DBX 160VU problems

Hi folks I`ve just had one of these land on my bench with a fault.

Basically the sound is breaking up, & sometimes you get a sound like a cap ramping up sort of like really slow motor boating. I`m thinking in terms of bad caps.

I`ve never had one of these on the bench before & I`m kind of busy at the moment. I wondered if anyone with experience with these could steer me in the right direction ? Any help appreciated.

Scope the output of the RMS-DC converter when there's silence at the input. -Make sure that there's no wandering DC voltage, which can feed through or modulate the audio.

The audio path itself is usually very simple in dbx kit.

The sidechain usually is comprised of an RMS detector, a threshold comparator and some DC processing. Look for DC related problems (when there's silence, there should be no DC. If you have noises that only happen in the prescence of noise, I usually send a low-distortion 1kHz sine through it, steady state, then listen to the output with 1kHz notched out. That tells a lot. The steady state should allow you to check for other wanderings in the RMS-DC detector, and the following threshold/ratio/attack/release/makeup area.

Noise? there's usually an unbalancing stage, a VCA with I/V follower, then a balancing stage, and that's your entire audio path. Sometimes noise can be injected at the CV, so meter and scope round there before you assume that it HAS to be in the audio path... if you're sure that it's in teh audio path, then there can only be a couple of suspects.

Tha audio path is in fact not unlike the GSSL path as a classic example. The DC sidechain (and feed-forward model so beloved of dbx :roll: ) is where it differers.

Keith

Thanks for that info Keith.

When you put the 1KHz sine wave in & then notch it out is a graphic good enough to do this or do I need something a bit more precise like a little dipper ?


Does anyone have a diagram of the board layout which shows where the parts are on the PCB ?

You can get a brand new one for $385 plus ship on evilbay, just in case your labor starts to total that thing out.

cj

If the problem is in the RMS converter which is the main fault that I have seen in the 160VU they are usually easily fixed. You can cut the clear sealer holding the metal can to the PCB with a sharp blade around its edge. Then unsolder the the PCB pads under it. And remove it from the Main PCB. All of the ones I have seen have a small PCB held in under the metal can with just two small blobs of hot melt glue which are easy to remove. The Small RMS PCB has a couple of very easy to get IC's and a transistor on it. We usually change the IC's and everything it back to normal.

Interesting to hear that the RMS converter isn't too complex. I remember hearing that the difficulty in cloning the old DBX 160VU was the RMS box, but maybe the VCA is the obstacle. Maybe that thread was from the old site, as I can't find it now. Any ideas, I have always loved the VUs, but never have the change to pick up a couple.

andrew