Faulty 128K +2A

[1024MAK]

So, can you please confirm that the +5V rail is between 4.75V and 5.25V.

Then can you please test the following pins of the Z80 CPU using your logic probe.
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
6 CPU clock (3.5MHz signal)
16 /INT (should be pulsing)
17 /NMI (should be high)
18 /HALT (should be high)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be high)
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 A15. This will give an idea of which memory the CPU is accessing.

Z80 pin-out here.

Mark

[walde123]

So, can you please confirm that the +5V rail is between 4.75V and 5.25V.

Then can you please test the following pins of the Z80 CPU using your logic probe.
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
6 CPU clock (3.5MHz signal)
16 /INT (should be pulsing)
17 /NMI (should be high)
18 /HALT (should be high)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be high)
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 A15. This will give an idea of which memory the CPU is accessing.

Z80 pin-out here.

Mark

5V rail is at 5.15V

6 seems fine
16 pulses
17 is high
18 is high
19 pulses
21 pulses
22 pulses
24 is pulsing although very slowly, low LED is barely visible
25 is high
26 is high, reset pulls it low as expected
27 pulses
28 pulses
A0-A15 are pulsing, high LED on A14 is very dim

One thing I noticed is that when the screen does the thing where it turns into red stripes instead, the wait signal starts to pulse more intensely, to the point where the low light gets flickery, high LED still looks solid to the eye though. Not sure how good this cheapo eBay probe is anyway. Should I pay attention to the intensities more?

[1024MAK]

The custom Amstrad gate array chip briefly pauses the Z80 CPU via the /WAIT pin when the CPU tries to access the RAM chips that store the screen contents if this happens during the time that the Amstrad gate array chip is getting data to produce the screen display.

The Z80 control signals all appear normal, so the Z80 CPU looks to be alive. So far, I don’t see anything that indicates that the Amstrad gate array chip is faulty (but it’s not possible to be certain yet).

Which board version do you have?

Tell me more about the two ROM chips that are fitted, including the type numbers (maybe post up a photo).

Mark

[walde123]

The custom Amstrad gate array chip briefly pauses the Z80 CPU via the /WAIT pin when the CPU tries to access the RAM chips that store the screen contents if this happens during the time that the Amstrad gate array chip is getting data to produce the screen display.

The Z80 control signals all appear normal, so the Z80 CPU looks to be alive. So far, I don’t see anything that indicates that the Amstrad gate array chip is faulty (but it’s not possible to be certain yet).

Which board version do you have?

Tell me more about the two ROM chips that are fitted, including the type numbers (maybe post up a photo).

Mark

It's the issue 1 amstrad board

[Ast A. Moore]

Oh, so it’a a +2B. Could you take the logic board out and flip it over? See if there’s a bodge capacitor soldered onto the CPU (across pins 10 and 11, I believe).

[walde123]

Oh, so it’a a +2B. Could you take the logic board out and flip it over? See if there’s a bodge capacitor soldered onto the CPU (across pins 10 and 11, I believe).

No I can't see anything underneath except solder joints and traces (and the ASIC obviously). I could be wrong but I'm pretty sure it's a 2A, although both boards look remarkably similar when comparing the ones on here https://www.tomdalby.com/retro/sinclair.html

[Ast A. Moore]

No, those are both +2B boards; one is a Z70833 Issue 1 and the other is an Issue 4.

A +2A board looks like this:

[walde123]

Interesting, maybe the motherboard was replaced then or something since the bottom of the case has +2A embossed on it.

[Ast A. Moore]

Very few +2B boards were fitted into casings with “+2B” actually molded on the bottom. Almost all of them were Issue 4 boards, and of those, most—if not all—were sold in Spain, as far as I know. I guess Amstrad just had a pretty pile of the original cases and didn’t feel like throwing them away just because of a new board revision.

[1024MAK]

Timeline:

• ZX Spectrum 16K (rubber key)
• ZX Spectrum 48K (rubber key)
• ZX Spectrum +
• ZX Spectrum 128K
• ZX Spectrum +2 (grey, uses 9V barrel power connector), note some were made in the U.K. and some were made overseas.
• ZX Spectrum +2A (black) and ZX Spectrum +3, both using the same PCB, but the +2A without the disk drive control chips and the +3 without the components required for the built in cassette deck. However, as Amstrad were probably running down existing ZX Spectrum +2 (grey) stock, not many of this version of the +2A were produced.
• ZX Spectrum +2A (black) with the +2A case, but with the later “+2B” PCB
• ZX Spectrum +2B case with the “+2B” PCB (not as many of these appear to exist compared to the large numbers of the above version).
• ZX Spectrum +3B, very few of these appear to have been sold in the U.K., but some exist in Europe.

Above information is my best estimate, but I welcome any corrections or extra information...

Mark

[Guesser]

Very few +2B boards were fitted into casings with “+2B” actually molded on the bottom. Almost all of them were Issue 4 boards, and of those, most—if not all—were sold in Spain, as far as I know. I guess Amstrad just had a pretty pile of the original cases and didn’t feel like throwing them away just because of a new board revision.

