When SALT goes bad

While trying out my new CoCo SDC with my Color Computer 3, I found some strange issues.  Symptoms included garbage on the screen while turning on, garbage after hitting reset, computer not initializing with any text.  Any cartridge inserted would cause the system to not display any video. I also noticed something odd, the usual double click of the cassette relay when power was applied was missing.  All I heard was a single click, and pressing the reset button did not generate any clicking of the relay.
I tried reseating the GIME, swapping out the memory card for a working 512k board, replacing the electrolytic capacitors over the entire board, and finally looking around for any anomalies that might explain why my Color Computer 3 was not working correctly.
The basics troubleshooting I had done to this point had gotten me nowhere, I needed to get out the service manual and a digital meter to see what was actually going on.  My first stop was to make sure that there was a good 5V power source for the board. As I turned on the machine and connected the meter, I was shocked to see a reading of almost 6.1v on the 5v line.  No matter how many times I checked, it was still 6.0V - 6.1V across different locations on the board. I also noticed that the SALT chip, the Motorola SC77527P, was heating up very quickly after the power was turned on.
Thankfully I had other Color Computer 2 and 3 machines to open and inspect.  I quickly found that other Color Computers had regular readings for voltage, and the SALT chip was not getting hot to the touch.  I knew that the SALT chip was challenging to l, and I started to get very concerned.

Following the service manual

I already knew that the 5v line was 6V, this quickly moved me to ‘D’ in the following flowchart.

Next was to check the Emitter and Collector of Q1, there was a difference of 10v to 6v.  Unfortunately, my computer was suffering from a defective SALT chip.

After opening 3 other Color Computer 2 models I had laying around I found the one I really needed, this unit had the SALT chip socketed, my doner was found!

How to remove the defective SALT chip

Removing the chip without damaging the Color Computer 3 board was going to be a challenge.  Here are some of the things that made it a success.
  1. Weller soldering iron with a fine tip and temperature control.  I preferred to use 800 degrees for this job.
  2. Lots of solder flux.
  3. Solder wick.
  4. Solder sucker.
To get started, you should clip out the old chip with a pair of dikes, this should reduce the chances of damaging the circuit board due to too much heat.  Notice pins 9-16 are clipped below int he photo.
Once all 16 pins are cut, you should be able to easily remove the bad chip.
Carefully remove the pins 1 by one and then clean up holes so the new part can be installed appropriately.

I was sure to install a good quality chip socket before using the new replacement chip, this was a lot of work, and I don’t want to be soldering this section of the board again.  Once I assembled the board and did a quick test, I could tell that things were back to normal. Hopefully, the 6v across the 5v line of the board did not cause any long term damage, I have not had the time to go through and test everything to be sure.  I can say that the CoCo SDC was working great, and I was quickly able to boot to Nitros-9 and try out my system again.

Bad SALT chip


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