Sinclair 16k RAM pack - how was it even working?

[Lardo Boffin]

I recently got a working RAM pack with a zeddy and while recapping it spotted the following:-

How was it even working? So many dry joints.

I resoldered the joints and replaced the caps. All good.

Ran it for about 30 mins and now it is dead. I have very little luck with these. Maybe I should leave the original caps in? But I assume they are part of the power circuit and will kill it if they fail? Maybe these things are just a little fragile?

[1024MAK]

Is the -5V present on the 4116 DRAM chips? That voltage should always be tested as soon as possible after powering up. A missing -5V supply is known to kill 4116 DRAM chips, as it is the bias supply for the substrate.

Mark

[Lardo Boffin]

I will check thanks Mark.

If the caps are ok what would cause the voltage not to be there?

[1024MAK]

The 16K RAM packs use an earlier version of the DC/DC transistor converter/inverter as used in the ZX Spectrum.

They draw power from the nominal +9V power pin on the edge connector and convert this to a semi-regulated +12V and a stablised -5V supply. 4116 (and equivalent) DRAM chips need a +12V supply for their internal circuitry, a +5V supply for the internal interface circuitry (for communicating with the CPU) and as I said, a -5V bias supply.

Causes of failure may be faulty capacitor(s), faulty transistor(s) or the coil having been damaged. If a DRAM chip failed and then went short circuit, this may then in turn kill the transistor(s), and then the loss of the -5V may cause more DRAM chips to fail...

I’ve not investigated or repaired the DC/DC transistor converter/inverter on the faulty Sinclair 16K RAM pack that I have, because the DC/DC transistor converter/inverter works. I think on mine, it is a failed ULA chip (it’s the later issue board).

Most people quickly give up trying to fix these and use a modern SRAM based solution instead. So if you do decide to continue, I wish you the best of luck. I will help where I can,

Mark

[dessony]

The 16K RAM packs use an earlier version of the DC/DC transistor converter/inverter as used in the ZX Spectrum.

They draw power from the nominal +9V power pin on the edge connector and convert this to a semi-regulated +12V and a stablised -5V supply. 4116 (and equivalent) DRAM chips need a +12V supply for their internal circuitry, a +5V supply for the internal interface circuitry (for communicating with the CPU) and as I said, a -5V bias supply.

Causes of failure may be faulty capacitor(s), faulty transistor(s) or the coil having been damaged. If a DRAM chip failed and then went short circuit, this may then in turn kill the transistor(s), and then the loss of the -5V may cause more DRAM chips to fail...

So the TS1500 has its own DC/DC transistor converter/inverter section, too! The TS1500 has own 8 DRAMs "4116" chips.

DesSony

[Lardo Boffin]

Most people quickly give up trying to fix these and use a modern SRAM based solution instead. So if you do decide to continue, I wish you the best of luck. I will help where I can,

Mark

Thanks Mark. I have a ZXpand so am ok for extra RAM it just would have been nice to have a working Sinclair RAM pack. Maybe next time...