[BC] Symetrix 528E mic processor problem

[Warren Shulz]

I have been overhauling about 14 of these 528e units with a build date of 1998 thru 2004.  For older ones bipolar and some polar caps are gone from heat.  Push button Shadow style  switches are erratic as can be.  You can bypass with jumpers 15 dB pad, bifurcate line/mic, and bifurcate symmetry in/out.  Remaining douse with Craig de-oxid.  Source is MCM.  It's a lube liquid you  eye dropper into pots and switches.  Restored factory coating that has long dried out.

Mic-preamp is an odd duck.  Early ones have thumping with cover off from room drafts!  Baffled me for days why the thing had thumping noise.  Not an issue with cover on.

2004 units re-did board mic-preamp layout to bring two bipolar inputs close to each other.  Mic input has a servo op-amp to reduce thumping which can rise to -52 dB below output level.  Typical noise is only -62 dB from operating level using -55dB input.  Not the best.  But distortion is less that 0.1 % THD or IM.  Decent  RFI.  16 panel trim pots and op-amps are unobtanium.  Square metric guitar amp style.  More Craig treatment. The mic gain trimmer will always be scratchy by design.

Bipolar surface mount can be replace with radial lead bi-polar from Digikey.  Foil is delicate and you will spend fair amount of time at it replacing failed surface mount bi-polar caps. You need an in ckt. Checker as removal is destruction. Also hi EQ poops out from capacitor failure.  Found several units pulled out of service with high EQ just not working until re-capped.

The 528e will pass a square-wave and you can check EQ fast.  It is a Swiss army knife for audio. It does not do well sitting still with audio passing through so many mechanical contacts.  Seems to be for a recording studio were you fiddle with adjustments every day.

As we live in a care taker mode you fix or do without.  Going forward you would want a processor that had no switch contacts or trimmers.  Maybe another day but not in the cards today.  I was able to leave them better than I found them but I doubt they will stay at peak performance as switch and trimmer pots fall back to erratic state. Time will tell.

Warren Shulz
WLS A/F Chicago

well now,,, it's time to take it apart and resolder connections on the board......
time consuming yes, but will fix all the intermittent problems.

bill

In a message dated 2/22/2012 5:35:51 P.M. Eastern Standard Time, thebeaver32 at gmail.com writes:
>Spraying these with contact cleaner does no good. The problem still recurs.