Measured bulb current draw, pleasant surprise, but purchased junk ammeter

 AMMETER PUT INTO CIRCUIT TO MEASURE ACTUAL CURRENT DRAW

I bought a 20A ammeter through Amazon.com from Uxcell, claimed to have a proper internal shunt and accurate to a reasonable degree. With the ammeter in the circuit, even with 44 bulbs installed it barely registered 1/2 amp draw. However, I was suspicious about the correctness of this given the dimming that happens with that many bulbs plugged in. I am returning it to Amazon since it doesn't work properly.

CROSS CHECKING AGAINST MY VOM AC AMP MEASUREMENT

My VOM has an AC current measurement but a max range of 200ma, not very useful for measuring more than a handful of bulbs, but it did give me a baseline for comparison. With four bulbs inserted, I measured almost 7V from the supply and 172ma of draw. 

The defective ammeter on the other hand is off by a factor of 4 to 5x. As a second check, I grabbed a second VOM, one with a higher range of AC current measurement, and put it into the test circuit with 10 bulbs installed. I should see 400+ ma of current draw if my other measurements are correct. I recorded 440ma with the unloaded voltage of 7.06 dropping to 6.89VAC when the lamps were lit.

Current consumed by 10 bulbs lit by Lamp Test

Populated the board with just 10 bulbs to do the measurement

Voltage while supplying 440ma to light the lamps

Unloaded voltage of the AC supply

Gate draw is going to be pretty negligible by comparison to the lamp current. The resistor voltage divider to each gate consumes about 0.2ma of current, 34ma for all 164 circuits simultaneously conducting. The thyristor adds some microamperes to the draw, which we can ignore. 

ACTUAL BULB CONSUMPTION DETERMINED 

That suggests that at full supply voltage in the IBM 1130 these will draw about 40-50ma apiece when lit. All 164 positions, if the same type bulb is used, would only draw around 7 amperes, well within the capability of the new beefier transformer that will arrive Tuesday. 

Some T 1/4 size bulbs have a spec of 200ma, others 50ma, and it is possible that I have a mix of both types in my supply. I have been measuring their cold resistance (10 ohms approximately) to verify lack of shorts, but I decided to set up a jig to test all bulbs for current draw and sort them into groups.

The consumption of the bulbs directly wired to the AC feed is different from their draw in circuit, in part because of the approximately 1V drop across the thyristor while it is conducting. I think that IBM specified 7.25VAC for its lighting circuit with that drop in mind, yielding about the nominal 6.3V at the bulb that matches the bulb specifications. 

The measurements confirmed the expectation that the entire board will consume between 7 and 8 amperes depending on the exact voltage inside the IBM 1130, with all lights illuminated simultaneously. 

NEW BULBS ON ORDER ARE HIGHER DRAW - NET EFFECT

If I determine that I have about 150 bulbs that draw 50ma and add 14 that draw 200ma, that would bump the board consumption up over 9 amperes but still within the short burst capability of the transformer that will arrive soon. I should look for a source of bulbs at the proper current, since each burned out bulb will force me to bump up the board load by 150ma, which would scale up the draw quite a bit over time.

As an alternative, if I could find 2114 bulbs, which are the type I already bought, that would be a preferable component to use with my console. In the end, I did manage to buy a couple dozen of the desired bulbs and thus can do a full up test of the board with Lamp Test once everything arrives here. 

Finished first version of bulb holder design for 3D printing

 DESIGN OF BULB HOLDER

This holder is designed to take a two position standard (.100" pin distance) header strip in the bottom, with the longer pins sticking up through the holes with a barrier between them. A wire lead incandescent bulb is placed astride the barrier, its wire leads wrapped around the two pins and soldered in place. The side walls keep the bulb protected and aligned. The fillet of the tops of the side walls helps insure that each holder slides into the opening in the 1130 console honeycomb cell. 

View from bottom where header is inserted

Drawing views

View from above where bulb is placed

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. 

Thinking about improvements for lamp holder for the incandescent control panel upgrade

 CURRENT LAMP HOLDER

I use mini incandescent bulbs with wire leads, which I wrap around a two position header strip and solder in place. On many of the lamps I built, I applied some kind of insulating goop to protect against shorts if the leads touch but it makes the lamp a bit ugly. These plug into two position sockets on the PCB, enabling easy replacement of any dead bulbs. 

ISSUES OR WORRIES

Because the bulb is simply suspended on its leads above the header strip, it is not held in exact alignment with the opening in the honeycomb matrix into which it has to fit. Bulbs can be cocked because one lead is a bit shorter than the other. They can flex over to the side as well. 

