1073 preamp static noise - help!

[kr2p]

I'm having trouble repairing a Neve 1073lb preamp and could do with some fresh ideas as I've tried everything I can think of. The issue is that the preamp makes a static/crackling sound, sort of like a fire. It makes this sound at all gain stages and the volume of the noise approximatelty increases with the gain knob - more info below. I don't have a schematic of the 1073lb specifically, but the original 1073 schematic is in the link below. I've inspected the PCB thoroughly and it appears that the two audio paths are exactly the same. The only difference between the lb version and old version (at least in the gain sections relevant to this issue) is that the switching on the lb is done by relays rather than directly. Also, there's no eq in the lb so everything to do with that can be ignored in the schematic.

https://www.technicalaudio.com/neve/neve_pdf/1073-fullpak.pdf
For those not familiar, a very quick rundown of how the 1073 works is that there are two preamp boards (283 and 284 in the schematic) which both apply a fixed gain. At low gain levels, less than 50db, the audio signal only passes through the 283 board and the 284 board doesn't do anything. Also, it is not the gain of the 283 board amplifier that gets adjusted, but rather the volume of the input signal is cut to varying degrees via a basic voltage dividing resistor circuit. So for less than 50db gain, the circuit goes

Input -> passive volume attenuation via resistors (value of resistors, and therefore attenuation, determined by the switch) -> fixed gain from 283 board -> output

At higher signal levels, >50db, the amplifier circuit on the 284 board is engaged before the amplifier circuit on the 283 board to provide the extra gain. The 284 gain is also fixed, and the volume is adjusted by attenuation between the two boards. So at high gain levels it goes

Input -> Fixed gain from 284 board -> Passive volume attenuation -> fixed gain from 283 board -> output

Sorry for the long post, I've tried to give all the info I have. It's been a real nightmare of a problem to solve but i'm hoping someone here might be able to offer some insight. Thanks in advance for your help!

 

[pucho812]

Have you reflowed solder?

 

[kr2p]

Have you reflowed solder?

I've reflowed in some of the "key" areas around the gain circuits, but there's so many parts I'm wary of damaging it more by being over zealous with the soldering iron.

 

[Ike Zimbel]

This is the 500 series module, yes? I had one of those in last year with a 48v switching problem, and in the end I elected not to work on it as there was just too much SMD stuff crammed in there. Intermittent crackling is often caused by hairline cracks in: solder joints, circuit board traces and SMD parts whose ends are secretly coming off. This is the kind of thing that you might find with high level magnification, or by testing the SMD resistors (and other parts, where possible) for off-value parts. Good luck...this is not a job for a novice.

 

[kr2p]

This is the 500 series module, yes? I had one of those in last year with a 48v switching problem, and in the end I elected not to work on it as there was just too much SMD stuff crammed in there. Intermittent crackling is often caused by hairline cracks in: solder joints, circuit board traces and SMD parts whose ends are secretly coming off. This is the kind of thing that you might find with high level magnification, or by testing the SMD resistors (and other parts, where possible) for off-value parts. Good luck...this is not a job for a novice.

Yes it's the 500 series module. You're absolutely right, the SMD parts are a bit of a nightmare to navigate.. Although I'm guessing the 48V must all be managed on the much larger, more crammed board that sits at the bottom when you open it up. At least the issue I'm dealing with is on the upper, smaller, board that's less densely populated. Also, luckily, the bit I'm dealing with follows the old schematic so I'm able to figure out what's what in there fairly easily rather than having to figure it out from scratch.

That's useful to know. I've examined the circuit board pretty closely but will have a closer look for cracks. I've measured all of the resistors in the audio path, and they're all bang on, so I take that to mean that they're all good. I should say though that the noise isn't intermittent. It seems to be pretty much constant the whole time.

 

[kr2p]

On reflection I don't think it can be anything to do with cracks/badly seated components in the audio path. Reason being that the line input and mic input follow completely independent paths to the gain stages, and they don't "join up" until after the relevant signal has been attenuated. But from this point onwards the gain is fixed. So the volume of the noise wouldn't be affected by the gain knob if the crack/bad component was after the paths joined up, and the noise would only be in one channel if the crack/bad component was before they joined up. But I find that it's in both channels and is affected by the gain knob.

Could there be something bad going on with the ground? I only say this because as far as I can see that's the only thing shared by both channels before the attenuation stage. What could it be and is there any way to test this?

I've attached an audio file of the noise as I move the gain knob from fully counter-clockwise to fully clockwise. You can hear each step. Note how the noise suddenly drops significantly at the very highest gain levels (it's still there but barely audible most of the time). I don't know if that helps diagnose or just makes things more confusing.

 

[radardoug]

It would be usefull for you to actually measure the signal to noise and provide us with that information. So apply 1 kHz to the input, adjust level at maximum gain so that you just clip the output stage, and tell us the output stage level.
Remove the tone, measure the signal to noise, and tell us that.

