16K RAM Pack photos

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Lardo Boffin
Posts: 1191
Joined: Sat Nov 26, 2016 2:42 am

16K RAM Pack photos

Post by Lardo Boffin »

I have just got hold of a dead JRS RAM pack so here are some photos of the internals.
9F6D6744-30DD-4421-B906-038AB6FB78D4.jpeg
C78216CE-410D-42D4-ADEE-78ADDF585847.jpeg
0F1527E9-860F-4F5B-9E26-EE1957CF632B.jpeg
B5F38567-BB03-4B24-B15A-BCAA1B042462.jpeg
3AA3428A-01A5-423F-B184-D4BA3BDC8F1B.jpeg
Last edited by Lardo Boffin on Mon Nov 18, 2019 6:24 pm, edited 2 times in total.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

Lardo Boffin
Posts: 1191
Joined: Sat Nov 26, 2016 2:42 am

Re: JRS 16K RAM Pack

Post by Lardo Boffin »

And the last one.
4ED18546-D6D4-4E4C-B5A7-4518A66AD5E0.jpeg
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

User avatar
1024MAK
Posts: 2897
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: JRS 16K RAM Pack

Post by 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
Posts: 1191
Joined: Sat Nov 26, 2016 2:42 am

Re: JRS 16K RAM Pack

Post by Lardo Boffin »

Hopefully this helps:

0C473DA3-598A-444D-8752-1E3A5683182F.jpeg
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

User avatar
1024MAK
Posts: 2897
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: JRS 16K RAM Pack

Post by 1024MAK »

Yeah, that’s exactly what I wanted. Thank you 8-)

Mark

Moggy
Posts: 1996
Joined: Wed Jun 18, 2008 2:00 pm

Re: JRS 16K RAM Pack

Post by 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! :lol:

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

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mahjongg
Posts: 170
Joined: Tue Nov 24, 2015 10:25 pm

Re: JRS 16K RAM Pack

Post by 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.

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1024MAK
Posts: 2897
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: JRS 16K RAM Pack

Post by 1024MAK »

Moggy wrote:
Wed Oct 23, 2019 9:22 pm
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! :lol:

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
Posts: 1996
Joined: Wed Jun 18, 2008 2:00 pm

Re: JRS 16K RAM Pack

Post by 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
Posts: 1191
Joined: Sat Nov 26, 2016 2:42 am

Re: JRS 16K RAM Pack

Post by 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.

9EBB4D83-971E-4633-9D1D-2985D2B4D1F2.jpeg
CEAC201D-3CEE-4277-B77D-FD4D9283A550.jpeg
3A6A237A-3D3C-42A4-B6E8-047CA564650B.jpeg
With added thumb:
137B46D3-6D62-48F7-B0FF-4E0E18746E58.jpeg
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

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