Called out to look at £15k worth of machine today - its older than I am but mechanically sound.
I've traced the problem down to a misbehaving SN7490N decade counter. Obviously these are no longer stock items having been long discontinued.
The 7490 is part of a 0-999 counter circuit - the three BCD switches are commoned towards +5v with a 220R resistor with the 1 / 2 / 4 / 8 outputs going directly to the Q outputs of the 7490 via diodes. When the BCD switches = 7490 count, the common side of the BCD switches is +5v, in all other cases its zero volts (or near as).
With the 220R resistor, thats around 22ma sink with a single input enabled (worst case scenario) - the 74LS90 will only sink 8ma according to the datasheet while the 7490 is happy with 16ma.
This *could* be why the original 7490 has packed up - being asked to sink 22ma may have been too much for it but the machine has worked without a problem for the past *cough* years.
I could increase the pullup resistor to 680R and try a 74LS but not sure how that will affect other parts in the circuit - its fairly complex.
Any suggestions ? I've heard back from the manufacturer who has said the board has long since been discontinued and they don't have scematics for them any more .....
Ta
I've traced the problem down to a misbehaving SN7490N decade counter. Obviously these are no longer stock items having been long discontinued.
The 7490 is part of a 0-999 counter circuit - the three BCD switches are commoned towards +5v with a 220R resistor with the 1 / 2 / 4 / 8 outputs going directly to the Q outputs of the 7490 via diodes. When the BCD switches = 7490 count, the common side of the BCD switches is +5v, in all other cases its zero volts (or near as).
With the 220R resistor, thats around 22ma sink with a single input enabled (worst case scenario) - the 74LS90 will only sink 8ma according to the datasheet while the 7490 is happy with 16ma.
This *could* be why the original 7490 has packed up - being asked to sink 22ma may have been too much for it but the machine has worked without a problem for the past *cough* years.
I could increase the pullup resistor to 680R and try a 74LS but not sure how that will affect other parts in the circuit - its fairly complex.
Any suggestions ? I've heard back from the manufacturer who has said the board has long since been discontinued and they don't have scematics for them any more .....
Ta