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Author Topic: DBX 165a help  (Read 3005 times)

BobSchwenkler

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DBX 165a help
« on: September 10, 2011, 02:40:38 pm »

Hey All,

I've got a pair of 165As that are not quite working right. They both seem to be exhibiting the same behavior. I've spent all day on one of them, so I'll just go into that one for the moment. Simply put, I'm getting excessive distortion of the signal when under compression in manual mode. Even with attack and release at 12:00 it's pretty noticeable. I don't have a working unit to compare, but I can't imagine I should be getting a sound like this at moderate compression settings. The distortion is somewhat light (as far as distortion goes), but still very noticeable on acoustic instrument material. Distortion pretty much goes away with A/R fully CW. Release affects it the most, as it normally would be expected to; with release at its highest, attack makes little or no difference in distortion. Attack and release constants are performing as they ought to on the scope.

The best I can tell, the issue is related to the manual time constant circuitry. In auto mode I hear or see no noticeable distortion.

Things look cool coming out of the rectification and threshold circuitry. At the output of the manual A/R circuitry there's some significant sketchiness on the scope which looks like it relates to the distortion I'm hearing. It's a very particular looking distortion, not simply rounded off waveforms. There are spiky looking fluctuations when signal goes over threshold, and under heavier compression things continue to look worse, almost like a ramp wave in a sort of way. I could take a photo if it might help diagnose.

Schematic:http://www.dbxpro.com/product_downloads/Schematics/165A%20Schematic.pdf

I swapped VCA with one sold as working from eBay. I don't have a way to absolutely make sure it's perfect, but it gave no different performance than the original.

I've swapped all the amps in the SC path, new diodes and transistor in A/R section, swapped all caps and tested all resistors in the A/R and following (LP filter?) section. It still is not working correctly.

If anyone has any other ideas I'm all ears, cuz I'm starting to run out of them (ideas, not ears).

Thanks!

BobSchwenkler

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Re: DBX 165a help
« Reply #1 on: December 02, 2011, 01:47:57 pm »

Just a check back in for anyone who might pass by this thread in the future. Simple answer, manual mode on these devices=distortion box. I'm really surprised at the design.

Bruno Putzeys

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Re: DBX 165a help
« Reply #2 on: December 05, 2011, 02:26:36 am »

Any sign of parts having been replaced? Particularly the LM311.
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BobSchwenkler

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Re: DBX 165a help
« Reply #3 on: December 05, 2011, 03:40:52 am »

Both units seemed almost entirely stock. The LM311s seemed pretty ok though the 308s showed a little bit of an aberration from what they seemed they ought to have been doing: the sidechain CV would settle nicely down until ~.5V where it would drop quickly to unity. I replaced them with TL081s and after resetting the release trimmer they performed nice and smoothly in this regard.

I got someone else with a known working unit to process some audio in manual mode, it gave the same results as both my units.
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