Wednesday, March 31, 2021

Placed 164 sockets on the backside of the PCB into which the lamps will be plugged


Since incandescent bulbs have a short enough life that they need to be replaced as a routine matter, I designed this upgrade for the IBM 1130 to allow easy swaps when bulbs are dead. The bulbs I chose are T small wire lead style, which easily fit in the honeycomb matrix unlike the original IBM part that is press fitted into each cell. 

My board provides sockets for all the lamps on the rear side. With all the bulbs inserted into the sockets, they are steered into the honeycomb matrix while the board is pushed flat against the matrix. Replacing bulbs is simply a matter of pulling the whole board back, unplugging the bad lamp and inserting its replacement. 


I chose a very simple socketing system for the lamps, since they are wire lead type. I cut header strips into two position lengths, wrapped the bulb leads around the top of the pins and soldered them in place. The PCB has header sockets, also cut to two position size, into which the header segments are plugged. 


It was a tedious session this afternoon, where I cut apart socket strips into two position segments, hand placed them on the rear of the board and soldered them in place. Doesn't sound tedious until you realize this is 164 repeats of those steps. 

Rear of board, covering the right side of the 1130 panel

Rear of board, covering the left side of panel, with sockets installed

Right side of panel with machine status lights

Left side of panel showing the contents of six major registers

Installed 164 Triacs and 164 resistors onto the IBM 1130 incandescent lamp upgrade board


The two hardest parts to install are the triacs and resistors. I expect that the best sequence is to put all the triacs onto the board, then add in the resistors. The triacs can slide around a bit as I maneuver them into place and might bump into the resistors. 


I added a bit of solder onto the large pads before applying the triac. This makes it easier to solder them into place and allows me to see the triac sink slightly as it makes a good bond. 

Using tweezers I placed the part in position and held it while applying heat and solder. Once the large heat sink of the triac was bonded to the large pad on the PCB, I soldered the ground and base leads to their small pads to complete installation of that triac.

I intended to have every triac perfectly aligned but no plan survives contact with the enemy, so there is a bit of variation in the placement. A very few are slightly rotated; more often the variation is vertical placement, if you call the side of the triac with two small leads the down side. 

First 96 circuits have triacs and resistors installed

The resistors were dead easy, even better than expected. I had none pop off the tweezers, none got lost, and they aligned very nicely with minimal work.

0603 sized resistor below left triac lead


I did discover a few errors on the PCB. Some were silkscreen layer issues - a few triacs still had their ICxxx label displayed and one row of signal names were placed above the lamp socket rather than the signal pin, but those are largely irrelevant. Others were missing connections that I discovered as I carefully checked connectivity after each step of soldering.

I was missing a connection between the gate pad of the Accumulator Register Bit 14 and one of the resistor pads. That was easily corrected with a small jumper. I also missed a connection between the large 'anode' pad for the Customer Engineer 4 triac and the lamp socket pad. This was also corrected with a jumper. 

Monday, March 29, 2021

PCB arrived, ready to begin component installation and checkout


After watching the progress of the box, I saw it delivered on Monday afternoon and arranged a time to drive over to pick it up that evening. After a bit of photography and video of the unboxing, I was ready to move onto the construction phase.


You can see the board as it arrived, with its component pads, holes for connectors and labeling - white resist with black lettering. Since I received five of them, I sent one to an east coast museum where they are restoring another IBM 1130 system. 

PCB ready for component installation

Closeup to see labeling of signal pins and detail work


Building this board involves placing and soldering the following on the board:

  • 164 surface mount 0603 resistors (6.8K)
  • 164 surface mount triacs
  • 164 single header pins for logic signal inputs
  • 164 dual header sockets for lamps to plug into
  • 3 heavy duty wire terminal blocks for AC, ground and Lamp Test wire connections
Technique is important when soldering on surface mount components, if you want them to be well aligned and look neat. A reflow oven would make use of the solder surface tension to pull the parts into good placement, but I don't have an oven big enough for this 17 1/2" wide PCB. 

In addition, the sequence of installation is important, as the header pins and wire terminal blocks would get in the way of placement and soldering of the surface mount parts. I chose to solder on the triacs first, then added the resistors, before installing all the connectors. 

I will tin the large pad where the triac body will be soldered, but not the pads where the gate and cathode are attached. I will do this because I could mechanically hold the triac by the sides while I heated the solder to make the triac sink down and solder to that large pad. Holding it properly would put the gate and cathode leads in the right place for easy soldering.

