[Frank Stearns]

Past threads have discussed rebuilding a Soundcraft Delta 200B -- new
chips, recapping, new oversized PS, etc.

Initially the console seemed to sound pretty sweet, but lately I've been
noticing some real crunch in the sound.


Any thoughts on these questions:

1. Prior to getting the new PS, the old original PS did drop its positive
rail on two separate occasions. Fortunately, one of my very first mods was
to put LED pilot lights in the master module so that I could see rail
status. On each failed rail occasion the console was not up more than 1 to
1.5 seconds before I killed the power (and I then removed some modules to
reduce the load and allow the PS to come up).

I'm wondering if during two such instances any of the new caps were
reverse biased and damaged (perhaps the 1000 uf/6,3V in the summing
circuit); though I'm not even sure how much offset the OPA2134 chips would
generate. (I don't recall that the OP275s showed offset under this
condition -- been a while since I'd tested that, however, and have not
tested the 2134s.)

2. If I could get my hands on a distortion analyzer I could probably find
out pretty quickly what's going on; whether the problem is cumulative
throughout the unit or whether there's something specifically wrong, such
as in the summing. I don't have an AP (and renting one is a little pricy).
I'm wondering if the Neutrik Minilyzer and its companion oscillator would
have sufficient resolution in its distortion measurements.


Any other thoughts or ideas on what/how to check are appreciated.

Thanks,

Frank Stearns
Mobile Audio
--

[Scott Dorsey]

Frank Stearns wrote:
Any thoughts on these questions:

1. Prior to getting the new PS, the old original PS did drop its positive
rail on two separate occasions. Fortunately, one of my very first mods was
to put LED pilot lights in the master module so that I could see rail
status. On each failed rail occasion the console was not up more than 1 to
1.5 seconds before I killed the power (and I then removed some modules to
reduce the load and allow the PS to come up).

I'm wondering if during two such instances any of the new caps were
reverse biased and damaged (perhaps the 1000 uf/6,3V in the summing
circuit); though I'm not even sure how much offset the OPA2134 chips would
generate. (I don't recall that the OP275s showed offset under this
condition -- been a while since I'd tested that, however, and have not
tested the 2134s.)

That should be easy enough to test with an ESR meter. It's not likely
to be the big issue, though.

2. If I could get my hands on a distortion analyzer I could probably find
out pretty quickly what's going on; whether the problem is cumulative
throughout the unit or whether there's something specifically wrong, such
as in the summing. I don't have an AP (and renting one is a little pricy).
I'm wondering if the Neutrik Minilyzer and its companion oscillator would
have sufficient resolution in its distortion measurements.

Is the issue on both channels of the 2-buss? Is it on all strips? If
you use an aux buss, is it there?

Any other thoughts or ideas on what/how to check are appreciated.

I'd put a channel strip on extenders, run a 1 KHz square wave through
the thing, and look around with a scope. It should be nice and square
from beginning to end. No overshoot, nice parallel tops and bottoms,
no ringing and definitely NOT fuzziness.

My first suspicion would be an ultrasonic oscillation issue with those
high-speed op-amps. That's something easily enough found with a scope
and a square wave too.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[RD Jones]

Frank Stearns wrote:

1. Prior to getting the new PS, the old original PS did drop its positive
rail on two separate occasions. Fortunately, one of my very first mods was
to put LED pilot lights in the master module so that I could see rail
status. On each failed rail occasion the console was not up more than 1 to
1.5 seconds before I killed the power (and I then removed some modules to
reduce the load and allow the PS to come up).

The protection circuit in the Soundcraft power supply design
would have prevented this from happening. If one rail goes
down the other shuts off immediately. (I know from experience
this function works)
Was the protection defeated ?

rd

[Frank Stearns]

"RD Jones" writes:

Frank Stearns wrote:

1. Prior to getting the new PS, the old original PS did drop its positive
rail on two separate occasions. Fortunately, one of my very first mods was
to put LED pilot lights in the master module so that I could see rail
status. On each failed rail occasion the console was not up more than 1 to
1.5 seconds before I killed the power (and I then removed some modules to
reduce the load and allow the PS to come up).

The protection circuit in the Soundcraft power supply design
would have prevented this from happening. If one rail goes
down the other shuts off immediately. (I know from experience
this function works)
Was the protection defeated ?

This was the very cheap supply for smaller frame units - 1.5A 317/337
regulators ala the manu's app notes and that's is -- no cross-rail shut
down at all.

