Hi,
Going way back there were some details given regarding repair of bricked PCMs. I have had reason with the re-programming project to make use of this. I wanted to document some bits here as this efforts may make it necessary for others to have this ability...

First up, here is the schematic of the hardware:
Schematic.jpg

I want to focus on the USB <--> Serial cable for a moment. Cables using the Prolific chip and it's clones are a problem. Microsoft goes out of it's way to mark your driver obsolete every time you change anything or they do an update. You can continue with it if you don't mind fighting with Microsoft all the time. Even then, depending on if your cable is a clone of the prolific part (most are) the baud rate is WAY off in most cases. Remember that a 5% error in baud rate builds over the 10 bit word and by the last bit is 50%. Probably doesn't work or at best is unreliable.
I have found that cables based around the CMD340 chip set work a treat. No driver issues. Baud rate with less than 1% error. I did a hack test on the ones I got for some rates of interest:
1,890, 8,192 and 12,288. These are the rates useful for work on the LT1 PCMs. Using a frequency counter I sent 'U's to the port and found better than 1%. I believe most of the error is from my terminal software. These cables cost $5 and I am going to purge all the half working stuff I own.

Ok, so that is the cable side of things. The rest of the schematic is just pulling pins low to change the processor startup mode to bootstrap, pull up the data lines because wire or is default and so on. The connector to the board I use can still fit in the case. They are just wire wrap header pins (.025 square, 2wide by four long). Here is a picture:
IMG_1985.jpg
The connectors are not numbered on the PCB, nor is there a square pad to indicate pin 1. I selected the following for pin out.
IMG_1984.jpg
Last up is a picture of my setup for bootstrap
IMG_1981.jpg

With this setup and some software you can overwrite both flash and EEPROM. It's just opinion but I don't think FLASH parts should ever be socketed or replaced. Once the conformal coating of the board is broken or a socket added I think the reliability will be poor.

When writing code for the loader I needed this setup to recover the PCM... twice I am sad to say.

-Tom