| Electronics > Repair |
| ADVANTEST R9211B |
| << < (34/39) > >> |
| m k:
LS07 pin 10 is not rational if there are no other connections. It's old DIL package because other versions are limited, so its function is essential, for what ever reason. Only rational reason is its open collector output style, means that many output signals can be connected together. It's also possible that leftover drivers are used elsewhere, when main function is to connect bidirectional CPU pins. Can you find other LS07 even numbered pins from HC148 pins 1 to 5, 12 or 13? Is HC148 pin 13 shorted to ground? |
| stfjohn:
LS07 pin 14 connected to HC148 pin 4. Hc148 pin 13 not grounded. |
| m k:
Try to find from where HC148 pin 13 is coming. It's the pin putting CPU pins 24 and 23 down, our suspected problem situation. LS07 outputs must have pull-up resistors, check that they are present. Between pin 14 and other even numbered pins is similar ohms value. Try also find where LS164 output pins 3 to 6 and 10 to 13 are going. If the chip operation is normal serial to parallel thing all of those pins go side by side to somewhere. It's also possible that only one goes anywhere, like to HC148 pin 1, 3, 12 or 13. |
| stfjohn:
HC148 pin 123 connected to central pin R35 (4 x 472 J). R35 pin 1 (black marked) connected to Pal U58 pin 20. |
| m k:
That fits, pin 2 goes to 9914 pin 9(reply #156) that is OD(Open Drain), same as Open Collector for TTL. R35 is a resistor network, leftmost is a common pin, it has its own PCB marked square, so that you know which way it must be installed. Other four pins are 4700 ohm resistors to that common pin. So from common pin to any other pin you must be able to measure close to 4700 ohms. If one pin has much different value you can start thinking what is its cause. Too high means that the resistor is not there and too low means that somewhere is a possible short to +5V. Since our assumed problem is constant interrupt we are searching the situation where that pull-up resistor is missing. But if that incoming interrupt Priority Line comes through some other component it's possible that the situation turns around and we should search something that is constantly up. Luckily both sides of HC148 are active low, means that we are searching something that is constantly down. And since we have two CPU lines down we are mainly searching sources for HC148 pins 11, 12 and 13. Pin 11 goes to LS07 pin 10, so it needs a pull-up resistor for sure, one candidate is R36 right of R35. To be a symmetric design three resistors connect to those high side input pins of HC148. And those high side input pins connect to high side of output pins of LS07, pins 12, 10 and 8, but be methodical and check lower output pins also, pins 6, 4 and 2. So how are those HC148 pins 11, 12 and 13, their pull-up resistors and their signal sources? |
| Navigation |
| Message Index |
| Next page |
| Previous page |