Resurrecting a ZX81 - Step by step help appreciated.

Discussions about Sinclair ZX80 and ZX81 Hardware
rune
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Resurrecting a ZX81 - Step by step help appreciated.

Post by rune »

Hi guys,

Im looking to try and resurrect 3 ZX 81 boards - 2xIssue 1 and an Issue3. I have an oscilloscope, a decent multimeter and a bit of time to do this now.

All 3 boards have had the RAM desoldered and a 32K RAM socket added. At the moment I'm testing using a Sinclair RAM Pack. I have two boards with Pokemons CCB composite card attached, the other will have one added soon.

Attaching the scope to the video out I can adjust the CCB card and get a nice back porch displayed, but there doesnt seem to be any data shown (K cursor) on the trace and the screen remains white. I'll add images tomorrow.

The voltage at the regulator is 5V. What would the next step/thing to check?
Moggy
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Re: Resurrecting a ZX81 - Step by step help appreciated.

Post by Moggy »

Help is at hand so please hang on Rune whilst I use the bat phone.. :D

MAARRRKKK!!!!
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1024MAK
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Re: Resurrecting a ZX81 - Step by step help appreciated.

Post by 1024MAK »

Okay, bear with me while I step through the basics...

When you say that the RAM has been desoldered and a 32k socket fitted, are these sockets empty? Have you altered the PCB ready for a 32k byte SRAM chip? Which scheme and guides did you use?

Is the Sinclair 16k RAM pack a known working one?

Have you cleaned and inspected the edge connectors?

Is the PSU a known good one?

Do you have a good clock signal on pin 6 of the Z80 CPU?

Mark
rune
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Re: Resurrecting a ZX81 - Step by step help appreciated.

Post by rune »

Hi Mark

At least two of these boards did work at one point, but with the usual problems - cracked membranes, old ULA (very dark picture). I had other ZX81 that did work so I concentrated on those. Now its playtime and I'd like to try getting these working again.

The PSU is good. I got the diodes changed and it runs my other ZX81s OK.

The RAM pack did work and looks like its brand new. But there is a chance that its gone, so tomorrow I'll try using one of the 2 others I have that work.

I haven't cleaned the edge connectors but they look OK. I'll clean it and check the clock signal on the Z80 next (tomorrow).

I haven't altered the board yet for the new RAM and the 32K socket is empty. I plan to use the scheme that I used with one of the other ZX81s, but instead of lifting pins out the socket, I'll cut the tracks. But the first thing I think, is to get a cursor on screen using the CCB board.

Pics will be posted.

Cheers,

Dave
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amigaman07
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Location: Newcastle Australia

Re: Resurrecting a ZX81 - Step by step help appreciated.

Post by amigaman07 »

From my ZX Spectrum Experiences, it is Likely to be a DEAD or Very Sick ULA!

Always the 2nd thing to Die on a ZX Speccy, after the Low or High RAM!

Try the DUFF ULA in a Good Known Working ZX81.

(Not the other way round!)

Don't put your Good ULA in the Dead Machine, there may be a Reason your ULA Popped (if this is the Fault) and I would hate to see you Kill your Good ULA!!! :?

All the Best.

Darren (amigman07)
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amigaman07
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Location: Newcastle Australia

Re: Resurrecting a ZX81 - Step by step help appreciated.

Post by amigaman07 »

Oh, RE the Plug in RAM Packs.

These can seriously get the Wabbles, and can Kill ZX81's with the Bad Contacts.

A Good Trick is a Strip of Velcro Tape on the Back of your ZX81, and Plug Home the RAM Pack. When the Other side of the Velcro Sticks to the RAM Pack, it makes a Great STABLE Connection, no more Wabble! :lol: ;)

Darren (amigaman07)
rune
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Re: Resurrecting a ZX81 - Step by step help appreciated.

Post by rune »

Cheers Darren, I've tried another brand new RAM pack. No difference - Also tested across 3 ZX81s.

Hi Mark, heres some results of testing. Apologies for it being so long, but I tested all three boards.

