Amiga A1200 Real-time Clock Module (RTC)

The Amiga 1200 doesn't have a battery backed real-time clock fitted by default.  It's shown in the schematics as an optional item but was never installed in any of the board revisions to my knowledge.

Some trapdoor expansions have a battery backed RTC fitted but I don't have one as yet.  I opted for a small battery backed RTC that's installed on the clock header next to the Kickstart ROMs.  I went for the Individual Computers RTC as I particularly like the products Jens produces.

Here is the board...

A1200 battery backed real-time clock module

And here it is installed in an Amiga 1200 with a Rev 1A board.  This 1A board has the full width header pins installed so the board is installed using the right most pins.

A1200 with battery backed real-time clock fitted

Firing up the Amiga I was able to set the time & date and save it to the RTC with "setclock save".  AmigaOS was able to see the fitted RTC and read back the correct time/date with "setclock load".

However, the following day when I started up the Amiga, the clock had jumped back to an earlier date as shown below.  The 3.1 ROM behaviour appears to be loading the clock from the battery backed RTC module, but if it returns a default value because the hardware has reset, the OS instead uses the DATESTAMP of the boot disk instead, which in my case was the installation time of the hard disk drive device DH0.

Amiga setclock with failed real-time clock battery

As the 3V battery was brand new, I initially suspected the RTC had a failed oscillator and wasn't incrementing when the unit was powered off.  However, this is easy to test by writing the correct date/time to the RTC (setclock save) and then periodically reading it back (setclock load) and checking the OS clock with "date".  In this case the RTC was working fine but was losing it's settings when the Amiga was powered down which indicates a problem with the battery backup.

Removing the RTC board I measured good battery voltage (3V) but wasn't reading any backup voltage on the clock IC itself.  On closer inspection it appears the battery holder possibly wasn't soldered to the board properly as shown...

Re-solder the tabs on the battery holder

After the repair I was able to measure 2.4V on the clock IC (there's 0.6V drop across D1).  The Amiga now seems to remember its clock properly on a cold boot as shown.

Amiga setclock with correctly working RTC

Amiga 1200 Rev 1A Distorted Audio Output - Bonus Material

Here is some additional information related to the Amiga 1200 audio fix described in my previous posts

This is the PCB audio section with the completed modifications.  Note that R325 and R335 are on the bottom side of the board, the other modified components are on the top.

A1200 audio opamp and filter section (top)

A1200 audio opamp and filter section (bottom)

The following oscilloscope images show the audio output at the phono jacks on the rear of the unit after the final Rev 2 board modifications.  The Amiga is playing a 1 kHz test tone at maximum volume.

The first image is with the low pass filter disabled (power LED dim).  The second image has the low pass filter enabled (power LED bright).

Low pass filter disabled

Low pass filter enabled

The Amiga schematics for each board revision are available online.  For reference here are the Rev 2 schematics for the audio section (Paula) with the changes from the Rev 1A board highlighted.

A1200 Rev 2 Schematics for audio section