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Old 28th July 2006, 07:18 PM   #1
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Question Symmetry acs-1

I need help to repair an electronic crossover Symmetry ACS-1
designed by John Curl.
Many IC inside but numbers erased: Secret schematic !!

If someone know the IC numbers or have a schematic.
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Old 28th July 2006, 08:00 PM   #2
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You can find some general information in my article on
phase coherent crossovers at www.passdiy.com

As I recall, the Symmetry is a 12 dB/oct adjustable frequency
low-pass and the high pass network subtracts that differentially
from the input to get a high pass function.

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Old 31st July 2006, 04:19 PM   #3
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Default Thank's M. Pass

I have already read your articles on your DIY site.
Very interesting !

But this crossover seems to be a little special: IC's numbers are erased but i can almost read one : The big M for Motorola and the number is like MPO6600 or MPO8800 with others (6) eight pins DIP IC's. Two of them are LF356. The others are erased and are working with the MPO's.

If someone know about...
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Old 31st July 2006, 05:17 PM   #4
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HS, you are correct. The big chips are MPQ devices from Motorola that consist of 4 transistors. 2 npn and 2 pnp. These are the high beta types. I have some, somewhere, and can sell them to you, if you actually need them. The other device, an 8 pin DIP, is a special pin out dual fet, that is not made anymore. I might have these as well, but it would take some research. Unfortunately, I don't have a schematic for this crossover, at this time. Mine was lost in the firestorm in 1991. Perhaps someone has made a schematic and can put it up for view. I sure hope so. The actual circuits are simple transconductance amplifiers, much like a traditional first and second stage shown here, very often. There is no output buffer.
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Old 2nd August 2006, 04:46 PM   #5
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Default Thank you M. Curl

I have found the MPQ6600 IC on internet.
I take some measurements on these IC's and they seems fine.

I explain the problem if you can remember the schematic.

On low pass, i have a phase shift between the two channels on
X1 only: i have no longer the same crossover frequency.
On X10 it is OK.

On high pass, i have a hum on one channel only at X1 and X10.
I have a little phase shift on X1 at the extreme right of the
frequency pot. At X10 no phase shift but the hum.

If you have an idea.....
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Old 2nd August 2006, 09:32 PM   #6
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Sounds like loose connections or bad solder joints.
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Old 5th August 2006, 03:32 AM   #7
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Default Thank's M.Curl but I have already checked

solder joints on pc board and the problem is not there.

I was thinking at the Fet IC's because fets give stranges problems sometime: maybe it is oscillating but my Hitachi 20MHZ
scope cannot see...

Or maybe the old style diode or current source next to each MPQ6600?

Again, thank's for your help.
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Old 5th August 2006, 04:00 AM   #8
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The constant current diodes should be about 2ma.
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Old 10th August 2006, 03:43 PM   #9
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Default Thank's M.Curl for the diode info.

If possible can you give the 8 pins DIP IC number next to the MPQ6600.

I have checked the current source and it is ok: i have swapped parts.

I have always hum on that output!

Thank's
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Old 10th August 2006, 11:46 PM   #10
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Still sounds like bad connections.
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