Fixing inherited Sinclair ZX81

[mikeyp]

So the first step is to use a multimeter to test the output voltage of the 7805 voltage regulator. Be very careful that you don’t slip and short anything out. The black (negative) meter lead/probe should go on the metal heatsink tab of the 7805. The red (positive) meter lead/probe should go on the right hand leg of the 7805. The voltage should be between 4.75V and 5.25V.

Mark

Are you sure the TV sintonizes the proper channel?
If you have a simple voltimeter check voltages at all integrated circuits, that includes the 7805 regulator that should be about 10v with the computer turned on at the input and 5v +/-10% at the output, you can check the other four ICs in the link at 1024MAK signature..
If you are not sure about the TV I suggest you to make a simple transistor composite mod that can be clamped with crocodiles to the modulator leads and GND just to have one less thing to think about during diagnosis.
Sorted this . I see all the IC are soldered so you can't easily exchange them to test with another ones.
Do you have a rampack? Using one known as good will rule out the ram as is will override de internal ram completely.
Then you can check for continuity and shorts all the D lines and A lines between ULA and Z80 and rom and RAM. Search for the schematics on line. Some of the buses have 470ohm resistors and others 1Kohm ( you won't see dirext continuity but the corresponding resistance).
All this can be done without any complicated instruments.. then you will need an oscilloscope or at least a logic probe.
There are little things to test only with a comon multimeter and with not much reliability as some of the Z80 control lines not being dead or the reset working

On the 7805:
Sinclair transformer ourput is 12.46V
Regulator output is 5.074V

The TV is an old black and white TV with manual (screwdriver) tuning. I have explored the entire range from static > black > static again. The Spectrum uses the same tuning frequency and does output an image across the range so I can use that as a refernce for behaviour. When I power off the ZX81 it returns to static.

The Z80:
Between pin 11 and 29 is 5.042V

The ULA:
Between pin 40 and 34 is 5.044V

D2364C ROM:
Between pin 24 and 12 is 5.036V

2114L2 RAM Chips:
Between pin 18 and 9 is 5.033V on both

I do have a RAM pack but all is unknown status as I have never seen it working. When plugged in, the red LED on the RAM pack turns on but the screen still turns black. Not hugely helpful sorry.

I'm out of time for now but will check the D and A lines later and will report back. Thank you again.

[mikeyp]

I found a schematic, probed with my multimeter and all the D and A lines check out. Checked the A lines to the ROM also. I probed the other lines between the Z80 and the ULA. They are ok too.

[1024MAK]

A good schematic is here. Don’t worry about the French TV Video circuitry, that does not affect the operation of the digital part of the system.

Okay, so things we now know:
The input supply is fine. The 7805 voltage regulator and the +5V rail is fine. Power is getting to all of the chips. Using an external RAM expansion makes no difference to the symptoms. You get a completely black image on the TV which turns to static when you power down the ZX81. All the chips on your board are directly soldered in, so even if you had replacements you currently can’t swap them over.

Ideally now we would switch to using a logic probe, logic analyser or an oscilloscope. You have already said that you don’t have a ‘scope, and they cost a lot of money (the cheap types are not of a high enough specification to be useful) so not worthwhile getting just to try to repair one item. However, logic probes typically cost around £10 to £15. And logic analysers are also reasonable affordable (you want one that has at least sixteen channels/inputs).

It is also possible to make your own very cheap logic probe for only the price of a pint or two. Ask if you want details.

However we can still use a multimeter to get more information. Note that we are primarily looking for activity. Most of these signals should be changing at a rate far too fast for the meter to display (and even if it could, your eyes and brain would be too slow to read it), so the meter will display a kind of average voltage. Unfortunately with only a meter, it’s not always possible to tell if the voltage is actually an invalid logic level (‘stuck’ somewhere between 2V and 0.8V) or the meter averaging out a fast changing signal that is alternating quickly between valid logic levels (logic one/logic high should be between 2V and 5V, logic zero/logic low should be between 0V and 0.8V).

Note: do all the following tests without any expansions fitted and with the machine powered on.
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.
To help, I give the expected result if you were using a logic probe or a ‘scope. Where a signal name has a slash (/) in front of it, this means that it is active low. That is, normally it is logic high, but pulses low to signal a condition. Pulsing signals will normally show on a meter as a voltage between 2.5V and 5V.

What voltages do you get on the following pins of the Z80 CPU?

