Unknown mods to 505B board

[lowrybt1]

Hello, I'm working with a 505B board which has some mods that don't seem to listed in the SAMS manual (or I'm just misreading the schematics). There are three stand-out differences between the board I have and the manual:

1. There is an 8-pin ICL7660CPA switching voltage regulator installed where the (empty per SAMS) pads for U1H are (adjacent to U1I). There's also a cluster of capacitors and a linear voltage regulator installed immediately adjacent to the ICL7660. Underneath U1H a capacitor has been added along with jumper wires that appear to connect to the added linear voltage regulator and to a resistor installed on the top-side of the board.

2. Over by U5C, a jumper wire has been installed between pads adjacent to the crystal and the SN7400N installed in U5C. (I can't tell if the jumper wire actually connects at all to the crystal. Another jumper wire is installed between pads adjacent to U5F and U6C.

3. Last but not least are jumper wires that run from U4F (SN7404?) to U2C (DM74148N). On the backside of the board, underneath and around U2C and the SYNMON 600 ROM are a few loops of jumper wire.

Pictures are attached. Any help identifying what these are all about will be most appreciated.

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20220101_190458.jpg (3.71 MiB) Viewed 2108 times

[Mark]

#1: It's hard to see where the connections go based on the pictures, but the ICL7660 seems to be configured for -V generation. The 7808 seems to be the V+ input, so it seems it's configured as a -8V voltage generator. The OSI schematics show PS1, a negative DC-DC convertor for RS-232, the ICL7660 could be used to replace that or used on a system that didn't have -9V on the backplane to provide +/- voltage for RS-232.

Not sure what the other mods have done... It's possible to change disk based to BASIC-In-ROM based by changing SYN600 addressing (and adding BASIC ROMs to memory map). Just a thought.