The Sinclair QL Forum

The Sinclair UK700 and UK1200 PSUs are unregulated types, so at anything less than full load, the output voltage will be greater than 9V. 10.5V to 11.5V with an unexpanded ZX81 connected is normal.

7805 voltage regulators typically have an output tolerance of 4% or 3%. The +5V should be within +/-5% so between 4.75V and 5.25V. So 4.85V is within spec. but often the output is closer to 5V.

Issue one boards only have two electrolytic capacitors, one across the 9V nominal supply (only used by the 7805 and the connection to the edge-connector) and the 1uF electrolytic used for the reset circuit for the Z80. Issue three boards often have a second electrolytic capacitor on the 9V nominal supply.

The only components that should run hot are the heat sink for the 7805 and an original ULA chip. Everything else should run cold, cool or warm.

I don’t think the vLA81 uses the 6.5MHz ceramic resonator. Obviously an original ULA will, but if the Z80 clock pin (pin 6) is getting a 3.25MHz clock signal, then the ULA oscillator is okay. I don’t recommend testing ULA pin 35, as even the slightest loading can affect the operation of the oscillator.

With a original ULA, a white screen often means the Z80 is not running the ROM program (RAM fault, ROM fault or Z80 fault). In this case, you will not get all the proper sync pulses.

Mark

OK, the input to the ULA on pin 35 is not looking good. I can see a small signal but it's hovering around the +5V mark. I wonder if R4 has died. This is supposed to be an 8K pull-down resistor for the clock. The clock on pin 14 is fine.

P.S. It's actually supposed to be 18K (poor scan of the schematics) and that's what it is. Not that removing it makes any difference. Of course, without a reference for how the signal should look it's tricky to determine what's right and wrong.

1024MAK wrote:With a original ULA, a white screen often means the Z80 is not running the ROM program (RAM fault, ROM fault or Z80 fault). In this case, you will not get all the proper sync pulses.

Mark

Ah, but the strange thing is that I'm getting the 'K' cursor but not the frame sync. The data and address buses are seemingly ticking along normally. I'm wondering if it's an interrupt issue.

The PIN 14 clock on the ULA is fine, as is the CPU's, which is derived from the same source.

OK, I've got it working with the original ULA.

When I reseated the CPU last night I didn't notice that the /RD pin wasn't in the socket! I'm amazed that the machine worked at all!

P.S. The vLA81 still doesn't work so I'll have to contact Charles.
P.P.S. The keyboard membrane has finally failed too much to trim. I'll have to try to find the replacement I put into a "Safe" place, where ever that is!

I managed to have a chat with Charles on Facebook and after a test he asked me to do it's confirmed that the CPLD is dead.

Anyway, it's alive with the original ULA...

Happy memories of my first computer.
I even liked the keyboard!

Cheers,
Norm.

NormanDunbar wrote:Happy memories of my first computer.
I even liked the keyboard!

Cheers,
Norm.

Yup, ZX81 was my first computer too...all the way in the US. I bought it mail order from Sinclair's Nashua, NH office. Before the pandemic I got to drive through Nashua to work several times a week all these years later :-/ I actually liked the ZX81's look a little better than the TS 1000 as Timex's nameplate substitute wasn't quite as nice as the raised red lettering.

NormanDunbar wrote:Happy memories of my first computer.
I even liked the keyboard!

Cheers,
Norm.

Mine too. Mine arrived while I was off work ill back in 1981. Helped pass the time and, via Spectrums, got me into a lifetime of QLing!

I had mine arrive when I was working in a boatyard and living on a boat at the time. My colleague and I used to hijack the waiting room TV at nights, in a Scottish Winter, with the heating off, working through the manual and learning to program.

Happy days.

Except for the cold, we used to have to get up early and break 4 inches of ice from around the boats to stop the hulls being damaged. Fibreglass is fair weather boating!


College followed, a 16K Spectrum, then a job, and finally a QL when the prices fell to 200 quid.


Cheers,
Norm.

The only niggles I have with DataServe Retro's replacement ZX81 keyboard membrane are that the keys are white rather than silver/grey and the keys are slightly too far towards the front by a mm or so, oh and it's very-very slightly (<1mm) too large back to front in places. Otherwise, it works well.