OB-8 power supply, and problems starting

[jh]

Hi Paul, and diy-list,

in the recent power supply discussion you mentioned the bad design
of the OB-8 power supply.

Now, while I have made a lot of mods to my beloved OB-8, and while
I think I know the sound generation part of it by heart, I never cared
too much about the PSU and the digital part - until now.

The reason for this mail is that my OB-8 starts to show strange behaviour
from time to time, and more often of late. It would come up normal when it's
powered on, work for a couple of hours, and suddenly most LEDs go out
and the selected patch won't be there anymore. Looks like it has switched
to manual mode, with a random set of parameters. 
If I turn it off and on again, a great light show of all LEDs would start, and after
a few seconds it's as it was before (i.e. unusable). Turning it off and on
a few more times then brought back the normal setting, finally, and the 
patches not ereased from memory - all fine.

A couple of possible reasons come to mind, like bad contact of ICs in sockets,
an ageing battery, or PSU problems.

Your recent mail about the PSU has caught my attention, and I took a closer
look on the PSU - for the first time, actually. And what I saw in the schemos
and on the board did actually frighten me. No reverse diodes across the 
voltage regulators, the tiniest rectifiers you can imagine - I fear if one component
dies it would draw a lot of other components into overvoltage death ...

So I have a couple of questions - I vaguely remember some of you *do* have
experience in that:

(1) Are the faults I have seen typically for one of these problems, sockets,
battery or power supply ?

(2) Apart from this specific problem - If I want to improove the PSU, what
am I supposed to do: Will replacing the rectifier in the 5V digital supply
be enough (This would draw the highest current), or are all three of them 
undersized ?

(3) Would adding "backward" diodes across the regulators, and reverse diodes
on each regulator output (to short a possible voltage inversion) be safe enough,
or is a crowbar circuit (thyristor etc.) across the CEM supply voltages recommended, too ?

Thanks in advance for any helpful hints. I have only seen this mint condition
(so far ...) OB-8 until now, so I really lack experience with dying ones -
and I would rather intervene before its too late.

JH.


PS.: There is *one* thing about the OB-8 PSU that is not so bad, btw.
        When I had serious problems with undervoltage in our previous house
        (something like 170V rather than 230V), the OB-8 was the one
        synth that would work until the end. Guess that undervoltage safety
        was the only thing they designed into that PSU ...