Mac Plus with high -12.9v rail post recap and service

[captaineos]

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.

 

[tony359]

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?

 

[captaineos]

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