Low Voltage on a Mac Classic

[Dog Cow]

My Mac Classic's screen shrinks and jitters a bit when there's disk access. Lately, the hard drive has been spinning up for a few moments when first powered on, but soon gives out. When I remove the memory expansion card, the hard drive is fine, but I don't have enough RAM to boot into System 7.1.

I think that the problem is low voltage coming from the power supply.

I measured the +5 and +12 lines from the floppy disk port. My multimeter is analog, so I couldn't get a precise reading, but the +5 looked like it was just above 4 volts, and likewise, +12 looked like it was around 11.

My question: what should I be looking at or adjusting on the analog board?

 

[uniserver]

unplug the harddrive, and power it with external psu, then solder the psu wires right to the power switch so it goes on and off with the computer.. that is what i did with my classic. those vintage hard drives draw a hunk…

http://www.microcenter.com/single_product_results.phtml?product_id=0285941

these kits always come with a harddrive psu, for 11 bucks cant go wrong.

 

[nvdeynde]

First ESR test all capacitors on the Analog board. It's possible there are bad one's.

For the voltage, adjust the variable resistor PP1 on the analog board. It is small and can be found at about the center of the board. Clockwise will increase output voltages. Be careful, it is sensitive. Only use this if bad elements have been repaired/replaced on the board, otherwise you risk damaging the machine.

Probably your hard drive is suffering from stiction of the heads and so draws too much power from the PSU.

What are the voltages with the hard drive uplugged ?

Take Care,

Nico