Hi everyone, thanks for this forum. I have a Macintosh Plus, the analog board (international version, works in 120V mode) is broken, I repaired it, and now I can see the floppy disk symbol when I boot it. The current problem is that the screen brightness is too high when cold start, and returns to normal after a few minutes, and the problem will not reappear after restarting immediately, and it will only reappear after shutting down for several hours. The brightness adjustment potentiometer on the analog board is always effective, and the brightness knob under the screen is always effective (but the effect is not obvious when the brightness is too high). Now I have to turn down the brightness potentiometer on cold start and adjust it back after a few minutes.
Later I found out that it was because the voltage output by the flyback circuit or transformer was too low during cold start (normal is -50v/+330v, cold start is -30/+270V), which caused the negative voltage on the gate that controls the brightness to be not negative enough, making the brightness too high.
I would like to ask if you have any suggestions on this issue? Thank you very much for your help!
Notes
*I repaired the analog board, and the previous faults and repairs are as follows:
1. Common problems: replaced the broken RIFA capacitor, repaired the slightly corroded circuit due to the explosion of the RTC battery, re-soldered the connector pins and large parts, and replaced all electrolytic capacitors (NP 50V 3.9uF replaced with 250V 5uF because I only have this type)
2. Cannot start, no response: replaced the startup resistor R35 (corresponding to the US version R42)
3. It starts up and it Flup-Flup-Flup, and occasionally it can start up for a while, the output voltage is high (up to 15V/7V) and cannot be adjusted: the optocoupler U3 and the surrounding circuits are normal, and R53 is changed to 2Ω (corresponding to the US version R49//R47)
4. The edge of the screen is slightly jittery: the fault disappears after replacing the electrolytic capacitor.
*Non-native English speaker, using Google Translate, I apologize for any unclear expression.
postscript:This problem has been solved and is kept as a reference case for everyone's repair:
After inspection, the problem may be caused by the leakage of RTC battery fluid, which caused the wire connecting the brightness potentiometer under the screen to leak to GND (the line starts from the potentiometer and goes 2/3 of a circle along the edge of the motherboard, which may be 2 feet long). The resistance changes with the temperature and humidity of the motherboard (drifting between about 1M~3M), Since the leakage resistor is connected in parallel with the brightness adjustment potentiometer,reducing the negative voltage that controls the brightness of the CRT. Later, the copper foil was cut off and connected with a wire to repair it.
Later I found out that it was because the voltage output by the flyback circuit or transformer was too low during cold start (normal is -50v/+330v, cold start is -30/+270V), which caused the negative voltage on the gate that controls the brightness to be not negative enough, making the brightness too high.
I would like to ask if you have any suggestions on this issue? Thank you very much for your help!
Notes
*I repaired the analog board, and the previous faults and repairs are as follows:
1. Common problems: replaced the broken RIFA capacitor, repaired the slightly corroded circuit due to the explosion of the RTC battery, re-soldered the connector pins and large parts, and replaced all electrolytic capacitors (NP 50V 3.9uF replaced with 250V 5uF because I only have this type)
2. Cannot start, no response: replaced the startup resistor R35 (corresponding to the US version R42)
3. It starts up and it Flup-Flup-Flup, and occasionally it can start up for a while, the output voltage is high (up to 15V/7V) and cannot be adjusted: the optocoupler U3 and the surrounding circuits are normal, and R53 is changed to 2Ω (corresponding to the US version R49//R47)
4. The edge of the screen is slightly jittery: the fault disappears after replacing the electrolytic capacitor.
*Non-native English speaker, using Google Translate, I apologize for any unclear expression.
postscript:This problem has been solved and is kept as a reference case for everyone's repair:
After inspection, the problem may be caused by the leakage of RTC battery fluid, which caused the wire connecting the brightness potentiometer under the screen to leak to GND (the line starts from the potentiometer and goes 2/3 of a circle along the edge of the motherboard, which may be 2 feet long). The resistance changes with the temperature and humidity of the motherboard (drifting between about 1M~3M), Since the leakage resistor is connected in parallel with the brightness adjustment potentiometer,reducing the negative voltage that controls the brightness of the CRT. Later, the copper foil was cut off and connected with a wire to repair it.
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