Repairing/restoring a few old D&R comp/gates

Hello,


For a few years already I have this rack with 4 D&R " compr/lim. II" and 3 "noise gate II" in it.

Crackly pots, one broken off, a big wiring mess inside,... I actually only ever used channel one as it turns out to be a great drum room smasher. I've promised myself to get them back to a good working state one day and I consider 2011 to be a good day 


comp circuit: http://www.d-r.nl/Service Parts/Compressor-Mono.PDF
gate circuit: http://www.d-r.nl/Downloads/NoiseGate-Manual.pdf
pics: http://www.gyraf.be/diy/dnrcompgate/


There's a discrepancy between comps 1-2 and 3-4 as seen in my pictures.
3-4 have a few components removed (the blue thingies) and newer caps (470/40v)
There's also a mix of older and newer resistors



Short of replacing all caps and pots on these boards, are there any other things I could or should change? (opamps?)

Also, these are unbalanced units and it is in no way clear to me at what level they operate - any need for change in that department or shouldn't I bother?





Thanks in advance


Herwig
(diy newb - can't read circuits, knows a bit about components, can solder, persistent - I searched the forum/net for answers, if I'm asking questions that have already been answered, please let me know and point me in the right direction - thank you)

component side of one of the gates:


component side of comp 3:


component side of comp 1:



More pics at the location in post #1

D&R was known here in the Netherlands as: Dutch Rubbish...  ;D
Because they used the cheapest components at the time, don't know how the situation is nowadays...
The technical design is in general not too bad, but the mechanical construction and the quality of pots and switches is the biggest problem.

The 'blue thingies' are 100nF capacitors connected across the + and - supply voltages. It's better to put them in.
The units are unbalanced and can be used at levels around +4dB. (1.23V), but will also work at 'consumer equipment level' (-10 dBv). The output stage can deliver > +20dBm into loads with an impedance of 10K.ohms. (TL074, +/- 15 or 18 Volts supply voltage.)

Thanks for that info.

Are blue caps the C304 and C305 in the circuit (p5 of the manual, 0.1/63 caps?) - again, circuit newbie here...

Would there be any benefit in changing the TL074's for something else?

Also I know the gates have the reputation of being 'oise 'ates ... any opinion there?


I'll start with the caps and new pots and see how that goes.



Herwig

0.1 uF/63V is fine. 0.22 uF/63V is also fine...
Because they are decoupling caps for the supply voltage, their values are not critical.
As long as the voltage they can handle is higher than 20 Volts...
Replacing the TL074's won't give much improvement; the internal operating level is rather high so noise should not be a real problem. The TL074's are even nowadays not too bad.
If you still would like to try other pin-compatible Quad-OpAmps, try to replace them with MOSFET types, because bipolar OpAmps might have an influence on the circuit. (Bipolar OpAmps have higher bias currents.)
To get a real improvement, you would need a different design. Unfortunately you can't change an old car into a Rolls Royce...
My advice: replace the pots and leave it as it is! Anything else you would do to it, is a waste of money.