7386 Power Booster

Content by William R. Walsh, original here.

Planar
VRAM Installation
OS Thoughts
Make A Power Connector


What Is The 7386?

The 7386 is a planar upgrade for Model 25 PS/2 computers, marketed by IBM to the educational market.

I know of one other upgrade for the Model 25. Made by Reply Technologies, the Model 25 Reply Board offers true 486 performance for the IBM PS/2 Model 25 computers. Whenever you hear about upgrading the Model 25, the Reply board will undoubtedly come up at some point.

To say that the 7386 is not common is an understatement. I have the only ones I have ever seen or heard of. As rare as it is, the 7386 is a pretty decent upgrade for the Model 25, which came in three types: an 8086, a 286, and a 386sx. The 7386 adds to that a 386slc, and possibly also a 486 processor equipped system.

The 7386 is not a standalone Model 25--it is a unit that was upgraded from any of the other two models, 25 or 25-286, but not the 25SX, which has a totally different system base layout.

I have four of these...got 'em from my local high school.

The 7386 upgrade does not go into the 25SX! The 25SX's layout is very different from a 7386 planar's layout!


Planar

7386 Planar

J7 IDE Hard Disk
J8 PS/2 Floppy
J15 Hard Disk Power Connector
J17 Monitor Connector
JP? External speaker?
JP3 ?
JP4 ?
JP6 ?
JP7 ?
JP8 Memory bank selection?
JP9 ?
JP10 ?
U4 Intel i8042
U6 24.0000 MHz OSC (for an FDC)
U9 Acer M5105 A3E
U24 IBM Video BIOS
U25 BIOS (Phoenix A386S)
U26 IBM 386SLC-20 CPU
U30 Math coprocessor socket
U31 14.3181 MHz OSC (RTC and ISA bus)
U32 40.0000 MHz OSC (CPU)
U33 OPTI 82C283
U43 OPTI P82C206
U44 Tseng ET4000AX
U45 Music TR9C1710 RAMDAC
U46, 47, 50, 51 VRAM upgrade sockets
U48, 49, 52, 53 512K VRAM (4x256K 70ns)
X1 32MHz OSC for CMOS Clock
Y17A 14.31818 MHz OSC

Notes:

Keyboard and mouse connectors are reversed from the way they are normally installed on a PS/2.

VRAM: 256K 70ns DIP - 4x soldered, 4x socket for upgrade

This board's got more jumpers than the earth has people--almost. Apart from JP3, which removes the power on password (in case you forget it) none of the others are documented, and I have no idea what they are for.


Installing VRAM

Easy as drop it in and go. Chips must be 70ns or faster--slower ones do not work and cause lock-ups. Make sure the chip orientation is correct, or you could damage the VRAM chips, planar, or both.

Results: You won't see a speed improvement. You will, however, get the ability to run higher resolutions.

WARNING: Though the 7386's SVGA chipset can do 800x600 (and beyond, up to 1280x1024) you will do serious damage to the monitor in your Model 25 if you try to run any mode outside of 640x480 at 60Hz (V) and 31.5kHz (H)!!! The only reason I know what the 7386 board is capable of is because I put one in a Model 30-286 and I used an external, multisync monitor to find out!

Color depth is another story: Upgrading the RAM theoretically gives you the ability to run a higher color depth at 640x480 and beyond (see note above). I tried running 640x480 at 64K color depth, and it wouldn't work under Windows 3.11. All I got was an error, stating that "The video driver could not provide the mode requested." and then I was dumped back out to a DOS prompt.

I do not know at this time what is placing the limitation on the video--the RAMDAC runs up to 1280x1024 (with 16 colors), and the video BIOS has support for that mode, so why wouldn't it have support for high color modes, just as it (seemingly) does for high resolution? The Windows 95 drivers don't help solve this problem either. From the looks of things, the ET4000AX chipset does not support high color modes. Only Tseng ETxxxxW32 chipsets support those modes. A transplant of a BIOS from a W32i based card got my 7386 doing high color, but it was not stable.


Thoughts on operating systems

Windows 3.1x and DOS seem to be the best bets. The system performs very well with either, as would be expected.

OS/2 would probably work acceptably well, but I've haven't tried it and don't know.

Linux--No. I tried to install Linux and it just kept locking up during the install process.

Win95--As a thought, I took out the last remaining 7386 that I hadn't done anything to yet. Wired up an IDE hard disk (650MB), installed a "Diskmangler", and ran Win95 setup. Works just fine, but a little slowly. A CPU upgrade would make a big difference, I am sure.

WinNT--Workstation 4.0 is the only version I've tried. It ran, but did it ever beat the devil out of the hard disk with swapping. Plus, it was S-L-O-W!

Win98--Yes, but it is even slower than WinNT on this thing.


Make A Power Connector

The 7386 planar upgrade has support for both the hard disk that came installed in the Model 25 and 25-286, and a standard IDE hard disk.

However, the 7386 has an odd miniature power connector. An adapter can be made, as shown here.

What you need:

  • (1) 3.5" Device Power Connector (the small ones)
  • (1) 5.25" 4 Pin Molex Device Power Connector

Both should have wires attached already. I would recommend getting these from a dead PSU.

  • Some Electrical Tape
  • Wire Stripping Tool

A soldering iron is not really needed, but if you want to solder the wires together...

DISCLAIMER: I take no responsibility for any disasters that may occur if you actually do this! If you don't feel comfortable doing this, then don't! One “BZZT!" and you could burn something important!

What to do:

  1. Strip the wires...about 1/2 inch from each wire should do it.
  2. Join the wires. This is the tricky part, as the 7386's on-board power connector is wired with 5 and 12 V “swapped” from the way it is normally wired. To elaborate. If you go by the wire color, and wire what would normally be 5V to the 5V at the point of joining the two cables together, it will really be 12V and plugging that into something would let the magic smoke escape very quickly! See the picture.

Making a power connector

Of course, this assumes that you put the 3.5" drive connector over the 7386 planar pins so that the small piece of plastic that comes out of the 7386 planar pins goes into the "bevel" that is cut into the 3.5" floppy connector. If you do it the other way around, then you will not need to swap the wires.

Making a power connector


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