Dead ZX81 Issue 3 :(

[Ziggy]

Sad tale

I have a faulty zx81 issues3 that i cannot get going and it's becoming very frustrating.....

What is the problem ?

At the moment the screen outputs 8 vertical bands - alternate grey & black.

At this stage all ics are socketted.
I removed the ROM , CPU, ULA and inserted sockets.
I removed the RAM and inserted turned pin strips to form sockets (for either 2 x 4 bit or 1 x 8 bit memory). The link L1 is open as it was originally 2 Ram Chips.

I checked all chips by installing in another issue1 zx81 - all function correctly -the 'K' comes up no problem in the other ZX81.

I initially replaced all capacitors on the pcb. I used ceramic discs in place of 5 47nF ones. Same values -should not matter.

(It's a long story - Initially, When the chips were socketed,I had a white screen - no cursor. The only IC in a socket was the ULA. Tested with another good ULA - no good

I removed the CPU and socketed it. Tried a different cpu (good one). No go!

I gradually removed all ics and socketed the complete pcb. Still no go , even with proven CPU/ULA/ROM/RAM.

Examining the pcb, I traced two nicks in tracks:(which i may have done myself !).

A: ULA pin 9 to pin19 cpu. Fixed with soldering a little wirewrap wire.
B: Track joining pin 12 1st RAM chip (IC4a) to Pin 11 of IC4 (single RAM). That should not have done any harm, since I had two ram chips installed. It simply left pin 11 of that empty socket detached. I repaired it with a little wirewrap wire. Testd for continuity on both tracks - all ok.

I traced every track on the pcb from ULA to CPU to ROM to RAM through resistors etc., with magnigying glass and help up to a light also. No other breaks detected. i checked all data lines for possible shorting with a multimeter. Found no obvious shorting or solder splashs etc.

I tested all components on the pcb and am sure none is defective.

Currently, I have the 8 vertical bands showing.

The only anomaly I can find at present is that pin 33 CPU Address 3 (A3) is not showing a signal. All the other Address lines A0 - A10 are clocking away nicely. Tested with a dualtrace scope.

Thinking that A3 was being pulled down by something, I removed the RAM/ROM chips. No difference. A3 still flatlining.

I've reexamined all the tracks again - following those tracks is a bit difficult, but I sure I established continuity and could trace no shorts on the interconnections along A3. I checked for shorting on all socket pins - no good !

The CPU tests ok in another ZX81!

At this point, I think I should just take a hammer to it

Any suggested strategy would be appreciated.

Do you ever give up and let the patient die . No !

DenisM

[PokeMon]

At the moment the screen outputs 8 vertical bands - alternate grey & black.

Hmm normally the ZX81 knows 2 colors only, white and black as these are the two voltage levels (sync would be the third) coming out from the ULA.
So is this the same color of the outer background ?
Is the picture synced and clear ?
Don't know wether your TV show if there is a valid (synced) signal.
Is it a steady screen or moving to any direction ?

Do the 8 stripes have the same size ?
So would it be 4 chars white, 4 chars black and so on as the active screen area is 32 chars (which would match to 4x8).
How much memory do you have installed ?

Do you have an oscilloscope ?

[1024MAK]

What type of display are you using (CRT TV, LCD TV, other...)?
Please post pictures of the displayed image and of the Zeddy PCB. When taking photos of the Zeddy, use macro mode.
What type of external PSU are you using (Sinclair/other)?
Have you tested the voltage of this PSU? What is the ripple level on this PSU? Test using a 'scope on ac range. If no 'scope, but you have a good multimeter, test on ac mV range.

Mark

[Ziggy]

See pics .



it's difficult to get an image from the screen, but you will see the detail in this pic.
The vertical columns are in turn showing as stacks of black horizontal bars.
The columns are all the same size save for the rightmost column is only half the width of the others , but i think that due to the TV being
old and the pic is a little larger than it should be.

Ignore the black band across the screen - it's a feature of the camera speed i suppose ...not really there .

I have a dual scope oscilloscope. Hitachi V252 - old but reliable
The second pic is the video output signal at the modulator input.
The Tv is a portable color Phillips -set a ch 36 UHF - auto tuning - so i cannot do
any fine tuning on it. The video is steady enough.

I'm using a sinclair PSU , showing 13.45 Volts off load.

That PSU works with the other zx81s

Z.

[Ziggy]

PCB front and Back :





Z.

[Ziggy]

The saga continues...

I set up two zx81 side by side to check the clocks and took a pic while attached to pins 6 of CPU on both of them.
The top trace is from the good zx81

Looks like I have an anomaly. Could the bad zx81 be clocking faster then the good one ?

See pics :





I'm not that experienced with scopes, but it seem to me that I have different clock rates between the good zx81 Issue1 and the bad ZX81 Issue3.

[Ziggy]

Hi guys,

This is very embarassing
Another senior monent

When I noticed the clock anomaly, I examined the components/area about the crystal and
realised that the capacitors C6 and C12 had been transposed when I reinserted them.

When removing the sockets, I also removed all capacitor to put in new ones.
I checked their values and they were good. I did replace most of them. Since i didn't have their values on hand, I reused
the original 47pF (4) and 100pF (1) capacitors. The damn things seem so alike - I mixed up the colors !
(Something in my brain told me that those color band caps were all the same value)

I've now removed and swapped C6 and C12 and all is well. I have a 'K' cursor...

Sorry for all the waffle..... But you know it helped enormously to be forced to re-examine
the whole issue while presenting it here.

Thanks to those who responded.

Another ZX81 has come back to life

Ziggy / DenisM

[PokeMon]

I saw your pic of the video signal.
So there is no time resolution displayed but thought it was out of timing.
Looked like about 78us / 12.8 kHz horizontal frequency (assumed you had 20us/div).

But now it seems to be too fast. Which gives about 39us / 25.6 kHz horizontal frequency (assuming 10us/div now after your last post).

Good that you could solve it. What about the A3 signal you talked about in your first post ?

[Ziggy]

What about the A3 signal you talked about in your first post ?

I've not rechecked that, but i guess if it's working there's no need now.

I suspect that placing a smaller value cap in the oscillator circuit would have interfered with the clocking of the Z80
to the extent that it would not function correctly, especially as it seemed to increased the clock speed.
Maybe the Z80 could not take the increased speed and presented an erratic response.

I will re examine all those data /address clocks again - just to become familiar with what is the norm.

Thanks again

Z.

[Ziggy]

PS
At an early stage I swapped out the CPU when I established that the original cpu was
faulty by substitution. So I guess the original fault was the CPU, but I compounded it
by replacing the socket and cutting a track in the process

Z.