Sinclair ZX80 / ZX81 / Z88 Forums

I recently assembled a zx81plus38 rev1.10 board and used all hc logic. I also am using a commodore 1084 composite monitor.

Having several issues:

After I apply power the board starts up and I get my “k”. After several minutes the screen becomes all white or sometimes all dark on its own. I normally need to wait several seconds apply power and the system will restart and I will get my “k”. I’ve changed the 1uf electrolytic capacitor in the reset circuit and still having the same issues. Sometimes the screen will display the old type of loading with the white/black wavy lines.

On the “enter line” occasionally the characters will get wavy.
I double checked all the component values against the BOM and found no issues.

Not sure what’s going on if anyone has any thoughts would be greatly appreciated.

During any of these symptoms, does the system respond to the keyboard?

Mark

Mark

No there is no response from the keyboard. I change the 1uf capacitor again, as it’s part of the reset circuit and wrote a quick print program with scroll and it’s been running for 1/2 hour without any issues,,,



Best

If you have further problems, I suspect a poor connection, either a dry solder joint or poor socket connection.

The capacitor used for the Z80 reset input does not normally cause problems even on original ZX80, ZX81 and ZX Spectrum computers that are over 38 years old. As long as it has enough capacitance (so the processor has enough time to fully reset) and has a low enough leakage current such that the charge resistor (that connects to +5V/VCC) can charge the capacitor close to +5V.

Mark

You’re right about the capacitor. However the behavior did change now after about 10 minutes the screen changes to this

I read somewhere the original designer mentioned that he added logic to present a loading screen which looks like the attached picture. I double checked all the soldering joints and used deoxy to
Clean the sockets etc.

Any thoughts

Diagnosing a ZX80, ZX81 or clone is always tricky because almost the entire system needs to work in order for any meaningful display to be generated. This is because they use both hardware and the Z80 running software in both ROM (for the normal BASIC system display) and RAM for it to work.

Absolutely anything that stops the Z80 or which causes it to execute rubbish will result in a blank white screen. Or black on clones where the background is selectable.

Hence my next question is about what test gear you have (other than a multimeter)?
And can you program ( "burn") EPROMs or EEPROMs?

Mark

Thanks for the reply

I have an oscilloscope and an eprom burner.

Is there a diagnostic rom that I can use similar to the spectrum?

There isn’t a fully featured diagnostic ROM like that available for the ZX Spectrum. There is a simpler test ROM that can be used with a ‘scope.

Using your ‘scope, what signals do you get on the following pins of the Z80 CPU?
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
6 Clock (3.25MHz approximately)
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Mark

Sorry for the delay but I got another rev1.10 board and soldered a new board together. With same ic chips and the video now works when hooked by to a composite commodore monitor

Now I have a new problem I cannot save and load and programs via tape recorder. I tried both the Sinclair branded cassette tape and a ti99/4a tape recorder and both work when hooked up to the ti99/4a

It appears to save and load I can hear the squalk noises but nothing loads into memory

I using a simple 2 line program with a print statement and a go to statement. I checked and soldered in the appropriate capacitors on the board

Any help or confirmation from someone that theirs works would be grateful best to everyone

Glenn

Hi ghhann,
Could it be a ground loop which causes interferences? Does mean, if you connect two chinch cables load/save to a recorder, only one GND must be connected to the recorder!