Monday, September 29, 2014

Repaired the delay board and prepared to use it to replace the original time delay relay in the disk drive


I ran out to a local supply shop to pick up the surface mount timer chip then installed it into the delayed on relay board that was broken when I received it. The bad behavior continued. I then traced the circuit to figure out what they were attempting and from that to determine what was wrong.

It appears that the through holes near the potentiometer don't have enough clearance around the internal ground plane, so that some boards are shorting one of those pins, causing the circuit to be reconfigured into the astable (cycling) mode.

I planned a way to wire around it - removing some components and connecting them differently to set up the right circuit wiring. When I wired in the planned component values - 1000uf capacitor and 100K resistor - the timing came in close enough to target that I don't need to tweak the part selection.

Resistor and capacitor wired around the flawed circuit board traces
The target is 90 seconds before the relay turns on, and the board as it sits activates after 100 seconds. That is close and on the safe side (longer means more time for disk platter to adjust to the air temperature inside the drive).

This replaces the original, malfunctioning relay in tight quarters in the AC power box of the disk drive. I attached the relay board to the mounting bracket removed from the relay. It fits in the intended space.

Relay mounting bracket used to hold delay board in place
Before I install this, I need to put together a resistor voltage divider to drop the 48VDC input down to the 12V that the relay board uses, cut the spade lugs off the existing wires and attach them to the terminal blocks on the board. Once that is done, it can be put into place and the drive closed up.

I had to spend most of today preparing for my business trip tomorrow morning, but am pleased that I was able to revive the delay board and establish the behavior required to support the disk drive disk loading sequence. When a cartridge is in place and the switch thrown, the motor begins spinning the cartridge  and energizes our delay circuit. We hold a signal line low until the 100 seconds has expired, then disconnect the line from ground. That signal is normally grounded through the relay and only allowed to pull up high after the delay.

The disk drive electronics won't pick the disk head loading solenoid until this signal goes high, along with other required conditions such as adequate rotational speed. Now that I have a good time delay to raise that signal, it will be fully operable. More importantly, the covers can be put in place and the annoying door closed up.


I spent a good hour with the FE maintenance and theory of operations manuals, plus the illustrated parts catalog, to discover the mechanisms I need to lubricate and check in order to get the print wheels striking the paper with enough force to leave a clean character image. No time to begin the cleaning, but that will be first up when I return from my trip.

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