Amstrad ALT-386SX

Resources

Amstrad User Group
User Manual (archived from the page above)

There is no Service Manual available on the Internet. If you have one and want to share, please let me know.

Internals

Motherboard

Somehow I forgot to make pictures of the motherboard. So this is only front of a dirty board after dismantling the machine. Now the computer is all assembled.

BIOS entry key is F2 during boot or CTRL-ALT-S from DOS prompt.

Power Supply

Rubycon RPS-07

ROM/RAM Module

Graphics Card

ISA Interposer Board

Hard Disk Drive

80 MB Sony SRD3080C-50. HDD Type in BIOS is 24.

Floppy Disk Drive

TEAC FD-235HF

Display/Keyboard Adapter Board

LED Panels

Display & Backlight Inverter Board

Display

Epson EG9005-F-LS-2

Keyboard

Battery

Reassembly

Fixing ALT-386SX

Phoenix BIOS Beep Codes

When BIOS boots, it reports errors of the power-on self tests in a form of audio beeps on the built-in speaker. The meaning of the error code can be found on the BIOS Central web page.

Checking the Configuration Switches

There are two sets of switches at the back of the computer. For first boot it is probably best to set the smallest memory size and internal display. When there is less memory available than configured, BIOS will report a beep error code. OFF = switch is UP, ON = switch is DOWN.

SW1SW2SW3Memory Size
OFFONON512KB
ONOFFON1MB
OFFONOFF2MB
ONOFFOFF4MB
SwitchMeaningONOFF
SW4Page modeAlways ON
SW5Wait statesAlways OFF
SW6Clock8 Mhz16 Mhz
SW7DisplayColorB&W
SW8SerialEnabledDisabled
SW9PipelineAlways ON
SW10Drive B: TypeAlways OFF

Recapping Power Supply

All electrolytic capacitors used in the Rubycon power supply are of poor quality and they all leaked. The effect was that the output voltages were much lower than required. Replacing the capacitors restored the correct voltage levels.

Checking SIMM Connectivity

When there are memory problem reported by the BIOS, it makes sense to check the SIMM connectivity to the chipset, especially when there was a battery leak that could have damaged the PCB traces.

SIMM modules are organised in Columns (0, 1 – also called banks) and Rows (1, 2).

Stored data bits are divided between rows: bits 0-7 are stored in row 1 and bits 8-15 in row 2. Therefore two SIMMs must always be inserted into row 1 and 2. Column 0 is used as a lower memory location and must be always populated with SIMMs, as it is used to boot the machine. Column 1 is optional and may be used for a memory extension.

SIMMs are wired to four chips on the motherboard, VL82C202 (Memory Controller), VL82C203 (Address Buffer), VL82C204 (Data Buffer), VL82C205 (Page Mode / Interleave Controller). Some of the signals are connected through a resistor soldered to the bottom side of the ROM/RAM board. If a resistor is present, when checking the connectivity of the traces, please locate and use the connection after the resistor, not at the SIMM.

SignalFrom
SIMM Pins
From
SIMM Column
From
SIMM Row
Through
Resistor
To
Chip
To
Chip Pins
Data Bits 8-15D0-D7*1VL82C204D8-D15
Data Bits 0-7D0-D7*2VL82C204D0-D7
Column Address Strobe/CAS01R337VL82C205CAS0L
Column Address Strobe/CAS02R338VL82C205CAS0H
Column Address Strobe/CAS11R339VL82C205CAS1L
Column Address Strobe/CAS12R340VL82C205CAS1H
Address Bits 0-7A0-A7**R321-R328VL82C203MA0-MA7
Address Bits 8-9A8-A9**R329-R330VL82C202MA8-MA9
Address Bits 10-11A10-A11**n/c
Write Enable/WE0*R331VL82C205RAMWA
Write Enable/WE1*R332VL82C205RAMWB
Data Parity OutQP*1VL82C204MDPOUT0
Data Parity OutQP*2VL82C204MDPOUT1

Pinouts of the chips used in the table above:

Fixing a Battery Leak Damage

Leaking CMOS battery can cause severe damage to the board. It can oxidate and dissolve traces and vias.

