ZX81 - White Screen No Cursor

Discussions about Sinclair ZX80 and ZX81 Hardware
Moggy
Posts: 1759
Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 - White Screen No Cursor

Post by Moggy » Tue Mar 26, 2019 3:40 pm

It's a 16k pack, but if the chips are being run out of spec I may just turn the juice up.

I only use this pack on my forth machine but as I have found a way to use the xpand+ with h4th I may switch permanently to that, which I run at 7.5v as the 2 regs on the rear of the xpand glow in the dark when used with the Sinclair pack but are luke warm at 7.5v. :lol:

ZeddyRepair
Posts: 12
Joined: Sun Mar 24, 2019 1:26 pm

Re: ZX81 - White Screen No Cursor

Post by ZeddyRepair » Tue Mar 26, 2019 7:19 pm

So Ive already bid on and won an offical zx81 rampack before reading your post :-/ ...lets hope its ok.

Ive tested the requested pins now 3 times powering off the zx81 inbetween each test. Test 1 & 3 produced the same readings Test 2 produced diffent on 4 of the pins i think.

ZX80 Chip Test 1:

Pin 4 - A14
T1_4.jpg
Pin 5 - A15
T1_5.jpg
Pin 16 - INT
T1_16.jpg
Pin 17 - NMI
T1_17.jpg
Pin 18 - HALT
T1_18.jpg

ZeddyRepair
Posts: 12
Joined: Sun Mar 24, 2019 1:26 pm

Re: ZX81 - White Screen No Cursor

Post by ZeddyRepair » Tue Mar 26, 2019 7:21 pm

Pin 19 - MREQ
T1_19.jpg
Pin 21 - RD
T1_21.jpg
Pin 22 - WR
T1_22.jpg
Pin 24 - WAIT
T1_24.jpg
Pin 25 - BUSRQ
T1_25.jpg

ZeddyRepair
Posts: 12
Joined: Sun Mar 24, 2019 1:26 pm

Re: ZX81 - White Screen No Cursor

Post by ZeddyRepair » Tue Mar 26, 2019 7:23 pm

Pin 26 - RESET
T1_26.jpg
Pin 27 - M1
T1_27.jpg
Pin 28 - RFSH
T1_28.jpg

ZeddyRepair
Posts: 12
Joined: Sun Mar 24, 2019 1:26 pm

Re: ZX81 - White Screen No Cursor

Post by ZeddyRepair » Tue Mar 26, 2019 7:27 pm

TEST 2 produced same on all other pins apart from the ones bellow

Pin 4 - A14
T2_4.jpg
Pin 5 - A15
T2_5.jpg
Pin 17 - NMI
T2_17.jpg
Pin 18 - HALT
T2_18.jpg

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: ZX81 - White Screen No Cursor

Post by 1024MAK » Tue Mar 26, 2019 11:26 pm

I’ve had a brief look at the waveforms you posted. Apart from the signals on A14 and A15, the Z80 looks like it may be okay.

Mark

ZeddyRepair
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Joined: Sun Mar 24, 2019 1:26 pm

Re: ZX81 - White Screen No Cursor

Post by ZeddyRepair » Wed Mar 27, 2019 8:39 pm

Kind of good news then i guess. I did think the A14 & A15 lines looked a bit strange. Ill assume the Z80 is good and wait for the ram pack to test further.

ZeddyRepair
Posts: 12
Joined: Sun Mar 24, 2019 1:26 pm

Re: ZX81 - White Screen No Cursor

Post by ZeddyRepair » Sun Mar 31, 2019 10:33 pm

So the Ram Pack arrived and is now fitted. The screen now shows more than just a white screen as pictured below, although sometimes it still just shows the white screen. It looks almost like its showing the cursor but its just repeated and scrabled.
Screen.jpg

ZeddyRepair
Posts: 12
Joined: Sun Mar 24, 2019 1:26 pm

Re: ZX81 - White Screen No Cursor

Post by ZeddyRepair » Tue Apr 09, 2019 11:40 pm

Update - I have managed to buy another working issue 3 zx81. I have tested the rampack on this and this also appears to work correctly giving a longer white screen before the cursor appears when it is connected and powered on.

So im guessing since the rampack didnt fix my issue the memory is good & from the Z80 traces that i took with the scope that this is good.

I did check the capacitors on the new board and compared to the non working board. I noticed Capacitor C5 on the non working has a much higher esr reading than the good board. Could this cause my issue?

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: ZX81 - White Screen No Cursor

Post by 1024MAK » Wed Apr 10, 2019 1:57 pm

There are two electrolytic capacitors on a ZX81 / TS1000 board. One is across the nominal 9V line. As long as the 9V PSU has a good electrolytic capacitor, this capacitor is not essential.

The second electrolytic capacitor is part of the reset circuit. If the /RESET pin on the Z80 CPU goes low to high at power on (it’s a very quick transition), it’s fine.

Mark

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