Neve 1272 preamps randomly lose output

[swordfishtrombones]

I have 2 x 2 channel Neve 1272 clones that I bought from Revive Audio about 10 years ago. Right from when I received them, I have had the problem that any of the channels may lose nearly all of it's volume apparently randomly. It can happen to one channel and not the other.
The preamps have a pad switch, and phase switch, and a phantom power switch before the input transformer. There is a DI input that bypasses the input transformer.

Things I have tried:
Returning them to Revive Audio - soon after I bought them I sent them back to Revive Audio - they could not find anything wrong with them.
Bypassing the pad and phase switch - makes no difference
Balanced and unbalanced output connections my interface
Difference interfaces (tried about 4 or 5 over the years)

Something odd I did notice - I gave up on using them as mic preamps as they weren't reliable. I used one unit between a Radial J33 phono preamp and a stereo power amp and it worked flawlessly. The Radial has an output impedance of 600 ohms, whereas most mics I have used with it would be about 150-200 ohms.
I haven't ever been able to deliberately recreate the issue using a test tone from my interface, it has only ever happened when using a mic and recording something. Very frustrating!

I haven't been able to find any 1272 troubleshooting guides, are there any available?

 

[CJ]

The toughest thing about these problems is keeping the preamp in failure mode long enough to troubleshoot them.

What kind of output transformers does it have?

 

[swordfishtrombones]

The toughest thing about these problems is keeping the preamp in failure mode long enough to troubleshoot them.

What kind of output transformers does it have?

I'm not sure, I will open it up on Sunday and post lots more pics and info.

 

[Ricardus]

Why is there an identical post like the top post here in another thread from a different user?

 

[pucho812]

intermittent issues are usually solder related. Try tapping on components with an eraser, see if it goes into failure? Then reflow solder in those areas after turning it off and letting it sit for a minute

 

[JohnRoberts]

Why is there an identical post like the top post here in another thread from a different user?

it has already been reported... it appears to be the same user with a new second account...

JR

 

[swordfishtrombones]

Why is there an identical post like the top post here in another thread from a different user?

Apologies for the double post. I thought my original post hadn't gone through and then realised I already had an account so I reposted. Please ignore my other post!

 

[kags]

Does the failure happen only with phantom-powered mics? I've had a similar problem where a John Hardy mic preamp would fade out after some time and then randomly pop back in. I replaced some caps and that solved the problem. IIRC it only happened to phantom-powered mics, but it might have been more than that.

 

[swordfishtrombones]

Does the failure happen only with phantom-powered mics? I've had a similar problem where a John Hardy mic preamp would fade out after some time and then randomly pop back in. I replaced some caps and that solved the problem. IIRC it only happened to phantom-powered mics, but it might have been more than that.

I don't remember it ever happening with phantom powered mics...

 

[swordfishtrombones]

The toughest thing about these problems is keeping the preamp in failure mode long enough to troubleshoot them.

What kind of output transformers does it have?

The Output Transformers are labeled "mini 1166" and 1150. A quick google makes me think they are a clone of the Marinair LO1166.

There is no label on the input transformers, but they look a lot like these:

https://www.hairballaudio.com/catalog/parts-store/audio-transformers/ea-10468-input-transformer
Please see attached photos for more details.

​

 

[Ricardus]

There is no label on the input transformers, but they look a lot like these:

Those look like my Ed Anderson clones that I bought from Hairball (the ones in the link).

You can see the sticker that says MIC INPUT partially obscured by the zip tie.

 

[Ike Zimbel]

I have 2 x 2 channel Neve 1272 clones that I bought from Revive Audio about 10 years ago. Right from when I received them, I have had the problem that any of the channels may lose nearly all of it's volume apparently randomly. It can happen to one channel and not the other.
The preamps have a pad switch, and phase switch, and a phantom power switch before the input transformer. There is a DI input that bypasses the input transformer.

