You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
79 lines
3.0 KiB
Markdown
79 lines
3.0 KiB
Markdown
---
|
|
layout: post
|
|
title: "Building a COSMAC/RCA CDP1802 learning computer"
|
|
date: 2024-05-01 13:00:00 +0200
|
|
comments: true
|
|
categories:
|
|
---
|
|
|
|
# PCB making
|
|
|
|
## Printing the transfer
|
|
|
|
I tried several methods to prepare the toner transfer intermediate:
|
|
* baking parchment paper with double sided tape
|
|
* glossy magazine with scotch tape
|
|
* glossy magazine with glue
|
|
|
|
What seemed to work best is the glossy magazine glued to a sheet of paper strong enough to not have irregularities.
|
|
:warning: Don't forget to mirror the image before printing :facepalm:
|
|
|
|
## Preparing the board
|
|
|
|
Grain #500 sandpaper -> steel wool -> acetone
|
|
|
|
## Transferring the toner
|
|
|
|
* **1st attempt** (glossy paper taped to carrying paper, max heat, 1.5 min): **failure** the paper sticks to the PCB, no transfer
|
|
* **2nd attempt** (parchment paper glued to carrying paper outside of printing zone, max heat, 3 min moving across surface, let cool): much better, good enough for silkscreen but probably not for etching
|
|
* **3rd attempt** (glossy paper glued, pre-scrubbed with isopropanol):
|
|
* very good print, almost no defects
|
|
* transfer with acteone/alcool (3:8): total failure, did not stick to copper at all.
|
|
* parchment paper, taped to carrying paper on all sides (outside of printing zone), pre-cleaned with isoprop
|
|
* iron 2 min on each part of the surface, temp setting 2, lots and lots of pressure
|
|
* let cool completely
|
|
* transfer failed :-(
|
|
|
|
Looking for a laminator...
|
|
|
|
Laminator (Pavo Vancouver A4) / parchment paper : utter failure, almost no transfer at all
|
|
|
|
:check: **Great** result with stickers backing sheet + cloth iron (max temp) + lots of pressure
|
|
* press for 2 minutes without moving iron much
|
|
* shift weight to distribute pressure
|
|
* high pressure finish over details (iron side/tip)
|
|
|
|
Reference: [Youtube](https://www.youtube.com/watch?v=haqP8xhsYas&t=600s)
|
|
|
|
Fix broken traces with Lumicolor permanent marker
|
|
|
|
:warning: Missed a broken trace
|
|
|
|
## Etching
|
|
|
|
Etchant solution:
|
|
* 50 mL 23% hydrochloric acid
|
|
* 50 mL 12% hydrogen peroxide
|
|
|
|
Etches in a few minutes. **Reaction is exothermic.**
|
|
*Don't take it out too early.* The protection by toner and marker both work well.
|
|
|
|
# Soldering
|
|
|
|
Checking regularly for continuity, expected resistance values, and absence of shorts was key to success.
|
|
|
|
# Debugging
|
|
|
|
System worked on first try except for LOAD mode flip-flop.
|
|
The 4027 from AliExpress behaves in a weird way, as though J and K were reversed.
|
|
|
|
The expectation is:
|
|
* ~DMA-IN is cabled to ~Q (i.e. the active low DMA IN signal is inactive when Q=0 ~Q=1)
|
|
* at startup Q=0 ~Q=1
|
|
* upon a raising edge of CLOCK, the flip flop transitions to Q=1 / ~Q=0 / DMA IN is active
|
|
* upon a positive signal on RESET, the flip flop transitions to Q=0 / ~Q=1 / DMA IN is inactive
|
|
|
|
According to both the truth table and the circuit design, this is the behaviour that should be observed with J=1 K=0.
|
|
In practice this is the behaviour observed with J=0 K=1.
|
|
|
|
If I set J=1 K=0 and set Q=1 by presenting a positive signal on SET, then on a raising edge of CLOCK I see the flip flop going to Q=0 ~Q=1. |