REQUIREMENTS OF THE CIRCUIT
The relay coil is activated by +48VDC driven by the circuit below through the coil to ground.
The contacts are in the circuit below which switches +6V power to a bank of eight SCR circuits in parallel that act as latches to record when a brush contacted the commutator under the carriage control tape due to a hole in that channel.
The contacts must handle 6VDC and a total load of 435 ma if all eight SCRs are latched at the same time. At a minimum we need a SPDT contact plus a SPST (normally closed) contact, although practically that is implemented as a DPDT switch.
The relay will break the contact on one side of a pole before making the contact on the other side. This is essential for correct operation of the circuit as it must reset the SCRs but interrupting power at the moment it switches.
OPTION 1 - REED RELAY
I found a Panasonic relay (TQ2SA-48V) that supports 500ma contact current, is DPDT and has a 48V coil. The only downside is that it is a surface mount part, so I could have to create some kind of interface between the pins on the IBM PCB and the pads of this replacement relay. It is about $5 so cost is swamped by shipping cost. The real issue is the interface. If I need to build a custom PCB, with the current tariffs and shipping costs, I probably have about $30 to build the minimum batch of five boards.
OPTION 2 - INTELLIGENT POWER SWITCHES
A device such as the STM TDE1798DP switch can act as an SPST switch, handling 500ma at 6V. I would need two of these to replace the SPDT action of the reed relay, plus another lower power switch for the additional SPDT switch inside the reed relay. The STM part costs $8 each plus the other parts. More importantly, a means must be devised to achieve the break before make behavior.
If I use a capacitor to slow the rise of the enable signal to the gates, it delays the make. Because the intelligent power switch uses a differential comparator trigger with E+ and E- inputs, I can have an RC network drive the E+ so that it slows the rise with an inverter driving the E- nearly instantaneously. This gives me instant break and delayed make for each switch.
I whipped up a circuit using a diode, an inverter, a resistor and a capacitor to drive the intelligent power switch. These are fed by the 48V coil voltage through a voltage divider to produce 5V when on and 0 when off. An analog switch capable of handling up to 10ma is used for the additional contact since the circuit in the 1132 drives about 5ma through the switch.
The analog switch is about $5.25, the inverter is about $0.30, the diodes, resistors and capacitors all together don't come to $0.50, so the total would be a bit over $22 plus tax and shipping to substitute for the reed relay. I would use some kind of breadboard to wire it together and a way to mechanically anchor it on or near the PCB.
OPTION 3 - EBAY USED RELAYS WITH SOME ADJUSTMENTS
I found a relay on eBay that with a 5V coil but SPDT with 500ma current capacity in a DIP form factor. I could buy two, with tax and shipping, for $18.40. I would use a pair of resistors to drop the +48V coil supply for RR1 down to 5V and put the four parts on a breadboard/perfboard then mount it near or on the PCB.
DECISION MADE - GOING WITH OPTION 3
The relays are ordered. I have the perfboard on hand, thus I just need to work out the resistor values and solder it all together when it arrives.
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