Model II Type 1 HD Interface Cards

[Gazza]

Hi all
My Model II has never been able to boot TRSDOS II 4.x with it's original HD card but boots everything else OK.
26-4150 runs OK on the original card but because of a dud HD INIT aborts.
Got my paws on a couple of other cards that have factory mods to the boards and they boot TRSDOS II 4.x with zero problems.

Note the extra wiring on U39 a 74LS08 quad 2-input and gate. The inputs on this normally unused gate go to pin 2 U43 and pin 11 of the CTC.
The track that runs from pin 12 of U43 to pin 14 of the edge connector is cut and the output of the and gate is connected to pin 14 of the edge connector.

I've looked through a couple of technical bulletins and service manuals but can't find any reference to these mods. All I know is that it fixes the problem but I'd like to know the origin of the mods.
More research (beer) required.
cheers
Gazza

 

[pski]

Gazza, I took a look at some of my Type 1 HD Interface cards and I have one with the same U39 modification as yours. What is interesting is that on 2 other cards I have the same U39 mod but on the back of the card (see image below). I also cannot find any reference to this mod in known documentation. I suspect there are a set of HD tech bulletins somewhere out there that have yet to be archived that contain the details of these mods.

https://imgur.com/a/cvV6LN6

 

[Gazza]

Pete, same here. 1 with no mods, 1 on the component side and the 3rd has the mods on the solder side.
Tis a lot more going on inside these 8 inchers than a 1,3 or 4 and a lot more to learn.
No smoke - yet
Gazza

 

[pski]

Oh, ok, ignore my previous statement. We do have the HD Tech Bulletins already. I should have remembered this as I performed some of these a few years ago. I'm getting old, man.

Unfortunately, it does not contain this U39 modification which is unusual. The only mods I've seen that are not in the tech bulletins were factory only, hence no need to issue a bulletin. But, Gazza has a board without the U39 mod. Maybe that was a super early version of the board.

Gazza, there is a 4 digit code on the solder side of the board and I wonder what yours reads? It appears to be a date code. See the highlighted location in the attached image.

 

[Gazza]

Pete
A bit hard to read with my old peepers.
The unmodified board looks to be - 4581 with a sticker which I'm guessing may be the serial number - 001188
1582 - 008812
4782 - 015136
on the two modified boards.
The plot thickens
Gazza