Sinclair Spectrum 48k Plus

The Sinclair ZX Spectrum was, by far, the most popular micro in the 80's in Portugal. I got mine early 1984, is was a rubber keys 48k Spectrum, with an issue-2 motherboard. This was the first mass produced board (issue-1 models are very rare), and it has many reliability problems. My Spectrum was repaired many times throughout the 80's, and some circuits no longer follow the issue-2 schematics.   

My 80's issue-2 ZX Spectrum. Some of the repairs were recorded on the heatsink. The power and video circuits might have been rewired to be closer to later board revisions. 

Because of the unknown nature of its repairs, I am very nervous every time I use it. It it fails, it will be very difficult to recreate those circuits.

Given this, I recently acquired a ZX Spectrum Plus with a motherboard issue-3. This is a more reliable release, and I plan it to be the day to day usage machine.

ZX Spectrum plus in good cosmetic condition.

Motherboard issue-3. Relative to issue-2 boards, the voltage regulator and heatsink were moved to the back of the board, the power circuit is different, as is the video circuit that no longer has trimmers.
 

Inspecting the motherboard, I see that most ICs have date codes from 1984, yet the ULA is more recent, from mid 1986. I suspect that the board is from late 1984 and that the ULA was replaced at some point. This revision of the ULA usually shipped with issue 4 or later motherboards. The bottom case still contains an old sticker with the date 10/86. I think that was left by the technician that repaired it and replaced the ULA back in 1986. 

After verifying that there are no short circuits with the multimeter in continuity mode. I turned it on. And this is what I see (watch the video too...).

This does not look like a healthy ZX Spectrum!

It is obviously broken. The seller mentioned that the screen was a "little fuzzy"...What an understatement!

I touched all the ICs looking for overheating, a sign of an internal short circuit, and found none. Then I measured voltages at various points in the board. The 5v rail measured at the output pin of the 7805 regulator and at several ICs (ROM, 74 logic, etc) seems fine. However, there are no -5v or +12v voltages at the lower 16k RAM.

The symptoms visible in the video are consistent with lack of -5v and +12v. +12v is used by the composite video encoder (LM1889), leading to bad video signal. Both -5v and 12v are used by the lower 16k 4116 ram that contains the video buffer. They should not run for long like this: lack of these voltages is likely to damage the 4116 RAM chips.

Schematic of the circuit that generates +-12V and -5V from the regulated 5V rail.

I suspect transistors TR5 (ZTX213) or TR4 (ZTX650). They are used in the circuit that generates +- 12v, and -5v from the regulated 5v, and they are known to fail. So I ordered modern equivalent replacements, ZTX651 for TR4, and ZTX751 for TR5. Check this list for equivalents. 

View of original TR4 and TR5 near the keyboard connector.

After removing TR4 and TR5.

View of replaced TR4 and TR5.

After replacing both transistors the computer boots. Yet, as you can see in the video below, the boot sequence seems too quick. This suggests upper ram issues, it is probably working as a 16k Spectrum.

For more complete diagnostics, I plugged in my home assembled ZX Dandanator with the Alford's diagnostics rom. (Press SW1 on the Dandanator while booting to run the diagnostics rom). 

ZX Spectrum ready to power up, with a ZX Dandanator in the expansion connector. You can see that, for now, I am getting composite video from the modulator input using alligator clips.

The diagnostics confirm ram errors in the upper 32k, identifying IC21 as possibly faulty. This is a 32k x 1bit TMS4532 ram chip and it is responsible for bit 6.

Diagnostics screen showing failed upper ram tests.

So, I warmed up the desoldering station and removed IC21, then I replaced it with a socketed 64k x 1bit  4164 ram chip. This is a suitable replacement since the original TMS4532 chips are actually 4164  that failed validation in only one of the upper or lower 32k halves. Reusing slightly damaged components was one of the cost saving techniques used by Sinclair.

Nice, clean desoldering job, no traces or pads damaged. If you don't have a desoldering gun, the safest approach is to cut all legs of the IC and remove them one by one.

View of board with newly installed socket, populated with a replacement 4164 chip.

After replacing the bad ram, diagnostic run fine. 

Diagnostic tests pass now.

Now that the Spectrum is running, it is time to install a permanent composite video mod. I always start by trying the capacitor mod. It is very simple and usually works quite well with most modern TVs and old computers (not with the Timex Sinclair 1500, that is a story for another day).

 

View of the modulator before the composite mod. The two connections to the modulator, video and +5v, must be desoldered (or cut). 

Composite mod.  The resistor that connects to the video out connector is disconnected. A 100uF capacitor is added, and its negative lead connects to the center of the video out connector in place of the resistor. The positive lead of the capacitor connects to the composite video signal pad on the board.

And voila. The jailbars interference pattern is much reduced compared to the alligator clip hack.

Once the computer was running, I reassembled and tested the keyboard. As described by the seller, the keyboard membrane was damaged. In the past, I had some success fixing ZX-81 and rubber keys ZX Spectrums membranes using copper tape with conducting adhesive. Not this time, I was able to fix some keys, but not all, the membrane is too complex, with at least 5 plastic layers strongly glued together. New membranes for the ZX Spectrum+ and ZX Spectrum 128 are hard to make because of the complex many-layer construction that can cause capacitive noise. I ordered mine from RWAP software that makes excellent membranes and sells old stock retro parts at sellmyretro.com.

Old membrane after some failed fixing attempts.

While waiting for the new membrane to arrive from Europe, I decided to add reverse polarity protection to the Spectrum power input. Sinclair used inconsistent polarity in their computers, it is center positive for the ZX-81, but negative for the ZX Spectrum, Timex was just as inconsistent in the TS-1000 and TS-2068. Because of this, I had accidents that one time resulted in the destruction of the ULA in my old TS-1000 (Timex rebranded ZX-81) and another time completely fried a TS-2068.

Cut power track from the power plug. Remember that socket polarity is center negative. I double checked it was the correct track by measuring continuity to the 7805 voltage regulator input pin. After cutting I verified lack of continuity between the regulator and power plug. 

Diode protecting against reverse polarity.

My modification is very simple: I just cut the positive trace to the power plug, and bridge it with a power diode. This simple modification has saved my old precious rubber keys ZX Spectrum at least once.