair pulling it might entail. But to do this, you need effective diagnostic tools. A decent video source for testing the display is a good first step in the right direction. The PC Monitor Checker presented in this article doesn’t generate numerous PAL/NTSC colour signal patterns, nor does it possess the special functions found on commercial-grade video testers. However, it also doesn’t cost upwards of $1000 as do most of the off-the-shelf units. Instead, this is a fairly basic unit that generates vertical bars, which can AUGUST 1999  35 Fig.1: the PC Monitor Checker uses oscillator IC3a and appropriate decoding circuitry to generate the horizontal and vertical sync signals. IC8 produces the RGB signals for the EGA & VGA sockets, plus a video signal for the MGA socket. be fed to VGA, and Hercules MGA (monochrome) displays, as well as to composite-video monitors. On top of that, the PC Monitor 36  Silicon Chip Checker is inexpensive to build, is battery operated to make it portable and can be assembled in a few hours. All the parts, except for a rotary switch and two composite video connectors, fit on a single PC board, so the construction is really easy. Note that, in its present form, the unit is not suitable for testing flat-panel LCD monitors. Circuit description A complete schematic diagram of the PC Monitor Checker is shown in Fig.1. It consists of three sections: an oscillator (IC3a) with decoders for horizontal and vertical sync frequency generation, a sync section and an output section. Power is derived from a 9V battery which is connected to a 5V regulator (REG1) through switch Sla. The maximum current drain from the fully loaded unit is 15-20mA, so the battery should last about five hours. Alternatively, you could use a 9V DC plugpack supply. The unit has provision for VGA, EGA and Hercules MGA monitors, as well as composite video displays. Special Notice* Oscillator/sync frequencies. The circuit uses a crystal oscillator (IC3a & X1) to generate a 2MHz squarewave signal. This signal is fed to pin 3 of IC1a, part of a 4013 dual D-type flipflop, and then to pin 11 of IC1b. The 4013 divides the oscillator frequency to produce 1MHz and 500kHz square-wave signals, which are used to generate three horizontal sync frequencies and a 60Hz vertical sync pulse. In addition, the 1MHz signal is further divided and decoded by IC8 and used to produce the various video pulses. The sync section is divided into two sub-sections. One produces the horizontal sweep frequencies, while the other produces the vertical sync. Most common monitors use horizontal sweep frequencies in the 15kHz to 32kHz range, while 60Hz (or more) is used for the vertical sync. The 1MHz square-wave output from IC1a is also fed to IC2, a 4024 7-stage ripple-carry binary counter. The output of IC2 is then applied to MGA Socket (J1 ) 1 This project and article has been adapted with permission from an article which appeared in the May 1999 issue of the American magazine “Popular Electronics”. The original design did not include a PC board and so this has been produced by SILICON CHIP staff. Our prototype PC Monitor Checker worked well with a variety of VGA and MGA monitors and those with composite video inputs. The design also features a 9-pin socket for EGA monitors but when we tested it, it did not give colour bars with the two EGA moni­tors we were able to obtain. If you do not anticipate using it with EGA monitors, the relevant 9-pin D socket could be omitted. IC3b, IC4a, IC4b IC3c & IC5c, where the signal is decoded to provide three selectable (via S1b) signals: 15kHz, 20kHz and 32kHz. The selected output provides a fast VGA Socket (J2 ) EG A Socket (J5) Ground 1 Red Video 1 Ground Green Video 2 Ground 2 2 R. Intensity 6 Intensity 3 Blue Video 3 Pri. Red 7 Video 5 Ground 4 Pri. Green 8 H. Sync 6 Ground 5 Pri. Blue 9 V. Sync 7 Ground 6 G. Intensity These three tables show the pin connections for the MGA, VGA & EGA sockets. These are designated on the circuit as J1, J2 & J5 respectively. 