 

[blakeyboy]

Hmmm...strangely, that sounds like a mechanically suspect small transistor.
I'd be reaching for my freezer spray and heat gun at this point.......
I had one on the 340 output board in a re-issue 1081 a few weeks ago- sounded exactly like your clip.....

 

[kr2p]

Hmmm...strangely, that sounds like a mechanically suspect small transistor.
I'd be reaching for my freezer spray and heat gun at this point.......
I had one on the 340 output board in a re-issue 1081 a few weeks ago- sounded exactly like your clip.....

I've tried freeze spray quite a lot and it doesn't seem to do anything, and unfortunately I don't have a heat gun to try out. I will see if I can find a tech to help as I'm not sure if I can replace the transistors myself.

It would be usefull for you to actually measure the signal to noise and provide us with that information. So apply 1 kHz to the input, adjust level at maximum gain so that you just clip the output stage, and tell us the output stage level.
Remove the tone, measure the signal to noise, and tell us that.

I will do that as soon as I can. Unfortunately I've had to self isolate this week so can't do the test for another few days, but once I do I will post the results.

 

[dmp]

I'd also suspect transistors, as I had a 1084 build where the bc184 transistors we're the source of the 'popcorn'/'fire' noise. And the whole bag (bought new from mouser) was bad I think. Swapped with transistors from a new batch to get past it

 

[blakeyboy]

Neve went through patches of bad BC184's.....the 226 boards in the routing modules of the EMINeve at ICP
often make this noise- the other five EMINeves I've worked on never have......

 

[kr2p]

While I search for a tech, I scrubbed the board with some isopropyl alcohol to clean it up and for the moment the sound appears to have vanished... I'm not sure how this could have helped, it's most likely a coincidence, but I have my fingers tentatively crossed..

 

[kr2p]

As expected the noise has come back.. but perhaps the fact that scrubbing helped temporarily might suggest something?

 

[analogguru]

Can you elaborate what you mean by "static noise" ?
I mean if something is "static", it doesn't move, like DC, and therefore there is no "noise"..... or are you talking about noise which is similar to the sound of "static-DISCHARGE" ?

 

[kr2p]

Can you elaborate what you mean by "static noise" ?
I mean if something is "static", it doesn't move, like DC, and therefore there is no "noise"..... or are you talking about noise which is similar to the sound of "static-DISCHARGE" ?

Static noise like a static tv channel. I posted an audio file of the noise in post 6.

 

[CJ]

use to fix car stereos, bummer of a job, stuff falling in your eyes while you are upside down contorted like a circus act, trying to wrestle the stereo out of the dash, back hurting, hotter than heck in the summer, sweat dripping from every pore which makes the dash debris stick, then the thing breaks again and you do another R and R, what a pain for ten bucks,

so the 1969 Mercury Cougar had these radios, the board would crack from the heat, because the radio is right by the heater outlets, so you fix one trace and another breaks. after about14 times i am looking for a high cliff.

so the guy who i replaced comes in for a visit, and i tell him the pain i am in with the Cougar radio, and he says, "oh yeah, been there done that, go down to Rendazzos Auto Upholestry on First St San Jose and ask for a Mercury Cougar gripling kit for $2.95"

sure enough it did the trick. the kit was nothing more than a cable harness of 14 colored wires and a chart showing where to connect them to the circuit board, thus jumpering all the broken traces.

still hate those cougars,

 

[jokeramik]

How looks the waveform at a oscilloscope? How old ist the unit?
If it is old please change all electrolytics.
Best regards
jokeramik

 

[BenJBX]

As expected the noise has come back.. but perhaps the fact that scrubbing helped temporarily might suggest something?

Hey everyone, I'm cruising through existing threads researching a problem I have with an old Sony TA-88 hifi amp that has this kind of noise on the left channel. Same as the OP, if I deoxit the channel fader it goes away for a few hours then comes back again. The fader ground connection looks OK and now I'm trying to figure out whether this stil points to the fader or whether to start looking elsewhere e.g. a failing transistor. Anyone able to shed any light on this query I.e. is it significant if cleaning temporarily fixes static noise, and if so does it point me in a certain direction as regards fault finding?

 

[scott2000]

Starting a thread with Sony TA-88 might be better. Never know , someone could have one and is familiar with them and their issues but would not look in a Neve 1073 thread.
Here is a service manual I grabbed. Not sure if it's the one for yours?

Transistors do get wonky.

 

[BenJBX]

Cheers

Starting a thread with Sony TA-88 might be better. Never know , someone could have one and is familiar with them and their issues but would not look in a Neve 1073 thread.
Here is a service manual I grabbed. Not sure if it's the one for yours?

Transistors do get wonky.

Brilliant cheers, I will do that. I did some more poking around and ripple on the supply rails is bad so I'm going to work on the rectifier circuit to fix that first and see what effect it has on the static problem.

I have the service manual for the TA-88 already but thanks for posting it anyway.