Substitute relay arrived for 2501 card reader, tested and installed

RELAY R3 HAD AN OPEN COIL THUS REPLACEMENT NEEDED

The power supply box includes a function to keep the main motor of the card reader running for 15 seconds after the 2501 receives its last read or feed request via an -execute command signal. Relay R3 latches every time -execute command arrives but the voltage across the coil declines based on an RC circuit until about 15 seconds have elapsed, when the relay coil is no longer able to hold the connection. 

The contacts of relay R3 send a signal -motor hold sw back to the 1130 system's controller logic, which uses it to maintain or drop the -motor relay command back to the reader. If this isn't working, the motor will shut off instantly after every card is read, which is hard on the motor and disruptive to the process of reading. 

EXACT PART NOT AVAILABLE BUT LOCATED A RELAY THAT SHOULD WORK PROPERLY

The Sigma part number did not come up in any online search, but there were many similar relays from Sigma that I could choose from. I looked for the critical specifications to get as close to the original relay as possible. This included the voltage and current rating of the contacts, the voltage for the coil, the DC resistance of the coil and factors like the drop out voltage. 

The original relay had contacts rated for 2A of current, but that is extremely far above the actual signal currents used in the 2501 and 1130 system. Thus, a 1A part was very suitable. The voltages for the contact, coil, and coil drop out were identical. The resistance of the original relay was 10K but the part I found was 9K. 

The final difference was the mounting method - the hole locations and shape of the mounting foot. I will have to adjust for the difference to install this new relay into the power supply box. 

CONNECTED AND TESTED ON THE WORKBENCH

I connected the power supply box to my 220V step up transformer and used some resistors and jumpers to simulate the control signal -execute command to trigger R3. My VOM was hooked across the contacts of the new relay to let me hear the duration of the relay actuation. The circuitry around the relay includes a potentiometer where I can fine tune the time duration, which should help me compensate for any differences due to the substitute relay.

The relay activated cleanly when the control signal was given. The relay opened up after 5.5 seconds. Based on that, I began adjusting the potentiometer but the max delay I could achieve was around 7 seconds. 

I experimented changing the resistance of the time adjustment leg - the potentiometer plus a 10K resistor - to see if I could lengthen the hold. Even with an infinite resistance, the relay held for 13 to 16 seconds, varying a bit, which I deemed close enough to proceed. The specification for motor hold time is 15 seconds +/- 3, therefore we are within the proper range. 

The potentiometer and related resistor are out of the circuit. The relay coil appears to be draining power faster than the original did, or it has a higher drop out voltage, or both. An alternative solution would be a larger capacitor in the timing circuit, which would accommodate the higher current flow and lower its voltage at a slower rate. However, since the unit performs adequately now, I won't pursue this alternative.

MODIFIED MOUNTING METHOD TO INSTALL INTO POWER SUPPLY BOX

The original relay had a rectangular plate on the back with holes near the top and bottom. The new relay has a different shape and hole pattern, thus I had to take off the plate from the old relay, drill out a hole and use a screw to attach the new relay to the plate. 

READY TO PUT POWER SUPPLY BOX BACK INTO 2501 CARD READER

The lower compartment of the 2501 card reader holds the power supply box, a card cage for the SLT circuits and the connectors used to connect cables to the 1130 system. The power supply box fits in the compartment using slots in the box that match mounting hardware in the 2501.

A flat rail runs across the top of the compartment, from front to back. Slots in the top of the power supply box allow it to fit over the rail, with enough room in the slot to move further upward than its final mounting position. 

That permits the bottom of the power supply box to be swung over the top of a rectangular rail that sits across the bottom of the compartment, running from front to back. Square cutouts on the bottom of the power supply box allow the box to settle down onto the rail. This suspends the box from moving side to side in the 2501. A bolt goes through the bottom of the power supply box to lock it down in position on the bottom of the 2501 compartment. 

Once I do this I will be reconnecting all the cables and wires I had to disconnect in order to move the power supply box to the bench for testing and repair.