16K RAM Pack photos - please add yours

[Lardo Boffin]

I have just got hold of a dead JRS RAM pack so here are some photos of the internals.

[Lardo Boffin]

And the last one.

[1024MAK]

Lardo, could you please take another photo showing the tracks on the top side of the board that has the edge-connector, angle the camera so that the tracks in the area of the edge-connector are as visible as practical.

It looks like a conventional DRAM design. But uses an audio amplifier to generate the -5V rail. Not sure about the +12V rail, but I suspect they have done the same as Memotech and just added extra smoothing to the +9V rail (big electrolytic capacitors) to get around +11V DC for the 4116 DRAM chips.

Mark

[Lardo Boffin]

Hopefully this helps:

[1024MAK]

Yeah, that’s exactly what I wanted. Thank you

Mark

[Moggy]

That audio chip is one I've used in many a headphone guitar practice amp, I wondered what the hell it was doing there, thought it was some crazy onboard audio lash-up!

Any idea Mark as to why something like that would be used rather than something more conventional?

[mahjongg]

I wondered that too, for a second, when I saw it, then I realized Its probably used to generate the -5Volt from the Raw +9V supply by running it as an oscillator, then driving a diode doubler circuit to invert it to -9V and using a 5V1 Zener to smooth and regulate it. You can see the components to do that next to it. The LM386 is cheap, and can drive low impedance loads.

[1024MAK]

That audio chip is one I've used in many a headphone guitar practice amp, I wondered what the hell it was doing there, thought it was some crazy onboard audio lash-up!

Any idea Mark as to why something like that would be used rather than something more conventional?

This is about the sixth design of ZX81 RAM pack that I have seen. Each one uses a different method or different variation of design for obtaining a -5V supply. Some use a transistor oscillator driving an inductor like the Sinclair 16k RAM pack. Some use a CMOS logic chip and capacitors to do it. Some use an audio amplifier chip. IIRC, I think I have seen one using a NE555.

Basically the audio amplifier chip is configured to either operate as an oscillator, or is fed with a suitable AC signal. The power output stage then feeds a coupling capacitor. Which then feeds a charge pump (diodes and capacitors). Then a 5.1V Zener diode. You now have a low current -5V DC rail for your 4116 DRAM chips. Keep in mind that each (of the eight) DRAM chips only needs about 50uA (typical) each at -5V.

As to why, I can only speculate that either it was cheaper at the time, or maybe the designers did not know enough about designing reliable low cost inductive oscillators.

Mark

[Moggy]

Many thanks to you both for the explanation.

I would concur re the cheapness of the audio chip as I seem to recall they were a couple of bob each at the time so yes cheap and cheerful as they say.

[Lardo Boffin]

Slightly off topic but I recently got a 16K Jigsaw RAM pack with a lose cover so here are some photos in case they had not been posted before.

With added thumb: