Neptune mixer model 611 - Works for a few minutes then fades out and distorts

[Zonzi]

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 !

[jiroy]

MC1458P Datasheet ..

[stj]

probably the same as LM458
it sounds like a semiconductor heating up - probably an opamp
get some freezer spray

[eccerr0r]

mc1458 is lm1458 or dual 741. i'm surprised they still make amps with 741 and 1458's...

lm358 is different and will behave different, though pin compatible with lm1458.

make sure power supply to the op amp is still good when warmed up, especially since it was modded???

[Zonzi]

Thanks everyone for your answers. I'm going to get some freezer spray this morning if the store has some.

Originally Posted by eccerr0r

mc1458 is lm1458 or dual 741. i'm surprised they still make amps with 741 and 1458's...

lm358 is different and will behave different, though pin compatible with lm1458.

make sure power supply to the op amp is still good when warmed up, especially since it was modded???

This is actually a very old amp, last time it was serviced was in the early 80s but it's from 76 if I recall correctly.

So, in order to test the power going to op amp, what would be a good thing to do ? Check the big caps' ESR and capacitance ? Or try and get a voltage reading for the transformer ?

Sorry if my questions are basic, it's a first for me on several aspects ! Thank you again for your help

[Zonzi]

Sorry about the double post guys, but I've done some more testing and it appears that the voltage at VCC+ and VCC- of all op amps is exceeding the absolute maximum ratings.

All 4136 have an absolute max. voltage of 15V, or 30V differential ;

I'm reading 32V between VCC+ and VCC- of every op amp, and logically 16V between VCC+ and GND, and VCC- and GND. The power rails read the exact same values, so perhaps it's not an op amp problem but a general overvoltage ?

What do you guys think ?

[stj]

that could be bad smoothing caps or just because it was converted from 110v
the opamps will probably work o.k. at that voltage though.

[Zonzi]

So I got some freezer spray and tried it on the op amps, then on the caps, but it didn't do much. I must have used half a can by now lol.

I also noticed one of the caps is of the wrong value. Near the MC1458 (which seems to control the spring reverb send or return) there's supposed to be a .47uF cap according to the board, but the one soldered is a .047uF. Could it be causing this kind of issues ? All other caps seem of the proper value. I've also ordered some new smoothing caps just to be safe

[eccerr0r]

1 to 5 minutes is too long of a time constant for small capacitors like that, must be a heating problem or power supply issue. Worth swapping the Mc1458 as these are cheap, I have a whole pile of them here I have no idea what i'll do with...

btw, esr meter and capacitor meter probably won't help. An oscilloscope is what you need...

[Zonzi]

Alright we're getting somewhere ! I ordered replacements for all op amps as well as the smoothing caps and the wrong value cap. I'll let you know when they're swapped in.

I know an oscilloscope would be great but that's a bit beyond my financial means for the purpose I have for it. Maybe if I started making money for my audio repairs ! But right now i'd have a hard time justifying the cost.

In the meantime if you have any other idea feel free to drop it here !

[eccerr0r]

You can get a cheap atmel 1MHz scope for not much more than 2 or 3 esr meters cost, like less than $50 (usd) would be more than enough for audio work. Something like the DSO150 would help get your problem fixed.

[Zonzi]

Oh wow thanks that's good to know. I'll look that up !

[stj]

Originally Posted by eccerr0r

You can get a cheap atmel 1MHz scope for not much more than 2 or 3 esr meters cost, like less than $50 (usd) would be more than enough for audio work. Something like the DSO150 would help get your problem fixed.

1MHz?
you can do a lot better than that!
https://www.aliexpress.com/item/1005001969702720.html

[eccerr0r]

yeah money solves problems, but problems don't solve money. crappy 1MSPS scopes can be had for $35 or so.

[stj]

https://www.aliexpress.com/item/1005004972406356.html

[Zonzi]

These are all good suggestions, thanks for that. I reckon 1MHz is plenty for audio work but we'll see after my next paycheck yeah

[Zonzi]

Alright guys I have some news ! I've dropped in all the replacement op amps after trying them one by one. I'm happy to say the unit now works !

BUT, because nothing is ever that simple

First of all, channels 1 and 2 only work for half the course of the level pot, when going above noon they suddenly stop and no audio is to be heard. I wonder if the pots are damaged or if something else is to check ?

Second, I have tested all the old opamps compared to the new ones and here are the readings (tested from VCC to all other pins in diode testing mode)

Old opamps : VCC to Ins reads about .7V to .8V. VCC to Outs reads about .8V to .9V. VCC to the other VCC reads about .65V

New opamps : VCC to Ins still reads about .7V. The big difference is from VCC to Outs, which reads 1.35V consistently on every out. VCC to other VCC is same as with old opamps.

That's a big difference ! Does that indicate faulty opamps regarding the old ones ? I have no dedicated opamp tester so that's my best guess.


I'm taking any advice ! Thanks again guys !

[eccerr0r]

Pots frequently open after abuse, should be able to check resistance at all positions, if it opens at the same place as the silence, then likely the pot is dead.

Btw did you find a specific op amp that was causing the problems seen?

An op amp tester is a fairly specific tool, and an oscilloscope can tell a lot about op amp operation. I'm not sure about using diode check to test op amps but they do have transistors and diodes on the inputs and outputs. But there are more diodes and transistors that are not visible to the outside...

[Th3_uN1Qu3]

Op amp tester can be a simple sinewave generator - use your sound card or mobile phone. Add in some series resistance on V+ and V- so you can read current consumption and there you go.

[Zonzi]

Originally Posted by eccerr0r

Btw did you find a specific op amp that was causing the problems seen?

I did not, after replacing the 1458 I tried the 4136 that's on the output stage, then the ones on the input stage, and the problem kept coming back. Then I also changed the ones on the unused channels and it ran for an hour with no problem, before I shut it off and went to bed. But some of the old opamps read some weird stuff, two of them have non-matching readings across the pins.

I keep them all in a distinct bag in order to test them when I get an oscilloscope, or a resistor for that matter ! I have some lying around at my gf's house.

As for the pots I'm going to test them but that's awkward, in order to get to the soldered side of the PCB I have to unscrew each and every cover, nut and washer of the 34 pots lol, see you in a bit