issue 2 dc dc modification ??

[jojo]

Hi forum ,


Has anyone already implemented the "known" dc-dc modification for issue 3 to 4 on a issue 2 ??


just to assure it is possible before i slaughter my mint issue 2

[1024MAK]

You mean this?

Mark

[jojo]

Indeed ,


has anyone implemented that modification already on an issue 2 ??


jojo

[1024MAK]

I’ve got a issue 2 board that I intend repairing (it’s been robbed of some parts) that I also intend on upgrading the DC-DC converter section. But have not got any further yet. The weather has been mostly fine where I live, so I have been doing outside work.

I have also recently seen a photo of an issue 2 board that has had this modification done. But I can’t remember on which forum.

Mark

[jojo]

the only picture i have is this one ;




most likely from the same forum post you are referring too .

will source the components and give it a try .


= this modification looks totally different as other suggestions around on internet =

this is how the board looks right now , only C has been added between c34 and r59 ......

[1024MAK]

There is some variation in how the DC-DC converter modifications were done. Partly due to the circuit evolving over time (so what was done depended on what the current version of the modification was at the time) and partly because different service technicians each did their own thing. And because obviously the original parts are not all positioned in the same places between different issue boards.

See also this web page for issue 2 modifications.

Mark

[jojo]

the board i have has the dc-dc described by Phil .

i just verified it at component level.

[zx64]

An idea ...

All generated voltages go to the edge connector. So it should be possible to disable all internal voltage generation (just removing coil, 7805 and two transistors should be enough?) and create a modern DC-DC converter board which can be plugged into the edge connector.

[1024MAK]

Aye, that’s possible (in fact, you don’t actually need to remove the coil just the 7805 regulator and TR4). From an external PSU feed the +5V to the +5V rail, -5V to the -5V rail, and +12V to both the +12V rail and the +9V rail. The +9V rail needs to be powered in issue 3 onwards boards so that the loud speaker works and so that any expansions that use the +9V rail work.

Alternatively, replace the lower DRAM with a type that does not need -5V and +12V supplies. Remove the 7805 and replace with a Recom, Traco Power or similar modern DC-DC switching regulator. Remove TR4. Now link the +9V rail and the +12V rail. Now your ZX Spectrum can run off a single 12V DC regulated PSU.

Please note that with any of these modifications, any expansion that needs the 12VAC (often shown incorrectly as ‘-12V’) will not work.

Mark