Yet another help needed thread Spectrum +2 (grey)

[manxmann]

Hi Folks,

This is my first post so let me say a big hello before I start!

As a child of the 80's 8bit micro's hold a special place in my memory. During the day I never owned a 'Speccy', for my sins I was a C64 owner. On a whim of nostalgia I bought a very clean Spectrum 128k +2 (Grey) from an auction. The price was great however it was 'untested' which translates to doesn't work

Whilst very cheeky, perhaps as part of my very first post I could ask for some help diagnosing the issue. Let me describe to symptoms and detail the steps taken thus far.

The unit powers on and using the RF modulator I get the following garbled output on the screen:



Pressing the reset button causes some brief activity, again all garbage then the screen returns to the above.

I've checked the voltages as follows:

PSU : 10.4v
D20 : 11.63V
D21 : -12.74V
CPU Pin 11 : 5.04V
RAM : 4.68V (fairly consistent across all chips)
Edge connector - +9V / -5V / 9Vac all present

Given the above I'd decided my power transistors are still alive and well, am I miss guided?

I've swapped out the CPU for a known good unit, no changes. I socketed all the ram chips and 'mix them up' again nothing changes.

I left the machine running for 20 minutes to see if any of the chips get excessively warm, the ULA / CPU and TEA2000 get warm but not overly so. The missing ULA heatsink is me as I've removed and cleaned all socketed chips as part of my testing and decided to leave it off for the time being.



I've ordered one of the SLAM replacement ULA's but that hasn't yet arrived.

What else should I check/swap, any help very much apprecaited.

[RMartins]

The Power supply seems almost good, but if you haven't, checked the and replaced the electrolitic capacitors, you should eventually do it (unless someone already did it, i.e. if they are new).

You probably have RAM or bus issues (control signals).
You should also check the ROM.

ULA is also a possibility, but the patterns on screen do mean, that it's doing something at least, since we see horizontal and vertical lines.

The initial activity on reset, also means the CPU/clock is ticking, but something could be messed up.

I would advise you to get one of the available Cartridge like interfaces, were you can run independent diagnostics (not depending on the internal RAM to RUN, but do depend on the CPU ). This will provide you with some more useful info.

[1024MAK]

Hello and welcome

The +12V at the cathode of D20 is a little bit low. I presume you were on the cathode rather than the anode?
What do you get on the -5V at the anode of D18 or edge connector contact 20 component side?

What do you get on the right hand leg (pin 3, +5V output) of the 7805 voltage regulator? Be very careful not to let your probe slip and short out to an adjacent pin!

The +5V should be between 4.75V and 5.25V. There should be no significant voltage drop across the whole of the board. So can you please confirm what voltage you get on pin 8 of each 4164 DRAM chip.

The +12V should be between 10.8V and 13.2V, but in most machines is a lot closer to +12V.
The -5V should be between -4.5V and -5.5V

When you first switch on, what colour does the border show as? What actually happens to the rest of the screen.

When you press reset, does the colour of the border change?

Note that the border can only show white, red, blue, green, magenta, yellow, cyan and black. Whereas the rest of the screen can show bright versions of these colours as well.

Turn off.
Now press and hold the BREAK key. Switch on. What do you get now?

What test gear do you have other than a multimeter (which I presume is a digital type)? Do you have a logic probe? Any other test gear?

Mark

[Ast A. Moore]

When you first switch on, what colour does the border show as? What actually happens to the rest of the screen.

The problem with 128K machines is that the border is set to white way into the startup routine. By that time, the OS has already made several RAM tests, set up the system variables, and copied the paging routine to the RAM. Makes troubleshooting so much more difficult.

[1024MAK]

When you first switch on, what colour does the border show as? What actually happens to the rest of the screen.

The problem with 128K machines is that the border is set to white way into the startup routine. By that time, the OS has already made several RAM tests, set up the system variables, and copied the paging routine to the RAM. Makes troubleshooting so much more difficult.

Yeah, I know. It’s one thing (of many actually) I don’t like about the 128K ROM.

But I still would like to know if the OP is seeing any screen activity with the border, just in case the Z80 is getting that far before crashing.

Mark

[Ast A. Moore]

I still would like to know if the OP is seeing any screen activity with the border, just in case the Z80 is getting that far before crashing.

Indeed. A short video of a cold start or hard reset would be quite informative.

[1024MAK]

Also, is the displayed results consistent every time, or different every time (including, is the border always the same colour)... Especially when tried say five times in a row.

Mark