If you have or can get new chips, remove the existing ones, fit sockets (if there is room under that heatsink/cooler) and fit the new chips.
Okay, a few, okay, a lot of words on capacitors and ZX Spectrum computers and the recapping nonsense that has spread like a plague across the echo sounding interweb.Pygmalion wrote: ↑Thu Sep 21, 2023 1:46 amSince I am already ordering from AliExpress, should I also order some solid electrolytic capacitors like these
https://vi.aliexpress.com/item/1005002973596027.html
I assume that solid means polymer, therefore solving aging problem. They are suspiciously cheap, but with excellent reviews.
Do you have any indication that caps could be be culprit for at least some of my problems?
First off, all the ZX Spectrum (and for that matter, ZX80, ZX81, TS1000, QL etc.) boards were built long before the so called capacitor plague (the capacitor plague was due to capacitors being manufactured incorrectly). And despite the bull and propaganda from some people and organisations, Sinclair did not use substandard components. The original electrolytic capacitors were normal quality parts used throughout the electronics industry at the time. These capacitors do not normally leak. The ZX Spectrum issue one, two, 3 and 3B (plus some later) boards were built in the U.K. using parts available in the U.K.
Within the ZX Spectrum 16K, 48K and plus (+) machines, I class some of the electrolytic capacitors as critical components. This is because either they are subjected to heat (those under or near components that get hot, such as under the heatsink fitted to the 7805) or because some of them (those in the DC/DC converter part of the power supply circuitry), can cause other parts to fail, if one of these capacitors degrades or fails.
Replacement of all the other electrolytic capacitors is optional, they can be left until there are symptoms that indicate that they are life expired. By life expired, I mean that the capacitance has fallen or the ESR value has increased. Typically this is because the electrolyte has ‘dried up’.
For ZX Spectrum 16K/48K/+ models, capacitors that I consider critical are:
For Issue 2, 3 and 3B boards:
Capacitors C34, C44, C45, C46, C47 (where fitted), C50, C74 and C79 (where fitted)
It’s also worthwhile thinking about carrying out the power circuitry (DC/DC converter) modification (see here). Although this is more involved on issue two boards.
It is up to you what type of capacitor you fit, but normal quality electrolytic capacitors are fine. They should last typically twenty years or more (assuming you buy normal or good quality parts from a reputable supplier). Especially if the 7805 is replaced with a DC/DC converter / switching regulator to reduce the heat in the case.
With the fault that you have, assuming the +5V, +12V and -5V supply rails are reasonably free of ripple and noise (best tested using an oscilloscope or an expensive multimeter that has a wide bandwidth on the AC mV ranges, but if your voltages are within specification, this is a good indicator that the power supply circuitry is okay) it’s unlikely to be an electrolytic capacitor problem.
It’s fairly common for people on finding that their ZX Spectrum does not work, to replace all the electrolytic capacitors, only to find it did not make any difference.
Having said all that (sorry if it’s seen as a rant), as long as good capacitors are used, replacing all the electrolytic capacitors, if done correctly, doesn’t do any harm. Some say that it also improves the quality of the video output. But back when the Speccy was designed, people used 12” and 14” CRT TVs, so the quality of the video was not so critical compared to today when viewing on large high definition telly… And there are other things that can be done to significantly improve the video quality.
Mark
Standby alert 
