Whilst installing and carelessly adjusting my CF card in my A1200 with it switched on, I managed to short out some component on the motherboard via the shield which caused the machine to reboot and then come up with the following screen.

You can clearly see that blue is completely missing.

A healthy kickstart screen should look like this

I wasted no time getting to work after cursing myself for 30 minutes or so and feeling bad all night.

1st step was to figure out where the fault actually was, so I consulted the schematics to understand the design and get an idea of where to start.

I have clearly marked the important parts of the drawing by marking each of the 3 RGB colours over the signals. First of all, we have an AGA ( Advanced Graphics Architecture ) custom chip called LISA which generates the colour information internally amongst other things, this must be converted to analog via the video DAC which accepts 24 bits of colour information from LISA and then passes those analog signals directly to the 23pin D-SUB connector on the back of the A1200.

The same 3 RGB signals generated by the video DAC are passed to the Sony Encoder chip for composite signal and also RF out to a TV.

The issue was also reproduced on the composite out, this would confirm the encoder just gets its RGB signals from the DAC and to look elsewhere. Unlike the Amiga 600 design which uses no DAC but uses the same encoder to pass RGB to the 23pin D-SUB, the A600 video output design is similar to the video hybrid method used in the A500 for generating RGB.

Thankfully we have a similar problem here from RetroGameModz that I used as a reference which describes the troubleshooting process. I really liked his use of Protracker to trouble shoot the colour, this proved very useful.

In Protracker, I turned Red and Green signals to the absolute minimum and adjusted the slider for Blue and see no change at all.  So with my scope I verify the signals coming out of pins 25,26 and 26 of the video DAC.  These are labelled IOR,IOG and IOB respectively. I could see valid waveforms on both on IOG and IOR but a flat line ( low ) on IOB no matter where I positioned the blue slider in Protracker via setup from the main menu. This explains the lack of blue.

So either, the 8 bits for blue were stuck on the output of LISA or I had a bad video DAC.

I then attached my logic probe to 5v somewhere inside the machine and adjusted the slider in Protracker whilst probing R,G and B signals. To my relief my logic probe responded to each and every single one of the 24 bits coming out of LISA. So LISA was spitting out colour information but the DAC was obviously not going to deal with blue. I repeated the same measurement on IOB output of the video DAC this time with the logic probe which shows its held low regardless of the position of the slider, further confirming the lack of waveform there with the scope.

I wasted no time ordering some replacements.  3 x VP101-3BA manufactured by GPS/MITEL and some 44pin PLCC sockets.

So currently waiting for these to arrive from China, delivery time is 2 months.

And in preparation I couldn’t wait to remove the faulty component from the A1200.  First step was using a liberal amount of polyimide tape ( this stuff is expensive but will save you from grief ) to protect surrounding components from heat exposure, I applied some flux and then heat the chip to around 200 for 1 or 2 minutes, then raised it temp to 350 to finish off.

I had this cleaned up nicely. Now just waiting for the spare parts to arrive for a part 2.

References: Amiga 1200 Schematics.

https://www.amigawiki.org/dnl/schematics/A1200_R1.pdf

See here for part 2

Amiga A1200 video repair – part 2

I recently received this Apple IIe PCB from the U.S to try my luck on salvaging the 60hz IOU and using it in my International NTSC board in the hopes of getting colour which I did successfully in my previous post. This board was advertised as untested, so that normally means “DEAD” and I assumed so.

Taking one look at the RAM suggests there were issues in the past and most likely still issues. We have some original mT ( Micron ) / Apple branded DRAMs mixed in with the various replacements. Micron don’t exactly have the best reputation in regards to reliability so this is definitely a red flag. These were used by Commodore, Atari and obviously Apple as well. I guess they were cheap!

Based on the excellent condition of the board I was happy and confident in firing it up. It displays some kind of RAM error but it’s not very helpful, definitely not as helpful as the reporting in the Enhanced IIes built in diagnostic. I don’t have a choice but to go through and check each DRAM individually.

My TopMax helps me find 4 bad DRAMs of the 8. Three of which were Micron ( expected! ).

Dead RAM chips on the left clearly marked  to remind me that these are dead and not to be mixed up with working. These will come in useful for refining my Atari 8bit diagnostic tool.

I temporarily replaced the dead RAM chips with known good Micron pulls from my friends Atari 800xl. But I do have some NOS TMS 150ns chips that will be going in shortly.

Replacing the RAM confirms the fix.

It’s nice to have a second board ready to go if I need it, and the fact that it’s a non enhanced board with everything socketed is a plus.  I’ll be looking at converting this over to enhanced at some point via a enhancement kit which involves a 65C02 and newer roms for Apple IIc compatibility.