Probably bad ZX8401 / PCF1306P how to rule it out?

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bola_dor
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Joined: Wed Oct 02, 2019 5:32 am

Probably bad ZX8401 / PCF1306P how to rule it out?

Post by bola_dor »

Hi, this is a CZ Spectrum Plus
IMG-20210807-WA0040.jpeg
an Argentinian Clone of the ZX Spectrum 48+. Board is very similar to Sinclair's ISSUE 6A and uses the ZX8401 / PCF1306P multiplexor .
2021-8-8 2-13-7.jpg
2021-8-8 2-13-7.jpg (29.2 KiB) Viewed 1864 times
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:
20210808_020257_1_4.mp4
(1.06 MiB) Downloaded 151 times
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.
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
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1024MAK
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Re: Probably bad ZX8401 / PCF1306P how to rule it out?

Post by 1024MAK »

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
ZX81 Variations
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bola_dor
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Joined: Wed Oct 02, 2019 5:32 am

Re: Probably bad ZX8401 / PCF1306P how to rule it out?

Post by bola_dor »

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.
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Probably bad ZX8401 / PCF1306P how to rule it out?

Post by 1024MAK »

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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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