Memotech 16K Ram pack failure

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gseb
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Memotech 16K Ram pack failure

Post by gseb »

Hello,

I have a blank screen with my Memotech 16K Ram pack. It's the only things connected to the ZX81, jumper are set for master.

I have only 9V on pin 8 of the Ram ICs. As I understood, it should be around 12V.....

Can someone tell me which part is producing the 12V ? This is probably the problem from that ram pack...

Thanks a lot.

gseb
Posts: 15
Joined: Mon Jun 08, 2020 1:22 am

Re: Memotech 16K Ram pack failure

Post by gseb »

I checked and the 12V is coming directly to the ram from an MC14011B pin 14....

Do you think I should replace it ? I think so as it is generating the 12V.

Thanks

zx-heinz
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Joined: Thu Apr 27, 2017 8:56 pm

Re: Memotech 16K Ram pack failure

Post by zx-heinz »

Hi,
it is more reliably to check the -5V at pin1 of the RAMs. Some Memotech RAM don't generate the 12V but use instead of the 9V from the power supply. If the -5V is missing, you have to repair something. First check.

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1024MAK
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Re: Memotech 16K Ram pack failure

Post by 1024MAK »

What type and rating of PSU are you using to power your ZX81 and RAM pack?

Memotech 16k RAM packs need a Sinclair 9V PSU (either the UK700 700mA or UK1200 1200mA type), because they process the unregulated +9V (nominal) voltage (which is normally around 10.8V) to produce the “+12V” supply for the DRAM chips.

Replacement modern lightweight 9V PSUs are not suitable. You can however use modern lightweight 12V PSU. But do always buy from a reputable shop or supplier and always check the polarity is correct, preferably by testing it yourself with a multimeter.

The DRAM chips also need a -5V supply which is generated in the RAM pack.

There is a pin-out of the 4116 (and equivalent) DRAM chips in the chip-pin outs link in my signature below.

Mark

gseb
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Re: Memotech 16K Ram pack failure

Post by gseb »

I have on the ram ICs :

+4.95 V
-4.65 V

and about 9V (instead 12 v ?!?)

My PSU is a modern 9V one, it output just 9V. This is working fine with a sinclair 16ko Pack though the sinclair one may generate the 12V differently....

MC14011B pin 14 (where should be 12V) is also connected to the ZX81 edge port connector, on the ZX81 edge connector there is the 9V from the PSU.....

The VDD Memory pin (suposely 12V) is ditrectly connected to the ZX81 edge port connector (9V) and to the MC14011 pin 14.

Because of this, it seems not to me that the 12V is generated by the Memotech but rather that it used the 9V from my PSU through the ZX81 edge connector. Maybe it's not working because of my 9V PSU. The ZX81 deliver only 9V from my PSU on the edge connector. Probably it may work with an original ZX81 PSU because it output more than 10V and it's OK for the ram but 9V is not enough ???

I still have an original +2 grey PSU... I have to build an adapter to be use it with the ZX81 and see if the 16Ko will work with it... Or I have to replace all the rams with modified 4164 one, no need anymore for the 12V and -5V...

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1024MAK
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Re: Memotech 16K Ram pack failure

Post by 1024MAK »

Yes, in the Memotech 16k RAM pack, the +9V (nominal) unregulated supply goes to the “+12V” rail on the Memotech board. By processed, I mean that you will (should) find a large electrolytic capacitor connected to this supply rail to provide extra smoothing.

The tolerance of the +12V supply to 4116 (or equivalent) DRAM chips is 12+/-10%, so 10.8V minimum.

Because the original Sinclair ZX UK700 (and UK1200) PSU are unregulated, unless used at their full rated current, the output voltage will be higher than the number on the label.

Sinclair 16k RAM packs use a different method to supply the +12V feed to the DRAM chips. These have a transistor based DC-DC converter (similar, but not the same, to that used in the ZX Spectrum).

Mark

zx-heinz
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Re: Memotech 16K Ram pack failure

Post by zx-heinz »

gseb wrote:
Thu Jul 02, 2020 12:15 am
My PSU is a modern 9V one, it output just 9V. This is working fine with a sinclair 16ko Pack though the sinclair one may generate the 12V differently....
Could you try with an original PSU? Cause of 9V<10.8V, the instructed minimum for 4116.

gseb
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Re: Memotech 16K Ram pack failure

Post by gseb »

I do not have an original PSU, just one from +2 but not the plug to do an adapter.

I took the way to build the Memotech PSU universel.... I removed all 4116 ram. I tested them all and they are all OK.
I will keep them for something else.

I will install 4164 (with the small hack to correct the pinout) on the Memotech and it will be OK for all PSU. I think it's the better way to corect Memotech mistake....

It means, all parts needed for the 12V and -5V can be removed ?
Can someone tell me all the not needed parts I can remove ?
Or is there a complete schematic of the 16ko somewhere ? I could not find one....

Thank you

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1024MAK
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Re: Memotech 16K Ram pack failure

Post by 1024MAK »

I’ve drawn most of the schematic.

See this post ;)

About using 4164 DRAM. I think with only a small modification, you may be able to fit the 4164 DRAM chips without having to do a modification for each chip.

Mark

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1024MAK
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Re: Memotech 16K Ram pack failure

Post by 1024MAK »

Image

Remove the following components:

D2, D3, ZD1, DZ2, R4, C3, C4, C5, C7, the wire link in D4 position (or other component if no wire link is fitted).

Pin 1 is now disconnected on all DRAM chips from the former -5V supply.

Connect what was C7 positive connection to the +5V supply at a convenient point, e.g. at the electrolytic capacitor that is on the +5V rail.

Before powering up, it is very important to use a multimeter on the resistance range to confirm that there is no connection between the +9V pin on the edge connector and the +5V supply.

Pin 8 is now disconnected from what was the”+12V” rail and is now connected to the +5V rail.

Pin 9 being connected to +5V should not matter.

Please check that the correct voltage is present at pin 8 before putting your 4164 DRAM chips in the sockets (I assume you will be using sockets) and that there is no voltage on pin 1.

Mark
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