Faulty 128K +2A

[walde123]

Hey all,

I recently acquired a Spectrum +2A for a pretty decent price with the original box. The seller of course said it was in unknown condition but I thought I could tackle it but I'm at a bit of a loss.

Initially when I got the machine I had no SCART lead so I used RF, but I couldn't get it to tune to the TV so I disassembled it and saw that the RF modulator was flopping around because the solder joints had cracked and one of the wires had snapped, easy fix and I got a display. Then I was presented with a black screen, occasionally with some purple stripes, so I started looking for faults first simply by testing the temps of the chips, and burnt my fingers on 2 of the 4464 RAM chips, which turned out to be shorted.

I ordered a SCART cable and 4 replacement RAM chips and now I'm getting this pattern:



I left the machine running for around 5 minutes and the screen changed to this, seems to happen every time:



It started out having the entire screen striped like the top half, then they slowly started to disappear one by one from the bottom up and in the end I was left with those thin vertical red stripes like it has on the bottom. After resetting the machine still has the thin red stripes and after a power cycle it's back to the checker board pattern.

Not really sure where to go from here as I don't own an oscilloscope or have any other suitable replacement chips which is why I'm asking for help any help is appreciated, I want some opinions from people who actually know something about the speccy unlike me before I start throwing chips at it

[1024MAK]

Do you have a digital multimeter? Do you have a logic probe?

Mark

[walde123]

Do you have a digital multimeter? Do you have a logic probe?

Mark

Yeah I do have both, never used the logic probe though as I got it for my C64 but ended up not needing it

[llopis]

Well, the +2 doesn't have much else to go wrong, so chances are it's a bad ASIC (which is a bummer).

Just to make sure, you can run the ZX Diagnostics ROM and see if it reports anything. While you're at it, remove the two ROM chips just in case they're hugely shorted and bringing the buses down.

[walde123]

Well, the +2 doesn't have much else to go wrong, so chances are it's a bad ASIC (which is a bummer).

Just to make sure, you can run the ZX Diagnostics ROM and see if it reports anything. While you're at it, remove the two ROM chips just in case they're hugely shorted and bringing the buses down.

I sure hope it's not the ASIC, but yeah I really don't have much of a clue where to go from here. Z80s seem to be available for pennies even to this day so I might as well get a couple spares and try that but other than that I don't really know what to do. I guess one or both of the ROMs could also be bad, but considering they don't heat up much I'd assume they're not shorted at least. Seems really difficult to get spare ROM chips anyway since I couldn't find any or even the pinout for that matter if I wanted to burn my own.

Don't have a diagnostic cart either but I guess I could get one assuming it wont be too expensive. I also recapped the machine as I happened to have a kit but no dice, wasn't expecting it to be the issue but it was worth doing anyway

Just making sure, it isn't necessary to have the keyboard connected to get to the startup screen right? I did try it with the keyboard on at one point with the same result but I obviously don't want to pull those ribbons off any more times than absolutely necessary

[Ast A. Moore]

I started looking for faults first simply by testing the temps of the chips, and burnt my fingers on 2 of the 4464 RAM chips, which turned out to be shorted.

. . .

I ordered . . . 4 replacement RAM chips and now I'm getting this pattern:

Is that after replacing the RAM ICs you were suspecting to be faulty? What happens if you swap the RAM chips on board randomly? Does the pattern change at all?

[walde123]

I started looking for faults first simply by testing the temps of the chips, and burnt my fingers on 2 of the 4464 RAM chips, which turned out to be shorted.

. . .

I ordered . . . 4 replacement RAM chips and now I'm getting this pattern:

Is that after replacing the RAM ICs you were suspecting to be faulty? What happens if you swap the RAM chips on board randomly? Does the pattern change at all?

Well, I have some 15NL and some 12NL chips since I ordered a whole bunch and if I only use the 12NL chips in I get the same checker pattern but instead it's sort of split into 3 like this:



No idea if it has any meaning but I'm guessing it's something to do with the faster RAM. Other than that I haven't noticed anything with swapping them around and pressing on them doesn't change anything so it doesn't seem like a bad connection.

The original 2 RAM chips were pretty much completely dead shorts and it was pulling the 5V rail down to something like 4.2 or 4.5 volts if I recall correctly, who knows if that damaged any other chips in the process but the aftermarket PSU I have didn't cut out because of it. Now the voltages seem fine after getting rid of the bad RAM.

[Ast A. Moore]

What happens if you take all the RAM ICs out, power on the machine and reset it a few times?

[walde123]

What happens if you take all the RAM ICs out, power on the machine and reset it a few times?

It started out with a white screen with the normal grey border, resetting doesn't seem to do much but it starts up much faster than usual which makes sense. After a few seconds of running the screen fills with flickery stripes for a few seconds and then the screen goes completely black and stays that way, even after resets and only does it again after a proper power cycle.

Interestingly if I touch the RAM sockets it causes random stripes to appear for a brief moment every time I touch them which I guess the system interprets as some kind of a signal.

