FIXED Spectrum +2 grey No Z80 M1 when Z80 M1 is good

[Rabs]

In case this helps anyone, as it certainly confused me for a while (which is not difficult ).

Working on another Spectrum +2 grey with a number of bad RAM ICs, a bad multiplexor and a bad TR9.

Anyway that is not what confused me.

What confused me was that I was not seeing a good signal on the Z80 M1 pin.

I know that some Z80s have a fault on the M1 signal and hence I guess this is why the Retroleum X-ROM Board has an LED for M1 activity (invaluable test tool by the way ).

But, even when I replaced the Z80 with a known good CMOS Z80, I still had no M1 activity.

Turned out that D16 had also failed (my multifunction tester thought it was an inductor), so not only was the Z80 not providing an M1 signal but D16 was also pulling M1 high.

So a double fault. Anyway just incase this helps anyone, if you have a Z80 with a known good M1 signal and still see no M1 activity, check D16 and D17.

A Z80 with no M1 but also a faulty D16.

M1.JPG (13.87 KiB) Viewed 4593 times

A Z80 with M1 activity and a good D16.

[1024MAK]

Well done with the fault finding and repairs

The purpose of D16 and D17 is to try to protect the Z80A...

For D16 to die likely means an interface/expansion was connected/disconnected on the edge connector while the power was on.

Mark

[Rabs]

For D16 to die likely means an interface/expansion was connected/disconnected on the edge connector while the power was on.

Mark

Thanks and interesting info.

I guess this is quite a common fault then?

[1024MAK]

On the adjacent edge connector pin to the pin for the /M1 line is a "12V AC" supply pin. This is not really a normal 12V AC supply, but rather, the rather large voltage pulses from "the coil" in the DC/DC converter circuitry. As such, it's far more dangerous to 5V semiconductors than people think!

Now, I don't actually know how many machines get damaged by this. And I can't prove it.

The vast majority of interfaces/expansions don't use the /M1 line. And none of the circuitry in a Spectrum uses it either. Hence the damage can occur without being noticed. Only when the current owner tries to use an interface/expansion that needs a working /M1 line is a problem noticed.

What I can tell you is that to the best of my knowledge, more ZX Spectrum computers are found to have Z80A chips with non-functional /M1 lines than any other Z80 based home computer. The repair workshops back in the 1980s often found this problem with ZX Spectrum computers. Note that the earlier models did not have the two diodes on the /M1 line.

Mark