Things I have tried:
Returning them to Revive Audio - soon after I bought them I sent them back to Revive Audio - they could not find anything wrong with them.
Bypassing the pad and phase switch - makes no difference
Balanced and unbalanced output connections my interface
Difference interfaces (tried about 4 or 5 over the years)

Something odd I did notice - I gave up on using them as mic preamps as they weren't reliable. I used one unit between a Radial J33 phono preamp and a stereo power amp and it worked flawlessly. The Radial has an output impedance of 600 ohms, whereas most mics I have used with it would be about 150-200 ohms.
I haven't ever been able to deliberately recreate the issue using a test tone from my interface, it has only ever happened when using a mic and recording something. Very frustrating!

I haven't been able to find any 1272 troubleshooting guides, are there any available?

If the 1/4" DI jack is in the signal path between the mic input and the line output, the switch contacts failing on those jacks are a really common problem, across many different makes and models of mic pre-amps. The next time the fault occurs, try running a plug in-and-out of that jack several times and see if it clears up. If it does, clean the jack contacts, or better yet, replace the jack with a top quality, gold part.
"Beware the switch you never use!" Me.

 

[swordfishtrombones]

If the 1/4" DI jack is in the signal path between the mic input and the line output, the switch contacts failing on those jacks are a really common problem, across many different makes and models of mic pre-amps. The next time the fault occurs, try running a plug in-and-out of that jack several times and see if it clears up. If it does, clean the jack contacts, or better yet, replace the jack with a top quality, gold part.
"Beware the switch you never use!" Me.

The DI runs in parallel with the output of the mic input transformer, I don't see how the jack being faulty would do it.

 

[Khron]

Where's that piece of schematic from?

 

[swordfishtrombones]

Where's that piece of schematic from?

I drew it

 

[swordfishtrombones]

This is how the Mic input appears to be wired. The Pad and phase switching is done with 2 x Omrom G5V-2 24V relays. These have a conact resistance of 50M max.

https://omronfs.omron.com/en_US/ecb/products/pdf/en-g5v2.pdf
I'm wondering if maybe the Relays could be causing the issue? A bit of leakage could mic the two out of phase signals and decrease the level.

I also noticed that the input transformer wiring is different to how JLM audio does it, JLM run the primaries in seriers instead of parallel...

http://www.jlmaudio.com/JLM1272hotrodmod.pdf

 

[swordfishtrombones]

Just noticed another potential issue with the relay circuit. The pad / phase circuits are wired to the NC (normally closed) contacts of the relay. This means a loss of 24VDC on the coil engages the pad or flips the phase.

The external switches for pad and phase control the 24VDC to the coil of each relay. These external switches are open when the pad or phase invert is NOT engaged. When you close these switches, the 24VDC at the relay coil is sent to ground, preventing any current passing thorugh the relay coil.

I think this is potentially problematic. The logic seems backwards to me, if we assume that usually we have the pad and phase inverter off, then the pad and phase invert should be wired to the NO (normnally open) of the relay.

I guess the use of relays is to avoid sending the signal to front panel switches, but the unamplified output of the input transformer goes though a pot on the front panel anyway. Perhaps it would be better to just have the pad and phase inverter on a few DPDT switches.

 

[Ike Zimbel]

The DI runs in parallel with the output of the mic input transformer, I don't see how the jack being faulty would do it.

I really wonder about this. With that arrangement, if you had, say, an SM-57 (or any dynamic mic) plugged into the mic input and you plugged a P-Bass into the DI input, you would hear the bass come out the microphone's diaphragm as well as the output of the pre-amp.
I have no idea of your technical chops, so please don't take this the wrong way, but if you metered the circuit to come up with your diagram, it will look like the mic transformer is in parallel with the jack, unless you have a jack plugged in to open the switch contacts. EVERY mic pre with a DI input that I have ever seen has the DI in series with the transformer output, and interrupts that path with switching contacts.

 

[swordfishtrombones]

I'm not sure what you mean about the jack? While it is a 6 contact, stereo jack, only the Tip and sleeve are connected. The Ring and all of the switched contacts aren't connected to anything. Plugging a just a cable into the DI jack itself wouldn't change the circuit, though it might add some noise by taking the signal path outside of the chassis.



I agree that in series might make more sense, but in this scenario I'm never using the DI input.