[Ast A. Moore]

then the screen goes completely black and stays that way, even after resets

After a reset, does the border always turn white (non-bright white, that is) every time, if only briefly? Not the best test for a 128K machine, but still.

The thing is, I’m not convinced yet that this is a gate array problem.

[walde123]

then the screen goes completely black and stays that way, even after resets

After a reset, does the border always turn white (non-bright white, that is) every time, if only briefly? Not the best test for a 128K machine, but still.

The thing is, I’m not convinced yet that this is a gate array problem.

Yeah the grey border always appears and never changes, only the center of the screen goes from bright white to stripy to black

[1024MAK]

The machine does not need the keyboard to be fitted. Unless you press a key, it does not even know that there is a keyboard connected...

Mark

[walde123]

Did some testing with a logic probe, not entirely sure what to look for but here's something that I found out:

The thing that seemed the most likely issue to me with my limited knowledge is the 74HCT273. The common clock (CP) pin appears to be permanently high and all the Q output pins are stuck low. The D input pins appear to have activity but according to the datasheet if I understood correctly the output pins only output based on the clock cycle, and well it seems that the clock isn't present on the chip.

So assuming I'm onto something here I'm going to guess the 74HCT273 might be bad? Considering the clock seems fine for the rest of the system.

[Ast A. Moore]

Interesting, but that’s just a flip-flop that buffers the data lines to the printer port. It gets its clock from the gate array, so if it is indeed stuck high, then the gate array is at fault.

Of course, it is also possible for the flip-flop to fail so spectacularly that it pulls that line permanently high and also does a number on the data lines. It’s safe to desolder that chip entirely and see if that makes a difference.

[walde123]

Interesting, but that’s just a flip-flop that buffers the data lines to the printer port. It gets its clock from the gate array, so if it is indeed stuck high, then the gate array is at fault.

Of course, it is also possible for the flip-flop to fail so spectacularly that it pulls that line permanently high and also does a number on the data lines. It’s safe to desolder that chip entirely and see if that makes a difference.

well, I guess I'll try that, need to get some more sockets before I try refitting it though. I was trying to locate the clock pin for the 40077 but the only 40077 schematic I was able to find seems to be for the +3 and it has 4 ground pins in one corner but I can only locate 3 ground pins scattered all over the chip in my case so it seems to me there's multiple variations but I can't find one that matches mine.

Though I'm still not sure if that clock signal would be something that would stop the machine from working entirely

[Ast A. Moore]

need to get some more sockets before I try refitting it though.

Unless you’re planning to use a printer, you can live without that IC.

I was trying to locate the clock pin for the 40077 but the only 40077 schematic I was able to find seems to be for the +3 and it has 4 ground pins in one corner but I can only locate 3 ground pins scattered all over the chip in my case so it seems to me there's multiple variations but I can't find one that matches mine.

Not sure what you’re trying to say here. All the schematics will show you the exact same pin assignments, since the actual gate array is the same, regardless of the logic board revision. And yes, the ground pins are pretty much scattered around the chip—pins 15, 40, 66, and 90—and not grouped in one corner. (They may be in the schematic, of course.)

I'm still not sure if that clock signal would be something that would stop the machine from working entirely.

You mean the one for the 74HCT273 flip-flop? No it wouldn’t. The machine would work fine without it.

[1024MAK]

The most comprehensive schematics for the +2A, +2B and +3 machines are on this site

Mark

[1024MAK]

The clock input for a 74HCT273 (as used in these machines to provide a printer port) does not work like a clock line for a CPU.
The clock input for a 74HCT273 is used only to load the flip-flops with new data. So unless a printer is in use and the computer is trying to send characters or graphic data to the printer, the clock input for the 74HCT273 will be static.

It’s unlikely that this part of the circuitry is causing your problems.

Mark

[walde123]

need to get some more sockets before I try refitting it though.

Unless you’re planning to use a printer, you can live without that IC.

I was trying to locate the clock pin for the 40077 but the only 40077 schematic I was able to find seems to be for the +3 and it has 4 ground pins in one corner but I can only locate 3 ground pins scattered all over the chip in my case so it seems to me there's multiple variations but I can't find one that matches mine.

Not sure what you’re trying to say here. All the schematics will show you the exact same pin assignments, since the actual gate array is the same, regardless of the logic board revision. And yes, the ground pins are pretty much scattered around the chip—pins 15, 40, 66, and 90—and not grouped in one corner. (They may be in the schematic, of course.)

I'm still not sure if that clock signal would be something that would stop the machine from working entirely.

You mean the one for the 74HCT273 flip-flop? No it wouldn’t. The machine would work fine without it.

Okay yeah I was just looking at the schematic thinking it had the proper pinout on it. I'll have to check tomorrow and just get rid of the flip-flop for now and do some further troubleshooting

[1024MAK]

Okay yeah I was just looking at the schematic thinking it had the proper pinout on it. I'll have to check tomorrow and just get rid of the flip-flop for now and do some further troubleshooting

Read my posts just above...

Mark

[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).