Faulty ZX81 logic probe results

Discussions about Sinclair ZX80 and ZX81 Hardware
Orac
Posts: 4
Joined: Sat Dec 15, 2018 10:33 am

Faulty ZX81 logic probe results

Post by Orac » Mon Dec 31, 2018 12:04 am

I have a poorly ZX81, nothing but a white screen, I’ve logic probed the IC’s, here are the results:

Logic Probe Results

H=High
L=Low
BH=Buzzing High
X=Nothing Lit


Z80A

1- X
2- X
3- X
4- X
5- BH
6- H
7- H
8- H
9- H
10- H
11- H
12- H
13- H
14- H
15- H
16- X
17- H
18- H
19- H
20- H

21- H
22- H
23- H
24- H
25- H
26- H
27- H
28- H
29- L
30- H
31- H
32- L
33- L
34- L
35- L
36- X
37- L
38- L
39- X
40- X


RAM

H
X
L
L
X
L
L
H
H
H
H
L

H
H
H
H
H
H
H
H
H
L
L
H


ROM

L
X
L
L
L
L
H
H
H
H
H
L

H
H
H
H
H
X
X
H
X
X
L
H

ULA

L
L
L
H
H
L
L
X
L
H
X
L
H
H
H
H
H
X
H
L

H
BH
H
H
H
H
H
H
H
H
H
H
H
L
H
L
L
L
L
H


Any clues as to why it’s not working?

dessony
Posts: 227
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Location: Indiana, USA

Re: Faulty ZX81 logic probe results

Post by dessony » Tue Jan 01, 2019 8:19 am

Any corrosion shown on your ZX81 motherboard? Try a little emery cloth sandpaper. Try giving these ICs some "pullings out" and "pushings in" their IC sockets to break oxidization and set up good contacts. Retest.

DesSony

Orac
Posts: 4
Joined: Sat Dec 15, 2018 10:33 am

Re: Faulty ZX81 logic probe results

Post by Orac » Tue Jan 01, 2019 11:55 am

I’ve done all those things.

Lardo Boffin
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Joined: Sat Nov 26, 2016 2:42 am

Re: Faulty ZX81 logic probe results

Post by Lardo Boffin » Tue Jan 01, 2019 12:35 pm

While trouble shooting my poorly rebuilt zeddy I did benchmark logic probe tests on a healthy Z80 and ULA:-

viewtopic.php?f=7&t=2592&start=130#p32197

In my case it was a dodgy transistor. Not sure which one as I replaced them both.
Are you able to chip swap with a working zeddy?
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Various Spectrum 48ks plus a DIVMMC future and SPECTRA

dessony
Posts: 227
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Location: Indiana, USA

Re: Faulty ZX81 logic probe results

Post by dessony » Wed Jan 02, 2019 1:03 pm

I concur with Lardo Boffin on the transistors. How are these two transistors on your motherboard? Can you retest Z80 CPU"s Clock pin, which is number 6, with your logic probe?

DesSony

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

Re: Faulty ZX81 logic probe results

Post by 1024MAK » Wed Jan 02, 2019 3:23 pm

The Z80 CPU needs a square wave signal known as a clock signal on pin 6 in order to do anything. This is on pin 6 of the CPU. In a ZX81, this should be a 3.25MHz square wave signal. Without this signal, the Z80 CPU will do nothing. So the RAM, and the ROM will also do nothing.

Beware however, that some logic probes struggle to correctly show signals that are greater than 2 to 6MHz. So test again and if your probe has a pulse detector mode, use this. A 3.25MHz signal should light both the red and green (high / low) LEDs, although their brightest may not be equal. So look carefully.

The clock signal for the Z80 comes from the ULA via a NPN transistor (which inverts and boosts the signal). The relevant output pin on the ULA is pin 14. If there is no 3.25MHz square wave signal here, then either the ULA chip or the 6.5MHz resonator is faulty. Alas, the hard to get ULA is often the cause of the fault, it having failed. If the ULA on your board is in a socket, the quickest test, if you have another ZX81, is to swap the ULA chip into the other ZX81, AND to swap the ULA from that machine into the board you are working on. Then see if the fault follows the chip.

Mark

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Ed in SoDak
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Re: Faulty ZX81 logic probe results

Post by Ed in SoDak » Sat Jan 05, 2019 5:28 am

I built a logic probe from a kit. While trying to use it and learn, I find I'm not very logical! :oops:

Mark is right. Just reading all pins without learning the underlying reasons for them won't find the problem. You need to understand what each chip is doing and what lines should be high/low/pulsing to send the proper signals on to the next chip or portion of the citcuit. Signals will change as the sequence steps through starting up to operating and processing.

Where this halts in a failed condition, when some line is low but is supposed to be high, or some other error, then you can narrow it down. For myself, that pretty much means someone plowed the road for me and listed out the relevant steps for me to more or less blindly follow. I have no clue at all by looking at a chip on the board or its data sheet what a logic probe should say when the power is turned on.

A few years back I bought a Radio Shack circuit trainer to learn this stuff. It starts with simple flip flops and takes you step by step. Instead, I mostly just built the "fun" circuits like the radio, metal detector and broadcaster and never did make the leap to actually learn ICs. It would make a good New Year's Resolution for me!
-Ed

Orac
Posts: 4
Joined: Sat Dec 15, 2018 10:33 am

Re: Faulty ZX81 logic probe results

Post by Orac » Mon Jan 28, 2019 2:13 am

Thank you all for your time and input on this, sorry for the delay in replying.

I’ve replaced the resonator and TR2. Testing showed TR2 works fine out of circuit.

I didn’t however replace TR1 but I have more transistors, so I will swap that out and see what happens.

I have one of those replacement ULA’s from Andy Rea in case I need it, in fact I’ll probably go with it anyway.

I’ll let you know how swapping that transistor goes.

Thanks again!

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1024MAK
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Re: Faulty ZX81 logic probe results

Post by 1024MAK » Mon Jan 28, 2019 8:29 am

TR1 forms part of the /WAIT circuit. This is connected to CPU pin 24. It is unlikely to be this transistor that is faulty.

Mark

Orac
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Re: Faulty ZX81 logic probe results

Post by Orac » Wed Feb 06, 2019 7:09 pm

I ended up replacing TR1 and TR2 because I had the transistors lying around, in the end I believe it was the Z80A.

I’ve fitted a new Z80 and an Andy Rea ULA.

It’s back to life and the picture is rock steady on composite video.

Fully working and looking good.

I’m so happy to not just buy a working one but take a broken one and fix it, save it from the bin.

Thanks for your help.

Now to find another machine in need of help!

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