U Wot?

That seems completely backwards

[Ast A. Moore]

That seems completely backwards

How so?

[Guesser]

There's loads of UK Issue 4 +2Bs (I don't have numbers but they're not uncommon based on what pops up on facebook etc), it's Z70830s in +2A cases that are rare and only seem to show up from Spain (those and the +3Bs).

[Ast A. Moore]

Well, I’ll be. I knew the +3Bs were rare, and never encountered an Issue 4 +2B board myself. From what I read, I concluded that they were as rare as the bottom cases with the “+2B” molded on them.

[walde123]

I was away for the weekend but I kept pondering on it and since everything seems to check out so far I'm wondering if one of the ROMs is bad. Considering they seem near impossible to find/too expensive I was wondering if it would be possible to burn my own.

I found some 27C256-150 EPROMs which seem to match the pinout of the speccy ROMs and I've been looking for an excuse to get an EPROM burner for a while so if that's possible to do then it would be a pretty cheap thing to rule out excluding the cost of the burner.

[Ast A. Moore]

Considering they seem near impossible to find/too expensive I was wondering if it would be possible to burn my own.

I found some 27C256-150 EPROMs which seem to match the pinout of the speccy ROMs and I've been looking for an excuse to get an EPROM burner for a while so if that's possible to do then it would be a pretty cheap thing to rule out excluding the cost of the burner.

Not only is it possible, it’s also incredibly easy, if you have the right tools. Have a look at the beginning of this thread for more info.

[Guesser]

Yep, a 27C256 is perfectly compatible. What's more some Spectrums came from the factory containing windowed EPROMs if I recall correctly.

[Ast A. Moore]

What's more some Spectrums came from the factory containing windowed EPROMs if I recall correctly.

Definitely 128Ks/+2s. Not sure about +2As/+3s.

[walde123]

Considering they seem near impossible to find/too expensive I was wondering if it would be possible to burn my own.

I found some 27C256-150 EPROMs which seem to match the pinout of the speccy ROMs and I've been looking for an excuse to get an EPROM burner for a while so if that's possible to do then it would be a pretty cheap thing to rule out excluding the cost of the burner.

Not only is it possible, it’s also incredibly easy, if you have the right tools. Have a look at the beginning of this thread for more info.

Well, in that case I'll see if I can make some. Unless someone has any other troubleshooting ideas I guess I'll have to wait for a burner to arrive until I can update on things

[Ast A. Moore]

Unless someone has any other troubleshooting ideas I guess I'll have to wait for a burner to arrive until I can update on things

Does the machine pass the RAM test every time now?

[walde123]

Some time has passed and I've got my hands on a TL866-II plus EPROM programmer. I haven't got any replacement EPROMs yet but I tried reading the ROMs from the Speccy and since it seemed to fail detecting PIN 1 which apparently is not connected in the ROM so I bypassed the pin detection and ID check and was able to get the contents of the 40093U ROM and the first half of it matches perfectly to the +2A/+3 v4.1 ROM 2 (+3DOS) found on here: http://www.shadowmagic.org.uk/spectrum/roms.html

I'm going to assume the other half of the ROM is fine too since the first half checked out but when I tried reading the 40092U ROM it just gives straight 0s, using the exact same settings as for the 40093 with simply swapping the chip in the socket in between attempts, so seems like to me that the 40092 is toast? or maybe I'm missing something. Anyway, I guess now I'll need to wait for the EPROMs to arrive so I can burn a replacement, I'm guessing there's some pin modification to do as well for using the EPROM since they expect 5V on pin 1?

[Guesser]

No you don't need to modify anything, it just plugs straight in.

[1024MAK]

To read the ROM chips from these machines, it’s best to manually select any brand/make of 27C256 EPROM. But only try to read from it, never try to write/burn to it.

Each of the two physical ROM chips contain two 16k byte ROM images. To see the other ROM image, get your programmer to read from 0x8000 (32768) onwards.

Just to add to what Guesser said... A 27C256 or 27256 EPROM chip can directly replace a ROM chip in these machines. No wiring or other changes are needed.

Mark

[walde123]

To read the ROM chips from these machines, it’s best to manually select any brand/make of 27C256 EPROM. But only try to read from it, never try to write/burn to it.

Each of the two physical ROM chips contain two 16k byte ROM images. To see the other ROM image, get your programmer to read from 0x8000 (32768) onwards.

Just to add to what Guesser said... A 27C256 or 27256 EPROM chip can directly replace a ROM chip in these machines. No wiring or other changes are needed.

Mark

Yeah I figured that would be the case, the machine is alive now so in the end the 40092U was bad, really glad it wasn't anything worse since in the end I only lost a few euros on some ram chips with some spares, and the eprom was only a few cents excluding the fact that I had to spend 40€ on the eprom programmer. Huge thanks to you and everyone else who helped me out. Still need to actually see if it can do anything but the startup screen is definitely a huge improvement over the garbled mess