Particularly low voltage on a Classic II board

[Secret Chimp]

Hi guys, I've bugged some of you on IRC pending my account but figured I'd pick non-lurker's brains. And yes, I have Larry Pina's book on the way but I have no idea when I'll get it (Amazon used seller).

Anyways, the analog board on a Classic II I picked up is putting out particularly low voltages - just under 10 volts on the 12 volt supply and 3.9-4.1 volts on the 5 volt side. This is testing directly off of the hard drive power connector with the drive itself unplugged and the logic board unplugged. I tried fiddling with the PP1 variable resistor and it didn't make any difference (and no it was not already cranked all the way down, the machine has not been opened before).

Does this still suggest only a recap should be necessary, or is some other component (or components) possibly out of spec as well?

 

[James1095]

At risk of sounding like a broken record, yes, replace the caps in the power supply. Usually the big bulk filter on the input is ok, but the smaller electrolytics in the output section will usually be bad. If that doesn't solve the problem there could be something else wrong, but most of the time it will. When the filter capacitors fail, the output gets really noisy/spiky instead of being a smooth steady voltage.

 

[Secret Chimp]

Well the filter caps are probably good on this one then. I didn't note much of any gain or drop between initially turning it on and after having warmed it up for 10-15 minutes or anything in between.

 

[James1095]

While bad capacitors usually perform better as they warm up, it's still entirely possible that they're just too far gone. Even if they are not the problem currently, it's a good idea to replace them unless you have an ESR meter and can verify they are in good health.

The fact that the adjustment makes no difference does suggest something else is up though. Check closely for cracked solder joints at the pot or nearby components. Sometimes these can be hairline fractures between the pin and the solder that are *really* hard to see. Try poking and prodding with an *insulated* stick while monitoring the voltage and look for spikes. Also check the pot itself, it's possible to have a cracked track or broken wiper.

Has anyone ever found or created a schematic for these power supplies? That would help with suggestions. I do need to re-cap mine at some point at which time I can poke around for likely culprits. Normally one of the outputs will be fed back through a voltage divider and compared with a reference. One output is regulated directly and the others follow by being wound on the same transformer.