Thanks very much for that, been adjusting for about 30 mins but with no real success - I think the problem is the wooden support that the plate is screwed might not be quite level and this is removing any benefit from tweaking the level screws on the plate - might longer screws/springs help to resolve this do you think?
The z=0 position of the wooden platform assembly is set when a limit switch in the back left of the printer is triggered. When you home the head, that's what determines z=0.
You're going to need to adjust the limit switch to be a bit lower down, so that the wooden platform stops a little lower down. That will then give you the room to loosen the screws at the back a bit more to raise the platform at the back, so it's no longer grounding out on the springs.
The way I would do it is to turn the printer off, lower the bed away from the head (by turning the z-axis coupler by hand) and adjust the screws at the back corners of the bed to a middle point in their range. Then manually move the head near to the back left spring, and turn the z-axis screw by hand, to raise the bed, until the bed is just about touching the bed (eyeball it, doesn't need to be perfect). Then, without moving the z-axis screw any more, move the head to the back right screw, and adjust the spring until the bed and head are just touching. Then move the head to be near each of the front screws, and adjust those. If you platform assembly is drooping a lot (it sounds like it might be), then you may not be able to screw the front screws out far enough to still keep tension on the springs - in which case, you'll need to screw both the back ones in a bit more, and start over, adjusting the z screw to bring the platform back up until it just touches the head again. The idea is to end up with the bed roughly level, so the head touches the bed in each corner, and still have some adjustment room in the four springs.
Then you need to adjust the position of the upper z-limit switch, so that it just clicks with the bed in this position. Loosen it slightly, adjust it, and then gently re-tighten (it's easy to strip the threads on the switches).
Now you can power the printer on, and auto home it. Now the wooden platform is in it's true z=0 position, and you can now level the bed more accurately. Heat the head, and then again move the head to near each screw in turn, and use a piece of paper as a feeler gauge. Tighten the screw until you can slide the piece of paper freely without it catching the head. Then loosen the screw slowly until the piece of paper just starts to catch on the nozzle. Then loosen it another 1/8 turn to account for the thickness of the piece of paper. Repeat at each corner in turn, and then recheck the first corner, to make sure the adjustments didn't throw it out.
BTW, my printer is of a similar age, and one thing I found was that the washers that sit on top of the springs under the bed are too small, so that they catch in the keyhole slots, and throw the bed off. Before leveling the bed, it's a good idea to press down on the washers through the keyhole slots (the UM hex screwdriver is good for this), and just make sure the washers aren't caught up in the slots. I added some slightly larger washers on top of the existing ones. That stopped them getting caught, and made adding and removing the bed, and re-levelling a much easier experience.
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