Sinclair QL RAM Replacement

A QL with the top case removed, showing the main board (photo: Ewx, CC BY-SA 3.0)

The Sinclair QL has 128 KB of RAM on sixteen 64K × 1 dynamic RAM chips, IC1 to IC16, arranged as two banks: IC1 to IC8 and IC9 to IC16. Within each bank every chip holds one bit of each byte, so each data line is shared by one chip in each bank. When a chip fails, the QL's start-up memory test shows it as a white or green screen instead of the copyright message, and Sinclair's Service Manual gives a method for finding the failed chip from the pattern on the screen.

Symptoms

What the screen shows at switch-on

Screen	What it means
A fine pattern of green, white, red and black dots ("tweed") runs down the screen, then the copyright message appears	The RAM test has run and passed
Plain white screen	The first part of the RAM test failed: the RAM could not store ones, zeros and a random pattern
Plain green screen	The second part of the test failed: data did not stay in the RAM. Either the RAM is not being refreshed or an address or data line is shorted
White or green with very narrow vertical stripes or a vertical pattern of dots	One bit is faulty: a failed RAM chip, or a short on a data or address line
After RESET, the tweed pattern shows for a while and then the screen turns white	A failure in the second bank of RAM
A geometric pattern such as red and green vertical bars	The ZX8301 is working but the processor is not running the ROM code. The CPU clock may be missing, the CPU may be absent, or the RAM may be faulty
The display becomes erratic after the QL has warmed up	Sinclair's fault table says to check the RAMs

A black screen with the power light on has several causes; Sinclair's list starts with the supply voltages and the ZX8301 (IC22) before the RAMs. Tony Firshman of TF Services adds that the QL's own memory test does not find every RAM fault, and recommends the Minerva replacement ROM, whose memory test reports the first faulty address.

Safety

There is no mains voltage inside the QL; the highest voltage on the board is the 44 V peak-to-peak AC feed from the power pack. Use an earthed, low-power soldering iron and do not overheat the pads when desoldering. Take static precautions when handling the chips.

Tools and parts

• A multimeter.
• A probe lead that can be connected to ground.
• A 16-pin IC test clip with a known-good 4164 attached, if you want to try the piggyback test.
• Replacement 64K × 1 dynamic RAMs. Sinclair's parts list gives the Hitachi HM4864P-2, with the TMS4164-15NL, MCM6665AP15, MK4564N15, UPD4164C-3, MSM3764-15RS and HYB4164P2BD as alternatives.
• A soldering iron, desoldering braid or pump, and a cross-head screwdriver.
• Optionally, a Minerva ROM for its memory test.

Procedure

Checking the supplies

1. Open the case: remove the four 5/16-inch screws along the front edge and the four 1¼-inch screws along the rear edge, but not the two screws below the Microdrives, and lift the top half. It stays connected by the keyboard ribbons and LED leads.
2. Power the QL and measure the +5 V output at the + side of C42 (5.0 V ± 0.15 V, no discernible ripple) and the +12 V output at the + side of C38 (12.0 V ± 0.25 V). Sinclair's fault finding starts with these voltages when the copyright screen is missing; if a rail is out of limits, fix the supply first (see Sinclair QL Voltage Regulator Replacement).

Reading the screen

1. Switch on and watch the screen from the start. If the copyright message does not appear, switch off and try two or three times: Sinclair notes that a QL can lock up at start-up and look dead.
2. Match what you see against the table above. A plain white or green screen means the RAMs are working in general and the fault is a short on an address or data line or a single faulty chip.
3. On a green screen, look over the RAM area for solder bridges between pins or across the address and data tracks, then further afield, for example around the processor. If you check with an ohmmeter, remember that readings can be misleading where the meter feeds back through the CPU.

Finding the faulty chip

Part of Sinclair's Issue 6 circuit diagram: on each 4164, pins 2 and 14 are joined to one data line (DB0, DB1, DB2 for IC1, IC2, IC3)

Sinclair's Issue 6 layout, showing where the sixteen RAM chips IC1 to IC16 sit on the board

This works when the screen shows narrow stripes or dots.

1. Connect one end of a probe lead to ground.
2. With the QL showing the faulty pattern, touch the probe on the data pin of each RAM in turn and watch the screen. On the QL's 4164s the data input (pin 2) and output (pin 14) are joined to the same data line.
3. Each touch puts a vertical black line or lines on the screen. When the black line falls on the faulty stripes or dots, you have found the data line.
4. The data line is shared by one chip in each bank, so the fault may be in either: for example IC1 or IC9. Sinclair's example is to change IC1, because it is in the memory-mapped area of the store.
5. As a rule of thumb, a long vertical stripe points to a short on a data line, and dots point to a RAM fault.

Which chips share each data line

Data line	First bank	Second bank
DB0	IC1	IC9
DB1	IC2	IC10
DB2	IC3	IC11
DB3	IC4	IC12
DB4	IC5	IC13
DB5	IC6	IC14
DB6	IC7	IC15
DB7	IC8	IC16

Confirming without desoldering

1. Sinclair suggests a "test IC": a known-good 4164 on an IC test clip, clipped over each suspect chip in turn. Sinclair says this method is not guaranteed to work but can save time spent changing chips unnecessarily.
2. If you have a Minerva ROM fitted, its memory test reports the first faulty address, from which the chip can be identified.

Replacing the chip

1. To work on the underside of the board, take it out. Release the screw that holds the regulator IC35 to the heatsink and lift the heatsink away, keeping the screw and its washers. Remove both Microdrives (three pan-head screws each; do not confuse them with the countersunk motor screws), then the reset button, the loudspeaker leads, the two pan-head screws near the expansion connector and the loudspeaker, and the push-fit Microdrive extension bung. Ease the board off its locating dowels, lifting it at the expansion connector end.
2. Desolder the faulty RAM without overheating the pads.
3. Fit the new chip with its notch the same way round as the others in its row.
4. Inspect the board for solder splashes; Sinclair warns that hairline splatter can measure several ohms and give misleading symptoms.

Reassembly

1. Refit the board reset-button end first, then the screws, the Microdrive extension bung, the loudspeaker leads and the reset button.
2. Refit the heatsink and the IC35 screw with its washers.
3. Refit the Microdrives. The drive in the left-hand (MDV1) position has an RF shield over its ULA; check that it is in place. Straighten the bare ends of each drive's ribbon cables and push them fully home.
4. Reconnect the keyboard ribbons and LED leads and close the case.

Testing

1. Switch on. The tweed pattern should run down the screen and disappear, followed by the copyright message with the F1 (monitor) and F2 (television) options.
2. Press RESET and check that the screen does not turn white after the tweed pattern, which would point to the second bank.
3. If a Minerva ROM is available, let its memory test run; Firshman notes that its test data varies, so it finds faults the standard test misses.

Revision notes

Both the pre-Issue 6 and Issue 6 boards have 128 KB of RAM on IC1 to IC16. The data-line table above is taken from the Issue 6 circuit diagram.

Related pages

• Sinclair QL
• Sinclair QL Maintenance Guide
• Sinclair QL Troubleshooting Guide
• Sinclair QL Service Manual
• Sinclair QL Voltage Regulator Replacement
• Sinclair QL Serial Port Repair
• Sinclair QL Keyboard Membrane Replacement
• Sinclair QL RGB Port Repair