Wednesday, June 14, 2017

All boards working on IBM 1130 upgrade for light panel


All boards are built and I began live testing. Lamp test works properly but for some reason I was not getting the bulb to light with the signal pin at an acceptable voltage. A single instance of the circuit built off board works, so this is a matter of interaction among circuits that I have to address. 

In the 1130, the lamp test line is hooked to all SCR gates with 6.8K resistors, while the individual signal inputs are hooked to the gate directly. The 1130 wiring has 6.2K resistors in series with all signals, thus it appears that the SCR gate is hooked to a voltage divider between the +3V logic signal and the ground level that lamp test is normally holding. 

About 1.5V goes into the SCR gate which should conduct. When lamp test is pulled up from ground to +3V level, the SCR gate conducts. The fact that I don't see the lamp lighting with the input signal is troubling. It only works when the voltage is boosted to 3.26V with lamp test floating or 3.9V when lamp test is at ground.

To hook this into the 1130, I have to accept the constraints of that system. Signal inputs are somewhat less than 3V to fire the lamp, AC supply to the SCR is 7.25V and the serial resistance with the signal inputs is 6.2K. Thus, it seemed the current boards wouldn't work. 

I moved the bulb over one position and the results were completely different! I seem to have a rogue SCR or a flaw in that one circuit. I will now populate bulbs in as much of the board as I can, set the input signal voltage to less than 2V and spot any positions that don't work as intended. Moving bulbs around will let me check out the entirety of each board.

I completed both small board checkouts. Three positions didn't work properly and need to have components replaced, but the remaining 59 worked properly with both 1.41V signal and 1.41V lamp test voltages applied. I am off to the CHM to work further on the power supply regulator card I am happy that the circuit is sound and these should work well when installed into the 1130 panel.

Tonight I only had time to test a portion of the big board, since it will require about 20 setup and test operations to move 5 bulbs carefully through all 96 circuits. I may need to float the lamp test line when not active, rather than grounding it as the 1130 currently does, since the grounding will drive the signal voltage to about 55% of its value at the SCR gate. Floating will provide all 100% of the signal voltage to operate the thyristor.


We worked on the voltage regulator card but were ultimately stymied by the lack of any 028 and 036 transistors on hand. We have sourced them and can continue with the repair next week. 

No comments:

Post a Comment