8 Ground 7 B. Intensity 10 Ground 8 H. Sync 13 H. Sync 9 V. Sync 14 V. Sync negative-going pulse that is applied to 555 timer IC7. This IC is wired as a monostable and is used to generate the horizontal sync signal. Note that the selected output is also fed back through IC10c (1/6th of a 4069 hex inverter) to provide the reset signal for IC2, which then starts counting over again. The output from IC7 appears at pin 3 and is buffered by parallel inverter stages IC10a, IC10b, IC10e & IC10f. The resulting horizontal sync signal is then fed to pin 13 of the VGA socket (J2) and to pin 8 of the EGA socket. The horizontal sync signal for the MGA socket (J1, pin 8) is derived directly from pin 3 of IC7. Because the counter and decoders do integer division only, the 15kHz sweep frequency is really 15.15kHz (ie, divide by 132). That’s not a problem. Adjusting the horizontal sweep on older monitors produces a good lock while in VGA monitors, the sweep is automatically/internally adjusted, within certain limits. The horizontal sync signal is another story. Every monitor that was tested or researched appeared to have different sync time periods that range from 5-20µs, with most hovering at the greater time period. The retrace time determines how much picture is displayed horizontally. Potentiometer VR1 can be adjusted to produce sync widths of about 10-25µs. Now let’s see how the vertical sync signal is derived. In this case, the 500kHz output from IC1b at pin 13 is fed to IC6 (a 4020 14-stage ripple-carry binary counter) at pin 10. The binary counter then produces several output signals that are applied to IC9a (half AUGUST 1999  37 pulse widths seem to be unique for every monitor and ranged from 75µs to 1ms. In some of the monitors tested (MGA and composite types), a dark horizontal space appeared at the top and bottom portions of the screen. With the newer VGA types, however, the vertical size of the picture is adjustable and the spaces could be eliminated. The vertical sync signals from IC11 are directly applied to pins 14 & 9 of the VGA and EGA sockets, respectively. The signal from IC11 is also inverted by IC5f to produce the vertical sync signal for the MGA socket (pin 9). Monitor outputs Fig.2: take care when installing the transistors on the PC board. They are available in two different packages and the pin connections are different. of a 4012 dual 4-input NAND gate). This NAND gate decodes the signals, producing a positive pulse through IC5d that is fed back to the reset input of IC6 at pin 11. The fast negative-going pulse from IC5e is fed to pin 2 of 555 timer IC11, causing it to generate a 220µs wide, fixed vertical-sync pulse. Like the horizontal-sync pulse, the vertical-sync Many older model monitors, along with a few newer models, use the composite format. This format uses a serial signal that’s composed of video, vertical sync and horizontal sync. The video signal “rides” on top of the peak sync signal level in between the sync pulses. The entire signal is approximately 1V peak-to-peak, with the sync level being about 0.2V and the video ranging between 0.5V and 1V. The video amplitude determines the intensity of the displayed picture. In this circuit, composite video/ sync is generated by first ANDing the horizontal sync signal from IC10d and the vertical sync signal from IC5f using IC3d. The combined sync signal is then inverted using IC5a and mixed with the video signal from pin 10 of IC8 at the base of transistor Q1. Q1 is configured as an emitter follower and provides composite video/sync to both J3 (an RCA jack) and J4 (a BNC jack). Although there are no longer many MGA (monochrome graphics adapter) monitors out there, the checker provides an MGA output at J1. All of the MGA-format outputs are TTL compatible except intensity. The intensity output mimics the video output but at Resistor Colour Codes  No.   1   1   1   1   5   5   1   2   1   4 38  Silicon Chip Value 100kΩ 47kΩ 22kΩ 15kΩ 10kΩ 4.7kΩ 1kΩ 330Ω 100Ω 82Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown red red orange brown brown green orange brown brown black orange brown yellow violet red brown brown black red brown orange orange brown brown brown black brown brown grey red black brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown red red black red brown brown green black red brown brown black black red brown yellow violet black brown brown brown black black brown brown orange orange black black brown brown black black black brown grey red black gold brown Fig.3: the leads to switch S1 and to the battery can be run using light-duty hookup wire (eg, rainbow cable), as shown here. Note, however, that the connection between the board and the BNC socket must be run using 75Ω coaxial cable. a maximum level of 0.7V. As with the composite video level, the greater the amplitude of the intensity signal, the