Sinclair ZX80 / ZX81 / Z88 Forums

Hello and welcome :D There is a welcome area if you want to tell other members something about yourself ;) The TS1000 is very similar to a ZX81 issue 3 board. If you browse or search the forums for ZX81 RAM upgrade or ZX81 memory upgrade, there are numerous others who have done this, and links and g...

This:

eccles wrote: ↑Mon Feb 10, 2020 11:26 pm What is the logic of the 12v/100mA test? Presumably if it trips the current limiter, then it's something on the 12V line shorting to ground?

Yes, to see if there is something else shorting out the +12V supply.

Mark

A coil with shorted turns tends to kill TR4 (as in cooks it until it goes short circuit)... What does TR4 show when tested? Or have you binned it? One other test before removing the coil. Is your current limited supply (is it a bench adjustable supply? If you have already said, sorry, please remind ...

So it does look like there is a problem with TR4, TR5 or the coil then...

Mark

Update

I can now confirm that pin 39 on the vLA module is not used (I asked the producer of the module). So the only thing a ‘scope will pick up here is noise

Mark

The ULA had been working, but suddenly quit. Could a failing capacitor in the -5V power circuit do that? Only the 4116 DRAM chips use the -5V supply. So normally a failure of the -5V supply should not affect the ULA. Unless a 4116 DRAM chip fails short circuit internally, in which case it is possib...

ULA pin 39 on a Sinclair/Ferranti chip is rather sensitive and the capacitance from a ‘scope probe can affect it. On some of the recreated modern ULA modules, often the crystal oscillator circuit pin is either unused, or used for an additional function, as these modules often have their own crystal ...

In your video, the Z80 is not running the start up code from the ROM. Before going any further, I have a bunch of questions: Which issue board is it (printed on the front edge)? What were the symptoms before you started work? What test gear do you own or have available? Have you tested all the volta...

Before you go any further with your “recapping”, you should consider bringing the DC/DC converter circuitry up to the final Sinclair version, which increases its reliability. See my blog page for details . Then concentrate on getting the voltages on the +12V and -5V rails correct (with no 4116 DRAM ...

On the 4A, remove TR4 from the board, and don’t fit any of the 4116 DRAM chips. Then power up the board and see what the current draw is now.

Mark