TS1000 white screen but no K

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junkman
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TS1000 white screen but no K

Post by junkman »

Hello all,

I am new to the Sincair world. I have a little TS1000 that is in great shape and as far as I can tell never opened. I checked some voltages around the board and all seems OK. I will keep looking.
The Z80a seems alive and is counting up on all its address lines. I have what I think is a normal looking video signal on the ULA pin 16. See attached at 10us.
ULA pin 16
ULA pin 16
I have tried plugging in the 16 ram pack and still no K. I tried hooking to a modern TV and a real 13" black and white 1970s GE CRT analog set. No K.

I started poking around with my Oscope and I noticed this weird (to me) signal on D0,1, and 7. The highs seem to be pulsing between 4 and 5 volts. Here is a youtube video of it. https://www.youtube.com/watch?v=3uUvFVL5b_Y
Unfortunatly I do not have another machine to swap chips with. I only have a Z80 (not A) as a spare CPU and don't think it would be safe to put in since the clock lower. I will hunt for a real Z80A in the meantime.

Here is my board.
IMG_1846.JPG
Any input is welcome. Thanks!
junkman
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Re: TS1000 white screen but no K

Post by junkman »

I dug around past topics and learned much.. I found the re-drawn schematic by Ron Reuter. I have some more info to bring to the table

I tried doing the SAVE "P" with a soundblaster speaker hooked up to the MIC port and fastmode commands blind. No signs of life.

I see the Clock coming off 14 of the ULA. I see the clock on the Z80. If I put my Oscope 1X probe on Xtal pin 35 of the ULA the screen goes out of sync.

I ohmed out the Address and Data bus with a DVOM at the CPU and edge card connector. I found no shorts to ground.

I swapped in a known working Z80A and the system acted the same.

The weird 2 level signal is on the Z80/ULA side of the data bus. If I sample on the RAM/ROM side of R9-R14 I only see the taller signal. My guess is something on the RAM/ROM side is pulling the data bus to 0 and that is why I am seeing 2 different levels on the CPU/ULA side?

Thanks

EDIT:
I desoldered the ram chip. The machine acts exactly the same and is still doing the same pulsing on the data lines I posted in the youtube video. I am thinking maybe bad ROM :?
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1024MAK
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Re: TS1000 white screen but no K

Post by 1024MAK »

A white screen often is due to non-functional RAM. But can be caused by any of the chips failing in a ZX81 or TS1000.

As you have tried an external RAM pack and have changed the Z80A CPU, hopefully these have been eliminated.

Before jumping in and desoldering anything else, what signals do you get on the following pins of the Z80 CPU?
Note that these tests assume that there is RAM present in the system.
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
16 /INT (should be pulsing)
17 /NMI (If the NMI generator is on, this will be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.

Next, test the /CS (also known as /S or/CE ) pin on the RAM chip. For the 2K SRAM chip, this is pins 18 & 20 (this is the actual chip pin numbering, not by counting the PCB pads, they are not the same as the PCB can take a 28 pin device). This does not apply if an external RAM pack is plugged in.

Test also the ROM chip /CS pin, this is pin 20 on the ROM chip (again, not by counting the PCB pads).

Z80, RAM, and ROM pin-outs are here.

A good schematic is here (but shows French TV output circuitry).

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
junkman
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Re: TS1000 white screen but no K

Post by junkman »

Thank you for the detailed reply. I will go through and note each of these pins.

In the mean time this is PIN 13 of the ROM chip. https://www.youtube.com/watch?v=NlXft5Q3ex8&
0-3.8 volts. This is in real time like the above video, Its not a memory scope. It is like the machine is looping.

I took apart RAM pack and checked the voltages. They all seem normal.
Will the ZX81 work with out the internal RAM chip and only the RAM pack?

I will report back with my finding on the pins you asked for. :arrow:
Mustermann
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Re: TS1000 white screen but no K

Post by Mustermann »

External RAM pack usually disable internal RAM by pulling /RAMCS to high.
So I would expect no difference if internal RAM is missing and external RAM is attached.

