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| Mac Plus with high -12.9v rail post recap and service |
Posted by: captaineos on 2024-05-23 18:43:01
Hi everyone
International board
Mac Plus that boots but has slight screen wobble and -12v rail high leading to sound issues.
If sound is being produced e.g in a game it's like the speaker is at 50% volume then 100% volume every second or so.
I have done extensive work on it including but not limited to :
- Near full desolder and resolder of solder joints including the interconnect pins on the logic board
- Full recap with ultra high quality parts
- Ultrasonic clean of interconnect cable
- Dexoit F5 to potentiometers
Replaced (international board)
CR20, CR5, CR15
and
Q2 The HOT.
The +5v rail is perfect at 5.0001v and the +12 is a little hot at +12.4v.
Turning the voltage adjust potentimeter doesn't really affect the positive rails but sends the minus 'up' to -13.6v!
Asking becuase I'm about 8 hours in and need to take a break from hunting around. |
Posted by: tony359 on 2024-06-02 14:34:25
Hi. A bit late to the party here!
I think the Mac Plus I recapped has the same -13ish line. I don't think that line is critical and it's usually a bit off.
The voltage adjustment though MUST adjust the positive rails! I'm not sure that that is your problem with the sound but there is definitely something off there.
I did clean the voltage trimpot and re-adjusted it and it definitely affects everything - but the idea is that you set 5V and the others will be ok. The Plus PSU WILL need correct loads or those voltages won't be accurate - but you're testing with the MoBo so that should be ok.
You say the screen is wobbly, do you have a scope to check those lines? Can you check the voltage potentiometer with a multimeter to confirm it still works? |
Posted by: captaineos on 2024-06-04 16:36:39
Problem solved by reading the USA schematics and replacing the respective zener (CR19 designation on 240v model). It was leaky but testing fine in diode mode |
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