Wrapping up testing of 2501 card reader power supply box - one bad component discovered

INVESTIGATED THEORY ON HOW TRANSFORMER PRIMARIES ARE CONNECTED

My speculation on how the US transformer might be connected to the five terminals of terminal block PSTB1 was validated. I isolated windings and measured DC resistances. I removed the jumper between terminals 2 and 3 which did indicate there are two sets of primary windings, as I suspected. 

I altered the configuration to 230V, to match my workshop, and reverified the output voltages delivered. Since the transformer is ferro-resonant, thus crudely self regulating, the differences in the voltages produced were less than the delta from 208 to 230, but they did move in the correct direction. 

TESTING MOTOR CONTROL CIRCUITS

The motor is started by activation of the contactor relay K1, based on the signal -Motor Relay coming from the 1130 system controller logic. This works by grounding the input signal, thus powering the coil to switch on the contactor. I connected and disconnected ground to terminal 10 of PSTB2 where the signal arrives. The contactor clunked in and out based on the connection I made. A VOM confirmed the conductivity of the contacts when K1 was activated.  

As the power for the motor is applied via K1, it is connected to the normal winding of the motor. The starter relay K2 connects the start winding through an R-C link to shift the phase of the AC, sufficient to start rotation of the motor. Relay K2 has a time delay then switches off the start winding. I did not test this yet as I need to connect the motor first. The current draw of the motor starting is more than my bench 240V supply can deliver, thus I need to replace the PS box in the card reader and hook it up to the IBM 1130 in order to test the last piece - startup of the motor. 

Relay K3 is a time delay which delivers a signal back to the 1130 controller logic -Motor Hold that indicates that the motor should continue to be powered by asserting -Motor Relay. The relay is activated by relay K4, which switches on for each -Execute command from the 1130 controller logic. Execute is issued to feed or read a card. 

The relay K3 coil is provided with 24V from the brief activation of relay K4. The 24V charges a capacitor which continues to hold the coil activated but an RC network is draining the charge. The timing is set up so that the coil of K3 will drop out after about 15 seconds - as the voltage declines below some threshold. 

Thus, each time a card is read or fed, the hold relay activates for 15 seconds and causes the controller logic to hold K1 active so that the motor runs. During normal reading of a deck of cards, relay K4 resets the hold relay K3 repeatedly so that the motor is running continuously, but after any pause of 15 seconds the motor will stop. 

I tested relay K4 by connected terminal 9 of PSTB2 to ground, as this is where the -Motor Relay signal is connected. Each time I did the relay switched and the VOM indicated a good conductivity for the contacts. 

TESTING K3 FAILS

That would have also validated relay K3, as it will receive 24V every time K4 switches. I didn't see or hear the relay switching. I hooked the VOM to the contacts to see if it was working, but nothing changed. I then hooked up the VOM to the coil of relay K3 to see what voltages it was seeing.

It was presented with 24V when K4 activated and ebbed so that after 15 seconds it dropped to about 3V where K3 probably would have dropped out. However, no action and the voltage should have dropped across the terminals due to the current flowing through the coil.

I then used the VOM to test the resistance of the coil. Infinity! Open circuit. The wire entering the wrapped coil was good on each side, so the break is somewhere inside the coil of the relay. The part is a Sigma 41 F-98326 - a single pole dual throw relay with a 28V coil with the ability to switch 2A at up to 75VDC. 

This is excessive given it is just grounding a single line going into an SLT card in the 2501 card cage - it is pulled up to about about 20V when the relay is open and pulls that to ground when activated with just a few milliamps of current flow. Finding a substitute just has to support contact operation at milliamps of current with a voltage under 30V. 

LOOKING FOR REPLACEMENT RELAY

I partially disassembled the relay thinking that if I could find another Sigma relay with a 10K coil I could swap them. 

I checked eBay and did google searches, but nothing matched the exact relay. I can find many Sigma relays with similar specifications. There were plenty with contacts rated at 7.6VDC, 16VDC, or 22VDC but I need to have one at least 28V rated since it is looking at 24V or a bit more. 

The other issue is the resistance of the coil - which determines how much it drains the voltage on the capacitor in the time delay circuit. The schematic indicates this is a 10K ohm coil, however the relays I am finding on ebay are 1K or 2.5K typically. 

I found a Sigma relay with a slightly different form factor and current rating. This was also a 28VDC coil, 9000 ohms which will activate at 2.4 ma which is below the 24VDC level presented by the closing of relay K4. Thus it will activate and its current draw is only 11% higher than the nominal part. The contact is rated at only 1A but that is well above our maximum draw. 

It doesn't mount the same way, but I can work up a mounting method inside the PS box. I will buy this and verify it works properly when wired into the circuit. 

REMAINING TO TEST

I need to test the motor and its start relay K2, but that has to wait until the PS box is back inside the 2501 card reader cabinet. I feel pretty good about the condition of the PS box and the card reader. Once the time delay relay K3 is replaced I can move on with my testing.