18-01-2021, 08:43 PM
I was reading in the HWNB and found a section about improving key switch reliability - one thing it mentions is increasing RN1 to 33kΩ ... Section A, Chapter 5.
In my 32K it is 4.7kΩ ... worth replacing? There's still one or two I'm not happy about, and I have to type quite slowly in general - just assumed it was because it was slow (2MHz and BASIC 5.10) ... ?
To improve the reliability of the keyswitches on the microbee, a resistor network, RN1, has to be changed.
This resistor network is in front of IC4 under the keyboard frame. It is used as a pull-up resistor on the input lines of IC4 and does not appear on the old circuit diagram. It is on the 1248-4, the latest circuit diagram.
The resistance value of this network is 3k3 or 4k7, which is too low. The resistance has to be increased to 33k (or near to this value); a 33k network or eight 33k resistors will do.
To check the value before removing, measure the resistance with a multimeter. From the bottom of the base board, measure between pins 1 and 10 at each end of the network.
<snip about replacement procedure>
If you have a machine with faulty keyswitches, it is advisable to check to see if the correct value is fitted for RN1.
In my 32K it is 4.7kΩ ... worth replacing? There's still one or two I'm not happy about, and I have to type quite slowly in general - just assumed it was because it was slow (2MHz and BASIC 5.10) ... ?
To improve the reliability of the keyswitches on the microbee, a resistor network, RN1, has to be changed.
This resistor network is in front of IC4 under the keyboard frame. It is used as a pull-up resistor on the input lines of IC4 and does not appear on the old circuit diagram. It is on the 1248-4, the latest circuit diagram.
The resistance value of this network is 3k3 or 4k7, which is too low. The resistance has to be increased to 33k (or near to this value); a 33k network or eight 33k resistors will do.
To check the value before removing, measure the resistance with a multimeter. From the bottom of the base board, measure between pins 1 and 10 at each end of the network.
<snip about replacement procedure>
If you have a machine with faulty keyswitches, it is advisable to check to see if the correct value is fitted for RN1.