Spectrum +2 DivMMC Stopped Working

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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rdhcook
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Spectrum +2 DivMMC Stopped Working

Post by rdhcook » Mon Mar 23, 2020 12:16 am

Hi,

My Spectrum +2 (grey case) was working great with a DivMMC. Then recently it would take a couple powercycles for DivMMC to boot into ESXDos. Today I get nothing but a garbled pink screen when using it with the DivMMC.

When in this state the reset buttons on the +2 and DivMMC also don't work. When I remove the SD card the spectrum works as normal. I tried a different SD card but got the same result.


Image

[BTW: The bottom of the screen is cut off because this is USA TV]

Any pointers on how to diagnose what has gone wrong?

Thanks
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rdhcook
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Re: Spectrum +2 DivMMC Stopped Working

Post by rdhcook » Thu Mar 26, 2020 11:24 pm

I think that the thread below is very similar to my issue, which makes me think the Z80 has a faulty M1 pin. But I don't really know how to test it.

viewtopic.php?f=22&t=1152&p=16635&hilit=m1+z80#p16635

Also, can I safely install any 40pin Z80 cpu bought online?

Thanks

Rich
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Firefox
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Re: Spectrum +2 DivMMC Stopped Working

Post by Firefox » Fri Mar 27, 2020 2:44 am

Any Z80A or better (Z80B... etc) should be fine.

They're made by several manufacturers, so may have different markings to the one you have in your Spectrum.

Be careful you're ordering a Z80A CPU, not a Z80 SIO, Z80 DART, Z80 DMA, or one of the other peripheral chips made to go along with the CPU.

Should cost about a fiver.
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rdhcook
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Re: Spectrum +2 DivMMC Stopped Working

Post by rdhcook » Sat Mar 28, 2020 5:30 pm

I bought this Z80 for $5 which should arrive next week. If it works I'll let you know, if not I'll probably have more questions. :D

https://www.digikey.com/products/en?key ... 84C0006PEG
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rdhcook
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Re: Spectrum +2 DivMMC Stopped Working

Post by rdhcook » Tue Mar 31, 2020 3:07 am

The new z80 works fine but I get a similar result when the DivMMC is plugged in. :(

The image is slightly different and occasionally the reset button changes the border colour to white, black, blue.

Image

Here are the mods from the previous owner.
- Buck converter
- Sound amplifier
- Recapped
Image

Please help, Where Time Stood Still took 11minutes to load!?
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1024MAK
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Re: Spectrum +2 DivMMC Stopped Working

Post by 1024MAK » Tue Mar 31, 2020 9:03 am

Okay, a bunch of questions...

Have you cleaned the edge connector? If yes, describe how you did it.

On the female edge-connector socket on the DivMMC, is the locating peg still present?

Without the DivMMC connected, does the machine work okay with both the 128K BASIC editor (can you enter and run a simple ‘hello world’ program) and with machine code games loaded from tape (or another suitable audio source)?

With the DivMMC connected, but without the SD card inserted, does the machine work okay with both the 128K BASIC editor and with machine code games?

Do you have another suitable working ZX Spectrum to try the DivMMC on?

The reset switch on the machine resets the Z80 CPU (and also the chip that controls the banked memory). The screen picture that you posted indicates that it is likely that the Z80 CPU is either not running, or it is executing random data instead of running the code in the ROM. On power up, or immediately after the reset button is released, the Z80 CPU should run the ROM code from memory location zero (0x0000). Anything that prevents the Z80 CPU from running the ROM code could be causing this. When a DivMMC is connected, the Z80 CPU will also try to run the ROM code in this device.

The ULA in a ZX Spectrum generates a display image regardless of what the Z80 CPU is doing (or not doing), but if the Z80 CPU has not written any data to the area of memory where the screen data is located, the ULA will display the data it finds there, which will be random data. Alternatively, in some cases, if the Z80 CPU is running random code instead of the ROM code, it may trample over the screen memory area writing garbage data.

Mark
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