Max
ZX81 issue 1 near to original state
ZX81 issue 1 56k internal ram with battery backup, UDG, CHR$128 and WRX enabled
Minstrel 3 ZX81 clone 64k with battery backup, UDG, CHR$128 and WRX enabled
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1024MAK
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Re: TS1000 white screen but no K

Post by 1024MAK »

The design of RAM pack that could expand the RAM to 2K, 3K or 4K uses the internal RAM. 16K and above RAM packs normally just disable the internal RAM. As far as the machine is concerned, there is no difference between a disabled internal RAM and a missing internal RAM if a 16K (or above) RAM pack is plugged in.

And I have a ZX81 that has no internal RAM chip, it works fine when I connect a RAM pack to it.

Do however ensure that the edge-connector contacts are clean.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
junkman
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Re: TS1000 white screen but no K

Post by junkman »

Pin Function
16 /INT (should be pulsing) -CHECK
17 /NMI (If the NMI generator is on, this will be pulsing) -CHECK

18 /HALT (should be pulsing) No its mostly staying high as far as I can tell but my scope is old and slow
This is at 10us 1/div
HALT pin
HALT pin


19 /MREQ (should be pulsing rapidly) -CHECK
21 /RD (should be pulsing rapidly) -CHECK
22 /WR (should be pulsing) -CHECK
24 /WAIT (should be pulsing) -CHECK
25 /BUSRQ (should be high) -CHECK
26 /RESET (should be high, except for a very short time after power is applied) -CHECK. Also unplug power and you can see it smoothly fall because of the capacitor.
27 /M1 (should be pulsing) -CHECK
28 /RFSH (should be pulsing) -CHECK

A14 is going pulse/low at the same rate I am seeing these other weird signals. Unfortunately I can't run a dual trace because my CH2 on my scope decided to die today.
A15 is going high/low at half the speed.

ROM /CS pulsing/stoping at the same rate as A14

RAM CS/
This is at 20us 1v/div . https://youtu.be/XJgSFefhvf4
Again looks like it is at the same rate as 14
Last edited by junkman on Sat Feb 22, 2020 10:01 pm, edited 1 time in total.
junkman
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Re: TS1000 white screen but no K

Post by junkman »

And here is A14
10 uS 1v/div
https://youtu.be/fWbaPbRgbFE

Thanks
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1024MAK
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Re: TS1000 white screen but no K

Post by 1024MAK »

If A13 is pulsing at twice the speed of A14, then that is a good indication that the Z80 is not running the code from the ROM. So either the ROM chip is bad, or there is a problem with the address bus, or the data bus.

As well as testing the address and data bus lines to 0V/GND, it’s also worthwhile inspecting the board (both sides) for anything that can short out adjacent PCB tracks/pads. Test also the address and data bus lines to the +5V/VCC rail, to any adjacent tracks/pads and continuity between the Z80 / ULA and the relevant RAM, ROM chip pins / expansion (edge-connector) contacts.

If you still can’t find any cause, then it would appear that your ROM chip has died. It’s hard to get hold of Sinclair mask ROM chips now. You have now have a choice...
Either using a 27 series EPROM (2764 or 27C64) and cutting tracks or bending pins to adjust the wiring, (this will need programming in a EPROM programmer, or ask for someone to program (burn it) for you,
Buying a “ROM EL-2364 FLASH EPROM Chips (replacement for 2364 ROM)”,
Or hunting for a pin compatible EPROM chip. If you do the latter, you also need to find a EPROM programmer that can cope with these old chips.

Although the videos and photos of the signal traces may not look particularly good, from what I can tell, all have been within valid TTL logic levels (logic low is 0.8V or below, logic high is between 2V and 5V). If the ULA thinks the Z80 is getting screen data, it will override the data bus on the CPU side and drive all eight data bus lines to 0V (which is the Z80’s NOP instruction code).

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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1024MAK
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Re: TS1000 white screen but no K

Post by 1024MAK »

Oh, I forgot to say, if the Z80 CPU never executes a HALT instruction, the HALT pin will stay high. The code in the ROM does contain a HALT instruction.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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