With both Spectrum’s working I decided on a few upgrades before calling it a day. Both of the cover plates are beyond fixing so I decided to buy replacements. The keyboard membrane on the Issue 4 also decided to give-up during the final stages and needed replacing. Both of these were easy to fit and only took a few minutes.
I decided to upgrade both with switched power regulators. These are much more efficient and remove the need for the heat-sink. Spectrum’s are well known for getting warm so anything you can do to reduce this will help future-proofing
With that done I declared them both finished, don’t they look good 🙂
On first inspection the Issue 2 showed a picture with the typical Spectrum coloured border and some flashing bars in the central paper area. The voltages all looked ok, with the exception of a low -5v line. Plugging in the diagnostics cartridge wasn’t much help as it didn’t do anything…
I decided to tackle the low -5v line first by replacing transistors TR4 and TR5. This was a simple job and resulted in a slightly better -5v line, but not by much. With the diagnostics cartridge not being much help I first swapped the ULA, as this was socket-ed, to see if this had any effect. It didn’t. Knowing that the first thing the Spectrum bios does on boot is set the screen border to white I suspected the Z80 CPU or the bios. The clock signal was ok, so the CPU should start, and the diagnostics cartridge would by-pass the bios, and that didn’t work, so I was left with the Z80.
Replacing the Z80 was a bigger job, it wasn’t in a socket and as a 40 pin chip was the biggest de-soldering job I had tackled. Armed with my new de-soldering station and some solder wick I started on the 40 pins.
With a new socket and Z80 CPU I powered it up, and it did … ish
The diagnostics cartridge started but hung after the first few steps without outputting a report. At least now the cpu was executing the code from bios (white border on boot). I tried the ULA from the Issue 4 board and the and the diagnostics cartridge executed correctly…. oh no. The ULA is the hardest component to replace, off to ebay. Meanwhile the diagnostics cartridge indicated that all of the lower DRAM and a couple of upper RAM chips were toast 🙁 What has this board been through?
I decided to replace the lower DRAM with a modern replacement board. It’s known to be problematic and a replacement board wouldn’t need the -5v or 12v supplies. With the number of components that this board was going to need replacing it was never going to be ‘original’. To do this I needed to remove all the lower DRAM chips and replace with sockets.
I decided to practise my de-soldering and remove all the lower and upper DRAM chips, makes fixing future issues easier. With this done I replaced the lower DRAM with a modern board from Retroleum and the upper ram with new 64K chips. The final piece was a Nebula replacement ULA, again from Retroleum.
With everything replaces, almost literally, I had a working Issue Spectrum.
After checking the power rails it was obvious that the 5v regulator wasn’t working, so first action was to replace it.
With the regulator replaced I hopefully tried the power again. This time all the power rails looked good, but still no picture. After checking with my multi-meter and oscilloscope everything looked good. Clock signals fine, memory address and data lines ok , ULA video output ok….. I was at a bit of a loss.
While thinking the problem over I decided to apply the composite video mod in the hope that when it did work at least it would look better.
With that completed, I plugged in once again to started diagnosing the problem, and surprise …. I had a picture. So the problem must be with the analog video out.
I wasn’t out of the woods yet, I had the typical memory fault colour bars on boot. Using my new diagnostics cartridge quickly identified the two upper DRAM chips that were to blame.
The upper DRAM was already socket-ed making replacing very easy.
With the fix complete it was time to play some games 🙂
Still some cosmetic fixes, but that’s a later post.
Over the last couple of months I have been watching YouTube channels of people fixing various retro computers, mainly Commodore 64’s, BBC B’s and ZX Spectrum’s. Like many others, my computing career started with the Spectrum, so armed with nostalgia, some self taught electronics knowledge and a soldering iron I decided to take up the challenge and bring a non-working Spectrum back to its full gaming prowess.
Where to start?
I didn’t want to buy a working Spectrum, where’s the fun in that? So I hit Ebay looking for ‘untested’ or not working listings. There are lots listed, the problem became identifying the best of the bad ones and not paying over the top for what could become a costly project. I set myself a limit of £35 and started bidding. I lost out on quite a few from the usual last minute automated bids, but eventually won two.
In the post
The Spectrum’s arrived quickly with a random selection of interfaces and game tapes. The general condition of the units was ‘dirty’, but that was to be expected, with various scuffs and scratches on the keyboard face-plates. All of which can be sorted with cleaning and/or replacement parts from the various retro/hobbyist websites.
Now to see what state they were actually in. I had watched far too many hours of YouTubers diagnosing faults so I had a good idea where to start, visual inspection first then check the power rails for the correct voltages.
Opening them up
Starting with the visual inspection I opened them up to find an Issue 2 (1982) and Issue 4 (1984) vintage.
Electrically, both looked fairly ‘clean’, with only one noticeable fix; the Issue 4 board has had upper memory chips replaced at some point in the past.
The Spectrum has a couple of peculiarities which it shares with some of the other computers of the same era. The power supply is provided by a centre negative power jack; this was the first thing to check, didn’t want to start by causing any more damage than was there to start. Multiple voltages – internally the Spectrum uses 5v, -5v and 12v, if these are incorrect it can cause damage, especially to the lower DRAM chips.
Following a quick check with my multi-meter, the Issue 4 Spectrum was completely dead – 0 volts everywhere except from the input source. The Issue 2 was a more hopeful with all of the voltages looking OK – although the -5v was a little low, but within specs.
Optimistically plugging the Issue 2 into a TV, with an analog tuner, resulted in a fuzzy, but recognisable Spectrum border, but cyan not white, with black and white bars in the centre. I decided to make the standard composite upgrade before further diagnosing to make the job easier, and less fuzzy (shown below).
Next blog, diagnosing the issues and hopefully fixing.
I started my computing with a ZX Spectrum 48k. I never really noticed the ZX80/81 and I don’t remember any of my friends having one. But they both played a significant part in kick-starting home computing. For this reason they were next on my hit-list for acquiring, off to eBay. As with the ZX Spectrums the prices vary significantly, especially if they are known to be working. What I hadn’t realised was the price ZX80’s are going for. I decided to set my sights on getting a ZX81 first.
It didn’t take long to win two auctions, the first for a ZX81 console, the second for a 16k RAM pack; I remember these being legendary for falling out and resetting the console.
I started with opening it up and doing a visual check, the immediate issue was the keyboard. Something had obviously happened to it with ‘ripples’ all the way around and one corner peeling up. On opening it up I also found that the membrane connection to the motherboard was also snapped. There was no saving this keyboard, luckily new ones are available for £10.
The voltage checks and it all looked good so I decided to try powering it up. I plugged it into my analog tv and turned it on. The boot ‘K‘ was sort-of displayed, although the TV was obviously having issues tuning the channel; but the good news was that it was working.
I decided to start the future-proofing while waiting for the new keyboard to be delivered.
The changes were to replace the 5v regulator with a switched regulator to reduce the output heat, replace all the capacitors and to add a heatsink to the ULA. The keyboard was fairly easy to remove with the help of a heat-gun.
The final mod was to upgrade the video to output a composite signal. This is slightly more involved that the Spectrum requiring a transistor and a few resistors. I created a prototype board to check the circuit, unfortunately I didn’t have any 33 Ohm resistors to used 3*10 Ohm in series for the prototype.
The results were a little bright, this is due to the lower value resistors being used. I decided to order the correct values and do a more professional job when they arrive.
I also replaced the keyboard, another 2 minute job, but it looks great and all the keys passed my first test.