A similar process, tinning one side of the pads for each resistor, will allow me to place the resistor well and solder one end. I expect this to be more fussy than the triacs because the means of holding the resistor is limited and it could easily pivot up leaving a vertical gap under the far side of the resistor. I think that I can push down on the far end while heating the near end solder to pivot the resistor back down in proper orientation. 

Sunday, March 28, 2021

PCB box approaches, on target for Tuesday arrival


The PCB flew through the shipping system, leaving Hong Kong within a day of beginning its journey in Shenzen. Expected delivery was Tuesday March 30th at that time, and as it cleared US customs in LA on Saturday, that seemed very realistic. Sunday morning it was in the DHL gateway in LA, thus still on track. 

Los Angeles is roughly 400 miles from my location, basically a few hours truck drive from there to a distribution center and then out on a van for delivery the next day. I expect it to be on the truck either overnight today or Monday in order to maintain the targeted delivery date.

Thursday, March 25, 2021

PCB jumps from 63% done and target of April 2, suddenly complete and waiting shipment

 This morning when I checked the boards were listed as 63% complete and this evening they jumped to 100%. It had appeared to me that the completion  percentage was simply incremented 1/16th per day based on the target of 16 days for production. This hinted that the board was contracted out to another fab, limiting the visibility that all previous orders had offered into the detailed manufacturing steps.

The PCBs must have arrived at PCBway or be ready for direct shipment from the actual fab plant, without advance notice of faster than forecast progress, thus the leap from 63 to 100 percent. Not that I am complaining, of course. Happy to have them arrive sooner, in fact. 

Once they are tendered to DHL I can get a better idea of the arrival time of the package, particularly once it passes through US customs next week. 

Wednesday, March 24, 2021

Components arrived as I wait for a seeming eternity for the PCBs to be completed


It seems like the proverbial watching paint drying, but I check in every day to see the glacial creep of the progress indicator for the production of my boards. Today it reached 57% on its journey to the predicted completion on April 2nd, after which they have to be shipped across the ocean. 


It wasn't worth the effort to harvest any of the surface mount 6.8K resistors, nor to remove the header pins and sockets. I now have several hundred 0603 sized resistors, rows of header strips and rows of socket strips waiting to begin installation on the PCB. The large wire terminal blocks also arrived this week.


In the interim, I am cleaning out excess possessions and donating or selling them off. Goodwill received over 20 large garbage bags full of clothing, for example. I sold a Tektronix 7854 oscilloscope and am in the process of selling a couple dozen plug-ins. 

I completed some work on the thru-wall air conditioner in the den, which involved carpentry and a lot of muscling tools and materials around. Just a matter of going through the long term list of tasks and repairs around the house, but it keeps me busy while I wait, wait and wait some more. 

Wednesday, March 17, 2021

Prep work while waiting three or more weeks for the new PCBs to arrive


I used my hot air rework tool to carefully remove all the triacs from the three existing boards, since these add up to a bit of money for 170 positions. In addition, of course, I unplugged all the socketed light bulbs and saved them.

Triacs on the bottom after removal from the PCB above

I made no attempt to save the header pins, sockets nor the 6.8K 0603 sized resistors as these amount to pocket change worth less than the time to remove them and clean them up for reuse.


Colored image of the left side of the board

I took the image from the PCB design tool and filled some sections with colors to approximate what my board will look like when it arrives. The resist layer is white and the text is black, so I didn't get this fully correct (the text in the image is brown). 

The grey areas represent the tinned surface where the Triac will be mounted. The black squares are the header pins where the signal wired plug in.  Smaller black rectangles show where the 6.8K resistors will be visible after soldering. The blue blocks are the beefy terminal blocks where I attach the power, ground and lamp test wiring for the entire board. 

The sockets for the lamps are mounted on the back side, seen here as yellowish white rectangles near each triac. If I had good 3D models of all the components, I could have used the tool to produce a 3D rendering of the board with parts mounted, but I don't have such files for the triac nor the wiring terminal blocks. 

Tuesday, March 16, 2021

Incandescent driver board now in fabrication at PCBWay with extended build time of 16-17 days


Back face (to rear of 1130) where components mount

The lamp socket is mounted on the bottom (front face), all other parts on top (rear face), which include the three 2 wire connectors for power and the 170 signal 2.54mm pins in addition to the triacs and resistors.

