TS1000 Diagnostic Help

[DeLorean_4]

Hi everyone,

I plugged in a powered TS2040 printer (Alphacom 32) to an unpowered TS1000 and am now greeted with a white screen. A printer at idle sends 5v at the 7th and 8th pins from the top right side of the edge connector on the computer.
These correspond to RD and WR. I imagine one of the neighboring pins must have gotten 5v as the printer connector was being hooked up.

For the first day following the incident, I was able to get about 20 seconds of fully functional life every minute or so, but now it's a white screen 100% of the time.

My TS1000 has a ZX81 issue 3 board and was previously working perfectly well with a vLA81.

I probed some critical pinouts and made the following observations on the Z80 (I swapped with a spare and there was no difference):

- INT signal is presumed low (noise between 0-0.2v)
- NMI signal is presumed high (noise between 3.3-4.2v)
- HALT signal is presumed high (noise between 3.6-4.4v)

From my research, it would seem that either my ULA, ROM, or RAM could be bad (Not very specific... I know). Would anyone here have any advice on how to narrow down the search?

I also ran a quick experiment with all combinations of ROM and RAM installed and removed. INT, NMI, and HALT were all the same as before (low, high, high). The only difference was in the picture quality of the blank screen.

Removing the vLA81 resulted in very clean INT/NMI/HALT high signals.

The voltages from my stock regulator seem to be within acceptable range: 11.3v, 0v, 5.15v
I get a clock signal from my CPU

[DeLorean_4]

Some more info:
CE on my RAM chip is high, which means it's on standby which makes sense since the CPU is outputting a HALT signal.

CS1 (Chip select) on the ROM is oscillating. They look like square waves on a larger time scale, but up close are more like an over-damped fall followed by a less-damped rise with some overlap. I don't quite understand this output. I would have thought it would output a stable logical high or low signal.

Looking at the datasheets of the various chips makes me think they should have withstood the 5v from the printer for the brief instant it was being connected. My TS1000 was not powered when it got zapped, so it's not like I could have shorted any of the edge connector contacts with 11.3v from the PSU (which would have certainly fried something). I'm starting to really think the problem is with the vLA81.

Can anyone think of any reasons I'd be wrong? Any more tests I could run?

[msknight]

Do you have any chips to swap out and test with?

[msknight]

I put a spreadsheet together of the pin outs/responses that people advised should be. It's on the page here - https://msknight.com/bbc/zx81-logictest.html - in Open Doc format. That might help.

[1024MAK]

All the control signals are active low, including /INT, /NMI, and /HALT.
Hence their normal inactive state is logic high.

With all waveforms in a digital computer, the edges (rise and fall) will not be nice and clean due to stray capacitance, stray inductance, limitations with the output drivers in the chips and maybe the bandwidth of your 'scope and maybe your probe (are you using the X10 setting?).

A Z80 when it encounters a HALT instruction, will continually re-read the same memory address until either an interrupt is received (for /INT they have to be enabled) or the processor is reset.

I do think it is most likely that your vLA81 may be damaged.

The problem is that when the printer was powered, when the edge-connector was being mated, the pins at 5V may have made contact before the 0V/GND. If some other pins were also making contact, current could have flowed via the internal semiconductor junctions in the vLA81 damaging them.

Mark

[Moggy]

Last time I ruined something in this manner was because the locater tab was missing from the interface I was using and so when coupled up to the zeddy was a space or two offset from the norm, and ended up frying my ULA. Does the Alphacom have its locater tab in place?

Just a thought.

[DeLorean_4]

Do you have any chips to swap out and test with?

No. I really wish I did though : )

I put a spreadsheet together of the pin outs/responses that people advised should be. It's on the page here - https://msknight.com/bbc/zx81-logictest.html - in Open Doc format. That might help.

Thank you! That document is terrific. I'll take a harder look at it tonight and tomorrow and compare it to what my machine churns out.

With all waveforms in a digital computer, the edges (rise and fall) will not be nice and clean due to stray capacitance, stray inductance, limitations with the output drivers in the chips and maybe the bandwidth of your 'scope and maybe your probe (are you using the X10 setting?).

Yes, my scope and probe are at 10x.

Last time I ruined something in this manner was because the locater tab was missing from the interface I was using and so when coupled up to the zeddy was a space or two offset from the norm, and ended up frying my ULA. Does the Alphacom have its locater tab in place?

Yes, it still has the locator. I think I started inserting the connector by first putting in the outboard end and inserting the remainder in an arc but with slighly incorrect alignment.

[msknight]

The ULA is, unfortunately, the most common thing to die, at least partially. Mark also thinks it's likely what's failed in yours.

vRetro sells modern replacements - https://www.vretrodesign.com/products/v ... eplacement

If you're in the UK they are available here - https://www.retrorevivalshop.co.uk/prod ... placement/

I've replaced them in mine, because of their tendency to just give up; and because they were only made for the ZX81, getting another second hand one won't likely gain much.

[1024MAK]

Question: apart from the ULA/vLA81, are any of the other chips in sockets?

Do you have another TS1000 or ZX81? If yes, which chips (if any) are in sockets?

Do you have any other computers based on / using a Z80A?

Do you have an electronic breadboard, some modern LEDs, thin solid core wire (off cuts of telephone or network cable for example) and a selection of resistors? If yes, a very basic test circuit for the Z80A processor can be built, called a NOP tester.

Do you have a RAM pack? When this is plugged in, it overrides the internal RAM.

Mark

[Moggy]

The ULA is, unfortunately, the most common thing to die, at least partially. Mark also thinks it's likely what's failed in yours.

vRetro sells modern replacements - https://www.vretrodesign.com/products/v ... eplacement

If you're in the UK they are available here - https://www.retrorevivalshop.co.uk/prod ... placement/

I've replaced them in mine, because of their tendency to just give up; and because they were only made for the ZX81, getting another second hand one won't likely gain much.

Just my experience others may differ.

To be fair Michelle I've never known the original ULAs to just give up or even fail due to the much vaunted yet unproven myth of "heat damage" , rather they bite the bullet due to human intervention such as plug/unplugging an interface when the Zeddy is powered up, shorting them out whilst working on a powered up board etc or an already faulty interface innocently plugged in taking them out.
Duff Memotech RAM modules were a bugbear in my case.

My Zeddies are used daily some of which have the same ULAs since I got them over thirty years ago, and Iv'e also found that the newer replacement types are far less hardy than their Ferranti counterparts and take less to destroy them than the originals.

It's a shame Andy Rea is no longer making his versions which at £15 a pop were good value, next time I see him I might just give him a prod.