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Help with Issue 6A voltages check

Posted: Thu Dec 05, 2019 1:19 am
by lbaeza
Hi there

This is Luis, a fan of the ZX Spectrum from Chile, South America. This is my first post.

Currently I am working on fixing the bad volts on my ZX Spectrum Issue 6A. For that matter, I have removed all RAM chips, replaced both TR4 and TR5, and replaced all electrolitic capacitors. After this, I have measured the voltages using a voltmeter, with its black terminal connected to the Spectrum heatsink, and its red terminal connected to the following RAM pins, obtaining these measures:

IC 13 Pin 1: -5.13 V
IC 13 Pin 8: +12.28 V
IC 13 Pin 9: +4.98 V
IC 13 Pin 16: +3 mV <--- bad?

IC18 Pin 8: +4.96 V
IC18 Pin 16: +3.6 mV <--- bad?

I have my doubts on the cases indicated with an arrow, since these pins are connected to ground and in theory they should measure 0V. However,in my case, the voltmeter measures a few mili-volts.

If by any chance you own an Issue 6A and a voltmeter, please measure Pin16 on both IC13 and IC16, and share your measures here.

Thanks in advance,

Luis.

Re: Help with Issue 6A voltages check

Posted: Thu Dec 05, 2019 9:18 am
by Ast A. Moore
Never measure voltages across the points on any PCB that are so far apart. If you want accurate readings, place the negative (black) probe directly on Pin 16 of each IC.

What made you start repairing the machine to begin with? Was it faulty? What were the faults and what signs did you observe before attempting to repair it?

Re: Help with Issue 6A voltages check

Posted: Thu Dec 05, 2019 11:57 am
by 1024MAK
lbaeza wrote:
Thu Dec 05, 2019 1:19 am
I have measured the voltages using a voltmeter, with its black terminal connected to the Spectrum heatsink, and its red terminal connected to the following RAM pins, obtaining these measures:

IC 13 Pin 1: -5.13 V
IC 13 Pin 8: +12.28 V
IC 13 Pin 9: +4.98 V
IC 13 Pin 16: +3 mV <--- bad?

IC18 Pin 8: +4.96 V
IC18 Pin 16: +3.6 mV <--- bad?

I have my doubts on the cases indicated with an arrow, since these pins are connected to ground and in theory they should measure 0V. However,in my case, the voltmeter measures a few mili-volts.
Hello Luis and welcome to the forum :D

When using test equipment it’s wise to understand what it is actually doing and the principles of electrical and electronic systems.

In the real world, using normal materials, all conductors have some resistance. Copper is one of the best conductors there is, but even copper has some resistance. As electrical current flows through the conductor, the resistance of the conductor (although very small) causes a small voltage to appear across this resistance. So if you connect a sensitive voltage meter (voltmeter) with one test lead at one end of a circuit (conductor) that is carrying a significant current and then connect the other test lead to the other end of the circuit (conductor), a sensitive voltmeter will show you this voltage (often called ‘voltage drop’).

A voltage drop of the order of 3mV to 3.6mV is nothing to worry about given that the printed circuit board (PCB) in a ZX Spectrum does not use a ground plane and therefore some of the 0V/GND/ground tracks are not particularly wide.

Mark

Re: Help with Issue 6A voltages check

Posted: Thu Dec 05, 2019 2:36 pm
by lbaeza
Ast A. Moore wrote:
Thu Dec 05, 2019 9:18 am
What made you start repairing the machine to begin with? Was it faulty? What were the faults and what signs did you observe before attempting to repair it?
Hi

The usual stuff: garbled characters on screen in multiple colors, blinking...unable to get to the Sinclair Research welcome message...

Kind regards,

Luis.

Re: Help with Issue 6A voltages check

Posted: Thu Dec 05, 2019 2:39 pm
by lbaeza
1024MAK wrote:
Thu Dec 05, 2019 11:57 am
A voltage drop of the order of 3mV to 3.6mV is nothing to worry about given that the printed circuit board (PCB) in a ZX Spectrum does not use a ground plane and therefore some of the 0V/GND/ground tracks are not particularly wide.
Hi Mark

Thanks for the clarification, so there's nothing to worry about then.
I will proceed and install sockets for the rams and give it a try.

Kind regards,

Luis.