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AW: TM PORTOROSE FAILING
Mike,
The reeds will be read by the chip marked with 74HC244 near the connector, it is a 8 octal buffer, which allows the 68020 proc to read the 8 reeds. As we can see the leds on the board flashing the two chips 74HC374 are working. One of them is a octal flip flop who clocks in 8 lines to the board (leds and reeds) and the other is a octal flipflop who clocks in the other side of the leds.
In order to read the reeds or show the leds (8 at a time) the octal flip flop is set who clocks in 8 lines to the board (leds and reeds) is clocked and then enabled through OE. In order to set the octal flip flop who clocks in the other side of the leds should have the OE disabled. It is a simple matrix led/reed board.
Anyway, the system is working, but does not read the reeds. So the first defective thing could be this chip. Replace it with an new 74hHC244. And check the system. If it is socketed this is easy. If not, your repair man can check if the chip is getting OE signals to 1OE and 2OE. If this chip gets the signals, he should check if the 8 signals are connected to the reeds (matrix wise). If the chip is faulty solder it out and replace… But you can check if the chip is working during power on. Or your repair man can.
If the 74HC244 is working,but does not receive the 1OE and 2OE signals, the next chip to check is the 3to 8 decoder marked 74HC138. This must be the address decoder. If I am guessing correctly, three of the outputs are connected to the mentioned 74HC244 chip and the two 74hc374 (to the CP and OE signals). Maybe some inverters are in between. I would suspect this chip to replace next, however,as the leds are working only the signal from this chip to 1OE and 2OE for the74HC244 could be at fault. PS, I checked, pin 9 from the 74hc138 is driving the 74hc244. If pin 9 is switched high the 74hc244 puts the reeds (8 at a time) on the data bus for the 68c020..
My top suspect is the74hc244, and secondly the 74hc138. But I am for 95% sure it is the 74hc244.
Lastly the problem could reside in the led/reed board or the cable.
Greets and maybe a US$ 5 repair...
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Grusse,
Ruud Martin
Geändert von Ruud Martin (20.07.2016 um 22:39 Uhr)
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