Progress on IOB6120, 1130 Extender and a few small disasters

IOB6120 ASSEMBLED AND INITIAL TEST SHOWS SBC6120 WORKING

The header strips arrived, I soldered them onto the FP6120 and IOB6120, then put the entire stack together. That is, the large front panel FP6120 has two daughter boards mounted on the rear, the SBC6120 (PDP-8 replica) and the IOB6120 for additional input-output functionality.

I provided power and watched the machine go through its power on self test with blinking lights on the panel. To go any further, I will need to hook up a terminal emulator and see the prompts from the built in monitor. That will allow me to confirm that the IOB6120 is found and eventually, once I flash the firmware onto the board, that the new board is working properly.

I last used the FP6120/SBC6120 for a Vintage Computer Festival West conference when I was showing teletypes. In order to support the teletype interfaces I was using, I had to make modifications to the SBC6120 interface chip. I added a slide switch that would reverse VCC and ground to a couple of pins on the HD6240 UART chip. I don't remember why or what this does. 

Before I proceed, I have to reverse engineer the change, then decide how I put the system back to original condition. Once done, I can try to connect the terminal emulator to the replica.

BUILT IMPROVED MOUNT FOR 160 PIN CONNECTOR FOR 1130 EXTENDER BOX

I made use of some metal standoffs and a plastic project box to hold the 160 pin connector outside of the extender box chassis, allowing room for the wires to run inside and be connected without the connector body getting in the way. I marked off and drilled the holes, cut out the rectangle to clear the connector body and assembled everything. 

New mount for the 160 pin connector

WIRED SIGNAL LINES TO THE PCB INSIDE EXTENDER BOX

I was able to route and screw down the wires from all 76 of the signals that run over the SAC feature cable to the IBM 1130 system. I will lace or add wire ties to make things a bit neater, once I verify that it all operates properly. 

Beginning the connection of all signal wires

MINOR DISASTER - TWO WIRES TORN OFF CONNECTOR REAR

I discovered that two yellow wires from the rear of the 160 pin connector had been torn loose, likely during all the manipulation to wire up and mount everything. I should be able to access the rear of the connector and hopefully make a repair without needing to buy the pin removal/mount tools for the connector. 

One of the wires that was broken off the connector

SOLDER REFLOW PLATE POWER ON TEST - DISASTROUS RESULTS

The solder reflow plate takes 12V as an input and can draw up to 5A when heating for reflow. Its logic is driven through a linear power regulator that produces 5V to feed the ATMEL ATMEGA328P microcontroller and the LMT85LP temperature sensor. 

As you may remember from previous episodes, this version of the board, recommended by the Great Scott youtube channel, came without a bill of materials. The base project that it claims to have tweaked does have a BOM, but as I discovered it is deficient in several ways. First, it points at parts number from a China supplier and not the manufacturers code or other international supplier numbering. Second, it had gross errors, such as specifying 0402 size capacitors to mount on 1206 sized pads. 

Well, when I powered the board up and began checking power supply values, I soon found that 12V was present on the rails that should have been 5V. The cause was quickly found to be a PCB layout that required a special 'reversed' pin version of a chip but the material in the BOM was for the normal layout. This resulted in routing the 12V straight through to the sensitive parts downstream.

I believe that I have fried the ATMEGA328P controller and the temperature sensor. I may have also blown the .91" OLED panel, but I will wait to see that after I get the rest of the board operating. I removed the offending version of the voltage regulator, which you may remember was also the wrong form factor for the PCB pad spacing, but have not yet removed the microcontroller because there are so many tiny parts very close to it. 

I will build a set of diverters to block the hot air from reaching the capacitors and resistors that sit next to the main chip, while still allowing me to melt the bonds and remove it. I have a few days until the replacement parts arrive before I need to remove the old one, so it can be done at my leisure. 

REORDERED PARTS THAT WOULD HAVE FAILED, PLUS PROPER POWER MODULE

I did find the correct part number for a power regulator for reversed layout and that fit the pads on the PCB. I also ordered all the other damaged parts. I should have it all towards the end of next week.