I recently had to repair a Bally Centaur -35 MPU board and found that the 9602 (U16) clock wasn't working. 9602's aren't easily come by where I live so I investigated an alternative and found that I could get a 74HC4538 to work with minimal modification and no hacking of the PCB !

NOTE! This modification also requires that U15 (MC3459) be changed to a TTL type 7437. The HC/HCT chips cannot properly drive a MC3459 at U15. A 74LS37 shouldn't be used for U15 because it doesn't have enough output drive capability.

The HC/HCT 4538 dual monostables are pin compatible but need pins 1 and 15 grounded and a 10K to ground (pull-down) at the pin 9/pin 4 junction. I'm yet to confirm the modification with the HCT version but it should be OK.

To maintain the approximate 500KHz. clock rate (I confirmed this with a CRO),C14 and C15 are changed to 120pF from 470pF. The extra 10K resistor is needed to ensure a "low" logic level on that particular connection when the clock is first powered up. Without this resistor it is random whether the oscillator starts or not because of an indeterminate logic state on the CMOS input.

R4,R5 and R10 stay as they are.

Here's a diagram of the clock circuit with the changes shown in red:

Here's a photo of the top side of the printed circuit board showing a 74HC4538 in a new turned pin socket and the two replacement capacitors (polystyrene) I used. They don't have to be polystyrene,that's what I had on hand at the time:

Here's a bottom side view of the PCB showing the added 10K resistor and the two new connections:

HINT: When removing the dead original U16 , or any other IC for that matter , it's best to snip off each individual pin from the body of the chip with fine cutters. Then remove the "carcass" and finish by removing the remainder of each pin separately. By doing it this way , it minimises the risk of PCB damage.

rgp Gruntcake 28th. February 2009

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