Building an Amiga 600 Junior

The Amiga 600 is the smallest of the Amigas (apart from the CD32, but that doesn’t have a keyboard), which makes it the ideal non-AGA Amiga to have on my cramped desk. However, with the Amiga 600 being over 30 years old now and Commodore having used terrible quality capacitors in its construction, most Amiga 600s have leaking capacitors and a motherboard that has suffered the consequences - corroded PCB traces, solder joints and components.

I bought an Amiga 600 on eBay that was in very nice external physical condition, but it wouldn’t boot. Replacing the reset circuit capacitor didn’t fix it either. Inspecting the board, I could see a lot of corrosion around many of the surface-mount capacitors. The board was beyond repair, at least with my skills. I searched online for replacement motherboards, and I found out about the Amiga 600 Junior. This was just what I was looking for - a brand new PCB that I could transplant components from my poorly Amiga 600 onto! I am lazy, so I ordered the Amiga 600 Junior PCB with auto-placed passive parts. It has nearly all of the 1206 and SOT23 package parts installed, but you will need to install a few yourself.

FYI, 1206 package looks something like this:

…and SOT-23 package looks something like this:

The only passive parts you’ll have to install yourself are:

A 1kΩ resistor (0805 package) on the underside of the board
Five diodes (DO-213AB package)
One inductor (1206 package)

I have not used composite or RF out on an Amiga since I got a 1084S monitor around 1990, so in this build I went with RGB-only. I will look at installing RGB2HDMI at some point. The CXA chip (CXA1145 or CXA2075) must be installed for RGB out to work - just the chip itself - no need for delay lines, capacitors, etc. I removed and installed the required resistors and capacitors for the installation of a CXA2075, but I installed a CXA1145. If I want to enable composite output in future, I just have to swap out the CXA chip.

Using the Mouser basket mentioned on the Junior product page as a basis, I ordered all the parts I needed from Digikey. Note that the Mouser basket has a mistake in it - it contains 2 x 74HCT244PW chips, which are SOT-360-1 package chips - a lot smaller than the 74HCT244D (SOT-163-1 package) chips that the Amiga 600 uses.

Some Amiga components are obviously not available nowadays, and have to come from a donor Amiga motherboard. I used the following components from the donor board (most were removed using a hot air station):

Power connector
Power filter coil
CPU
Denise
Paula
Gayle
CIA x 2
Keyboard MPU
512K Chip RAM x 2
3Mhz oscillator
74HCT244D x 2 (only because I ordered 74HCT244PW by mistake)
DB23-female Disk drive port
DB23-male RGB port
CXA1145 RGB to composite encoder

I could have also reused all of the connectors too, but as I said, I’m lazy, so I bought new ones. That PCMCIA connector is surprisingly expensive, though! I’m amazed that the Amiga 600 keyboard connector is available to buy new.

Here’s a Digikey basket containing the parts I used: https://www.digikey.co.uk/short/953mfmbf

The basket excludes a few capacitors that are required for using a CXA1145. If you use that basket to place an order, please double-check that it contains the parts and quantities you need.

I could not have built this “new” Amiga 600 without help from John "Chucky" Hertell’s excellent post about his Amiga 600 Junior build. I followed John’s post closely, though he started from a point where ALL passives were installed. So I started by installing the remaining passives.

First up was R102 - the tiny 0805-package 1kΩ resistor on the underside of the board:

Next was the inductor (U21 was installed later - I took these photos out of sequence):

Then the five diodes, which you can see here (U7 and CN6 were installed later):

Then the power connector and filter:

And the RGB and LED connectors. Now the board looked like this:

From this point on, I just followed John’s post. One thing that John mentioned is that Denise is tough to solder with the 28Mhz oscillator in place, so during my build I did a temporary installation of the oscillator using some legs that I cut from some new through-hole capacitors. Before I installed Denise, I snipped the long legs and removed them from the board and the oscillator. Once Denise was installed, I installed the oscillator properly.

I used John’s invaluable DiagROM for testing at various points in the build (as per his blog post). I didn’t have a DB25-to-DB9 serial adapter, and wanted to build one myself out of the donor Amiga board’s parallel port (DB25-female) and an old DB9-female port I had lying around. I looked at this page for information on how to build the adapter, but I found that it contains an error. It says that DB9 pin 2 is TX and pin 3 is RX, which is the wrong way round. The correct adapter wiring is:

DB25-female pin	DB9-female pin
2 (TX)	2 (RX)
3 (RX)	3 (TX)
7 (GND)	5 (GND)

I bought a generic RS232 to USB-A adapter that looked like this:

Windows 10 recognised it straight away with no issues - it contains a CH340 chip, which as of July 2024 is still supported by Windows I guess.

The Junior board allows you to fit through-hole capacitors or SMD ones in some locations. I used through-hole wherever I could, though for C303 and C304 (under the floppy drive), I had to stick with SMD for space reasons.

And finally, here are a bunch of pictures showing how the build progressed: