Refurbing an issue one zeddy

[siggi]

My Zeddy has also a heatsink glued to the ULA:

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Siggi

[Moggy]

Yes but at least it's exposed to the surrounding air where it might... DO SOME BLOODY GOOD!!
All it needs now is a fan and it's complete.

Something like this perhaps bolted to the side of that construction site on the right.


Only joking siggi, that is one cool set-up!

[gammaray]

Just to make myself the black sheep... Heatsinks lower the temp of the underlying chip. Q=h A dT

Q is constant, h is approximately constant, increase in A results in a decrease of dT...


UNLESS BOUND UP IN A PLACE WITH NO AIR EXCHANGE FOR CONVECTION!

(added) Then the dT goes way down and the A needs to be very large. And the temperature still does not decrease much...

So put a holes or slots on the back side of the case on the inclined face which will look ugly. There are slots over the heatsink inside which can draw in convective flow.

Or get a switching power regulator.

[1024MAK]

Heatsinks lower the temp of the underlying chip.

UNLESS BOUND UP IN A PLACE WITH NO AIR EXCHANGE FOR CONVECTION!

Welcome to the point of the last heatsink discussion! That started here and went on for a fair while...

Heatsinks work by lowering the thermal resistance to the surrounding air, and hence the case temperature of the chip is lowered. But if the heatsink is in a small enclosed space like a ZX81 case, with little or no air flow, the heatsinks effectiveness is severely reduced. For more, or to continue the heatsink discussion, go to this thread

Mark

[1024MAK]

I have an issue one zeddy that I thought I would try and refurb (it's one of too many I bought recently but this one is cosmetically too poor to sell on with good conscience).

I am planning on:-

1) Changing the regulator - done this before but want to do it 'properly' this time though! Last time I cut off the legs of the old one and soldered onto those.

You've seen how I did it on one of my ZX81 boards. I did it like I showed in the photo in the other thread so that it would fit in a ZX81 case okay. If using a case with greater clearance, you can fit the switching regulator as intended by the manufacturer.

2) Recapping

The only two electrolytic capacitors on a ZX81 / TS1000 board are C3 (22uF 16V originally fitted, but I recommend using a 25V or 35V rated replacement) used for decoupling the nominal +9V rail. And C5 (1uF, 16V or 50V or 63V) that provides the power on reset to the Z80 CPU. Both C3 and C5 are not critical as in the actual values can have a wide tolerance, and failure of either will not do any damage to any other parts. So I only replace these if I see symptoms of failure. Failure of C5 prevents the Z80 CPU from properly resetting.

3) Possibly fitting an internal RAM upgrade. Although the current chip is not socketed so this may be beyond my skills.

The modern SRAM chips need a 28 pin socket. I showed how I did an issue 1 board here
Note that my mod is designed to allow a further modification to 32k bytes. Hence the cutting of the +5V track, then fitting a wire link across it.

Dave Curran of Tynemouth Software shows how he does an upgrade on an issue 1 board here.

So take your pick. They all do the same. Expand a ZX81 to have 16k bytes of internal RAM.

I have a question about item 2 - recapping. How do I know the alignment of the existing caps? Below is a photo but the caps only have an arrow on the side rather than positive / negative. I don't want to fit them the wrong way round!

If I was to guess I would assume that the arrow is the positive side but hopefully someone knows for certain?

Answered above by other members
Most common (and modern practice) is indeed the arrows which include the negative "-" sign. Some older capacitors have either a red dot next to the positive terminal, or a black dot next to the negative terminal, or a "+" sign next to the positive terminal.

Mark

[1024MAK]

So having talked the talk about some capacitors using a "+" symbol, here are some pictures...

IMG_4437.JPG

Mistreated ZX PSU photo 1

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Mistreated ZX PSU photo 2

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IMG_4439.JPG

Another ZX PSU

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Mark

[Lardo Boffin]

As ever thanks for all the info Mark! Chips are in the post.

[Lardo Boffin]

I have an issue one zeddy that I thought I would try and refurb (it's one of too many I bought recently but this one is cosmetically too poor to sell on with good conscience).

I am planning on:-

1) Changing the regulator - done this before but want to do it 'properly' this time though! Last time I cut off the legs of the old one and soldered onto those.

You've seen how I did it on one of my ZX81 boards. I did it like I showed in the photo in the other thread so that it would fit in a ZX81 case okay. If using a case with greater clearance, you can fit the switching regulator as intended by the manufacturer.

3) Possibly fitting an internal RAM upgrade. Although the current chip is not socketed so this may be beyond my skills.

The modern SRAM chips need a 28 pin socket. I showed how I did an issue 1 board here
Note that my mod is designed to allow a further modification to 32k bytes. Hence the cutting of the +5V track, then fitting a wire link across it

Mark

How much clearance does the regulator need above it? Looking at the tracopower TSR 1-2540 it looks (without detailed measurements) like there is a reasonable amount of clearance above it. As an aside 'properly' was probably a bad choice of words...

Also aside from a PCB drill (which I don't have) what is best to cut tracks with? Looking at the two 16k options yours looks the easiest to follow so I think I will go that route!

Lardo

[1024MAK]

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IMG_4474.JPG

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As the modern switching regulators run cool, it's okay if they touch the case. As long as the case fits without stress on the parts.

I mostly use a PCB drill with a cutting disc. A Dremel would be okay if it has a speed control. It's far easier to control the cutting if you can slow the disc speed down from the high speed these typically run at. If you don't slow it down, it is hard to control and you risk it flying off and damaging something else.

Other options are a sharp craft or Standley knife - make two parallel cuts about 2 mm apart. Make sure the knife cuts through the copper track. Then use a very fine flat bladed screwdriver to lift the track between the cuts so that there is a clear gap.

Or use a very fine file, edge on to file through the copper track.

Mark

[amigaman07]

"cosmetically too poor to sell on with good conscience"

Looks in Better Condition than My Timex 1000 !

Whats this about "HEAT SINKs"???



Darren in OZ