Desperado
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I have a signal waveform in this ic and when i press reset, the signal disappear...
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Desperation with Sharp MZ-80A ?
- Thread starter Desperado
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Desperado
Veteran Member
- Joined
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Desperado
Veteran Member
- Joined
- Nov 25, 2017
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- 6,827
Commodore 64 or Amiga ?
daver2
10k Member
Sorry, I meant IC19 pin 6...
The pulse may be so short that you can't see it. But your logic probe should in pulse catch mode.
The power on reset capacitor is C12. Shorting the two pins of this capacitor together should result in IC18 pin 1 going from high to low.
Dave
The pulse may be so short that you can't see it. But your logic probe should in pulse catch mode.
The power on reset capacitor is C12. Shorting the two pins of this capacitor together should result in IC18 pin 1 going from high to low.
Dave
daver2
10k Member
Those two large ICs with 'rusty' legs are responsible for the bulk of the video production...
Cleaning and fixing the pins may have fixed the machine...
Dave
Cleaning and fixing the pins may have fixed the machine...
Dave
Desperado
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But i have always high signal on cpu pin 26
daver2
10k Member
Why don't you check the video signal from the logic board (using your oscilloscope) and compare it to the photographs you took previously of the same signal to look for changes?
How do you KNOW you only have a high signal on CPU pin 26? If the low pulse is short (a few tens of milliseconds say) how will you ever see that I wonder - unless you use your test equipment correctly.
Dave
How do you KNOW you only have a high signal on CPU pin 26? If the low pulse is short (a few tens of milliseconds say) how will you ever see that I wonder - unless you use your test equipment correctly.
Dave
daver2
10k Member
Here are the tests you need to perform in sequence:
Solder a normally open push-button across C12 (22uf). This is the power-on reset capacitor. Power the machine up. Monitor IC17/4 (7414) with your logic probe. This should be LOW. Push and hold the button. IC17/4 should go HIGH. Release the button. IC17/4 should go LOW.
Monitor IC13/26 with your logic probe. It should be HIGH. Press and hold the button. IC13/26 should go LOW. Release the button. IC13/26 should go high.
That concludes the test of the power-on reset circuitry.
Dave
Solder a normally open push-button across C12 (22uf). This is the power-on reset capacitor. Power the machine up. Monitor IC17/4 (7414) with your logic probe. This should be LOW. Push and hold the button. IC17/4 should go HIGH. Release the button. IC17/4 should go LOW.
Monitor IC13/26 with your logic probe. It should be HIGH. Press and hold the button. IC13/26 should go LOW. Release the button. IC13/26 should go high.
That concludes the test of the power-on reset circuitry.
Dave
daver2
10k Member
To test the manual reset button.
Put your logic probe on the positive (+) end of C31. The signal should be HIGH. Press and hold the manual reset button (the one on the computer NOT the one you soldered in previously). The signal should go LOW. Release the button. The signal should go HIGH.
Monitor IC19 pin 6 with your logic probe set to capture pulses.
The signal should be LOW. Press and release the manual reset button. You should observe a caught pulse on your logic probe.
Move the logic probe to IC13/26 (still in pulse capture mode. The signal should be HIGH.
Press and release the manual reset button. You should also observe a caught pulse.
Dave
Put your logic probe on the positive (+) end of C31. The signal should be HIGH. Press and hold the manual reset button (the one on the computer NOT the one you soldered in previously). The signal should go LOW. Release the button. The signal should go HIGH.
Monitor IC19 pin 6 with your logic probe set to capture pulses.
The signal should be LOW. Press and release the manual reset button. You should observe a caught pulse on your logic probe.
Move the logic probe to IC13/26 (still in pulse capture mode. The signal should be HIGH.
Press and release the manual reset button. You should also observe a caught pulse.
Dave
daver2
10k Member
Back to the Z80 signals IC13 (with your logic probe).
Measure the following points:
Pin 17 (/NMI) should be HIGH.
Pin 25 (/BUSRQ) should be HIGH.
Pin 16 (/INT) should probably be HIGH - but it may also have a varying signal on it.
Dave
Measure the following points:
Pin 17 (/NMI) should be HIGH.
Pin 25 (/BUSRQ) should be HIGH.
Pin 16 (/INT) should probably be HIGH - but it may also have a varying signal on it.
Dave
daver2
10k Member
Back to the Z80 signals IC13 (with your oscilloscope).
Measure the following points:
Pins 18, 19, 20, 21, 22, 27 and 28.
Most of them should have varying signals on them. Some of them may have static signals on them. If they are varying, say so. If they are static, state whether they are HIGH or LOW.
That should keep you busy for a short while...
Dave
Measure the following points:
Pins 18, 19, 20, 21, 22, 27 and 28.
Most of them should have varying signals on them. Some of them may have static signals on them. If they are varying, say so. If they are static, state whether they are HIGH or LOW.
That should keep you busy for a short while...
Dave
daver2
10k Member
Can you also check IC40 - that it has no rusty or damaged pins.
Dave
Dave
daver2
10k Member
Just to re-iterate, all repairs follow a common process - no matter what CPU they have:
1. Ensure safety of the mains side of the power supply unit.
2. Check out the power supply rails. Check for the correct DC voltage, ripple and noise (the latter two measurements using your oscilloscope). Ripple using AC coupling with a timebase setting set appropriately for your mains supply frequency (50/60 Hz) and double this (100/120 Hz). Noise by increasing the timebase slowly. All measurements looking for the peak-to-peak voltages exceeding the specifications. For a +5V rail this would normally be 4.75V to 5.25V. However, I would want to ensure tighter limits/tolerances than these.
3. Ensure the clock circuits are all working using your oscilloscope to measure them.
4. Ensure the power-on-reset and manual reset (where fitted) are operational.
5. Check out the key pins of the CPU.
Please write these down and put them on your wall...
Dave
1. Ensure safety of the mains side of the power supply unit.
2. Check out the power supply rails. Check for the correct DC voltage, ripple and noise (the latter two measurements using your oscilloscope). Ripple using AC coupling with a timebase setting set appropriately for your mains supply frequency (50/60 Hz) and double this (100/120 Hz). Noise by increasing the timebase slowly. All measurements looking for the peak-to-peak voltages exceeding the specifications. For a +5V rail this would normally be 4.75V to 5.25V. However, I would want to ensure tighter limits/tolerances than these.
3. Ensure the clock circuits are all working using your oscilloscope to measure them.
4. Ensure the power-on-reset and manual reset (where fitted) are operational.
5. Check out the key pins of the CPU.
Please write these down and put them on your wall...
Dave
daver2
10k Member
We are trying...
Dave
Dave
I have to second your comment.