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Symetrix SX208 compressor issues: Threshold circuit failure
Old 27th January 2013
  #1
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4 Reviews written
🎧 10 years
Symetrix SX208 compressor issues: Threshold circuit failure

Hi Gang. Scored a Symetrix SX208 as-is with a blown fuse and ground coupling resistor on the BA4560 op amp controlling the Threshold circuit ('R10' and 'U3' within the attached schem link). This op amp also looked oozy so I removed it, added socket and popped in an LM358 equiv. Upon firing up I finding that U3 gets super hot (as if in runaway mode), however the fuse hangs on and all other circuits/components function as expected. I've replaced D1, R9 and individually lifted C3 and C44 with no difference. With U3 installed I don't find much fluctuation in bias at pin 5 (pretty flat around 0VDC); with U3 removed I see -VDCs from 9 to 12 ish. I'm stumped.

Where should I be looking to resolve this runaway condition (if it is in fact in runaway)...U1 coupling caps C26; C27(tantalums), Q1 failures (MC3346P), or ??? All suggestions greatly appreciated. TIA 'Slutz

http://www.symetrixaudio.com/kb/SX208_sch_Rev1B03.pdf
Old 29th January 2013
  #2
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4 Reviews written
🎧 10 years
Well, as it'd turn out I went ahead & replaced Q1 which I think fixed my problems...at least U3 isn't melting down. What I find now is that the threshold circuit must be slammed hard...well into clip territory, in order to see it flop through it's level settings ( a fixed -10 dB), otherwise it stays forever in an over threshold state. It might be due to characteristics within the LM354N vs. BA4560...really don't know. Awaiting new 4560s from the Orient which will take several weeks, and so can't experiment further until those arrive. Meanwhile, can anyone suggest what proper bias levels should be? Currently I have every trim pot set to 0Vdc at the center lug, but not sure if this is optimum or not.

Again thanks for your insight & guidance.
Old 29th January 2013
  #3
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🎧 10 years
you need to align the detector amplitude trim and the threshold trim controls.

0V on trim wipers is not "optimum" obviously .
Old 31st January 2013
  #4
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4 Reviews written
🎧 10 years
Thanks, Doc. I'll start on some research right now for I'm a little fuzzy on what you mean by 'alignment'. Is there a reference I should adhere to? Meanwhile I'm off to Google...
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