Resolution of Run lamp issues

DETERMINING CAUSE OF RUN LAMP FAILURE TO LIGHT

Since we saw that a direct ground input to the 3819 card does illuminate the lamp, the issue is further back in the machine. It was important to trace connectivity back to the gate which generates the condition which should light the lamp.

A card in slot G7 does most of the logic surrounding the Run lamp and the usage meter. It is the direct source of the signal that is routed to the 3819 card to make the lamp illuminate. I pulled the card and did a bench test of its functionality. The type was 3556, specific to this function. 

Simplified ALD page

The main source for turning on the lamp is CPU activity, but it will also activate if the disk drive is busy doing a seek; even if the CPU is in a wait state, the meter is also turning. Another interesting part of the circuit is where the CE key switch, if turned to the CE position to accumulate time on the CE usage meter rather than the customer usage meter, also blocks turning on the Run lamp. Thus, even if the CPU is executing instructions and the CE meter is rolling, the RUN lamp will not illuminate. This blocks the usage meters in the printers and card readers from running. 

The gates at the bottom right are the ones that directly turn on the RUN lamp, through the connection to the 3819 card. My initial check indicated that the gate is not producing a full high output as it should. This however could be a fault in the signal path to the 3819 card or a fault in that card itself, but I will first check the new card for correct operation. 

The actual circuit on the 3556 card that roll the meter is:

The circuit on the 3556 card that controls the RUN lamp is:

My workbench set up the card with an oscillator simulating the phase A master clock signal and slide switches to set up the various inputs that should control the operation. The 3556 card begins once the three signals from the top left of the ALD page are combined, so I have a switch to indicate that the CPU is working. Another switch indicates whether the CE key is activated or not. A third switch acts as the disk access busy signal. 

What I expect is to see the usage meter output B02 pull to ground when I have either the run condition or the disk access signals turned on. This does not have a pullup resistor on the 3556 card, thus I will have to add one for my testing. I should see it fire off the signal for a minimum of 400ms and continue until the driving conditions go away, with an additional 400ms thrown in for good measure. 

The RUN lamp output B07 should mirror what is happening on B02 as long as we don't activate the CE key signal, otherwise it should be blocked. 

Observing the actual card behavior, I found no output on either gate. There is a pin on the card (B13) that shows the trigger of the oneshot and that was working properly. When the conditions were proper for having the Run lamp and usage meter working, it was active. 

The part of the card circuit on the lower left, with inputs D04, D05 and D06 and output B05 is not used but the remainder is what implements the ALD section you see above. The output of the upper right transistor is what should produce a timed pulse from the circuit along the bottom from the middle to the right. That timed pulse will be visible on B13 and is redriven for more power where it is connected to one of the diode inputs for the B07 output and also to the OR input of the B02 output. 

I had a good signal going into the gate for B07 but nothing coming out. When I began probing the gate on the card (it is in one SLT module on the card) I found that it was not working properly. With a valid input when the run conditions were correct, it was not conducting properly to produce the output.

REPAIRING THE RUN LAMP

I grabbed another SLT module of the same type (451) from a donor card in my spare parts and swapped it on the card. Testing showed that this fixed the issue, with the card now generating the output on B07 that I expected.

I put the oscilloscope on the signal to validate the existence of the 400ms one shot output. Everything looked great so I put the card back into the 1130 and powered up. The Run lamp now glows exactly as it should.