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rosco_m68k RAM expansion build guide

Four extra megabytes of static RAM for the rosco_m68k, built by you from a bare PCB. Five soldering stages, lowest parts first, then fitting and a memory count at boot. The same guide is available as a PDF.

U1RAMU2RAMU3RAMU4RAMU5RAMU6RAMU7RAMU8RAMIC1 DECODER
4 MBstatic RAM
8 × 512 KBAS6C4008 chips
1–13 MBbase, by jumper
122 × 104 mm381 solder joints
Download PDF

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Read first

Before you solder

Tick the kit off against the packing list, then build in the order of this guide. Low parts go first, so the board always lies flat while you solder.

Which way round

Every map shows the component side with the long bus connector J1 on the right. Keep your board that way and "left", "right" and "up" in this guide always match what you see.

You need

Temperature-controlled iron with a fine tip, solder 0.6–1 mm, flush cutters, small pliers, multimeter, masking tape. For repairs: flux and solder wick.

Three parts that can go in backwards

NOTCH PIN 1

Chips and sockets

The notch marks the pin 1 end. Match it to the notch printed on the board.

Here: every notch points down.

STRIPE= MINUS LONG LEAD= PLUS

Electrolytic caps

The long lead goes in the hole marked +. The stripe faces away from it.

Here: both stripes face down.

FLAT FACE= FRONT ROUND BACK

Transistor

The flat face of the body sits on the flat of the printed outline.

Here: the flat face points down.

Every joint

Good

A small cone that wets both the pad and the lead.

Bead on the lead

Solder never reached the pad. Add flux, reheat pad and lead together.

Bridge

Two pads joined. Draw the excess off with wick, then check the gap.

The routine

  1. Heat the pad and the lead together.
  2. Feed solder into the joint, not onto the tip.
  3. Take the solder away, then the iron. Hold still until it sets.
  4. Trim the lead just above the cone.

Static

The memory chips and the decoder are static-sensitive. Leave them in their packing until stage 5 and touch something grounded before you handle them.

1
Stage 1 of 5

Resistors

5 parts
10 joints
10K100K4K7 ×3
Fit nowAlready fittedLater
ValueQty4 bands5 bandsPositions
4K73yellow violet redyellow violet black brownR3–R5
10K1brown black orangebrown black black redR1
100K1brown black yellowbrown black black orangeR2

How

  1. Resistors have no polarity. Bend both leads at the body, press the part flat on the board, solder, trim.
  2. Finish one value before you open the next bag.

Read the bands from the end where they are grouped. The last band, set apart, is the tolerance.

Look-alikes
10K and 100K differ only in the third band: orange for 10K, yellow for 100K. Measure both before you fit them.
2
Stage 2 of 5

Capacitors, transistor

10 parts
21 joints
Q1
Fit nowAlready fittedLaterNo label on the map = 100nF
PartQtyMarkedPositions
100nF ceramic9104C1–C9
2N3904 transistor12N3904Q1, flat face down

How

  1. The capacitors have no polarity. Seat each one low on the board, solder, trim.
  2. Q1 last: flat face down, matching the printed outline. Push it down until it stands about 3 mm above the board, no lower.
  3. Solder the three legs of Q1 quickly, one at a time, and check for bridges: the pads are close together.
Which way
The flat face of Q1 points down, towards R1 and R2. Turned round it will not work.
3
Stage 3 of 5

Sockets

9 parts
276 joints
203232323232323232
Fit nowAlready fittedLaterNumber = pin count
PartQtyPositionsWhich way
32-pin socket8U1–U8notch down
20-pin socket1IC1notch down

How

  1. Drop the socket in, notch over the printed notch. Solder two opposite corner pins.
  2. Look from the side. Not flat, or notch wrong? Reheat a corner and fix it now.
  3. Solder the remaining pins. It is easy to skip one in a long row.
No chips yet
The sockets stay empty until stage 5. All 9 notches point down.

