Sinclair QL

Good looking Sinclair QL.

For a while, the Sinclair QL was a mythical machine, the next big thing bringing 16-bits to the masses! In the early to mid 1980's, young readers like me, were salivating at its specs in computer magazines such as Your Sinclair or Crash. I played with Mega Basic in my ZX Spectrum because it was supposed to be similar to the QL. Anyway, while the QL launched years before the Atari-ST and the Amiga, and 2-weeks before the Apple Macintosh, it ended up being mostly a flop. Still, it gathered a community of users that maintained and expanded it throughout the following decades.

For a while I had been trying to get one. They are rare in the US, where I am, and they are usually quite expensive in online auction sites. I was always outbid. Until one day I saw a listing from Canada. Shipping a QL from Canada to Europe, where most of its fans are located. is probably expensive, so I had reduced competition for the auction and I won it.

Powering Up

My QL did not include a power adapter. I had to find a way to power it. The QL requires ground, 9V DC, and 16V AC. The 9V DC is used as input to various 7805 5V voltage regulators that power most of the chips and the Microdrive (each Microdrive has its own 7805). The 16V AC is used by a 7812 and a 7912 linear voltage regulators to generate +12V and -12V respectively. AC voltage adapters are heavy, and hard to get these days. I always try to avoid using them.

In a first attempt to check for signs of life in the computer, I plugged only 9V DC and ground, ignoring the AC input. I saw some signal in the Monitor but is was quite fuzzy (sorry, I forgot to take pictures). Then I realized that the Motorola MC1377P, that converts the RGB signals from the ULA into composite PAL video, requires +12V to operate, so I really need the other voltages.

I then decided to bypass the AC part of the power circuit, and feed DC voltages directly to the input pin of the 7812 and 7912 regulators (after the diodes that bridge them from the AC signal).

To get all the required DC voltages (9V for the 7805s, +14V for the 7812, and -14V for the 7912), I used a LM2596S 3A DC-DC converter to get 9V from a laptop power supply (16-20v) ~$1 dollar, and  a separate positive/negative  DC-DC converter that I had lying around to generate 14 and -14V. If I did not have this converter, I would have used another LM2596S  for the +14V, and yet another, wired in "reverse" and connected with diodes, to generate -14V. (Or I would have finally bought a pico-psu). 

DC-DC voltage converters that I used to convert a laptop 19V DC power input into the 9V and +/-14V for the QL linear voltage regulators. (The background schematic is unrelated).

View of the wires connecting to the voltage regulators.

Video

To get a video signal I could either use the RF output, or the monitor port. I am nervous about using the monitor port, because the signals connect directly to ULA 8301 without any kind of buffering. Thus it is very easy to accidentally fry the ULA 8301 (this is a common failure). I also don't like to fiddle with the TV trying to sync on an RF channel, so I decided to tap the composite signal directly from the input to the RF modulator using alligator clips. This is often a signal good enough to debug the machine. Later I'll worry about getting a better signal.

First Test

After powering the QL I got the following image:

This is good. The fact that I am getting a video signal means that the ULA 8301 is probably fine.

The mostly white screen means that the CPU is good enough to run the memory test routine (and that the boot ROM is fine too). The bad news is that the memory test does not finish. The vertical stripe suggests a RAM problem. The QL contains 128k of RAM implemented as 2 banks, each containing 8 4164 (64k x 1 bit) DRAMs. The vertical line is consistent with either a broken RAM chip, or, much less likely, a problem with the bus transceiver (74LS245). I checked the voltage of the output pin of each RAM chip, and I got 5V on all chips excepts the chips corresponding to bit-2 in each bank (IC3 and IC11), those measured around 2V. I desoldered the 3 chips and installed sockets. I could have tested one chip at a time, but since I had to warm up the desoldering station anyway, I decided to remove all three in one go.

Board after installing sockets for the bus transceiver (IC21, 74LS245, on top near a ROM), and the 2 RAM chips, IC3 and IC11, (under the bus transceiver).

As expected, the problem was one of the RAM chips. After replacing it, I got to the boot screen.

Keyboard

I tried the keyboard but many keys were not responding, this means that the keyboard membrane was damaged. I inspected it, and I looked for failure patterns that might hint at a broken trace in the pigtails that connect to the mainboard, but I did not see any. Ordinarily I would spend some more time trying to repair the membrane, but since I was about to order a ZX Spectrum plus membrane, I also ordered a new QL membrane to save on shipping costs. I got both membranes from RWAP software.

Next Steps

My QL included a Microdrive cartridge with the Quill Wordprocessor, and it runs. But, I need to find a way to load other software.  I do have some cartridges, a Microdrive, and an Interface one for the ZX Spectrum. But my Interface one seems to have a bad ULA. I'll need a different solution to load software.