One lamp position from the design

I only needed traces on the rear facing (top) of the board, plus two interior layers that are beefy 2 oz copper to carry the 7.5VAC to the top pin of the lamp connector and the ground through a via to the triac lower right lead.

Circuit I designed

Board circuit does not include transformer, switch nor the 6.2K resistor to signal. The board will have the higher current MCR706AT4G triacs installed. The 6.2K resistor is part of a group mounted inside the chassis of the 1130 system, the transformer is in the power supply area of the chassis and the switch is just under the cover of the 1130 with the other five Customer Engineer switches. 


PCBWay has sponsored PCBs that are shown on the CuriousMarc YouTube channel which will include a future episode of the 1130 control panel improvement. I just uploaded the files to and contacted our representative to have them mark it as paid thus beginning production. 

Sunday, March 14, 2021

Design of the new PCB for a better incandescent lamp mounting in the IBM 1130 display panel


The board, if fully deployed with all eight CE lamps and the Synchronous Communications Adapter lamps, can light 170 bulbs simultaneously. With a bulb spec of 200ma steady state, that comes to 34A of current with the bulb at its peak voltage. I will use RMS voltage because the filament is fed by AC and that is the most appropriate measurement to calculate the power used. With the 1130 lamp circuit fed by 7.5VAC, the board will dissipate some 255W across its area of just over 74 square inches - a power density, if we cheat and assume even bulb spacing, of about 3.4W per square inch. 

The traces between the triac and bulb socket as well as from the ground plane to the Tbulb socket need to carry 200ma, which is easily handled by the traces I drew. Signal current when lighting the lamp is around 0.63ma, thus trivial. Lamp Test mode with all 170 lamps illuminated would require about 107ma.

The remaining current to accommodate is the connection of power and ground wiring from the 1130 to the PCB. These have to handle 35A across them. Thus, if each is fed with dual terminals (17.5A load) a plated hole of 1.6 mm will be adequate. In normal practice, without Lamp Test turned on, a typical system draw is probably down around 5A to 10A. I chose to specify a 2 oz copper for the 4 layer PCB which makes this design even more conservative. 

I put in about 12 hours of time drawing all the traces, placing components, composing the silkscreen text and verifying the design. One side of the 2 pin header socket that attaches to the bulbs will be connected to the 7.5VAC inside layer simply by the design tool leaving the copper of that layer in contact with the plated hole that the socket is soldered into. However, that is not visible in the normal view in the tool with all layers stacked, so I have to turn off the upper layer and observe the AC layer to ensure that an insulating hole is not reserved around that pin of the socket. 

Saturday, March 13, 2021

Performing checkout of incandescent lamp improvement to IBM 1130 display panel


The PCBs with their installed lamps are placed on the honeycomb matrix to position the bulbs behind the assigned legends on the panel face for each signal. As an example, the ACC(umulator) register on the left has 16 lights in its row, the leftmost representing bit 0 and the rightmost is bit 15. 

The big PCB is installed on the left of the panel (as viewed from the front of the computer) and displays the contents of six major registers - IAR, SAR, SBR, AFR, ACC, and EXT - Instruction Address, Storage Address, Storage Buffer, Arithmetic Factor, Accumulator and Accumulator Extension. 

One small PCB is installed on the right and displays Operation Register, Operation Flags, Index Register, Interrupt Levels, Cycle Control Counter and Condition Register. The other small PCB is installed in the middle and displays the T clock states, X clock states, arithmetic unit control states, eight CE selectable displays for debugging, and the status of the Synchronous Communications Adapter (not installed on my computer). 

Prior to installation, I hooked AC and Lamp Test signals to the boards, verifying that individual lights can be turned on or off and that all lit up with Lamp Test set to +3V. The large PCB and one of the small PCBs (for the middle position) worked just fine. 

Unfortunately, the rightmost PCB appeared dead. Neither lamp test nor individual signals applied produce any lit bulbs. I also had intermittent problems with the other boards as I moved the power wires around. 


I detached it and started to check continuity between the power wires and the important points on the board. There was not continuity from the ground power connection to the ground points on the board. Other boards would sometimes not light but this was fixed by flexing the pin bringing ground or AC to the board. 

