Hi there !
This is my first post out there, glad to be part of the community.
I was gifted an old analog mixing unit (from the 70s) by a good friend. It's a Neptune Audio Mixer model 611, 6 channel mixer with :
-6 XLR inputs wired hot to 3, and 1&2 to ground (which is already a weird start. You can see this in Picture 3, and all sockets are what I understand to be common ground, as there's a long wire connecting all XLR grounds together. Besides, all Hot are connected to the other pins via a cap with the marking 24 102k)
-6 unbalanced HI-Z Jack sockets with no ground wiring
-1 MAIN IN, 1 MAIN OUT, 1 FX SEND, 1 FX RETURN, 1 MONITOR IN and 1 MONITOR OUT jack sockets, all wired without any ground connection.
Each of the 6 channel also has a PREAMP OUT jack socket, which allows to have a clean direct out without effects.
And lastly it has a spring reverb module that works, no apparent problem, I'm just adding it for clarity.
It was built in the US 50 or so years ago, but as the previous owner is European (like me) the input transformer and the two capacitors have been changed to acommodate 220V.
Now here's the deal :
It plays nice for some time when I plug it in, sound is good (even if the OP Amps are not the best out there), but after a moment (1-5min) it suddenly fades out and stops emitting any sound. If I drive the input really hard it produces a weak, very distorted version of the input, but that's it.
Now it seems to me that the problem is on the output stage, because the same behaviour is apparent no matter which input is playing, but what do I know, right ?
I've done some basic measurements, none of the inputs seems shorted to ground, but I'm at a loss regarding hat to measure next.
I'm waiting for an ESR Meter and a Capacitor tester in the mail, but in the meantime if anyone has an idea what to test then, I'll be happy to follow your ideas !
PS : Some more info about the insides :
-All the OP Amps except one, are UA4163DC which have an odd pinout (VCC- on pin 7 and VCC+ on pin 11). They can be replaced with Texas Instruments RC4136 if needed.
-The only one that's not the same model as the above, is an MC1458P which I don't really know anything about.
All of their VCC pins read a good connection to their respective power rails. Would it still be a good idea to switch them out for new ones ? They're cheap around my place so it's no big deal.
I'll also add some pictures if I can manage to insert them properly, as I have no experience doing so on a forum.
That's it folks, I can't wait to hear if someone has an idea, because I really love this old unit and would be thrilled for it to get a new life, 50 years on !
This is my first post out there, glad to be part of the community.
I was gifted an old analog mixing unit (from the 70s) by a good friend. It's a Neptune Audio Mixer model 611, 6 channel mixer with :
-6 XLR inputs wired hot to 3, and 1&2 to ground (which is already a weird start. You can see this in Picture 3, and all sockets are what I understand to be common ground, as there's a long wire connecting all XLR grounds together. Besides, all Hot are connected to the other pins via a cap with the marking 24 102k)
-6 unbalanced HI-Z Jack sockets with no ground wiring
-1 MAIN IN, 1 MAIN OUT, 1 FX SEND, 1 FX RETURN, 1 MONITOR IN and 1 MONITOR OUT jack sockets, all wired without any ground connection.
Each of the 6 channel also has a PREAMP OUT jack socket, which allows to have a clean direct out without effects.
And lastly it has a spring reverb module that works, no apparent problem, I'm just adding it for clarity.
It was built in the US 50 or so years ago, but as the previous owner is European (like me) the input transformer and the two capacitors have been changed to acommodate 220V.
Now here's the deal :
It plays nice for some time when I plug it in, sound is good (even if the OP Amps are not the best out there), but after a moment (1-5min) it suddenly fades out and stops emitting any sound. If I drive the input really hard it produces a weak, very distorted version of the input, but that's it.
Now it seems to me that the problem is on the output stage, because the same behaviour is apparent no matter which input is playing, but what do I know, right ?
I've done some basic measurements, none of the inputs seems shorted to ground, but I'm at a loss regarding hat to measure next.
I'm waiting for an ESR Meter and a Capacitor tester in the mail, but in the meantime if anyone has an idea what to test then, I'll be happy to follow your ideas !
PS : Some more info about the insides :
-All the OP Amps except one, are UA4163DC which have an odd pinout (VCC- on pin 7 and VCC+ on pin 11). They can be replaced with Texas Instruments RC4136 if needed.
-The only one that's not the same model as the above, is an MC1458P which I don't really know anything about.
All of their VCC pins read a good connection to their respective power rails. Would it still be a good idea to switch them out for new ones ? They're cheap around my place so it's no big deal.
I'll also add some pictures if I can manage to insert them properly, as I have no experience doing so on a forum.
That's it folks, I can't wait to hear if someone has an idea, because I really love this old unit and would be thrilled for it to get a new life, 50 years on !
Attached Files
O.k. then… unplug the power supply from the main board and put in a full size ATX PSU (yeeeesss! Exact that blown Powerman IP that I fixed a while back), plug it in and the old...
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