MOTM

When I power it up and it reaches a stable state it tends to stay  
stable for a long time after I power it off again - maybe an hour.  
Since I'd been using it last night for long enough for it to settle  
down I decided to hold off until this morning, to let everything  
return to it's "drifty" state.

I disconnected the 960 and measured the voltages and, sure enough,  
the negative is solid but the positive drifts, from +15.5 slowly down  
to +14.9. This is with about 150ma worth of modules connected on a  
PSU rated for at least 1000ma.

I was thinking that it had to be the 960 because the pitch of my  
dotcom vco - connected directly to the PSU - didn't drift in pitch  
while my MOTM vcos - connected to the 960 - did. But now I remember  
that the dotcom gets +5v, so perhaps that is stable and that's what's  
keeping it tracking correctly.

In the next few days I'll try and extract the PSU and have a good  
look at it, see what's socketed.

Matthew

On 23-Nov-09, at 10:38 PM, Paul Schreiber wrote:

>
> Recently I shuffled some units around, pulled the 960 out of the  
> system and
> put it back. I've noticed that now the + voltage takes an eternity  
> to settle
> after powering up. It will swing up to 15.6v and then drift  
> randomly between
> that and 14.8v for maybe 15 minutes before finally settling on  
> 14.9, while
> the - voltage heads right to -14.9 and stays there, rock-solid.  
> I've had a
> meter hooked up for the last few days trying to figure out the  
> pattern.
>
> I'm inclined to say that the problem isn't due to the PSU, which is  
> a dotcom
> NOS by Power One, because my dotcom VCO doesn't drift in pitch at  
> all but
> the two MOTM vcos do, together, and along with the + voltage. The  
> 960 is the
> only thing between the PSU and the MOTM units, so it's the only  
> thing that
> appears wonky.
>
> -------------------------------------
>
> This is very odd. It sounds like the Power One is unstable, the  
> LM723N on
> the Power One is 'wandering'. They have a built-in Zener diode  
> reference,
> maybe it is drifting around with temperature. You can try  
> disconnecting the
> '960 from the Power One, and see if it still wanders around.
>
> If it still does, you might get lucky and if it's in a socket you  
> can get
> new ones from Mouser/Digikey for <$1.
>
> If if does *not*, then:
> a) are you exceeding 80% of the rated current? You might have an in- 
> rush
> power issue. The +15V current draw is probably 2X the -15V draw.  
> Use the
> meter to check the *current* on each supply line.
>
> b) a new LM723N might still be the fix. If both of the parts are in  
> sockets,
> swap them and see if that fixes it (there may even be 3 on there if  
> there is
> also +5V).
>
> c) you can see if it really is the '960 by keeping it connected to  
> the Power
> One, but unplugging all the modules to it.
>
> Paul S.
>
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>

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