MOTM

Hi all,

Just had a question about the 960: are the bypass caps that are there
essentially a just-in-case precaution?

Bypass caps, from what I understand, are there to reduce noise. And
correct me if I'm wrong, but most of us that are using the 960 would
probably be also using a power supply with its own bypass caps.

Do the caps then only apply to certain of the power connectors, like
the one on the side of the caps? Or is it the case that if you
connect from a PSU to one of the .156 connectors - or any other spot,
for that matter - that it would pass the caps?

You'll have to forgive me if there's a schem out of there somewhere
that would explain this - I didn't see one.

thanks,
Matthew

a) technically, they are not 'bypass' caps, they are 'bulk' caps. A 'bypass'
cap is a small value cap (like 0.01uf) that is very close to an IC's power
pin. It is there to 'bypass' RF frequencies to ground cause by the lead
inductance of the part. The caps used in the '960 are there to reduce
voltage drop when wiring large cabinets, where the '960 is say 20-30" away
from the main supply (or if you are wiring say 2-4 of them up in a big
cabinet.

b) the caps are across the +-15V lines, so every connector "sees" the caps.

Both bypass caps and bulk caps are needed in 'real systems' to counter
effects of the 'real world'.

Paul S.

Just theorizing here: could a faulty cap across one of the lines cause swings in voltage?

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.

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.

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