Time delay relay for disk drive is bad - bought generic replacement

TIME DELAY RELAY IN INTERNAL DISK DRIVE FOR IBM 1130

The platter has to spin for about 90 seconds in order for temperatures to stabilize and any loose dust to be blown out of the cartridge before the heads can safely be lowered onto the surface of the disk. This is controlled by a time delay relay that is powered in parallel with the spindle motor relay. 

A time delay circuit inside the relay should cause it to take roughly 90 seconds before the relay pulls in, switching contacts that are wired into the drive electronics. The contacts are wired so that the normally closed contact is hooked to ground and thus the delay signal is at logic low initially. When the relay energizes this signal is disconnected from ground, thus becoming logic high to the drive circuitry.

When it goes high, assuming the spindle is still spinning at its nominal 1500 rpm, the drive electronics will activate the Head Load solenoid. This forces the backs of the heads down to the disk surface. They fly on an air cushion due to the 25 rotation per second spinning. 

With the heads at a point two inches in from the outside of the 14" platter, we have a 10" diameter circle thus about 31.4" of travel distance per rotation. The heads are moving 785 inches per second or roughly 20 meters per second around the circular track. A speed of 72 Km per hour is going to experience a lot of air resistance. 

The air is dragged along with the disk surface at the very small height that the disk head flies, which is what imparts the force to keep the head off the disk surface. 

The head load solenoid has a tab that depresses a microswitch when it has activated. The signal is used to switch on File Ready, the signal that informs the 1130 controller logic that the disk is ready to accept commands to move, read and write. 

REMOVED AND TESTED HOOKED TO 48V BENCH SUPPLY

I disconnected the wires and unscrewed the relay from the AC box of the disk drive. Hooking it up to my bench power supply, I supplied 48V and started a stopwatch to time the activation. Immediately, I noticed that the relay was drawing quite a bit of current, consuming about 36W of power during the time it should be simply charging an RC network to turn on a transistor in 90 seconds. 

The relay never activated and the power draw remained steady long after the time interval was up. I could feel some current flowing in the coil - insufficient to pull the armature down but enough that if I assisted the armature it would hold in the activated position. 

DISASSEMBLED AND INSPECTING COMPONENTS

The relay has a rectangular box on one side which contains the circuitry. It consists of a few resistors, capacitors, diodes, one transistor and one potentiometer. It has five wires running to the relay coil and the terminals on the relay, which suggests that the coil of the solenoid is not a single monolithic winding. 

I removed key components to measure them without any interference. The capacitors measured fine. One resistor had drifted about 30% high and the other was on spec. Both diodes tested good on the VOM at least. The transistor voltage gaps looked odd, however. They were not regular silicon nor germanium junction gaps. 

Given the partial energization of the coil and the failure to change state after the time delay, the transistor may be the bad part causing everything, but I am not certain. The transistor and capacitors all had private label markings, making it impossible to correlate to a part number I can order. For example, the transistor is marked GE P-1901-28 and the bigger capacitor is marked 1068-6720. 

GENERIC TIME DELAY RELAY PURCHASED TO TAKE THE PLACE OF DEAD RELAY

Generic delay relays are available at really good prices, offering a variety of activation voltages and delay adjustment ranges. I found a part that will activate with 48V DC and can be set to 90 or 100 second delay. When it arrives I will connect it to the drive thus restoring its ability to load heads and become ready for use.