Recapping your Spectrum

If you don’t know what recapping your Spectrum means or why it’s important, I’ll first explain a few details.

Capacitors typically found inside a ZX Spectrum. These are C44 and C45, and are rated at 100uf at up to 16v.

The term ‘recapping’ means the process of replacing the electrolytic capacitors in your Spectrum. If you don’t know what capacitors are, they’re the (typically) blue or yellow cylindrical parts with a diameter of 8mm or so, with a lead coming out of each end (so, ‘axial’ capacitors, unlike ‘radial’ capacitors which have both leads coming out of the bottom of the can). Those in your machine might have already been replaced so the colour in your machine may differ.

Why is it important to replace them? Well, electrolytic capacitors get their name from their construction, which consists of wound layers of aluminium contained in an electrolyte solution. As these parts age, the electrolyte tends to dry out, which means that the capacitor is no longer effective and starts negatively influencing the circuit in which they operate.

Axial and radial capacitor types (axial on top). The negative lead of the radial capacitor is usually marked by an arrow, but if not the longer of the two legs is the positive.

In a computer like the Spectrum, where the operation of the DC-DC converter circuit is critical in supplying correct voltages to the lower RAM chips, aging capacitors can lead to the circuit failing, which in turn can potentially damage the lower RAM chips due to the loss of the +12v and -5v power rails.

One more point to note – unlike ceramic capacitors, electrolytic capacitors are polarised parts, so it’s important to make sure that they are installed the right way around, otherwise (1) the circuit most likely won’t work, (2) the component itself will be damaged, and (3) the knock on effects to other parts of the circuit could be disastrous.

Electrolytic caps have their polarity marked in two ways – the negative lead typically is indicated with arrows pointing in the direction of the lead, and also axial capacitors have an indentation near the top of the can indicating the positive end.


Selecting Capacitors

If you’re not confident about sourcing the correct capacitors for your computer, there are services like Retroleum who will sell you capacitor kits that contain all the right types for your computer.

If you want to choose your own, then there are a few points to note.

  1. Capacitors are primarily rated according to two measures – capacitance and voltage. When replacing capacitors, it’s important to choose ones with a capacitance that matches the one being replaced (1uf, 22uf, 100uf etc.). The voltage rating of a replacement must be at least the same as the original, but a 22uf part rated for 50v can happily replace a 22uf 16v capacitor. These do tend to be of a larger diameter though, so be careful to stay within 6-8mm diameter replacement parts if you want the job to look neat.
  2. If replacing parts on an Issue 1 or 2, the capacitors in the vicinity of the heatsink are subject to greater than normal thermal stress. Otaining capacitors with a temperature rating of 105 degrees celsius (instead of the standard 85) for these positions is recommended.
  3. It is better to choose good quality capacitors rather than cheaper equivalents, they will have a longer life, and be more reliable –  some extra money spent now can prevent problems down the line. Vishay BC capacitors are preferred by a lot of repairers as they are good quality and are also blue in colour, so are a good match functionally and aesthetically for the original Gelec caps. Other good capacitor manufacturers include Nichicon, Panasonic, Hitachi and Rubycon but there are doubtless others, so don’t consider the above a definitive list. If in doubt, perform some research on a particular capacitor manufacturer before buying.

The capacitors used in a 48K Spectrum vary by issue, but a kit containing the following values will suit all 48K Spectrum types:

  • 1uf 63v axial (x3)
  • 4.7uf 63v axial (x1)
  • 22uf 25v axial (x7)
  • 100uf 25v axial (x2)

Some machines will have a radial capacitor installed near C34 (north east end of the PCB) as part of a service modification, a 4.7uf 63v radial capacitor will replace this.

Replacing the capacitors

This subject has been covered many times by many other sites. Rather than try and replicate the same old process here, I’ll simply offer some tips that I find useful.

Snip out old capacitors: Instead of desoldering the capacitor entirely, I find it useful to simply snip the old capacitor from the PCB using a small pair of side cutters. It’s then very easy to remove the remains of the lead from the through hole on the PCB. Use a desoldering pump for this job as solder wick will just drive you mad!

Add more solder: This might sound counterintuitive, but adding some fresh solder to a joint before desoldering can help in removing the existing solder, by making the combined mass flow more easily, and also by adding some heat into the joint. This can be particularly useful for parts attached to ground planes, like the negative end of C65.

One by one approach: If you’re new to this job, I recommend replacing one capacitor at a time, i.e. remove one capacitor, keep it on your workspace while you’re replacing with new, then testing your work by powering on the machine. This approach will make it easy to narrow down where you might have made a mistake during replacement. Don’t forget to check the area for shorts caused by old bits of lead and solder splashes before testing, and of course remember to snip the excess length from the leads of the new capacitors – this can easily short out if forgotten!

Don’t believe the silkscreen: On Issue 2 PCB’s, capacitor C46 is installed back to front with regard to the silkscreen on the PCB (as the polarity markings here are actually wrong). It’s good practice to note which polarity the old capacitor was installed in, and ensure you refit the new one in the same orientation.

Easy on the temperature: Sinclair PCB’s are delicate things, so don’t be tempted to turn the temperature up on your iron to do the job quicker. Use no more than 300 degrees celsius, preferrably 250 degrees.

Help, I’ve got an Issue 1 Spectrum

Issue 1 PCB Capacitors
Issue 1 PCB with capacitor values annotated. Click for larger version.

If you’re lucky enough to have an Issue 1 Spectrum, you may be torn between keeping it in orignal, ‘as-new’ condition, or replacing the capacitors to ensure that when it is powered on, the risk of a failure causing damage to other components is minimised.

Personally I would advise replacing the capacitors if there is any possibility of the machine being powered up, as in a worst case scenario, faulty capacitors causing a DC-DC converter failure can potentially then induce internal failures in the 4116 RAM.

Since these IC’s run internally at 12v, internal shorting of the voltage rails to data/address lines or even the 5v rail could potentially destroy the increasingly rare 5C102E ULA. (I have seen this happen, thankfully to a more common PCB issue.)

Unlike other 48K issues, the Issue 1 has a good smattering of radial caps on the board as well as axials. Worse still, some of these do not have silk screen markings.

In this case, the ‘replace one at a time’ advice given earlier is doubly important to ensure that correct polarity is maintained by the replacements.

Cleaning Up

Once you’ve replaced all the capacitors, give the PCB a good final clean with some isopropanol alcohol (IPA, available from Maplin) and an old toothbrush. Once you’re happy that the board is clean, reassemble your machine and bask in the knowledge that your machine will happily run for another 20 years or so!

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