FIXED Spectrum +2 grey RAM fault but RAM is good

[Rabs]

Hi, struggling with what should have been a simple repair for a friends ZX Spectrum +2 grey.

Retroleum diagnostics is indicating IC31 bit 1 faulty.

IC31 removed and tests ok.

Socketed and new good 4164 RAM installed, still same.

IC31 continuity to ULA ok

IC31 and RN2 to Z80 ok

Scope signals on D1 show data.

Any ideas much appreciated

[Rabs]

The other hints maybe that the DiagROM continues and the upper RAM passes but then it displays a warning regarding bits stuck on port 254 which never clears (I don't have a keyboard connected).

Also the ZX Diags just reports RAM FAIL and nothing else, although sometimes some text flashes quickly a across the middle of the screen before RAM FAIL but to quick to read.

[Paul]

To quick to read?
In this case I usually take a video of the process and review it slower

[Rabs]

Nice tip thanks.

[Rabs]

Not that interesting but still great tip

[1024MAK]

Is there, at any point any screen corruption?

RAM in positions IC25 to IC32 provides the RAM for the screen display. If IC31 / bit 1 was faulty, or there was an open circuit fault in the D1 data line, you will get screen corruption.

Did you test to both pin 2 and 14?

The other hints maybe that the DiagROM continues and the upper RAM passes but then it displays a warning regarding bits stuck on port 254 which never clears (I don't have a keyboard connected).

The ULA does control the RAM in positions IC25 to IC32. The I/O port 254 (0xFE) for input is used for the tape input and the keyboard. Having no keyboard connected is no different to no keys being pressed. Hence bits 0 to 4 will be high due to pull-up resistor pack RN4.

The same port when used for output controls the border colour, the bleeper and the tape output. But the program would have no way to know if these outputs are working (or not).

Mark

[Rabs]

Hi, I checked continuity of both pins 2 and 14 back to the ULA.

This is the bit I don't quite get. So in the 128K I guess there is no upper RAM as such since each bit of the 64K is provided by the 4164s. So if the lower RAM test is failing why is the upper RAM test passing?

I have also cleaned and reseated the ULA, removed the ROM and swapped the Z80 just in case and no difference.

[1024MAK]

There are still two groups of eight DRAM chips. Each group provides 64K bytes. The group IC25 to IC32 is the "lower" RAM as far as the ULA is concerned. The other group of DRAM is the "upper" DRAM.

This is the reason why half of the RAM is contended and half is non-contended.

Mark

[Rabs]

Got it thanks.

Signal shape on D1 is very different to other data lines?

D1

D0

Something is clobbering the signal.

I removed the ROM so not that. Joystick Controller?

[1024MAK]

Without context (which device is driving the data bus at any point in time) it's rather hard to make a determination just on a 'scope waveform.

The most important thing is: when being driven, are the levels within the valid TTL logic level ranges? See here for logic level range details.

When the bus is not being actively driven, the stray capacitance will cause the voltage level to curve. After a while it will settle to the non-driven level.

If there is a device that is acting as a partial short circuit, it could be any chip connected to the data bus.

Which gets me back to my earlier question. Is there any visible corruption on the screen?

Mark