LATCH PCB
The large printed circuit board that is attached to the left rear frame of the 1132 printer does multiple things, but its main purpose is to detect the holes in the carriage control tape as the paper moves up in a skip operation. It uses SCRs to latch when a brush makes contact with the metal roller upon which the carriage control tape loop moves. These are then released by a relay once the paper has moved to the next line position - a signal +Carriage CB is generated when reaching each line.
The normally closed terminals of the relays will feed the +6V supply to the SCRs which will latch on if they receive a positive voltage on the input gate. The relays have to activate to release the SCR(s) that latched. The use of SCRs allows the various tape channels to be detected at any point during the movement of the carriage, even if they don't occur at the same instant or if they disconnect before we reach the next line position.
I can test each circuit by first applying the +6, -3 and ground power to the board, then applying a +6V value through a 1K resistor. I can sample the output which will be at ground when unlatched or at 3V when the SCR has fired. I then shut off power to clear the SCR state before the next one it tested.
The board is wired in through edge connectors but I can attach clips to the parts on the board to perform the test. For example, -3V is attached to an appropriate component such as one side of R18 on the PCB.
The board contains three reed relays RR1, RR2 and RR3 that are used to manage the carriage skip and space signals plus related logical conditions.
RR1 turns on when the program starts a carriage skip and remains on until the program issues a stop skip. While RR1 is on, it keeps the SCRs powered to record any carriage holes they pass and sets the +Space Interrupt Allow to low. Normally, as the carriage reaches the next line it allows a space interrupt, but during a skip we block that.
RR3 turns on when the microswitch on the carriage closes, indicating we have reached the next line of the paper. Its only purpose is to activate relay RR2. Thus RR2 stays on for the duration of the contact interval, lagging RR3 by a brief amount in both turn on and turn off.
The two relays are used to form an edge detector for the carriage microswitch. The logic diagrams and schematics for the 1132 are particularly horrific to decipher, with circuits spread across multiple pages as seemingly isolated chunks. I grabbed a few sections and moved them together just to create a somewhat clearer circuit for the reader to view.
The relatively long duration carriage microswitch pulls the left end of coil RR3 down towards -3V causing it to close the RR3 contacts. In the lower right, the RR3 contact will energize RR2. Then the RR2 contacts on the top will open up, so that the carriage microswitch is isolated and the output is pulled up to +12V by the 6.8K resistor.
The output -Carriage CB drops to a low voltage with the voltage divider of the 6.8K, 1K and 240 ohm resistor pulling it down to a slightly negative level in the instant after the carriage microswitch closes. The contacts move to close in RR3 as it energizes. This then starts the contacts of RR2 to move as the RR3 contacts energize RR2. As soon as the RR2 contacts have opened, the -Carriage CB output is pulled back to +12V even though the carriage microswitch will remain closed for a while longer.
This creates a sharp edge pulse of about 10 microseconds. By comparison, the carriage microswitch is closed for 1500 to 2500 microseconds.
RESULTS OF THE TESTING
When I saw the rusty covers of the SCRs, while everything else on the board appeared pristine, I expected problems with the SCRs. However, they all latched and released exactly as they should.
I wanted to check out the reed relay circuits, thus I applied +12V to the coils and checked the conductivity of the contacts in both the energized and rest states of the relays. RR2 and RR3 were working perfectly. However, no response whatsoever from RR1. I pulled it off the board and checked directly. Open circuit for the coil.
FIRST TRY TO LOCATE A GOOD END OF THE COIL WIRE
I did a bit of excavating to see if the wire snapped off due to corrosion, allowing me to reconnect it. It didn't look promising. Several fragments of enameled coil wire broke off as I looked. I will need to find a replacement reed relay or create an equivalent circuit.
FUNCTIONALITY CONTROLLED BY RR1
Relay RR1 is only turned on and off by the signal from the processor that activates for a skip operation. It is not high speed with critical timing like RR2 and RR3 which define the edge detection pulse for every line that is reached by the carriage.
RR1 has three sets of contacts. Two of them form a break then make pair such that when the relay is changing state, both are unconnected for a brief instant. This is used with the circuit that resets the SCRs for detecting a brush passing over a hole in the carriage control tape.
When RR1 is not energized, the +6V to the SCRs is fed through RR2 contacts. Thus the SCRs have power but are interrupted by the carriage microswitch activation. This means that the carriage hole detection is reset at each line allowing the program to sense only the holes at the current position of the paper.
When RR1 is energized because we are in a skip operation, the SCRs remain continually energized. They are therefore collecting all the holes that are passed during the entire skip duration. When the program tells the printer to stop skipping, RR1 turns off.
The two sets of RR1 contacts ensure that the SCR is turned off when the skip starts and again when the skip starts. The bottom RR1 contact opens up as the relay begins to energize, but the top RR1 contact is not connected until a bit later as the relay fully energizes. Similarly, when the skip is stopped, RR1 breaks the top contact earlier than it makes the bottom contact. In both cases there is an interruption that lets the SCRs reset.
The third RR1 contact pulls the +Space Interrupt Allow line low when RR1 is energized.
I am relatively free to substitute relays or modern electronic switches for these functions, but have to provide the brief interruption of +6V when switching states. I will be thinking about the most straightforward solution so I can get back to restoring the printer.
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