276 pins in this stage: take a break halfway.

4
Stage 4 of 5

Headers, electrolytics

4 parts
74 joints
J1SOCKETJP1–JP3
Fit nowAlready fittedLaterNo label on the map = 100µF
PartQtyPositionsWhich way
100µF electrolytic2C10 C11long lead in + hole (up), stripe down
Header 2×31JP1–JP3short pins into the board
Socket 2×32 female1J1component side, square to the edge

How

  1. Electrolytics first: long lead in the + hole (up), stripe down. Push flat, solder, trim.
  2. The 2×3 jumper header: short pins into the board. Tack one pin, check it is upright, then the rest.
J1 carries the card
The 2×32 socket strip goes on the component side like every other part. Tack one pin at each end, check it sits flat and square to the board edge, then solder all 64: the card hangs from them.
5
Stage 5 of 5

Chips and jumpers

9 chips
no soldering
U14008U24008U34008U44008U54008U64008U74008U84008IC1 16V8 JP1JP2JP3
Fit nowAlready fittedLaterPosition and chip

First, with the sockets still empty

  1. Check every joint with a magnifier: beads, bridges, skipped pins.
  2. Multimeter on continuity across the leads of C10. A brief chirp is the capacitors charging. A steady tone is a short: find it first.

Then the chips

  1. Press each row of legs gently against the bench until both rows are parallel.
  2. Notch to notch (down), every leg in its hole, then press down evenly.
All the same
The eight AS6C4008 are identical: any one fits any 32-pin socket. IC1 is the programmed ATF16V8 decoder and goes in the 20-pin socket.

Address jumpers

For one card on a rosco_m68k, leave JP1–JP3 open: the card then starts at 1 MB, right after the main board's memory. Other settings are in the next section.

Last step

Fitting and first boot

  1. Power off. Unplug the main board before the card goes on or comes off.
  2. JP4 on the main board. Fit a cap on it. The card has no DTACK of its own: with JP4 shorted, the main board answers the CPU for it.
  3. Fit. Turn the card component side down and push J1 onto the 2×32 pins of J3 on the main board, or of a bus board slot. Every pin in a hole, none left over at either end.
  4. Beep test. Multimeter on continuity. The + lead of C10 to the +5 V pin of the main board's power connector must beep, and its − lead to GND. No beep: the card is turned the wrong way. Take it off.
  5. Power on with a terminal on UART A, as for the main board, and read the memory count in the banner.
Never skip the beep test
Turned the wrong way, J1 puts 5 V on an address line and can destroy chips on both boards.

Address jumpers

JP1JP2JP3Base
□□□1 MB
□□■3 MB
□■□5 MB
□■■7 MB
■□□9 MB
■□■11 MB
■■□13 MB

■ = cap fitted across that row of the header. A second card goes where the first ends: base 5 MB after a full card at 1 MB. On the r2 main board expansion space ends at $DFFFFF, so at 11 MB only 3 MB of the card are used, and at 13 MB only 1 MB.

What the banner tells you

MC68010 CPU @ 10.0MHz with 5242880 bytes RAM
  • 5242880: 1 MB on the main board plus 4 MB on the card. All good.
  • 1048576: the card is not seen: check that JP4 on the main board is shorted, then IC1 (notch, seating), that JP1–JP3 are open, and the joints of J1. If those are fine, check U1 and U2.
  • 2097152: the count stops at the second megabyte of the card: reseat U3 and U4, look for a folded leg.
  • 3145728: the count stops at the third megabyte of the card: reseat U5 and U6, look for a folded leg.
  • 4194304: the count stops at the fourth megabyte of the card: reseat U7 and U8, look for a folded leg.

The firmware counts memory upwards from address 0 and stops at the first gap, so the number points at the pair of chips that failed. Each megabyte is one odd and one even chip.

Still stuck?

Send clear photos of both sides of the card and the full start-up banner to Telegram or solderdemon.official@gmail.com.