brighter the picture. Here, the MGA video output signal appears on pin 10 of IC8 and is fed directly to pin 7 of the MGA socket (J1). In addition, the signal from pin 10 is fed to a voltage divider and buffered by emitter-follower Q5 to provide the intensity signal. This is fed to pin 6 of the MGA socket and also to pins 2, 6 & 7 (R. intensity, G. intensity & B. intensity) of the EGA socket (J5). The VGA signal is made available through J2 (a 15-pin D-type connector). The 4017 decade counter (IC8) divides the 1MHz square-wave from IC1a into three separate video signals: PRIMARY RED, PRIMARY GREEN and PRIMARY BLUE. These signals appear on pins 2, 4 & 7 of IC8 respectively. In the VGA format, video-colour intensity is determined by an analog representation of the signal level, with 0.7V representing the brightest illumination. For this reason, the RGB outputs from IC8 are fed to resistive voltage dividers to produce the correct levels, after which the signals are buffered by Q2, Q3 and Q4, respectively. Buffering is required because the VGA video source impedance should be approximately 75Ω. The sync signals are at TTL/CMOS logic levels and are applied to pins 13 & 14, as described previously. EGA monitors are now fairly rare. However, we have included an EGA output in case you ever do have to service one of these monitors. As before, the primary RGB colour outputs (which are TTL/CMOS compatible) are provided by IC8 (pins 2, 4 & 7). These signals are fed directly to pins 3, 4 & 5 of the EGA socket. The colour intensity is controlled by the output of Q5 at its emitter. This transistor drives the RGB intensity control pins (2, 6 & 7) which are connected in parallel. The voltage on these pins, approximately 0.7V, gives the maximum intensity. Construction OK; now that you know how it works, let’s put it together. Virtually all the components mount on a PC board coded 04108991. Only the horizontal frequency selector switch and the two composite video output sockets (RCA and BNC) are mounted off the board, on the front panel. Check the board for etching faults before installing any of the parts, by comparing it with the published pattern (Fig.4). If the board corners are square, they should be filed away using a round file, until the edge of the arc is reached. This is necessary AUGUST 1999  39 Fig.4: this is the full-size etching pattern for the PC board. It’s a good idea to check your board for etching defects by compar­ing it with this pattern, before mounting any of the parts. for the board to clear the corner posts of the case. Fig.2 shows the assembly details. Begin by installing the 27 wire links. Some of these are quite long, so you will not be able to use resistor pigtail offcuts. Instead, you should use tinned copper wire for the links but first, you have to straighten it. The procedure is to clamp one end of the wire in a vice, then stretch it slightly by pulling on the other end with a pair of pliers. The resistors can go in next, followed by the MKT and monolithic capacitors. This done, install PC stakes at the external wiring points, then fit the transistors, electrolytic capacitors, crystal X1, voltage regulator REG1, trimpot VR1 and the three “D” connectors. Make sure you solder both mounting lugs on each connector, as the 15-way unit uses them to link two ground tracks. The PC board has been laid out to suit 2N2222 transistors in the TO-18 (metal can) package. It’s also possible to get these transistors in a TO-92 plastic package but the two packages don’t have the same pinouts – see the base diagrams on Fig.1. If you have TO-92 transistors, the trick is to bend the base lead of each transistor towards the flat on its body. The transistor will then slot straight into the board. Take care to ensure that the transistor pin connections are correct; the circuit won’t work if you get them mixed up. The ICs can now be installed. Our prototype used IC sockets but we recommend that you solder the ICs directly to the board. Make sure that they are all correctly oriented and be sure to fit the correct device to each location. Final assembly As shown in the photos, the board mounts on the lid of the case, with the three “D” connectors protruding through one side. Use the board as a template to mark and drill the mounting holes, then 15kHz OFF COMPOSITE VIDEO MGA MONITOR SILICON CHIP 40  Silicon Chip 20kHz 32kHz secure it to the lid on 5mm standoffs. This done, sit the lid