Hi all, I have discovered the cause of Sanyo MBC-55x boards that experience disk slow-downs and may eventually lose their ability to access the floppy drives: Part of the circuitry that generates a fake time delay in providing a "head loaded" signal to the floppy controller in response to the FDC's "load the head now" signal becomes nonfunctional, and the floppy controller never thinks the drive is ready.
Symptom: Floppy accesses don't work at all or take a long time to start each time the disk spins up. Possibly this appears or gets worse as the system warms up.
Cause: Capacitor C9 is installed with the wrong polarity - even the board markings are wrong for the circuit. Tantalum capacitors like the one used are sensitive to reverse connection and fail instantly at higher voltages, but at lower voltage, apparently behave like this with slow damage accumulating on the capacitor.
Verification: Check the datasheet for the 74LS221N chip that the capacitor is connected to - the datasheet describes the correct way to connect the capacitor. I have attached a scan of the relevant Sams PhotoFact page that details this part of the circuitry so you can find the pins to measure. If you intend on doing hobby work with one of these machines, buy a copy of this technical manual from ebay - it's that good.
Resolution: Replace capacitor with a new tantalum or electrolytic 47uF, installing it with the correct polarity. Important - be sure the capacitor was installed wrong first as they may have fixed this in late revisions. I am going to do some tests to see whether it is workable to simply reverse the existing (damaged) capacitor - tantalums are known for some "self healing" properties but I don't know if it is effective once it gets to this point.
Additional: Also attached is an oscilloscope trace of the output of multivibrator circuit to floppy controller (pin 4 of U91 in yellow), and the capacitor timing node (pin 15 of U91 in blue) on a WORKING motherboard. On a non-working board, the blue line starts to rise, and levels out before reaching the trigger voltage. This signal is captured during the start of a floppy operation where the drive is spinning up. You don't need an oscilloscope to verify this - a voltmeter is enough to check that pin 4 on U91 is stuck low while the machine is hung up.
Side note, I never thought I'd find a design problem like this on this board. It blows my mind with the attention to detail they used on it.
Symptom: Floppy accesses don't work at all or take a long time to start each time the disk spins up. Possibly this appears or gets worse as the system warms up.
Cause: Capacitor C9 is installed with the wrong polarity - even the board markings are wrong for the circuit. Tantalum capacitors like the one used are sensitive to reverse connection and fail instantly at higher voltages, but at lower voltage, apparently behave like this with slow damage accumulating on the capacitor.
Verification: Check the datasheet for the 74LS221N chip that the capacitor is connected to - the datasheet describes the correct way to connect the capacitor. I have attached a scan of the relevant Sams PhotoFact page that details this part of the circuitry so you can find the pins to measure. If you intend on doing hobby work with one of these machines, buy a copy of this technical manual from ebay - it's that good.
Resolution: Replace capacitor with a new tantalum or electrolytic 47uF, installing it with the correct polarity. Important - be sure the capacitor was installed wrong first as they may have fixed this in late revisions. I am going to do some tests to see whether it is workable to simply reverse the existing (damaged) capacitor - tantalums are known for some "self healing" properties but I don't know if it is effective once it gets to this point.
Additional: Also attached is an oscilloscope trace of the output of multivibrator circuit to floppy controller (pin 4 of U91 in yellow), and the capacitor timing node (pin 15 of U91 in blue) on a WORKING motherboard. On a non-working board, the blue line starts to rise, and levels out before reaching the trigger voltage. This signal is captured during the start of a floppy operation where the drive is spinning up. You don't need an oscilloscope to verify this - a voltmeter is enough to check that pin 4 on U91 is stuck low while the machine is hung up.
Side note, I never thought I'd find a design problem like this on this board. It blows my mind with the attention to detail they used on it.