Paul D. DeRocco wrote:
> > From: d_thumim
> > ...adjust the resistors on the power supply. I don't have a
> > scope, just a handheld tester, but I thought I'd try. First
> > R17 until the LED came on, which allowed the machine to boot
> > up but not make any sounds. Then R9 to adjust the voltage
> > across P2-P3 - it started around 4.7, and with R9 turned all
> > the way I couldn't get it above 4.85V. The spec is 5.00V +/-
> > 0.01. Does that mean I need a new power supply? Is it
> > possible to still get parts for these things? I'm willing to
> > tinker a bit.
>
> If it can't get up to 5V, it either means there's something worn out in the
> supply, or that something elsewhere is pulling it down. My bet would be the
> filter capacitors in the supply are dried out, and that the 5V rail has lots
> of 120Hz ripple on it. If you had a scope, you'd be able to see this easily
> enough, but you might also be able to to test this by hooking some
> headphones across the 5V through a small capacitor (maybe 1uf to 10uf) to
> block the DC. The rail should be quiet, but you'll hear a nasty hum if
> there's ripple on it.
>
> It might not be a bad idea just to replace all the large filter caps, even
> if you have to hand-wire caps of a different physical form factor. They're
> notorious for drying out. It's doubtful that any impossible-to-get part
> (like the power transformer) is bad; the rest of the supply is just
> garden-variety stuff you can get from places like DigiKey.
Actually, the transformer itself does look a bit rusty... what is so
unique about this one that it can't be replaced?
> Since your memory contains nothing but garbage, you should also do the steps
> on page 6-5 of the service manual. You may find this brings things back to
> life--that is, if the membrane switches will let you.
Most of the membrane switches do seem to work, but alas STOP is not
among them which prevents me from getting past step 1. ∗sigh∗
Replacing those switches looks like a major headache. This is
starting to look more like a long term project than a little
tinkering.
Thanks for your help.
-- |)aniel