DBX 906 Flanger Repair

[tubejay]

I'd love to repair this myself, but I think it might be outside of my ability to do so. Does anyone know anyone who works on 900 series gear? This one is tough because of all of the power rails going into it. I have a bench power supply, but I think I'd need another just to power it on outside of the 900 rack.

Problem: It doesn't pass any audio when the circuit is engaged. I think the problem is in the audio path, because the oscillation circuit etc seems to be doing exactly what it should. I can adjust the controls on front, and if I crank up the output on my mixer/mic preamp, you can see the speaker actually moving along with the changes in the controls at a very low frequency, and it's slightly audible, though no audio passes from the input. So I think it's a problem with the audio portion of the circuit. I'm thinking maybe a bad transistor or opamp somewhere?

I tried a basic recap and that didn't change anything.

These things are pretty rare, so I'd love to find a way to fix it or even pay someone else to fix it. I just hate to see it go to the trash, when it's probably something simple. The schematics are readily available, I just can't really test voltages because I can't power the thing outside of the 900 rack.

I guess my next step it start pulling and testing transistors and opamps. It just seems like a lot of extra work and a total crap shoot to do it that way. Not to mention the strain of desoldering everything on the circuit board.

 

[emrr]

You can get the right vector cards and make an extender, and I think JLM may actually already make a 900 extender or adapter that could be modified.

 

[user 130681]

Whatever you do, do not change/test things willy nilly.
Use your oscilloscope and follow the signal path. Where the signal stops is you faulty part.
Otherwise it will never work again and an engineer that knows his way around these basic delay timers, will not be able to fix it for a reasonable cost to you.

 

[David Kulka]

I'm away from my shop until January but I have a test jig that I built for 906 repair and can service it for you after I return. PM me if you'd like to discuss. They are cool units but definitely tricky and time consuming to service, partly because of the sandwich configuration of the 3 PCB's and the short ribbon interconnects between them.

That said, the recap concerns me a little. It's easy to make a minor mistake with something like that that doubles the amount of repair time in the end. Hopefully that's not the case here.

 

[Jahroen]

Hi! You solved it? I'm working on a 906 right now and built jig with psu and audio in/out for testing purposes. The unit I'm working on did not pass audio through the circuit either, only some weird oscillation. Besides a few blown parts, I narrowed it down to the compander circuit, with a dead DBX IC in the RMS detector. I'm redesigning the ICs functionality and currently have a batch of small footprint PCBs with SMT components which fits very nice and the results look promising. This card with the compander and two expanders does only need +-15V and is quite easily to work with if you take it out of the 906 unit. Besides a bench supply, a function generator and a scope are mandatory. What you could check quite easily is the double BJT transistors QA1-QA8 with the diode function on your multimeter. Like all the chaps above already have mentioned, it's not a device to fiddle with if you are not a tech yourself.