There are two issue 1s and an Issue 3. I tested Pin 14 & 16 on the ULA and Pin 6 on the Z80 plus all 5v and 0v pins.

The second Issue 1 had a strange output on Pin 14 so I swapped ULAs and got similar results to the others. The first Issue 1 had a similar output on ULA Pin 14 and Z80- Pin 6. So Im wondering if I've made a mistake with the photo.

Can you look at the images and tell if anything is wrong/correct, which board shows most promise for a repair and where to go next?

Results are below

Issue 1(a)

Voltage Regulator
5V 4.88V

ULA
Pin 14 (CPU) Image
Pin 16 (TV) Image
Pin 34 (OV) 7mv
Pin 40 (5V 4.88V

Z80
Pin 6 Image
Pin 11 (5V) 4.88V
Pin 29 (0V) 3mv

ROM
Pin 12 (0V) .5mv
Pin 24 (5V) 4.86V

RAM
Pin 14 (0V) 2mv
Pin 28 (5V) 4.88V



Issue 1(b)


Voltage Regulator
5V 5.00V

ULA
Pin 14 (CPU) Image
Pin 16 (TV) Image

Changed ULA
Pin 14 (CPU) Image
Pin 16 (TV) Image

Pin 34 (OV) 0mv
Pin 40 (5V 5.00V

Z80
Pin 6 Image
Pin 11 (5V) 5.00V
Pin 29 (0V) 0V

ROM
Pin 12 (0V) 0V
Pin 24 (5V) 5.00V

RAM
Pin 14 (0V) 0V
Pin 28 (5V) 5.00V


Issue 3

Voltage Regulator
5V 5.01V

ULA
Pin 14 (CPU) Image
Pin 16 (TV) Image
Pin 34 (OV) 7mv
Pin 40 (5V 5.01V

Z80
Pin 6 Image
Pin 11 (5V) 5.01V
Pin 29 (0V) 7mv

ROM
Pin 12 (0V) 2mv
Pin 24 (5V) 5.01V

RAM
Pin 14 (0V) .5mv
Pin 28 (5V) 5.01V
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1024MAK
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Re: Resurrecting a ZX81 - Step by step help appreciated.

Post by 1024MAK »

So here are some results from two different working issue 1 boards.
ZX81 issue 1, ULA2C184E, ULA & CPU clocks.png
ZX81 issue 1, ULA2C184E, ULA & CPU clocks
(59.23 KiB) Downloaded 410 times
ZX81 issue 1, ULA2C210E, ULA & CPU clocks.png
ZX81 issue 1, ULA2C210E, ULA & CPU clocks
(53.93 KiB) Downloaded 410 times
Note that waveforms can look rather different, depending on the settings of the 'scope...
ZX81 issue 1, ULA2C184E CPU clock pin 6.png
ZX81 issue 1, ULA2C184E CPU clock pin 6
(36.88 KiB) Downloaded 410 times
ZX81 issue 1, ULA2C184E, ULA pin 16 & modulator input - sync pulse, DC range.png
ZX81 issue 1, ULA2C184E, ULA pin 16 & modulator input - sync pulse, DC range
(40.14 KiB) Downloaded 410 times
ZX81 issue 1, ULA2C184E, ULA pin 16 & modulator input - sync pulse, AC range.png
ZX81 issue 1, ULA2C184E, ULA pin 16 & modulator input - sync pulse, AC range
(44.68 KiB) Downloaded 410 times
ZX81 issue 1, ULA2C210E, ULA pin 16 & modulator input - sync pulse, DC range.png
ZX81 issue 1, ULA2C210E, ULA pin 16 & modulator input - sync pulse, DC range
(46.97 KiB) Downloaded 410 times
ZX81 issue 1, ULA2C210E, ULA pin 16 & modulator input - sync pulse, AC range.png
ZX81 issue 1, ULA2C210E, ULA pin 16 & modulator input - sync pulse, AC range
(45.28 KiB) Downloaded 410 times
ZX81 issue 1, ULA2C210E, A14 address line, measured on D7 cathode.png
ZX81 issue 1, ULA2C210E, A14 address line, measured on D7 cathode
(44.24 KiB) Downloaded 410 times
ZX81 issue 1, ULA2C210E, A15 address line, measured on D8 cathode.png
ZX81 issue 1, ULA2C210E, A15 address line, measured on D8 cathode.png
(44.62 KiB) Downloaded 410 times
ZX81 issue 1, ULA2C210E, Z80 WAIT pin.png
ZX81 issue 1, ULA2C210E, Z80 WAIT pin
(42.92 KiB) Downloaded 410 times
If you have hook type test probes, you can carefully fit them to some of the resistor, capacitor or diode leads:-
Image
Image
Image
Just be very careful that you don't short anything out. Especially nearby conductors, or to the grounded metal case of the modulator...

