Sinclair ZX80 / ZX81 / Z88 Forums

Hi, this is a CZ Spectrum Plus an Argentinian Clone of the ZX Spectrum 48+. Board is very similar to Sinclair's ISSUE 6A and uses the ZX8401 / PCF1306P multiplexor .
Sadly it doesn't work. Looks like ram is not being read.
I suspect the multiplexor is bad. No one IC is hot. And all are soldered.

I ran Brendan Alford's test from a rom based diag card.. and here's the result: It looks like ULA is good and all lower RAM ICs are marked bad in the border bars.

Lower RAM has an identical disposition as in ISSUE 6A board but only uses +5v and is good.

These machines are difficult to get. They made a very few of them. They are not different from others Argentinian clones but for the keyboard that is the same Spanish Spectrum + and the Spanish ROM. I would like to preserve it as original as I can but in working condition.
Before desoldering the ZX8401 is there a way to confirm the diagnosis? Everything but the lm1889 is soldered.

I can’t think of an easy way unless you have an expensive logic analyser with enough inputs to trace all the signals on that ZX8401 chip.

Because the ZX8401 does all the multiplexing for all the DRAM from the Z80A side, if this chip dies…

However, the ULA does not use the ZX8401 when reading the DRAM to generate the video display. So if it is possible to isolate (disconnect) the /DRAMWE line from the ULA (pin 4), and instead tie it to +5V via a 470 ohm resistor, this should prevent garbage continuously being written to the ‘lower’ DRAM. And may give a static screen of garbage instead. Power down/pause/power up five or more times so that you can compare the screens.

This may give you a clue.

Mark

1024MAK wrote: ↑Sun Aug 08, 2021 11:33 am I can’t think of an easy way unless you have an expensive logic analyser with enough inputs to trace all the signals on that ZX8401 chip.

Because the ZX8401 does all the multiplexing for all the DRAM from the Z80A side, if this chip dies…

However, the ULA does not use the ZX8401 when reading the DRAM to generate the video display. So if it is possible to isolate (disconnect) the /DRAMWE line from the ULA (pin 4), and instead tie it to +5V via a 470 ohm resistor, this should prevent garbage continuously being written to the ‘lower’ DRAM. And may give a static screen of garbage instead. Power down/pause/power up five or more times so that you can compare the screens.

This may give you a clue.

Mark

I'm afraid I don't have such instruments..
The /DRAMWE test will rule out a lower RAM and a ULA failure?
The lower RAM is made up of eight x MCM4517 DRAM chips,much less likely to fail, all together, than 4116 ones.. and ULA seems OK so far.. if performing that test means to cut a trace or the ULA leg, then I guess I prefer to desolder the ZX8401.. but I will follow that track just in case something is wrong in the board first..
I think I can get a NOS PCF1306P...

I Really appreciate your help Mark, thank you.

Well, it’s just an idea…

The video you attached is similar to what happens if no ‘lower’ RAM is fitted, with the ULA just reading garbage from the data bus (garbage because the ULA screen reads are not synchronised with the Z80 bus cycle).

It’s also possible that there is a fault with one of the other control lines to the DRAM chips, such as /RAS or /CAS.

For reading, the DRAM needs to see /RAS go low, then /CAS go low, then when either of these goes high, the access cycle ends. If either of these things does not happen, the DRAM will ignore everything and stay inactive.

Mark