Memopak 16k - in need of new caps etc. before being used?

[Lardo Boffin]

I have just bought a ZX81 which came with a Memopak 16k RAM pack. To be fair I'm not going to use the RAM pack as RAM I'm going to use it as a case for my ZXpand but I was wondering if should I wish to use it should it be recapped etc. first? I don't really know much about these!

I have to say the case is exceptionally well made on these. Very solid and professional! I can see why the ZXPand was designed to fit them.

Looking at the bare board one of the components looks a bit dodgy even with my limited knowledge. It has been slightly flattened!

IMG_5261.JPG

(1.81 MiB) Downloaded 341 times

IMG_5262.JPG

(1.66 MiB) Downloaded 341 times

[1024MAK]

If you are going to use it, yes, renew the two large electrolytic capacitors with the same capacity rating (in uF) and a voltage rating of 25V. Note that modern ones are physically smaller (assuming the originals are rated at 25V, but they may be 16V).

Test the -5V on one of the DRAM chips (type 4116) to confirm that the inverter / DC-DC converter is working. If the -5V is missing, is positive, or is between 0V and -3.9V, switch off immediately. Otherwise you risk damaging and killing the 4116 DRAM chips.

Mark

[Lardo Boffin]

If you are going to use it, yes, renew the two large electrolytic capacitors with the same capacity rating (in uF) and a voltage rating of 25V. Note that modern ones are physically smaller (assuming the originals are rated at 25V, but they may be 16V).

Test the -5V on one of the DRAM chips (type 4116) to confirm that the inverter / DC-DC converter is working. If the -5V is missing, is positive, or is between 0V and -3.9V, switch off immediately. Otherwise you risk damaging and killing the 4116 DRAM chips.

Mark

Thanks Mark. Is -5V on pin 1?

Lardo

[1024MAK]

Thanks Mark. Is -5V on pin 1?

Yes. I've just added details of the 4116 to this thread

Mark

[Lardo Boffin]

Thanks Mark. Is -5V on pin 1?

Yes. I've just added details of the 4116 to this thread

Mark

Thanks Mark. I had seen a schematic elsewhere but wanted to be sure I was looking at the right thing.

[Lardo Boffin]

I lifted one of the capacitors up and it is 470 u-M, 16V.

IMG_5271.JPG

(1.57 MiB) Downloaded 286 times

Is that something like:-

http://cpc.farnell.com/panasonic-electr ... dp/CA07282

One more ultra noob question (until the bits arrive and I have time to look at then anyway) - the red probe on the multi-meter will go on pin 1 - where will the black probe go on this board?

Thanks and Merry Christmas all!

Lardo

[gozzo]

i would say its ok to leave the big ones alone if they are physically ok, more critical would be the 3 small ones in the -5v generator circuit!

[1024MAK]

I lifted one of the capacitors up and it is 470 u-M, 16V.

Is that something like:- http://cpc.farnell.com/panasonic-electr ... dp/CA07282

Yes, however, compare the physical size of the existing (diameter and height [length]). If it will fit, a modern replacement rated at 25V or a 35V capacitor will last longer. So in the same range, this is a 16V compared to the next highest voltage available from CPC, here a 35V rated capacitor.

The reason that a higher rated voltage part is preferred is two fold:

1. The ZX81 PSU when off load (plugged into the mains, but with nothing connected to it's output) rises to 14V to 16V, although when connected to a load (the ZX81 [and Memopak]) this will quickly fall to between 11V to 13V,
2. A designer should always allow a good margin between the actual circuit voltage and the rating of a electrolytic capacitor, but back in the day, this was not done for cost reasons, as it was never expected that the equipment would be used for more than two or three years...

One more ultra noob question (until the bits arrive and I have time to look at then anyway) - the red probe on the multi-meter will go on pin 1 - where will the black probe go on this board?

On any 0V connection, or pin 16 (which is it's 0V supply) of the same chip. As pin 1 should be negative relative to pin 16, if using a digital meter, it should display a minus sign. If using a moving coil meter with a needle, you need to swap the leads around, as the needle will go off scale.

i would say its ok to leave the big ones alone if they are physically ok, more critical would be the 3 small ones in the -5v generator circuit!

My reasons for not automatically recommending these be changed is two fold, they are only being used to generate the very low current -5V line (so low power levels, and low heat levels), and it's likely that their working voltage is plenty high enough. The -5V output is zener diode controlled, so no risk of over-voltage. But, there is no harm in renewing them. The larger 470V 16V capacitors however, get a large inrush current every time the power is applied and are only rated at 16V. They need to add lots of extra smoothing to the unregulated "+9V" rail so that the 4116 chips get a "+12V" supply without too much ripple on it. Keeping in mind that the capacity and condition of the smoothing capacitor in the ZX81 PSU's will vary...

Mark

[gozzo]

just noticed that one of the big caps looks like it got squished , so that will have to be changed anyway, and may as well do both at the same time

[Papalapa]

I'm sorry to refloat this post after more than two years, but I have a question about the Memopak 16K

What the hell are the DIP switches on the back of the module used for?

I download the user manual and there's no mention on it, also I do not found any information in Internet.

Anyone has some information about it?