Frank
Mobile Audio

--

[Frank Stearns]

(Scott Dorsey) writes:

snips

2. If I could get my hands on a distortion analyzer I could probably find
out pretty quickly what's going on; whether the problem is cumulative
throughout the unit or whether there's something specifically wrong, such
as in the summing. I don't have an AP (and renting one is a little pricy).
I'm wondering if the Neutrik Minilyzer and its companion oscillator would
have sufficient resolution in its distortion measurements.

Is the issue on both channels of the 2-buss? Is it on all strips? If
you use an aux buss, is it there?

Any other thoughts or ideas on what/how to check are appreciated.

I'd put a channel strip on extenders, run a 1 KHz square wave through
the thing, and look around with a scope. It should be nice and square
from beginning to end. No overshoot, nice parallel tops and bottoms,
no ringing and definitely NOT fuzziness.

My first suspicion would be an ultrasonic oscillation issue with those
high-speed op-amps. That's something easily enough found with a scope
and a square wave too.

Thanks, Scott. As I recall, things have looked pretty clean, but I will
start a more methodical point-by-point check. I do have a lower-end scope
and oscillator, but will see what I can see.

I can lay out several modules on a spare loom on the bench so that makes
the process a little easier.

Frank
Mobile Audio
--

[[email protected]]

On Mar 17, 12:38 pm, Frank Stearns
wrote:
Past threads have discussed rebuilding a Soundcraft Delta 200B -- new
chips, recapping, new oversized PS, etc.

Initially the console seemed to sound pretty sweet, but lately I've been
noticing some real crunch in the sound.

Any thoughts on these questions:

1. Prior to getting the new PS, the old original PS did drop its positive
rail on two separate occasions. Fortunately, one of my very first mods was
to put LED pilot lights in the master module so that I could see rail
status. On each failed rail occasion the console was not up more than 1 to
1.5 seconds before I killed the power (and I then removed some modules to
reduce the load and allow the PS to come up).

I'm wondering if during two such instances any of the new caps were
reverse biased and damaged (perhaps the 1000 uf/6,3V in the summing
circuit); though I'm not even sure how much offset the OPA2134 chips would
generate. (I don't recall that the OP275s showed offset under this
condition -- been a while since I'd tested that, however, and have not
tested the 2134s.)

2. If I could get my hands on a distortion analyzer I could probably find
out pretty quickly what's going on; whether the problem is cumulative
throughout the unit or whether there's something specifically wrong, such
as in the summing. I don't have an AP (and renting one is a little pricy).
I'm wondering if the Neutrik Minilyzer and its companion oscillator would
have sufficient resolution in its distortion measurements.

Any other thoughts or ideas on what/how to check are appreciated.

Thanks,

Frank Stearns
Mobile Audio
--
.

Fuzzees are probably a non-linear opamp. Many stages in this design
are not stable with other than 1~3 mhz opamps. Local feedback caps are
needed on many of the stages and local .1 uf psu bypasses are also
needed. The line input stage with it's voltage follower in the
feedback loop is not stable with wide band opamps without a re-design.
If you re-install the TL072's does the grind go away? Just the EQ
stages need 10~22 pf feedback caps and .1 uf psu bypasses.

I have mine fitted with class A TI THS opamps that have heat sinks on
them. They are running at 300 mhz, but are entirely stable. If you saw
the 25+ parts installed on the rear of the pcb to insure stability you
would probably wonder if it's worth it. I get around .0008% THD+noise
with the AP while extending the audio bandwidth to -1 db at 200k hz,
line in to stereo buss out. This does require extensive mods to
accomplish but I find it worth it and very satisfing to use in place
of the computer mix, the delta is cleaner, wider, fatter.

Jim Williams
Audio Upgrades

[Scott Dorsey]

Frank Stearns wrote:

I'd put a channel strip on extenders, run a 1 KHz square wave through
the thing, and look around with a scope. It should be nice and square
from beginning to end. No overshoot, nice parallel tops and bottoms,
no ringing and definitely NOT fuzziness.

Thanks, Scott. As I recall, things have looked pretty clean, but I will
start a more methodical point-by-point check. I do have a lower-end scope
and oscillator, but will see what I can see.

A $10 Heathkit from a hamfest is fine. You don't need to be able to accurately
reproduce the ultrasonic trash, you just need to be able to see that the
trace gets fuzzy when it's present.

The old Tek scopes have a 1 KC square wave generator built in for calibrating
scope probes, which make them really convenient. Just make sure you have
the thing set up to see a nice square wave on the input to begin with.

I can lay out several modules on a spare loom on the bench so that makes
the process a little easier.

They may behave differently alone than they do when loaded by the master
module. And don't forget to check the master module. Pull all the
other unneeded modules out of the frame (talkback, oscillators, etc.)
so you don't confuse any issues.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."