Based on this, I came to the conclusion that my connections for wiring were inadequate and I would have to redesign the boards. In the interim I could patch around the problems to finish testing.


To get the PCB working properly, I tack soldered wires to several of the triacs ground pin and similarly connected to the AC side of several lamp sockets. This allowed me to verify that the rest of the board design was sound. 

Three boards in place showing some of the temp tack wiring


It would seemingly be an easy task to change the existing designs to provide for larger more robust connections for ground and AC. Alas, that would be true if I could find the design files for the boards anywhere on my computers or backup files. I couldn't. Four years after I designed this, and probably when I was hot on the trail of the color LED version, I must have deleted the design files. 


I started afresh, laying out the board. This time, I chose a single board 4 1/4" high and 17 1/2" wide, the full size of the honeycomb matrix that holds the display lights. I also selected much more robust connections for Ground, 7.5VAC and Lamp Test connections. The remainder of the design is proven and will be carried over. 

Thursday, March 11, 2021

Back home and starting on projects


The IBM 1130 computer has a display panel that sits above the console printer and shows the status of the major registers and important internal state of the machine, through a matrix of approximately 150 incandescent lamps that are individually driven by that many triacs. 

The earlier machines, such as mine, built small PCBs with 8 or 16 triacs that mounted 8 or 16 lamps. These lamps had to be pushed into a plastic matrix, but if any of the lamps was cocked at all then the assembly wouldn't seat. 

Original IBM design for lamp mounting and replacement

With six rows that are about a half inch apart, that meant six PCBs that had to be maneuvered to seat their lamps with little room to see or manipulate them. Thus, changing the burned out lamps became a tedious and time consuming activity. There was one column of six rows of the 16 lamp PCBs and two columns of 8 lamp PCBs, with a rats nest of wires carrying the roughly 150 signals plus power and lamp test wires to each PCB. 

Later models relocated the triacs to separate PCBs mounted on the rear wall, but there was still a maze of wires and lamps to be individually guided into the holes in the matrix. The bulbs were individual lead bulbs which were mounted in small nylon sockets that had holes for larger pins to press in. One pin was on the end of the triac and the other was wired to a common line on the PCB. Even the task of pushing a socketed bulb onto the pins was fussy work. 

I decided that I needed to build a more workable alternative for the display. Since each circuit had just a bulb, triac, resistors and wiring, with additional resistors, signal drivers, test switches and other components elsewhere in the machine, it would be easy to duplicate the functionality but using a larger PCB to sit across the face of the honeycomb matrix with lamp sockets at all the proper positions. 

Honeycomb matrix into which the lamps are inserted

Using smaller bulbs with a less unwieldy socket for replacement, I could simply guide a PCB onto the matrix to have all 16 columns by 6 rows of lights (or 8 columns by 6 rows) slide into place in one operation. I found a suitable triac, by which I mean one that will trigger reliably but could handle much higher power and voltage, that I could surface mount on the PCBs I designed. 

The AC power, common ground and lamp test leads are connected once to each of the big PCBs, while the circa 150 logic signals that are displayed push onto individual header pins on the rear of the PCB. The circuit would work exactly the same as the IBM original methods, but with a mechanically better design that is far easier to maintain. 

Test fit of one PCB onto the honeycomb

I soldered smaller individual lead incandescent bulbs to two pin headers that will plug into sockets on the face of my PCB at each light position. The one area that I remain unsatisfied with is the protection from the two leads twisting into contact with each other and blowing out a triac. On the original IBM design, this could easily happen and I lost a few triacs as a consequence, while wrestling lamps and the boards into place on the matrix. I tried some insulating goop but it looked ugly and was cumbersome to apply. 

original IBM bulb on right but went with left bulbs instead


Because of the remaining issue with bulb leads potentially shorting, I put the boards aside after building them. I didn't complete testing them because I then conceived of a alternative using full color LEDs. That would have the advantage of long life without bulbs burning out periodically requiring replacement. It had the technical challenge that reproducing incandescent bulb dynamics with LEDs is hard yet I wanted the console display to look just like the original while in operation. 

Recently I was contacted by a museum that is restoring an 1130 system. The console lamps where a mess due to the poor design and they were seeking an alternative. I decided to finish testing my design, so that if it worked satisfactorily I could send them the PCB design files and triac part number. 


