Difficulties having a full Lamp Test verification of the board

 EXPECTED CURRENT DRAW DURING LAMP TEST

The bulbs I am using draw 200ma at their rated voltage. With 164 eventually installed, that would total about 33 amps. Since the IBM 1130 lighting circuit is fed with 7.25VAC, the actual current will be higher, up to almost 38 amperes.

FIRST TEST SETUP

I used a bench power supply to provide 6.3VAC for the initial testing. That will light up individual lamps or small numbers of lamps to a satisfying brightness. However, it is rated at only 1 amp thus woefully inadequate to test the full board.

When I populated the board with 158 lamps and tried the Lamp Test, they all glowed, but very dimly. The power supply made a groaning sort of sound which was the transformer overloading, and when I checked the AC voltage it was down substantially due to the (over)load. 

By removing bulbs, I was able to confirm that Lamp Test will cause bulbs to glow as brightly as will logic signals, but only within the capacity of the power supply. That is, I could run up to about 10 lamps, which will draw 2A, and the brightness is nearly identical to a single lamp. Any more and the dimming becomes quite noticeable. 

SECOND TEST SETUP BEING PREPARED

I have bought a beefier transformer, one that is rated at 6A for 6.3VAC. Based on my experiences, I should be able to fully light 30 positions all day and could push up to around 60 without too much dimming. That is of course as long as I limit the time I am overloading and heating the transformer. 

I also bought an AC ammeter to insert into the line. I want to confirm the current being consumed by bulbs at full brightness. I should be able to measure the current demands of 30 bulbs, then scale it up by 5.47 times to estimate the full load on the board at 6.3VAC. 

THIRD TEST SETUP BEING CONSIDERED

As a third and final bench test, I will try to make use of a variac to produce 7.25VAC and satisfy the full current demands of the board. I am short about 13 bulbs, but I have a new supply of 50 lamps coming later next week. In that way, I will have the means to accomplish a full Lamp Test of the board, checking for issues and hot spots.

Since I don't have an isolation transformer, one side of the variac output, which has to be hooked to my common ground on the board, will also be tied to the wall supply neutral conductor. I shall need to test my bench supply that delivers the +3V for Lamp Test control to be sure that it won't be harmed by having its ground terminal hooked to wall neutral. 

I guess at worst case I could make use of two batteries to produce the +3V to feed the gate current of all those triacs. I project they will draw in the range of 60ma from the +3V control line to have them all firing, which is within the capabilities of a pair of D cells. 

The Variac that I own is only rated at 5A. I would need one rated at quite a bit higher level. It doesn't have to be rated at a full 40A, but can't be all that much less or the heating effect could damage the windings and the slider that sets the output voltage. I could easily live with a 30A unit, but it isn't worth the $75 to $100 it would take to buy one. I think that after the second test, I will have to hook this into the IBM 1130 and validate the thermal and full current behavior of the board while installed in the console and active.