The board can be cleaned with vinegar. Vinegar should not be left on the board for too long and can be removed with clean water (possibly with some baking soda added).

In order to fix the traces, the most damaged chips should be removed from the board to reveal more damage to the traces beneath the chips.

Now it is best to use some super fine grade sanding paper to remove the varnish and reveal all the traces.

Making photos of the board and using a paint application to draw multiple colors on top of the traces will tremendously help in checking the connectivity and fixing broken traces.

Traces can be fixed with a special ultra-thin wire. It is possible to solder the wire directly into the vias on the board. The mesh looks messy, but does the job. Before soldering the SMD chips back to the board, use PCB coating varnish to protect the uncovered traces. When all soldering is done, use more coating on all the uncovered traces and soldering points.

14 thoughts on “Amstrad ALT-386SX

  1. WOW! What a great amount of effort and kindness to share this. It’s helped me alot find many faults
    on the 386 board that I have been working on. Much credit you deserve for this. THANK YOU!

    Like

  2. I re-capped the PSU and got my ALT386 powered up but then the display went blank the following day.
    Still boots as I can hear the boot process and the floppy booting. Checked the orange power wires on the
    VGA board for CN502,CN503,CN504,CN506 and none of them have any voltage, most of the blue wires have no voltage either. No display from the VGA monitor port either. Thinking the VGA board may have a fault but can’t do much checking without the schematics.

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      1. Good afternoon Pawel, good to hear from you and thanks for the reply. The machine was working great after the caps were replaced along with the cmos battery and good clean of the board. No dip switches were changed, I have the manual BTW. Screen just started to flicker then went blank. Powering off I see the screen light up for a brief second. I think the display draws all it’s voltages through the ISA riser and into the VGA card. The main power board has all the correct voltages (5V 5V 12V -12V -19V GND GND). I may have to strip her down again (excuse the pun) 🙂 and have a poke around with my multimeter. I did notice that the large D1833 NPN power transistor on the VGA card was cold, but did have 12V on the collector was but not drawing any power. Your very clear photos have helped very much, thanks for those again.

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    1. Yes, she was tight inside (no pun intended) with all those cables. The manual is too many pages to scan and would take me forever and a day with the scanner that I have. If I can find it online, then I will post a link.

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  3. Very helpful, I believe I have now diagnosed mine with trace damage from a battery that didn’t seem to be leaking, and possibly a cap too. I very much appreciated your comments on FB and the link to here.

    Thank god for microscopes and people who have gone before!!

    Hopefully I’ll break out the bodge wires and get it running soon 🙂

    Like

  4. Finally have a map of my bad connections, seems that the 33uF SMT cap (C121) near the memory connector had also gone, distributing corrosive goo under U144, so not a dissimilar state to yours. I have a large scale image printed, and hope to get on with the repairs from tomorrow. Bodge wires are definitely going to be required!!

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    1. I finished the rats nest of bodge wires, luckily my corrosion wasn’t as bad as yours, and it now runs again!!

      Thanks so much for all the time and effort working through this yourself, it’s certainly made things much easier for me and without it I don’t think I’d have mine running again. It has tested my patience and soldering skills though!!

      Like

  5. Well, I had a brief diversion with my Alt-386, in that I lost 12v from the PSU. After a lot of troubleshooting,it turned out to be the short 1 inch cable from the PSU to the mainboard!! Some of the connectors in the 7 wire plug had failed, giving an intermittent 12v connection. It’s all repaired now, reassembled and fully running again 🙂 Once again, I appreciate your excellent site, without which I’d have probably given up! I has tested my patience and I really do know what you mean about it being a strange and awkward way to relax! I also relocated the battery, and the capacitor that killed it has been replaced with a tantalum, so no more leaking!! One thing of note is that the capacitor is part of the speaker circuit, so if the speaker volume is low, you know that the capacitor should be removed immediately 🙂

    Like

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