"Line jammed" usually means "the line appears to be stuck in the non-idle state".
Measuring resistances in circuit is hit-and-miss: in this one you will be seeing a lot of resistors in parallel, and also the protection diodes inside the chips...
The receiver circuit is probably easiest to debug by measuring voltages (vs 0V) - with the power on, of course. Try with the combinations of the network & IC93 fitted & not, and start by looking at IC93 pins 12 & 13. IC93 dying is fairly common, but if the data pair is unchanged with it fitted or not, then that's not the issue - it will be the receive side. (also a dead IC93 tends to stop the rest of the network working as well)
With the network connected, you should be able to see the bias voltage across the data pair: Pin 12 (D+) should be around 0.3-0.5V higher than Pin 13 (D-). If not, unplug the beeb and check the network. If you still don't have a good bias on the network, sort the network termination out (some machines are more picky than others about this being correct).
If all is well, then reconnect the network and move on to check the outputs of the receiver (IC94 pin 12, should be high) and the collision detect circuit (IC95 pins 5/12, I think should be low). On the other hand, with the network disconnected the receiver input pins (4 & 5) should be at the same voltage, as should the collision-detector inputs (IC95 pins 5 and 10). After which you should have a clue as to where to try next...
Measuring resistances in circuit is hit-and-miss: in this one you will be seeing a lot of resistors in parallel, and also the protection diodes inside the chips...
The receiver circuit is probably easiest to debug by measuring voltages (vs 0V) - with the power on, of course. Try with the combinations of the network & IC93 fitted & not, and start by looking at IC93 pins 12 & 13. IC93 dying is fairly common, but if the data pair is unchanged with it fitted or not, then that's not the issue - it will be the receive side. (also a dead IC93 tends to stop the rest of the network working as well)
With the network connected, you should be able to see the bias voltage across the data pair: Pin 12 (D+) should be around 0.3-0.5V higher than Pin 13 (D-). If not, unplug the beeb and check the network. If you still don't have a good bias on the network, sort the network termination out (some machines are more picky than others about this being correct).
If all is well, then reconnect the network and move on to check the outputs of the receiver (IC94 pin 12, should be high) and the collision detect circuit (IC95 pins 5/12, I think should be low). On the other hand, with the network disconnected the receiver input pins (4 & 5) should be at the same voltage, as should the collision-detector inputs (IC95 pins 5 and 10). After which you should have a clue as to where to try next...
Statistics: Posted by wiggy — Wed Nov 13, 2024 10:35 pm