Spectrum Computing

A broken track would only cause some of the keys to become unresponsive, the rest would still work.

Unless it's a control line to the ULA.

Mark

That could be the ULA at fault. Did it respond to the keyboard before?

Mark

What make of 'upper' DRAM chips are fitted? And which issue board is it? On issue two boards, for some makes of DRAM, this logic chip had to be modified so that the make of DRAM that was fitted would work correctly. Some issue 3 boards also used this system even though the wire jumpers on it could b...

Contention occurs only when the ULA (or gate array) detects a memory address on the address bus that is in the 'lower' RAM range OR an I/O operation that accesses or which may access the ULA (or gate array). Once the Z80 has reset, the address on the address bus will be 0x0000 which points to ROM. A...

Yes, cutting the data in (pin 2) and connecting it to 0V for the DRAM chip that provides D0 or D1 should be enough. BTW I got a bit carried away in my earlier post, I had 0x02 in my head (as that is what the code counts down from), so it's the chips that handles data bit D0 and D1 that are relevent,...

Which 'upper' DRAM chip have you removed?

If it's the chip that handles data bit D0, D1 or D2, that's enough for the ROM memory check to fail the 'upper' RAM, the machine should then try to operate as a 16K system.

Mark

You could try disabling the ‘upper’ (or expansion) DRAM. On this forum, the topic about this is here. Note however, that this disable test assumes that the DRAM chips respect the control inputs.

Mark

Also, just a reminder, CLEAR can be used in a running BASIC program, like so: 10 PRINT "one" 20 PAUSE 150 30 CLEAR 60000 40 PRINT "two" But keep in mind that CLEAR also clears the screen and clears any current variables in use. Hence if you have any data in variables, you need to...

When your own machine code is in control, you can move the entire area around RAMTOP down, as long as your code updates the relevant system variables and the stack pointer to the new values.

Mark

Yeah, why are you trying to save knackered DRAM chips? :shock: I strongly recommend cutting all the pins on the DRAM chips, then removing each pin individually. BTW, I set my iron to 340°C and my desoldering station to 380°C. Nearly all new DIL/DIP chips are manufactured with splayed out legs. If a ...

Is there any reason why RAMTOP is not being moved down? Also, if UDG are not being used, that area can be used.

Mark

Well, there will still be a memory access because the normal DRAM refresh still NEEDS to occur.

Mark

For those following along at home (or wherever else you are browsing this forum), the OP also opened a topic over on Sinclair ZX World here.

Mark

Hello there unhuzpt!

Fancy seeing you here

Mark

It's no good hiding here as well as posting elsewhere! I'll still find you!

Okay, best not to do anything else until I've had a chance to think about this...

You say you have a digital multimeter, do you know how to measure resistance, continuity and voltages?

Mark

The +2A/+2B/+3/+3B use essentially the same ROM code, hence work the same (more or less) when it comes to common features.

Mark

Thanks Mark, I saw that but it looks dead, and that's where I found the link to the website. You might very well think that; I couldn't possibly comment... I built that serial database one afternoon in the pub about five years ago, and ever since then I've been meaning to update it so that it's a b...

The topic about serial numbers wanted is over on WoSf here.

Mark

All these have the same functionality: PCF1306P, ZX8401, Amstrad part no. 40058.

Mark

The 'lower' and 'upper' RAM uses different multiplexer circuitry, but of course, if it's the PCF1306... I've not really taken much interest in working out what test patterns are being used in this version. However, common test patterns include (all in binary) 00000000, 01010101, 10101010, 11111111. ...

Alternating is explained above and can be seen in my video...

Multiple failure may indicate it's one of the multiplexer chips is the problem.

Mark

Here's a diagnostic board based on Dylan Smith's design running on one of my Speccys:




Mark

Yes there are things you can do. But first, what (if any) chips are in sockets? I'm presuming you can solder and desolder, and have a digital multimeter? Apart from the bench PSU, do you have any other test gear? Fault finding may involve either track cutting or cutting chip pins, are you okay with ...

Hi David, I see that you've made it here okay

Mark

You don't need a non-conductive screwdriver to adjust the two variable resistors ("presets" VR1 and VR2). A metal (but non-magnetic) screwdriver should be fine. Keep in mind that it's very small movements that are needed. Try not to touch the metal shaft though. Hold by the plastic handle....