Monday, December 10, 2018

Continuing populating DSKY display PCB and testing in stages


Installing components and checking as I go

I had a plan to install components in stages, checking operation as I went along. This caught errors really early when rework (and debugging) was easier. I laid down as many of the 80 surface mount LEDs as I had on hand - waiting on additional parts to finish up. I lack 14 white LEDs and just 2 green ones but the additional parts are already on order.

I moved on to installing the nine relay driver circuits. These have a PNP transistor, 10K resistor,  2K resistor, SMD relay and a 1N4001 quenching diode apiece. First to lay down were the SOT-23 sized transistors, next were resistors, the relay third and finally the diode. I completed one circuit first and tested it to be sure that it would work properly, which it did.

After the circuit was proven to work properly, I installed the remaining parts to enable all nine relay circuits. I left off the relays because I need room to add other circuit elements here, but will go back and insert them once the AGC input circuits are done

Eight of the AGC output circuits will take inputs from the keyboard PCB I have designed and convert them to AGC signal levels. That is, they will pull the AGC lines down to ground through a 2K resistor using the relay whenever the input signal is pulled down to ground. This is the same circuit used to feed signals from DSKY to AGC.

Next steps were to install all the transistors, resistors, and capacitors that comprise the AGC input and output interface circuits. These deal with the 28V logic levels of the Apollo spacecraft and match the impedance and other characteristics that the real DSKY presented to the Apollo Guidance Computer.

The first circuit was installed, 28V power was applied and I tested this with a VOM on the output pin and a 2K resistor to ground touched to the input pin, to simulate a real AGC input. What I need to see is the output sitting at approximately ground when the input is open, but when the input is pulled to ground my output should jump up to 5V.

Once I saw that the circuit worked exactly as it should, I continued installing the circuits one by one. By dinnertime I had five input circuits installed and verified. Continuing after I ate, I kept soldering four resistors, a capacitor and a transistor for each input.

The 0603 resistors are easy to solder down, as are the 1206 capacitors. The SOT-23 are a bit harder, not due to small size as much as their instability as I try to tack down the first of the three leads. It just slows me down, with sporadic incidents of transistors popping off the tweezers or tipping over as I try to place them.

I sped up considerably and had 12 of the 23 inputs completed by 6PM, although I had only tested circuits 1 to 5 so far. I discovered three of the circuits weren't working properly. Two were not responding to inputs but the third was delivering 22V to the output instead of 5, which would have toasted the input pin of the Arduino!

I quickly resolved the overvoltage issue - I had swapped the resistors for the voltage divider taking 28V down to ground with a tap at the output, thus delivered 22/28 instead of 5/28 of the power rail to the output.

I found a short in the input side of the circuit to a via on the board near the resistor, which resolved both of the dead circuits. Turns out that this board does not have tented vias - resist is not covering the vias so they have a plated surface to which solder can attach.

I realized that as I completed the inputs in the top half of the board, I can add in the relays that sit near them. I took the time to install relays that fit in the upper area where circuits 1 to 12 sit, since those are proven out already.

By mid-morning, I had completed circuits 13 to 18, leaving just five more before I was done with the AGC input section. Testing of the new circuits turned up a short circuit between 28V and ground which I had to correct before I could actually check out the input circuits I had wired.

The short was complex in that 28V shorted to +5V, which in turn was shorted to ground. Finding the 28 to 5 short was easy; it took longer to locate the remaining short across 5V to ground.

Fortunately I didn't have much hooked up to the 5V and ground rails, just one test LED limiting resistors, plus the relay driver circuits that had 50V tolerant transistors and other parts. That eliminated the risk that I blew a component by subjecting it to 28V. 

Still, it took a time to locate the short. With it corrected, I validated that circuits 13 through 18 worked properly. I moved on to solder on the components for the remaining five input circuits.

Unfortunately, after a few too many of the small SOT-23 parts sprang free from the tweezers and departed to parts unknown, I was forced to order a small stock of additional transistors before I could complete the input circuits.

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