JBL/UREI 7110 lacks something?

[Sowhat]

This JBL/UREI Model 7110 Limiter/Compressor came in the shop and I kid you not, the repair ticket says in the “Work to be done” section “No Bottom (LowEnd)”, that’s it. I know this customer and he has a recording studio… oh boy! pro audio!!! I want to trade some fixing for some mixing but that’s another thing. Forget about getting a more precise description from the client, it would only make things more confusing, trust me.

The manual and schematic are easy to get and I saw it had two test points named “A” and “B” for the +-18 volt rails which power the army of op amps on the board. I figured if one side was out that might account for this elusive lack of bass compression or is that limiting, or both, I don’t know if I even understand what that means… anyway, both rails checked out good and dandy so now what’s next?

I put some sine waves on the input at 0db and I read the sine wave from the output set to 0db with a scope and yes, indeed! there’s a sine wave passing through undistorted whether the thing is powered or not. When power is applied the led displays reflect the systems condition pretty much as you would expect. If I twiddle around with the threshold, detector, attack, release and ratio I get things to happen, I get loss of output db and things like that, things that look compressor like and limiter like but a scope image of an undistorted sine wave don’t really tell me much. I don’t know if I actually have a coherent technical non subjective check for this type type of anomaly or really what I might be looking for in the first place, does anyone have a clue on this and how to diagnose a problem like this?

I know it’s closing time Friday and I won’t get back to things until Tuesday but it always seems that I get a little problem child right at the end of the work week which gives me time to gobble up my days off mulling things over.

[The Dude]

This sounds more like operator error to me. Low end usually has the highest peaks, so it stands to reason that lows are compressed more. Does he have another unit so you can do some side by side comparison? Otherwise, I'd probably use a variable tone generator and sweep it while scoping output to see if indeed there is a loss of lower frequencies.

[octal]

Have seen dried out electrolytic coupling caps on consumer audio products cause lack of bass. Not sure if this particular product has many electrolytic coupling caps in the signal path. Follow the Dude's suggestion, sweep it while you have it set for minimum compression, the frequency response should be close to flat.

[Sowhat]

I'd probably use a variable tone generator and sweep it while scoping output to see if indeed there is a loss of lower frequencies.

I did sweep around from about 40 to 400hz and don't remember seeing anything that jumped out at me. I didn't have a strong game plan then so probably not much made sense to me at the time. I will definitely do some calibrated testing like this starting off on Tuesday but for now it would probably be a good thing for me to completely read the manual and study the schematic to find any insight into the circuits method of operation and like that. I guess if I really knew what's going on as far as electronic circuits in this thing I would be smarter at troubleshooting.

[Sowhat]

Have seen dried out electrolytic coupling caps on consumer audio products cause lack of bass.

Good idea! I'll scan through the schematic and see if there's any electrolytics serving as couplers. There are a few electrolytics scattered around the board, and some small 1uf or so ones which could be couplers but I think most of them are used for ripple filtration like in the power supply section. I'll need to check that out.

[Sowhat]

Sorry!!! I forgot to link you the manual and schematic which is traditional here at MEF...

http://www.jblproservice.com/pdf/Vin...110 manual.pdf

(Hope this link links)

[g1]

You might hear obvious defects if you try playing music though it, rather than sweeping tones. A dynamic signal should get more compressor action than a static signal will.

[Sowhat]

You might hear obvious defects if you try playing music though it, rather than sweeping tones. A dynamic signal should get more compressor action than a static signal will.

I stopped by the shop today for a few minutes, mainly to get out of the 105 degree F heat. I pumped some ZZTop through it and fiddled around with the settings and I got nothing that I could notice, Aside from acting like a volume control on the compressor side sometimes, both the bypass and the compressor sounded pretty much identical. I even tapped it with the scope but I couldn't see any obvious compression action, maybe if I hooked up that other probe and fire up channel 2 I could look at both the input and output simultaneously, maybe then I would notice something. I'll leave that for Tuesday, hope I can find that spare probe.

[Sowhat]

I've had another look, this time at pin 11 of U2 which is the control signal for the voltage controlled amplifier that's at the heart of this compressor. The scope trace showed me that all the controls are sending control information to the chip much as you should expect, you can even see the edges grow and get jaggy with the attack and release controls. You can see much the same type of thing with the detector set all the way over the peak detection. Most of all you see the proper type of behavior by the control signal for a compressor limiter, just what you should expect... oh yeah, I used some cover song I recorded recently.

As I feared I can't find my spare scope probe so I'm going to need to whip up something quick, I need to get a glimpse of input versus output because I can't hear a thing different between the two... maybe the VCA's blown? If that don't pan out I don't know exactly where to go from here, maybe it's time for a talk with the client.

[Sowhat]

I finally figured out a way I can definitively test something I can't hear and without the proper test equipment... I love when this happens! I broke out a Presonus Audiobox USB and USB'ed it into my computer running the Reaper DAW. I then plugged the compressors output into channel 1 of the Presonus and hooked the input to channel 2. I then fed the input sound files from my iPhone using the parallel input jacks both 1/4" and XLR on the compressor acting as a Y cord. After setting levels and creating 2 tracks in the DAW to record each input on a separate channel I shot some playback through it and recorded it on the DAW.

I did this several times and used a chunk of audio that had been severely clipped observing the resultant waveforms. I could now clearly track the amount and type of compression and or limiting I applied, mainly by observing the curve of those clipped sound file regions who's input was a straight across clip but after compression became curved in predictable ways. This showed that the VCA was indeed working fine.

I finally talked with the customer and he corrected me and said that he was getting nothing at all through the compressor, which ain't what the repair ticket said... I guess there some more people I need to have a talk with! I told him that the compressor works just fine and he was relieved, we discussed it and figured he had a bum cord or something in the studio of forgot to turn on the phantom power or something esoteric like that... He's still getting charged however. This took a while to diagnose even if the diagnosis is no fault found.