on top of the plastic case and mark the cutouts for the three “D” connectors. The cutouts can then be made by drilling a series of holes and filing to get the correct shapes. The front panel label can now be fitted, after which you can drill a hole for the switch plus holes for the RCA & BNC video output sockets. The wiring between the PC board and the front panel hardware can then be completed, as shown in Fig.3. Note that the composite video outputs sockets are wired using 75Ω coaxial cable. The cable braid at the board end is attached to an earth solder lug, which is secured by one of the EGA-socket mounting nuts. Finally, solder short lengths of red and black hookup wire to the battery holder (red to +, black to -). The other end of the red lead connects to the 4-position switch; the black to the appropriate PC pin on the board. Make sure that you don’t get the battery COMPUTER MONITOR CHECKER EGA MONITOR VGA MONITOR Fig.5: this full size artwork can be used as a drilling template for the front panel. Parts List 1 PC board, code 04108991, 148 x 85mm 1 plastic case, 158 x 95 x 53mm, Jaycar HB-6011 or equivalent 1 2MHz crystal, 10 x 3.5 x 13mm, Jaycar RQ-5268 or equivalent 2 9-pin right-angle PC-mount female “D” connectors (Altronics P3030 or equiv.) 1 15-pin high-density right-angle PC-mount female “D” connector (Farnell 210-535 or equivalent) 1 3-pole 4-position rotary switch 1 panel-mount BNC connector 1 panel-mount RCA connector 1 9V battery 1 9V battery holder 2 doubled-sided adhesive tabs 1 1kΩ horizontal PC mount trimpot (VR1) 1 220mm-length 75Ω coaxial cable 4 5mm spacers 8 3 x 10mm countersunk head machine screws & nuts 4 flat washers 1 solder lug This is the view inside the prototype. Note the insulation placed over the earth lead of the coaxial cable, where it attaches to the solder lug. leads mixed up, as there is no reverse polarity protection. The battery holder is attached to the inside of the case using double-sided adhesive foam tabs (available from most stationery suppliers). Testing Some precautions are in order when using the unit. First, it helps to know what kind of monitor you are testing so that you can select the appropriate horizontal sweep frequency. Second, always use the appropriate cable type with the required plugs for a particular monitor. And third, be sure to plug the monitor connector into the appropriate socket. Note that you won’t do any damage if you choose the incorrect socket. If you plug an EGA monitor into the MGA socket or vice versa (they both use 9-pin sockets), the monitor simply won’t work. There shouldn’t be any confusion when it comes to VGA monitors, since they have 15-pin connectors. As mentioned earlier, the checker does not produce elaborate test patterns. When it’s connected to a working composite-video monitor operating with a 15kHz horizontal sweep frequency, six vertical evenly-spaced bars of video should be seen. When testing MGA monitors, which have horizontal sweep frequencies of about 18kHz, set S1 to the 20kHz position – in this case, the monitor should display four to five vertical bars. Finally, EGA and VGA monitors have sweep frequencies that are automatically adjustable from 31kHz to 37kHz and are internally set. Set S1 to the 32kHz position for these monitors. Two to three groups of red, green, and blue vertical bars should be seen on the display and there should be evenly spaced dark regions between these groups. Note that the red bar in the first group may be slightly narrower than those in the remaining groups. This simply reflects the influence of the horizontal retrace time. Please note: circuit modifications to give more ideal scan frequencies are published in Circuit Notebook, SC November 1999. Semiconductors 1 4013 dual D-type flipflop (IC1) 1 4024 7-stage ripple-carry binary counter (IC2) 1 4011 quad 2-input AND gate (IC3) 2 4012 dual 4-input NAND gates (IC4, IC9) 2 4069 hex inverters (IC5, IC10) 1 4020 14-stage ripple-carry binary counter (IC6) 2 7555 CMOS timers (IC7, IC11) 1 74C4017 decade counter (IC8) 5 2N2222 transistors (Q1-Q5) 1 7805 5V regulator (REG1) 1 1N914 small signal diode (D1) Capacitors 1 10µF 16VW PC electrolytic 3 0.1µF monolithic 1 .022µF MKT polyester 1 .01µF MKT polyester 2 .0022µF MKT polyester 1 270pF 5% ceramic disc 1 100pF 5% ceramic disc 1 33pF 5% ceramic disc Resistors (0.25W, 1%) 1 100kΩ 5 10kΩ 2 330Ω 1 47kΩ 5 4.7kΩ 1 100Ω 1 22kΩ 1 1kΩ 4 82Ω 1 15kΩ AUGUST 1999  41