Sinclair ZX80 / ZX81 / Z88 Forums

Well my series 2 Spectrum died this evening, display started showing random squares, so turned it off and on again, seemed OK apart from 2 keys on the bottom row would not work. Turned it off again and removed the interface 1, now just get horizontal bars across the display

History - been sat in my Dad's loft for 20 odd years, found it the other weekend, needed a new keyboard membrane. Planning a composite video mod and recapping.

Where should I start to bring it back?

Does the border always end up white (power off / on at least five times)?

Can you post a photo of the image on the display?

Do you have a multimeter?

Can you solder?

Mark

On the soldering subject - here's one of my own designed MERG PCBs

Good news, of sorts, this evening, it booted. But it quickly deteriorated with the random squares on the display. B, N & M keys not working. External test rom plugged in and all ram checks out ok.

Working from home tomorrow so will get time to check voltages etc and see if any ram chips are getting hot etc. it certainly seems to fail as it warms up.

Right, done some testing.

Voltages as follows:

input from psu (standard Sinclair UK1400) - 10.33v
+5v - 4.82v
+12v - 11.57v
-5v - -4.72

voltage across big resistor next to ULA - 1.86v

here's some screenshots - note I occasionally get it to boot and I cannot get a white screen - it's pale blue no matter what I do with the tuning etc. Colours are not right
No RAM id getting hot, ULA is as expected, heatsink on regulator hot, but not too hot to touch

So it looks like the voltages are ok? what should I check next?

With regards to the keyboard the B, N and M keys are not working as follows:

Cannot get BORDER, BIN or B

can get * and BRIGHT

Same for the N and M keys

I can't get the B to register even if I type LOAD "B

for example

Let’s start with the “best to leave until later” problem. The blue tint is an analogue video issue. The issue one and issue two boards need their colour encoder circuitry calibrating manually by carefully adjusting preset (adjustable) resistors VR1 and VR2. But it’s best left until the machine is otherwise fully functional.

Unless you have already renewed the membrane, the non-functional keys may be the membrane. Or may be due to another problem. If you really want to test, use a piece of thin insulated wire (a wire from an off-cut from a fixed telephone cable or network cable for example) stripped at both ends obviously. Poke one end in one keyboard connector so it touches one contact, then poke the other end into the other keyboard connector. There are 40 combinations to test…

The voltages (+5V, -5V, +12V) are all okay.

Unless your board has an intermittent connection (socket, dry joint, solder splash, cracked/broken PCB track or similar), the most likely cause is a DRAM fault.

Is this a 16K or a 48K board? I can’t see evidence of a single “lower” RAM fault in the photos, leading me to wonder if it’s an ‘upper” DRAM chip that is misbehaving.

Did you leave it running the test ROM for a while? BTW, which test ROM is it?

However, it could also be a faulty ULA. Or indeed, any fault that causes the Z80 to execute random code rather than running the ROM code.

Mark

You're going to love this ......

it's working apart from the screen colours issue - no amount of twiddling seems to get things better, but it might be the TV? I have composite mod pcb to fit so hopefully that will sort things

So

this is embarrassing

when I was a kid I added a hardwired joystick connector to the pcb thus
Now the red wire had come unsoldered and may or may not have been in free air or touching something it shouldn't

anyway its removed and all is well, super stable Speccy.

I told you you'd love it

Keyboard is now a bit iffy on the column including 2, w, s, x and the corresponding column on the other side of the keyboard, but pressing the symbol shift key on the right hand side sorts things. I'll have a look to see if something is misaligned physically sometime.

Now to get the proper colours on the screen ....

and recap

and get a new lower case without a D connector shaped hole LOL