I want to test the brightness of the lamps, the ability to independently light them and the functionality of the Lamp Test switch that would cause all lamps in the display to light simultaneously. I chose one of the smaller PCBs to minimize the scope of the test wiring. The particular board I chose has 33 triac controlled light sockets, although 8 of them are for CE display lights I won't use initially so no lamp was installed in those sockets. 

The boards are powered by a 7.5VAC source with one of the leads tied to the logic ground of the system. Each logic signal that is connected to a triac is isolated with a 6200 ohm resistor, that resistor is mounted elsewhere in the machine. The triac gate is also connected through its own 6800 ohm resistor to the common Lamp Test wire. That wire is switched to either ground or +3V. 

The logic levels in an 1130 are 0 and 3V for binary 0 and 1 respectively. Thus, the driving gate in the machine delivers 3V or 0V through the resistor divider of 6200 and 6800 ohms to ground, making the triac gate either slightly over 1.5V or at ground. When the Lamp Test wire is switched to +3V, it makes every triac gate sit at a bit under 1.5 or up to 3V depending on the state of the driving gate. 

To properly test the board, I have to provide the Lamp Test wire at ground while connecting all the triac gates to what appears to be independent driving gates in series with a 6200 ohm resistor. I want many off and a few on, so that I can switch those wires around to prove that I can light specific lamps. 

I did this by using diodes in series with 6200 ohm resistors, three of them hooked to +3V and the remainder hooked to ground (with the diode direction set appropriately). This will appear to be 33 discrete driving gates delivering 3 or 0V. 

Once the individual lighting control is proven and the bulb intensity is judged sufficient, I will connect the Lamp Test line to +3V to verify that all the bulbs light up simultaneously. I soldered 33 resistors to 33 diodes, hooking one end of each of those pairs to a wire with a socket to fit my signal pins on the board. The other end of the pairs were wired together, 30 to a wire that will hook to ground and three together to a wire that hooks to +3V.

6.2K and diode to simulate each logic driving gate

I then needed to hook up an AC source. I chose my bench supply with a 6.3VAC, since in real operation that bulbs will be even brighter. If it works okay at 6.3 I should be good in the real world. Finally, I wired the wire for Lamp Test which I can move between ground and +3 power supply connections for that part of the test. The wire from the three leads that will light chosen lamps are connected to the +3VDC supply and the wire from the 30 dark lamp leads is hooked to ground. 

Power supply for 6.3VAC and 3VDC

With all that wired in place, I connected the leads with three lamps chosen to be lit, but obviously I can swap these around to check other lamps. The Lamp Test was set to off (ground). The +3V supply was turned on first and then the 6.3VAC supply was energized. The first tests judge lamp brightness and individual control by moving the leads around. The second test will move the Lamp Test wire to +3V and verify that all 27 lamps are illuminated. 


Individual control - the lamps I hooked to +3V lit, the others did not. Moving the hot leads moved the lamps that would illuminate. Test passed.

Brightness - The bulbs seemed bright enough, but I inserted the test board into the matrix to see what the front panel brightness would be. I was pleasantly surprised, with the panel indoors with ordinary room illumination I could read them just fine. 

Test of illumination of T7, X7 and W(ait) status lamps

Lamp Test mode activated - when I first switched the Lamp Test wire from ground to +3V, the lights lit for a brief moment then the whole board went dark. I disconnected everything to do some diagnosing, hoping that traces didn't burn or something else fail from the current of all bulbs lighting.

I discovered that the pin for ground to the PCB wasn't well soldered. After touching it up, the board passed with both ordinary and lamp testing mode. I checked the brightness with all active to see whether voltage drop would make them unacceptably dim, but again this passed just fine. In the image, two of my bulbs were folded under and not inserted properly into the matrix, thus they are dimmer than the rest, but that will be solved by proper insertion. 

P2 and ADD lamps didn't insert into matrix properly, but all light up


  1. I have to wire these boards into the 7.5VAC, Lamp Test and Common Ground lines. 
  2. I must set up holders to hold the board up in the right place and against the rear of the matrix of holes. 
  3. The logic lines have to be pushed onto the pins on the rear of the PCB in the proper places. 
  4. I will refine the PCB design slightly to improve the power connections to each board as that was the weak link. The new PCBs will be shared with the other museum and I will probably rework the components from the old PCBs to my new design before I button up the 1130 display panel.

I am quite pleased with the results of this project.