May 25 2018 0

Refurbishing a ZX Spectrum 16K (Part 1)

I came across a ZX Spectrum on an auction site that was listed as not fully working. Video out was working but the keyboard was not working.

Since I already wanted to try my luck at repairing a ZX Spectrum I decided to buy this unit and see how I would fair in fixing up this old puppy.

Usually when the keyboard on a ZX Spectrum has stopped working it is due to a membrane failure due to old age. No wonder since the connectors are very thin and is about 36 years old (assuming it still has the original membrane when it rolled out of the factory). So this might be a good starter project for me to get my hands dirty.

Inspection on the outside

After the unit had been delivered I did a quick visual inspection of the unit and the unit is in need of a little bit more repair then I anticipated but still no big issues

  1. Faceplate has several noticeable marks
  2. The rubber keys are pretty dirty and on some keys the printing has degraded
  3. Two of the rubber feet are missing
  4. ZX Spectrum logo has no white paint applied (maybe this was by default on the 16K model, will have to do some searching)
  5. The RF output might not have been modified to output a composite signal
  6. The unit has a 16K sticker on the back meaning it is the 16K version and not a 48K version
Missing rubber feet
Two rubber feet are missingTwo rubber feet are missing

Marks on the faceplate and it is getting loose at the corners
Marks on the faceplate and getting loose at the cornersMarks on the faceplate and getting loose at the corners

Issues one, two and three are relative easy to fix, no soldering is required for these. The last two (five and six) will require some soldering and if I want to turn this unit into a 48K model a bunch of ICs.

Based on what I could see through the extension port of the unit I noticed ICs in places where I was not expecting ICs, so maybe I am in luck and it has already been upgraded from 16K into a 48K model. Or I am out of luck and it is a 48K model operating as a 16 K model (see 48K Machines working as 16K on the Sinclair ZX Spectrum 16K & 48K page at DataServe Retro). I will know more after I have a look inside…

Inspection of the inside

Opening the case off a ZX Spectrum is really simple, just remove 5 screws from the bottom of the case and you can carefully lift the top and disconnect the tabs of the keyboard membrane.

ZX Spectrum motherboard issue 2ZX Spectrum motherboard issue 2

As I suspected from peering into the extension port all the ICs related to a memory upgrade from 16K to 48K are indeed present. One of the ICs (IC26) looks a bit weirdly placed (see the right-hand outtake in the image above), but since the machine boots without any issues I will just leave it as it is now.

Also appears I was correct in assuming the RF module had not been modified to output a composite signal (see the left-hand outtake in the above).

The next part to inspect is the keyboard. Since the faceplate was already loose on both corners it was easy to remove it completely and take out the membrane. At first sight it looks fine, but taking a closer look indeed confirms the wiring has snapped.

Broken membraneBroken membrane

Next question to get answered is it actually an 48K model or 48K model with only 16K working. Today I received another broken ZX Spectrum (motherboard issue 3B) having video issues (probably the next project after this one), but has a working keyboard (LO profile). So easy job to hook its keyboard to the unit I am now diagnosing.

Lo Profile Professional KeyboardLo Profile Professional Keyboard

Since the RF module has not yet been modified into an composite out, connected my ZX-HD HDMI interface (available at ByteDelight) to the extension port, so I can connect it to a modern TV or monitor. The ZX-HD HDMI interface also removes the need to modify your ZX Spectrum to be able to connect it to modern technology.

ZX-HD HDMI interface by ByteDelightZX-HD HDMI interface by ByteDelight

Using the command PRINT PEEK 23733 you can quickly perform a test. In the case of this unit I got the number 255 as a result, which means it is indeed operating as a 48K model . This will save me some time and effort refurbishing this ZX Spectrum.

Result Meaning
255 48K machine
128 16K machine
127 48K machine working as a 16K unit

I already ordered a new keyboard membrane and some rubber feet. I have not yet decided if I will replace the keyboard mat and faceplate.

To be continued…


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