Refinements to the PC Monitor Checker

The PC Monitor Checker published in the August 1999 issue can be modified to provide more appropriate scan frequencies for the various monitor types than the original circuit published in the May 1999 issue of Popular Electronics. Another modification produces the pre-ferred sync polarity for VGA & EGA monitors.

Fig.1: suggested circuit modification for producing the required horizontal scan frequencies.

Table 1 shows the common horizontal and vertical scan rates and sync polarities for the various monitor types. Obtaining the horizontal frequencies from the 1MHz clock signal requires the division ratios from IC2 as shown in Table 2.

The circuit of Fig.1 shows the suggested modifications which involve using a 2-pole 5-position switch (eg, DSE Cat P-7508). Note that IC5c is now unused and should have its pin 5 tied high (+5V) or low (0V). These modifications would enable the testing of monitors such as the Philips/Commodore 1084 which use 15,625kHz and 50Hz line and frame rates respectively, and can accept RGB and composite video inputs.

To produce a 50Hz vertical scan frequency, the spare gate in IC9 (4012) can be employed. Counter IC6 is presently wired to divide its 500kHz input clock signal by a factor of 8329, result-ing in an output of 60.03Hz. By connecting the Q9, Q10, Q11 & Q14 outputs of IC6 to IC9b, the resultant output frequency is 500,000/9984 = 50.08Hz. This modification is shown in Fig.2.