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Symetrix se400 dual parametric equalizer schematic

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  • Symetrix se400 dual parametric equalizer schematic

    I bought this Symetric SE400 Dual Parametric EQ back in 2003 (on ebay for $102) when the core of my band reformed for a few years to take another crack at putting some tunes together and doing an album, this time produced by ourselves. I had been working at BGW Systems when they went bankrupt, worked for a spell at Marshall Electronics in their Microphone Division in El Segundo, later worked at Westlake Recording Studios running their repair shop, and later still landing the gig at CenterStaging, LLC in Burbank where I still hang my hat.

    The living room at my apartment in
    Gardena, CA got turned into a music room, and I bought a Yamaha 02R Digital console, it having four ADAT Digital I/O cards, which eventually got routed thru Motu 324 interface into my DAW for recording. I made use of this equalizer in my bass channel for a while.

    I had done some clean-up work on the EQ when I was using it with a GK 1001RB bass amp at my church, and now using it with my restored Ampeg SVT4-Pro. This past week, I pulled the unit apart, having bought replacement caps for all the cards inside. While taking photos of the boards for my records, I found a few date codes. Latest one was on the side of the Gain pot of the I/O board, being 9230 (1992, 30th week). Date code on the NE5534A was 8130 (1981, 30th week), and on the frame of the power xfmr 8838 (1988 38th week). I’m guessing this was built in maybe 1993. The date on the schematic of the Symetrix 528 Mic Preamp Channel Strip was July 7, 1993.

    I’ve never found any schematics for this SE400, and from what I’ve read on the internet, nor has anyone else. Symetrix offered no help, it being long out of production. When I had the unit apart this past January, armed with the schematic of the 528, I found the parametric EQ circuit very similar to it, though with one less op amp stage in the EQ bands. This past week I finished creating the schematic for the I/O module, then loaded my copy of Protel DXP onto the laptop at work, having it on my DAW at home. So, for the past couple days, I’ve drawn up the schematic for the SE400 Dual Parametric Equalizer, just for fun, and have attached those two pages here.

    Symetrix SE-400 Parametric EQ Ch 1.pdf

    Symetrix SE-400 Parametric EQ Ch 2.pdf

    This past weekend, I took photos of the modules prior to re-capping the modules, and made use of the I/O foil pattern to aid in creating that schematic. As the electrolytic caps in this unit appear to be nearly 30 yrs old, and had polarized caps in the signal path, I replaced those with Nichicon MUSE Nonpolar parts. I forgot to order 1000uF/35V axial buss caps for the power supply board, and now after seeing DC voltages on the buss caps, I’d say replacing those will make more sense. The power xfmr secondary is 37.4VAC CT, and rectifying those 18.7VAC lines and charging them up, finding +23.1VDC and -22.5VDC on the buss caps, those seem low. I don’t recall hearing hum in the unit, but fresh caps should yield higher equal levels.

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    I did replace the green ‘chicklet’ mylar caps while I was at it, using metalized polypropylene parts from Wima and Kemet. I’ll later have a look at those parts on the network analyzer.

    When I did maintenance on this unit, along with my Symetrix 501 Compressor/Limiter as well as the CL-100 Compressor, I had lifted the audio circuit off of the chassis, floating them, with bypass ceramic caps (0.1uF/500V) to chassis, so the chassis’ were at AC Ground for the RF shield, and got rid of the HF buzz I was getting with the GK 1001RB patched into their loop. Some of that buzz was also from the nearby cheap light dimmer switch on the wall adjacent to the bass rig on stage.

    Patched into the Ampeg SVT4-Pro, it’s dead quiet.
    Attached Files
    Logic is an organized way of going wrong with confidence

  • #2
    Nice work!


    • #3
      In double-checking the schematics for errors, I did find a minor one. Ch 1, Low-Mid Frequency Band. R136 was used twice. As the numbers on these 'step-and-repeat' circuits were in the same sequence, the thought of having to change R136 thru R172 two times (doing the Ch 2 circuits as well) led me to cop out, changing the second use of R136/R236 instead to R173/R273. Of course, if this was in manufacturing, I would have changed all those numbers for the sake of getting the sequence right. No doubt, auto-sequencing of numbers in the program would have prevented it in the first place.

      So, here's the updated schematics:

      Symetrix SE-400 Parametric EQ Ch 1-A.pdf

      Symetrix SE-400 Parametric EQ Ch 2-A.pdf
      Logic is an organized way of going wrong with confidence


      • #4
        A couple days ago the replacement 1000uF/35V Axial Buss Caps arrived from Mouser, so I pulled the power supply PCB off the rear wall, removed the original buss caps, which turned out to be 470uF/35V units. After putting in the new parts, and adding the two protection diodes D5 and D6 around the outputs of the IC regulators to the input buss caps that I added to the schematic, my brain didn't register that I had them shown backwards. Those weren't on the power supply board until I replaced the buss caps. When I had powered up the unit, now with those diodes incorrectly in place, sure enough, unregulated buss voltage was being dumped onto the output of the regulators and onto the buss lines of the system op amps. OPPS!!

        This morning, having spotted that schematic mistake, I opened the unit up, and both verified the regulated supplies were no longer regulated. Did I kill all of the op amps? Didn't yet know. After correcting the installation of D5 & D6 protection diodes, I found the +15V supply regulator wasn't working, and replaced it. I had still been seeing dissimilar buss voltages with the new caps, and even replaced the four bridge rectifier circuit diodes, having had two measuring much higher on the DMM semi test mode. That didn't change anything....but now with the replacement 7815T regulator in place, both buss voltages are finally correct.

        After correcting a solder bridge that I managed to add during the swapping of the protection diodes, everything worked...until I cranked up the input level with the HPF engaged. I lost signal thru that stage, while everything else worked...just one of the two channels. Turned out to be a solder joint issue on one of the resistors I had moved to the foil side of the PCB to make room for those larger 100nF caps in the HPF. So, it survived the short-term excess supply voltages that got applied.

        I've updated the schematics and added them here, now at Rev B for the two channels.

        Symetrix SE-400 Parametric EQ Ch 1-B.pdf

        Symetrix SE-400 Parametric EQ Ch 2-B.pdf
        Logic is an organized way of going wrong with confidence


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