ZX81+38 Build Help

[DeadTed]

Hello everyone, I just finished my ZX81+38 V1.9 build and I cannot get a picture. I have checked voltages and I am getting 3.2Mhz on pin 6 of the CPU, not sure where to go from here. I have attached pictures of my board in case someone picks up something wrong that I have missed especially with the jumpers. I am using a 27c256 OTP EPROM and have copied 4 images of a standard 8k rom into it.

[1024MAK]

Hello and welcome

You don’t say what test gear you have (although you appear to have a multimeter).

As always, please double check the board for any missed solder joints, dry joints, solder bridges, solder splashes (which can be as fine as a human hair), or any other obvious faults including bent chip pins (for example bent under the chip rather than having gone in the socket contact), or chips fitted the wrong way round). Using a magnifying glass and a good light helps

When you say you can’t get a picture, do you mean you just get a white image, a black image or does the display/monitor/TV show a no signal message?

If you have a oscilloscope, logic probe or logic analyser, can you please test 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
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

[DeadTed]

ok so the TV says no video signal.

The following pins failed the test you specified, pins - 17, 22, 24, 25, 26

[1024MAK]

So as pin 26, /RESET is a control input to the Z80, if the Z80 does not receive a reset pulse, it will not function correctly.

You need to investigate this part of the circuitry. I don’t have a schematic to had right now.

Mark

[DeadTed]

Yeah pin 26 is definitely low after startup. I will review the schematic and see if I can make sense of what is happening. I have never done this before so at best I will be guessing hehehehe.

Hey, really appreciate your help

[DeadTed]

Just checked again 26 is fine but the others I previously reported are not

[acsi]

My eyesight isn't what it used to be but it looks like one pin on the 74HCT541 (I think that's the one (It's the chip in the middle next to the resister network) isn't soldered? I'm probably wrong though.

[DeadTed]

Thank you for the heads up, I rechecked all solder points and they are all fine.... I am going to take the time to do a scope pin by pin comparison between my original zx81 and the zx81 and post the results for any additional insights people may have.

[Fdiskitup]

Hi I'm new here too but we are having a very similar experience -
it looks like you are in the using 60Hz - is that intentional ? from timestamps it looks like you are with me on NTSC USA zone ?

I just got my ZX81+38 to boot today, I could not get it to work with the original TS1000 ROM. After 2 nights I gave up on that rom.

See if you get some screen activity with no rom plugged in - black and white bars / white flashes.

To get mine to work I reverted the jumpers JP8, 9, 10, 11 to the original settings, then made a new EEPROM SST27SF256 with 4 copies of the rom files that worked for me.

zx81 version 2.zip

(7.05 KiB) Downloaded 38 times

here's my rom that I know works - you will need to unzip, do you have a programmer ?

zx81 version 2.zip

(7.05 KiB) Downloaded 38 times

[1024MAK]

I just got my ZX81+38 to boot today, I could not get it to work with the original TS1000 ROM. After 2 nights I gave up on that rom.

Do you mean an original ROM chip or do you mean a copy of the data that was programmed (blown) into a EPROM/EEPROM/flash ROM chip?

And there are actually three different official ROM versions. As far as we are aware, none of the TS1000 computers have the first ROM version. The first version has the maths bug.

It’s important to know that there are differences in the pin-outs of the various ROM, EPROM and EEPROM/flash ROM chips.

Mark