Pin Function
6 Clock input (3.25MHz ‘square’ wave)
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 continuously high)
26 /RESET (should be continuously high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)
4 A14 Address line A14 (should be pulsing)
5 A15 Address line A15 (should be pulsing)

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.

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

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

Mark

[mikeyp]

Thank you again for your help here. I'd be grateful if you could recommend a logic probe or a logic analyser. Happy with 2nd hand on auction sites if need be. What sort of money would I be looking to spend on a suitable oscilloscope? I don't mind spending money on things which will get used in future. Being able to diagnose things like this is something I'm interested in.

Here are the measured voltages on the Z80 requested:
Pin Function
6 Clock input (3.25MHz ‘square’ wave) - 5.073V
16 /INT (should be pulsing) - 4.697V
17 /NMI (If the NMI generator is on, this will be pulsing) - 0.169V
18 /HALT (should be pulsing) - 3.673V
19 /MREQ (should be pulsing rapidly) - 0.293V
21 /RD (should be pulsing rapidly) - 0.169V
22 /WR (should be pulsing) - 0.539V
24 /WAIT (should be pulsing) - 0.320V
25 /BUSRQ (should be continuously high) - 5.065V
26 /RESET (should be continuously high, except for a very short time after power is applied) - 4.964V
27 /M1 (should be pulsing) - 4.999V
28 /RFSH (should be pulsing) - 4.995V
4 A14 Address line A14 (should be pulsing) - 2.305V
5 A15 Address line A15 (should be pulsing) - 2.215V

Oh, I also checked the 3 (original) capacitors with an ESR meter and they seem good.

[1024MAK]

Hi Michael

Sorry for the delay in my reply. Too many fingers in too many pies... plus work, DIY and real life...

With respect to the voltages, the clock to the Z80 on pin 6 does not look good. If the Z80 does not get a good clock, the various other pins on it do strange things. And indeed, that’s what some of them are showing, strange results.

The next step is to test ULA pin 14 and see what you get here. Also visually check ULA pin 35, the 6.5MHz ceramic filter (X1), R4 and C6. You are looking to see if they have any physical damage, for any short circuits, for poor or dry jointed solder joints, and for any broken PCB tracks. These passive components are all between the Z80 and the rear of the PCB. There may also be an extra resistor soldered on top of C6.

Mark

[mikeyp]

Please don't apologise for the delay, I'm really grateful for your time and advice and I must just be patient! I too have family and work and other things that must take priority.

ULA pin 14 is showing 0.111V

Visual and continuity checks around pin35, the ceramic filter, transistor, resistors and capacitors are ok. Quality of the soldering is absolutely terrible but as I said, continuity checks are ok. Everything looks in good condition and there is indeed an extra 1K resistor on C6.

UPS are supposed to deliver my logic probe tomorrow so I will revisit the above measurements with that.

Thank you again!

[mikeyp]

Logic probe has arrived. Results as follows. Not great.

Z80
6 High
16 High
17 Low
18 High
19 Floating
21 Low
22 Floating
24 Low
25 High
26 High
27 High
28 High
4 Floating
5 High

ULA
14 Low
35 Low

Nothing pulsing

[1024MAK]

Pin 35 on the ULA is not a normal logic level input or output pin, so that result is not surprising (also most test gear will load it and hence affect its operation, that’s why you will never see me ask for any one to test it).

I think your ULA is dead

Two last things, first, also test ULA pin 16. This is a output pin that can produce one of three voltages, again not a normal logic level output. See what you get with your meter and your logic probe.

Then, resolder all the ULA pins, especially pins 35 and 40. Also the legs of X1, R4 and C6. Just in case. If that does not work, then a replacement ULA or replacement ULA module will be needed.

One more thought, how warm or hot does the ULA get? Let it warm up for say ten minutes. Then use your finger (place in the very middle of the chip) and count until you feel pain...

Mark

[mrtinb]

Then use your finger (place in the very middle of the chip) and count until you feel pain...

And then go to the hospital to get your finger fixed

[mikeyp]

Sadly I think you're right.

Pin 16 3.093V Logic High

I have resoldered all ULA pins, X1, R4, C6. No change. The only pulsing is Z80 pin 12 and ULA pin 28, as before. I suspect something is spitting out garbage considering nothing else is active.

The ULA is hot to the touch after 10-30 minutes. It's unpleasant to keep the back of my finger on there more than 5 seconds.

I think a vla81 and a socket is called for...