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Modified (Jim Williams?) DBX 162 op amp problem
Old 13th September 2012
  #1
Here for the gear
 
jefferysmarshall's Avatar
 
🎧 10 years
Modified (Jim Williams?) DBX 162 op amp problem

Hi Guys,

I just purchased a DBX 162 that I believe has been modded by Jim Williams. I can see the remnants of his sticker on the faceplate and many of the standard IC have been replaced with high speed video amplifiers. The pots have also been replaced with aftermarket ones (which work very well actually). This sounds like the work of Jim Williams from what I have heard. Anyway, the guy I bought it from told me the right channel was very low output and distorted. I figured it would be a good project, so I bought it.

The first thing I did when I brought it home was build pigtails and clean some of the switches. I plugged it in and, interestingly enough, both channels worked fine. It actually sounded very nice. Pretty different from the other 162s I have heard and actually much different from any DBX compressor I have used. The sound was much cleaner. The highs were more detailed, the bottom seemed a bit tighter, and the mids seemed slightly more scooped compared to other DBXs I have heard. At this point I was pretty happy that this compressor was fully functional and sounded nice, because I was under the impression that channel two was not functioning properly.

The next day, I went to use it and channel two was low output and distorted. I opened it up and poked around, looking for cold solder joints or bad components. I swapped the VCA cards from channel one and the problem still remained on channel two. I finally located the problem. It was one of the 4081 video amps that replaced the original (maybe a TL081? Not really sure to be honest.) I swapped the 4081 op amp from channel 1 to channel 2 and the problem went away. I placed the bad 4081 in channel 1 and the same problem occured on channel 1. So I found out that my problem is a bad op amp.

When I purchased the unit, I was given a bag of old components that was sent to the original owner after it had been modded and recapped. I decided to pull a TL081 out of the old component bag and drop it into channel two. I powered it back up and it sounded great. The original DBX sound was back on that channel. The two channels sound much different now that they have different op amps. Both good, both different.

After that long story, my real question is this:
Is it okay to put the original op amp back in even though the unit has been modified? I am worried that since it was modded, the old op amp may cause other parts of the circuit to fail or not function correctly. Although it sounds good when I put the old IC in, I do not want to damage the circuit.

Also, since the circuit is modified, would you just replace the 4081 video amp with another one, or put the original ICs in?

Maybe this is a question for Jim Williams, as I believe he is the one who modified the unit, and he knows what he did to it better than anyone. I just thought I would ask here first and maybe he would join the thread as well.

Thanks for your help, and I hope I didn’t waste too much of your time making you read this.

-Jeff


Attached is a picture of the inside of the unit. Note the small ICs on the green boards. These are the 4081s.
Attached Thumbnails
Modified (Jim Williams?) DBX 162 op amp problem-dbx-162-inside.jpg  
Old 13th September 2012
  #2
Lives for gear
 
🎧 15 years
Hi
You should at least run this past Jim as he is 'responsible' in part for this unit.
Components are allowed to fail so he does not need beating up for that unless he knows of a potential 'hiccup' in which case a suggestion of alterations (from him) would be good.
Without knowing exactly which chips were swapped with what it is difficult to say if there would be a 'problem' with fitting some of the old chips back in, most likely not in terms of reliability but for example the 'video' amp will far outperform a 'sluggish' TL081 and it may well be other parts have been swapped to utilise the performance of the video amp which would make the TL0 seem 'worse' than original.
Matt S
Old 13th September 2012 | Show parent
  #3
Here for the gear
 
jefferysmarshall's Avatar
 
🎧 10 years
Quote:
Originally Posted by Matt Syson ➡️
Hi
You should at least run this past Jim as he is 'responsible' in part for this unit.
Components are allowed to fail so he does not need beating up for that unless he knows of a potential 'hiccup' in which case a suggestion of alterations (from him) would be good.
Without knowing exactly which chips were swapped with what it is difficult to say if there would be a 'problem' with fitting some of the old chips back in, most likely not in terms of reliability but for example the 'video' amp will far outperform a 'sluggish' TL081 and it may well be other parts have been swapped to utilise the performance of the video amp which would make the TL0 seem 'worse' than original.
Matt S
Thanks for the info Matt. You are probably right, I should run this by Jim to see if there could be any problem with swapping the ICs. Also, I hope my post did not come off as me complaining that Jim modded this compressor and now it doesn't work. That was not my intention at all. In fact, Jim I would say it sounds awesome with his upgrade. I completely understand that components fail, so I was not trying to blame anyone.

My only concern (besides the change in the sound of the unit) is that putting the old IC in could damage other components in the unit. I assume this is a possibility because when you change one part of a circuit, it can affect the rest of the circuit. However, having only a small amount of knowledge and experience in electronics, I am not actually sure if this is completely accurate.
Old 13th September 2012
  #4
Lives for gear
 
Ike Zimbel's Avatar
 
🎧 10 years
Hi Jeff,
Did you take a real close look at the soldering on the chip adapter? Maybe one of the pins isn't making good contact, which is why the fault is intermittent.
I'm sure Jim will chime in here shortly, but my $.02 is: You should be able to go back to the original chip, as long as it doesn't have significant DC offset. Often the upgrade process involves removing DC blocking caps, so if that was done, and it was there for a reason, you could have some unwanted bumps and thumps from DC.
Best,
Ike
Old 14th September 2012
  #5
Here for the gear
 
jefferysmarshall's Avatar
 
🎧 10 years
Ike, thanks for the reply. I actually did check continuity between the pins of the chip and the legs of chip adapter. I also checked for shorts. The contact seemed fine in both of these tests, so that is why I figured the IC had just gone bad. It was interesting that it worked for that short period though. Maybe it was a thermal thing? When it heats up, it stops working? I tested that theory and it didn't seem to make sense either. Whatever it is, I think I'll just replace it.


Jim, thanks for clearing up my issue. I did notice the MIT caps you mentioned. It seems like an interesting idea to try out a few of the different opamps you mentioned. After discovering how much of a sonic difference it made switching just one of them back to the TL081, I would be interested in hearing what some of the other opamps you mentioned have to offer. Maybe I'll have to pick up a few of each just to experiment. Thanks for the list of options. However, I do like how the one channel of the unit sounds right now with your mod, so I may settle on the 4081. I guess we'll have to see.

Thanks for the help!

-Jeff
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