Neumann U473 compressor repair help

[beatnik]

I have one of these Neumann U473 compressor module that has stopped working

The compressor/limiter function doesn't work anymore, neither does the gain reduction meter

The circuit is still passing a signal and the expander section is working fine that means that the VCA is working, I am very glad that's not the problem as it would have been very hard to find a replacement

I cannot figure out where the fault is, i presume it is somewhere in the detector section, but cannot understand how to trace this part of the circuit, it is rather complex

If someone worked on these units before and can share some suggestions i would really appreciate

original schematics attached

 

[psych60s]

Did you try to swap some ICs? Check if the voltages are as written on the schematics. I can't remember if these ICs are socketed, but if there are, swapping is easy!

 

[synthiaks]

Recap...

The electrolyths are old and some probably are leaky by now.

It's hell though on those 70s/80s Danner modules. They are as Compact as you can get with hole through. This makes them a Royal pain to do soldering work on.

Not to mention the fact that the West German Engineers LOVED Tantals back then.

 

[jensenmann]

+1 what Synthiaks said. I´ve refurbished a few 473 and the problem always were the caps except once a defective chip.

 

[beatnik]

i have left this one aside for the moment but will follow your advice

yeah these blue tants are trouble makers. they will go to the garbage

do you know which knob manufacturer used neumann for these modules ? I am missing one of the knobs for attack

 

[jensenmann]

Elma knobs. IIRC you can get replacements from don-audio.com

 

[synthiaks]

Im quite sure it will work again after recap.

If not, check PSU voltage, check onboard regulator, trace signal on OPs and transistor stages to get a feeling for where signal is beeing lost.

 

[beatnik]

psu voltages were fine and actually the audio signal has no interruption, i believe the problem is in the detector part of the circuit where the ac audio signal is rectified to dc

thanks for the knob link

 

[timtom]

I have one broken u473. I recapped all caps except of the 1uf tantalums.
A few weeks ago the compresser went into full compression. I replaced the 1uf tantalum (C1 on the second schematic page) and it was working again.
Today it started having the same problem again. I guess it's the sidechain circuit cause the unit is passing audio. I resoldered theC1 cap again.
Any idea?

 

[leadbreath]

Change ALL tantulums, they are notorious on these old German units!!!

 

[timtom]

In the end it was a defective IC (LF356 ) in the sidechain part.