Re: +2B with a non responsive keyboard
Posted: Wed Dec 30, 2020 11:40 pm
Yeah, take with a pinch of salt all the ‘recap’ everything that you find on the internet and YouTube. It’s not always necessary.
Yes, electrolytic capacitors do have a limited lifespan. So they may well need replacing eventually. Generally they only need renewing when symptoms of failure or degradation are noticed.
However, in critical applications, or where they are subjected to heat (either internal heating or external heating), then it’s wise to replace before failure, or as part of servicing.
However, in the 1990s, a new water-based electrolyte was being developed. An incomplete formula made it into the wilds. Some capacitor manufacturers produced capacitors based on this (incomplete) information. This resulted in the so called ‘capacitor plague". These ‘bad caps’ fail far sooner than they should. And they often leak electrolyte over the board which corrodes the PCB tracks. They often deform or burst. Unfortunately a wide range of equipment was affected between about 1999 to 2010.
The good news is that unless a Sinclair computer was recapped using these ‘bad’ capacitors, then they don’t normally suffer from leaking electrolytic capacitors. As they were made before the ‘bad’ caps were available.
However, the 16k/48k/+ machines do run rather warm. And they use a discrete DC-DC converter circuit based on transistors TR4, TR5, the ‘coil’ and various electrolytic capacitors. This DC-DC circuitry generates the +12V and -5V supplies that the ‘lower’ (4116 or equivalent) DRAM chips need. Loss of the +12V stops this memory from working. But worst, the loss of the -5V supply (while the positive supply voltages are still present) can result in one or more of these memory chips self destructing.
And as all these Sinclair computers are now over 35 years old, it’s wise to replace at least the critical electrolytic capacitors and the ones next to or under the heatsink for the 7805. Especially on issue one and issue two boards. But you may as well replace all of them while the board is out for servicing.
It’s less of a problem on the 128K machines (including the +2 and later models), because they use a different type of DRAM chip, that only need a single +5V supply. And these machines don’t get so hot inside and have better ventilation.
Later computers (some Amigas for example, and various PCs and laptops) and various other devices that may have been manufactured using ‘bad’ capacitors should have any affected or suspected capacitors replaced if you want them to continue to work and before the damage to the PCB becomes too bad.
Since then, modern good quality products should be okay, as no manufacturer should be using electrolytic capacitors made with the bad electrolyte.
However, the quality of the electrolytic capacitors available from some sellers on trading sites like eBay, is a bit of a gamble.
Mark
Yes, electrolytic capacitors do have a limited lifespan. So they may well need replacing eventually. Generally they only need renewing when symptoms of failure or degradation are noticed.
However, in critical applications, or where they are subjected to heat (either internal heating or external heating), then it’s wise to replace before failure, or as part of servicing.
However, in the 1990s, a new water-based electrolyte was being developed. An incomplete formula made it into the wilds. Some capacitor manufacturers produced capacitors based on this (incomplete) information. This resulted in the so called ‘capacitor plague". These ‘bad caps’ fail far sooner than they should. And they often leak electrolyte over the board which corrodes the PCB tracks. They often deform or burst. Unfortunately a wide range of equipment was affected between about 1999 to 2010.
The good news is that unless a Sinclair computer was recapped using these ‘bad’ capacitors, then they don’t normally suffer from leaking electrolytic capacitors. As they were made before the ‘bad’ caps were available.
However, the 16k/48k/+ machines do run rather warm. And they use a discrete DC-DC converter circuit based on transistors TR4, TR5, the ‘coil’ and various electrolytic capacitors. This DC-DC circuitry generates the +12V and -5V supplies that the ‘lower’ (4116 or equivalent) DRAM chips need. Loss of the +12V stops this memory from working. But worst, the loss of the -5V supply (while the positive supply voltages are still present) can result in one or more of these memory chips self destructing.
And as all these Sinclair computers are now over 35 years old, it’s wise to replace at least the critical electrolytic capacitors and the ones next to or under the heatsink for the 7805. Especially on issue one and issue two boards. But you may as well replace all of them while the board is out for servicing.
It’s less of a problem on the 128K machines (including the +2 and later models), because they use a different type of DRAM chip, that only need a single +5V supply. And these machines don’t get so hot inside and have better ventilation.
Later computers (some Amigas for example, and various PCs and laptops) and various other devices that may have been manufactured using ‘bad’ capacitors should have any affected or suspected capacitors replaced if you want them to continue to work and before the damage to the PCB becomes too bad.
Since then, modern good quality products should be okay, as no manufacturer should be using electrolytic capacitors made with the bad electrolyte.
However, the quality of the electrolytic capacitors available from some sellers on trading sites like eBay, is a bit of a gamble.
Mark