Also watch out that the dog does not run off with the spring hook type test probes and chew the end making it useless... :o :twisted:

Mark
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1024MAK
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Re: Resurrecting a ZX81 - Step by step help appreciated.

Post by 1024MAK »

Hi Dave

First, all the DC supply voltages are within specification. For the +5V supply it should be within +/-5% or better. The minimum is 4.75V and the maximum is 5.25V.

The photos of the waveforms on your 'scope would have been more useful if you had included the range settings (volts/div and timebase setting). In my waveform pictures, because I am using a digital 'scope, you can see the settings used (this 'scope can be, and is, configured to compensate for the X10 probes, so no need to do any mental arithmetic).

Also are you using a normal 'scope probe or a switchable type set to the "X10" setting? If you have switchable probes that have a "X10" setting, please use it. You will need to calibrate the probe if you have not already done this. Also, when set to the "X10" mode, the waveform voltage displayed on the 'scope will be less by a factor of ten.

Issue 1(a)
ULA pin 14 - clock out from ULA - this looks more like what I would expect on the Z80 clock input pin (pin 6), but maybe okay. With high frequency signals, the capacitance of the 'scope and the test lead will have an effect. The effect is minimised when using X10 'scope probes.
ULA pin 16 - video out from the ULA - this is fine :-)
Z80 pin 6 - clock input - this looks okay, assuming that the peak at the top is 4.5V or greater and that it falls to around zero at the bottom.

Issue 1(b)
ULA pin 14 - clock out from ULA - with the first photo, was the 'scope test lead ground connected properly to a ZX81 0V/GND?
ULA pin 14 - clock out from ULA - second photo. This may not look brilliant, but it is most likely okay.
ULA pin 16 - video out from the ULA - this is fine :-)
Z80 pin 6 - clock input - it is hard to tell if this is any good.

Issue 3
ULA pin 14 - clock out from ULA - it looks okay.
ULA pin 16 - video out from the ULA - this is fine :-)
Z80 pin 6 - clock input - it is hard to tell if this is any good.

So in the hope that the CPU is getting a good clock signal, next up, to see if we can get an idea of what, if anything, it is doing.
So can you test the following CPU signals/pins:

For reference here is a pin-out of the Z80 CPU.
Image

What signals do you get on the following pins of the Z80 CPU?
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 (should 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.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

There is no need to take photos of all these signals if they are doing what is expected.

Mark
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msellan
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Re: Resurrecting a ZX81 - Step by step help appreciated.

Post by msellan »

1024MAK wrote: Sat May 06, 2017 11:56 pm
Also watch out that the dog does not run off with the spring hook type test probes and chew the end making it useless... :o :twisted:

Mark
:lol: I thought I had problems with the cats jumping up on the table to "play" with the cables. But seriously, these are some amazingly helpful images and troubleshooting info. Thanks for posting that Mark.

Best,

-mark
3 ZX80s (1 UK issue 2, 1 US issue 2 & 1 US issue 1), 5 ZX81s, 1 TS1000, 2 Memotech 16k RAM,
2 ZX81 16k RAM, 2 TS 16k RAM, 1 ZX81 unbuilt kit, 1 TS2040 printer, 2+ ZX8-CCB and counting
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