Sinclair ZX80 / ZX81 / Z88 Forums

Hi,

Just got my Lambda 8300 up and running. Disabled the original composite and replaced with a nice ZX8-CCB module. It powers up, gives med the READY and a blinking cursor - if I let it run, after a while the screen goes black. The Lambda is a version modified to use an EPROM instead of the original ROM.

What I've done so far:
- replaced composite with ZX8-CCB
- replaced 7805 with a Traco
- removed all Chips from sockets, ULA, CPU and ROM - cleaned with iso and reinserted
- replaced the NEC Z80C with a Zilog

Not sure where to continue - maybe it's the EPROM? Sometimes the screen just goes black, sometimes it behaves a bit like a loose power connection

Any suggestions where to start?

It sounds like a bad connection, that comes when the machine gets hot. Or bad RAM, that is just discovered, when the machine is used.

Not many had these machine, so there might not be as much help, as if it was a ZX81 you hade problems with.

At least it looks like the ULA is working

1024MAK wrote: ↑Sat Nov 23, 2019 3:13 pm .

Mark what do you think? Maybe it's one of the ICs that stops working after beeing warmed up.

kolbeck wrote: ↑Fri Jun 02, 2023 12:48 pm At least it looks like the ULA is working

What happens after resetting when it goes to black screen? Does it then work again? Does it only stop working when you use it, or will it go black, when you just leave it there without using it?

Yeah, need more information. It does sound heat related.

Is the +5V rail stable and in spec?

The ULA runs off a separate supply derived from the input, not the +5V supply. So test the positive supply pin to the ULA.

Are all the socketed chips firmly pushed into their sockets? Have all the pins been visually checked to see if any missed the socket contact due to getting bent?

The Z80, is a known good one? Have you tried the original? Do you have another Z80 based machine to test the Z80 in?

Mark

1024MAK wrote: ↑Sat Jun 03, 2023 10:37 am Yeah, need more information. It does sound heat related.

Is the +5V rail stable and in spec?

The ULA runs off a separate supply derived from the input, not the +5V supply. So test the positive supply pin to the ULA.

Are all the socketed chips firmly pushed into their sockets? Have all the pins been visually checked to see if any missed the socket contact due to getting bent?

The Z80, is a known good one? Have you tried the original? Do you have another Z80 based machine to test the Z80 in?

Mark

I’ll test the +5 on the ULA. Yes all socket chips have been out and inspected and cleaned. I have replaced the NEC CPU with a ZILOG from a working ZX81

mrtinb wrote: ↑Sat Jun 03, 2023 10:04 am

1024MAK wrote: ↑Sat Nov 23, 2019 3:13 pm .

Mark what do you think? Maybe it's one of the ICs that stops working after beeing warmed up.

kolbeck wrote: ↑Fri Jun 02, 2023 12:48 pm At least it looks like the ULA is working

What happens after resetting when it goes to black screen? Does it then work again? Does it only stop working when you use it, or will it go black, when you just leave it there without using it?

It works again for a while, just leaving it, if I just power it on- can’t do the reset as the keyboard flatcables broke off

The ULA runs at around 9V IIRC.

So, if the DC supplies are okay, it’s got to be either a poor/intermittent connection/contact somewhere, or a problem with the ROM/EPROM. Or the Z80 is loosing its clock input. I’m assuming you don’t see any screen corruption, which is why you think the ULA is okay. No screen corruption would also indicate that the SRAM may be okay, but unfortunately does not prove it…

Mark

1024MAK wrote: ↑Sat Jun 03, 2023 10:46 am The ULA runs at around 9V IIRC.

So, if the DC supplies are okay, it’s got to be either a poor/intermittent connection/contact somewhere, or a problem with the ROM/EPROM. Or the Z80 is loosing its clock input. I’m assuming you don’t see any screen corruption, which is why you think the ULA is okay. No screen corruption would also indicate that the SRAM may be okay, but unfortunately does not prove it…

Mark

Yeah, the display - as shows in a previous post, are good - no corruption. I Will try with a new eprom later as Well.

Thanks for the input

As you are not inputting any program (as the keyboard is not working) you don't know if the RAM is ok.

You can "press" the keys without a keyboard, by shorting the keyboard matrix. Put a wire between one of the connectors for one side of the matrix 1-5, to the other side of the matrix 1-8. That makes 40 combinations, which is all 40 keys. There must be one other wire on the Lambda 8300 as well, for the reset line.