Some of the sockets are tilted over, rather than at right angles to the PCB since I didn't have a jig that assured proper alignment. This tilts the lamp plugged into the socket. 

This degree of variability makes the insertion of the PCB with all 164 lamps installed a bit fussy; the bend in various lamps may cause them to not slide into the honeycomb cell without some encouragement with a thin screwdriver. 

The leads can short together, particularly if the lamp is bent over because it fails to enter a honeycomb cell during installation of the PCB into the matrix. The goop is not a perfect solution to this problem.

POTENTIAL SOLUTIONS

I will go back over the board to manually realign any sockets that aren't perpendicular, reflowing the solder while I position the socket with a suitable tool. That part is easy to address.

It will be necessary to build a holder for the lamps, something that keeps the wire leads separated, holds the bulb upright and perpendicular, and ensures that each lamp is well aligned with the honeycomb matrix so they slide right in. 

I am exploring having them 3D printed, after I build a design that I feel will properly maintain alignment for all 164 bulbs. Over the coming day or two I will noodle around with various designs until I find one that is promising enough to have some samples made. 

Powered tests of the functionality of the board - everything working very well

 MAKING CONNECTIONS TO THE TERMINAL BLOCKS

The board requires a common ground between the SLT logic level DC (0 or 3V) and the 7.25VAC lighting power. The other leg of the AC supply is hooked to the top terminal block. Finally, the Lamp Test block on the bottom is hooked to a switch that ties it to ground in the normally closed position and flips the Lamp Test line to 3V DC when moved to the normally open position. 

NO LAMPS INSTALLED, VERIFY NO CURRENT FLOW AND NO AC ON SIGNAL PINS

With the AC turned on but no lamps or inputs connected, the board should not consume any appreciable current on the AC level - and it did not as it was less than one ma. I also checked that AC was not presented on any of the 164 logic input pins. 

SINGLE LAMP TEST OF LOGIC 0 INPUT, LOGIC 1 INPUT AND LAMP TEST

I inserted one bulb to verify operation of one of the 164 lamp circuits. With no connection to the input pin, the lamp did not light. With a simulated logic level applied the bulb lit up. This was done with +3V applied through a diode and a 6.2K resistor, matching the output driver inside the 1130 that provides the signals to the control panel. 

I flipped the lamp test switch, putting +3V on that control line, and my bulb lit with either logic 1 or logic 0 applied to the input pin. This represents a full test of the operation of one lamp position. I know that the basic circuit is working properly

MULTIPLE LAMP TEST VERIFYING INDIVIDUAL SELECTION AND MASS LAMP TEST

I installed five bulbs onto the board and switched on again. Nothing should light with Lamp Test off and no logic 1 applied to the input pins. None did. I hooked up three logic 1 levels to three of the circuits and left the other two at logic 0. Those three bulbs lit and the others did not. Finally, switching on Lamp Test caused all five bulbs to illuminate. Again, all good. 

Three signal lines switched to high, Lamp Test switch in off position

FULL UP LAMP TEST WITH ALL LAMPS INSTALLED

I built a quick and dirty tester for lamps to make sure that the wire leads were not shorting before I plugged them into the board. It was a bit of header strip in a breadboard with my ohmmeter in line to check the cold resistance of the bulb. I could distinguish between dead (open) lamp, shorted wires and a workable lamp using this setup. 

I inserted the board with all the lamps into the honeycomb matrix of the front panel. This would let me access the signal pins on the rear, protect the lamps inside and let me watch the illuminated positions from the front. With all lamps installed, I repeated my check to verify that none of the 164 logic input lines were seeing AC. This was key to protect the 1130 circuitry from damage when I install this upgrade in its control panel. 

I powered up and verified that no lamps were lit in the absence of an SLT logic 1 level on its input. I tested all the circuits to verify that they operated correctly with my logic 1 inputs on their pins. Finally it was time to flip the Lamp Test switch and verify that all the lamps lit up properly.

The current measured during steady state of the lamp test was hard to measure since my ammeter didn't support high enough current and none of my AC supplies have current measuring capabilities. 

Even worse, the puny AC power supply couldn't put out the current demanded by the lamps when all were on, thus the voltage dropped to about 5VAC and the lamps were all quite dim. I checked for signs of hot spots and was satisfied with the thermal performance of triacs and the board since everything was cool as can be, loafing alone under the dim version of the Lamp Test. 

Lamp Test switched to on position, power supply struggling to keep up

CHASING DOWN AND FIXING PROBLEMS DETECTED DURING THE TESTING

Accumulator Register Bit 6 did not light with the Lamp Test activated. The lamp lit properly for a logic 1 applied to its input pin, but did not react to Lamp Test. I traced this down to lack of connectivity from the 6.8K resistor to the gate of the triac. After a quick repair, I reinstalled a lamp to verify that ACC 6 does light under Lamp Test. 

Operation Bit 2 did not light with Lamp Test nor with direct logic 1 to the signal pin. This was a poor solder joint on the gate pad of the triac. Resoldered and tested.

Tag Bit 7 did not light with Lamp Test but did work with logic 1 on the signal pin. The 6.8K resistor was not connected properly. Resoldered and tested.

REC signal of the Synchronous Communications Adapter did not light with Lamp Test nor with logic 1, but did when I wiggled the socket. Poor solder on one of the pins of the socket. Resoldered and successfully tested.

Odd Status lamp was always illuminated even though logic 0 on the input pin and Lamp Test not activated. Found a bridge on the triac between the output 'anode' and the gate pad. Removed and replaced triac, then tested successfully. 

My original designs did not attempt to light the 8 Customer Engineer (CE) bulbs nor light the signals from the Synchronous Communications Adapter (SCA) since that feature is not installed on my machine. However, this new board is universal and includes those positions. However, when I built my lights - wire lead light bulbs soldered onto two position header strips - I didn't build the extra 16 needed to fully populate my board. 

NEXT STEPS

I have to build another 16 bulbs and install them into their positions for the CE and SCA displays. 

I want to work out a good fastening or clamping method to hold my board onto the honeycomb. 

It would be reassuring to find a high capacity AC power supply with current measuring capability and hook that to my board so that I can verify how it operates with the 148 currently installed bulbs. That should be close enough to the fully populated load of 164 to feel comfortable installing this into the IBM 1130.

The console should be put back together, my board wired into the 1130 with all the signal lines attached to the signal pins on the back. That will allow for a full power on test with the actual computer, the final certification of this design to replace the kludgy IBM incandescent lamp mechanism that came with the system. 

Anomaly tracked down to failed triac

 ANOMALY NOTED DURING RESISTANCE MEASUREMENTS

When measuring the resistance from the Lamp Test control line on the terminal block to the common ground, I found a surprise. It was 6.8K, the value of one of my resistors whose other end was somehow shorted to ground. All of the resistors should run from Lamp Test to the gate of its triac and there is no ground trace near these that could account for a solder bridge causing a short.

I checked the resistance from each triac gate to ground, which should have been infinite but was 13.6K for almost every triac - the resistance if its local resistor plus the mystery circuit whose resistor was sunk to ground. Eventually I found that the gate of the triac for Interrupt Level 1 lamp was at ground. No place where a solder bridge could have joined it to ground, leading me to conclude that the triac itself was internally shorted, gate to ground. 

I had reclaimed most of the triacs from my earlier boards to use here, they were not new other than 30 I still had in the box unused. Somehow this one was damaged and needed to be replaced. 

I whipped out the hot air rework tool and the soldering supplies, removed the triac and cleaned up the pads for installation of another of my reclaimed triacs (after testing it for a similar short of course). I set up a number of air dams to avoid loosening any adjacent parts when I unsoldered the bad triac. 

TESTING THE FAILED TRIAC

 Very puzzling indeed - with the triac removed, the short to ground was gone on the board but the triac itself didn't show a short. Nothing on or around the pad could have bridged to ground, so it must have been some failure inside the triac, perhaps some package stress that was relieved when I removed it. 

RECHECKING THE CIRCUIT WITH THE REPLACED TRIAC

I used a different triac, installed it into the position and checked over everything again.  All appears well with the board so it is on to the powered testing phase.

Finished build of the incandescent light upgrade to the IBM 1130 control panel

 LAST STEPS IN ASSEMBLY

There are only two remaining steps to finish installing all the components on this board. The first is another large tedious operation - placing 164 single header pins on the board for all the logic signal inputs. The second is quite small by comparison, installing the three wire terminal blocks that provide 7.5VAC, Lamp Test signal and common ground connections. 

By mid-day I wrapped up the signal pins and moved on to the terminal blocks. Since this board used 2 ounce copper pours for each layer, it has a very noticeably higher thermal mass than the usual PCB. It takes quite a bit of heat, with a wide tip, over a longer than usual time to flow solder properly through the pins and holes for the terminal blocks. 

VISUAL AND BENCH CHECKS COMPLETED

I carefully examined the soldering of all the sockets and pins by eye before starting tests with an ohmmeter. I then checked for zero ohms across the three power/control input terminal blocks. Finally I tested every socket and every signal input pin for signs of a short. Everything was good. It was time to apply some power and check things out.

Finished board ready for testing