Soundcraft console problem: no signal unless +48v momentarily switched on

[dasbin]

The subject pretty much describes the problem.
I've got a Spirit8-24 console where a few of the channels won't pass audio unless the phantom power is switched on momentarily. This seems to "kick-start" the preamp into operating again, though I'll have to do it again every 30 minutes or so.
Another symptom on these channels is that when they are in "non-working mode" (ie before phantom has been toggled), if you crank the gain you will get so signal until absolute maximum, when the signal will channel will first "pop" then pass a very quiet thin signal while at maximum pre gain.

Any idea what could cause this? I'm hoping for something I can fix myself.

It is possible that on some of these channels, phantom was inadvertantly engaged while an unbalanced source was connected (pins 1 and 3 shorted). I know this can sometimes cause damage. Could this be the cause, and if so, what would be damaged?

[Tim Farrant]

mmmm, sounds like the input stages have developed some sort of fault where the DC bias on the input transistors is a muck, and the phantom surge temporarily biases it up again. This is certainly a problem for an experienced tech to look at. I would suspect that electro caps may need replacing, or the actual input transistors may have been damaged, or maybe some bias resistors have gone open, difficult to tell with out having the thing here on the bench.

Good luck with it.
Tim

[dasbin]

Thanks Tim, that is very helpful. I will likely then just replace all the components you mentioned and hope for the best.

The problem is that I don't know any techs around here I can trust... I trust your probable diagnosis online more than I'd trust a half-assed tech I don't know poking around for hours and racking up a huge bill.

It's probably the stupid switch. If the switches are anything like the API boards then I would go in that direction. Your audio does pass through that double pole double throw used for 48V. Because if it works for a little bit, like a half hour then it's not going to be something too serious. Unless it's getting to hot, and shutting down, but that kind of protection is normally not afforded in studio gear. Intermittent problems are a bitch. Play with the switch.

[Matt Syson]

Hi
I would certainly go for 'excercising' the switch as you probably don't normally operate it often. you could pull the fuse out that supplies the 48 volt phantom then put moderate level audio through the input (as normal) then operate all switches on the channel and listen for any crackles or intermittencies. Repeated operation of phantom is probably not a good idea with the supply actually available, hence the suggestion for pulling the fuse (while doing the experiments).
Good luck with it
Matt S

[Mick Crash]

Sounds like it's indeed the switches. With the power off - try a small shot of Lexite PS contact cleaner from Chemsearch and exercise it. Lexite is the best contact cleaner I've ever used. That should fix it. If not repeat a few times. If it's still NG try replacing the switch. I doubt it's a transistor or chip. I do Soundcraft authorized on site service in NYC and never have had a call yet for your console though I've serviced many Ghosts, DC 2020's, Series 4's etc. I'm surprised there aren't any tech's you trust in Vancouver. Try the Soundcraft web site to find an authorized service tech if you need one. This should be a very easy fix. Good Luck!

[dasbin]

Thanks guys but it is definitely not the switches - they both tested fine.
I didn't really think that was it anyway considering the problem was not intermittent but rather very predictable and was acting like a capacitor or bias problem (having to "reset" the process after a predictable amount of time).
These channel strips are very easy to work on and I want to start swapping components, but unfortunately I can't get my hands on a schematic (they are expensive from Soundcraft) and I don't have a good eye for blocking up sections of circuitry in my mind.
If anyone can give me a heads-up as to where I should be looking it would be helpful. I can post a photo of the strip if necessary.

There are two 10uf/50v electros right by the input along with several resistors and ceramic caps. Any transistors are quite a bit farther down the board. There are 5 TL072's per channel but I am assuming they are for EQ/Aux/Summing/etc. and the preamps are discrete... right?

The nearest Soundcraft service centre is on the opposite side of the country and I need this fixed in a week so that is not going to happen.

[Mick Crash]

Sorry it wasn't the switches. Try the fundamentals first and check the power supply rails after the fuse resistors near the ribbons. Then check the voltages on all the TL072's. You should be getting about + and - 17 volts or so. Any significant deviations or missing voltage will point to the problem component(s). Soundcraft should be able to email you a .pdf file of the channel schematic. If I had it I would. If they can't or won't help, try Creation Audio Labs in Nashville.
It is very surprising that the same problem is happening on several channels so I still suspect something like bad connections, maybe loose ribbons. Or, maybe some outboard gear with excessive DC offset at the input is causing blowage when it's plugged in. Make sure you check all the basic stuff first before jumping to conclusions about blown components. Shotgunning (changing all suspect components) doesn't fix things when it's a dumb problem. Best of luck.

[Matt Syson]

hi
Assuming you have a multimeter I would measure voltages around the input transistors and the nearby TLO72 and jot them down while the channel is working (or on an OK channel) then leave it until it stops working and try again.
Swapping the TLO72 might possibly help if it has suffered some form of internal damage causing strange latch up problems. Move it to another place if you don't have a new one.
There again it may not actually be the input stage at all as a good kicking by phantom would probably wake up any poor connection in the strip, so listen out for ANY crackles on ANY control with maybe a low level tone into the channel at say 300 Hz. It could simply be dirty connectors especially in a smokey environment.
Matt S

[ulysses]

Don't just replace all those components at once. If you decide to use the shotgun approach, replace all the electrolytic capacitors first, then see how it works. This includes Aluminums and Tantalums. They are the most likely components to fail, so they're the first to be suspected. When they go open or get leaky or short out, they will shift your DC biases on your transistors, which sounds like the problem you're having. Squirting a little phantom current through a leaky capacitor maybe shifts the bias for a while until that charge drains away.
Resistors can be checked pretty easily with an ohmmeter, but you have to consider the effect of the surrounding components unless you're able to desolder one leg first. The input transistors might be matched for some parameter or other, and I wouldn't replace them unless you're sure there's something wrong with them.

[dasbin]

Funny thing - it seems to be working now. I didn't really do anything except take out the channels then plug them back in again for testing.
I have no idea how a loose hookup cable could cause such a bizarre symptom, but hopefully all is well now.

[Tim Farrant]

Originally Posted by dasbin

Funny thing - it seems to be working now. I didn't really do anything except take out the channels then plug them back in again for testing.
I have no idea how a loose hookup cable could cause such a bizarre symptom, but hopefully all is well now.

Well you do get some odd things in electronics from time to time. How old is the board? Some of the early Soundcraft mic preamps lacked protection diodes, and phantom spikes used to cause failure if the gain was turned right up, and you switched the phantom on.

I looked after this Allen and Heath live console for a hire company. GL4 I think it was, big 40 input. Anyhow, the mic preamps in it starting playing up, not just one or 2, but a good number of them. Traced the fault to faulty 4k7 resistors. Just about all the 4k7 resistors in the board (presumably from the same manufacturer) were failing and going open. And man, there were quite a few of them to replace!

Originally Posted by WHITEAUDIOLABs

...Your audio does pass through that double pole double throw used for 48V...

No, usually the phantom power switch preceeds the standard 6k8 feed resistors onto the input legs. Audio does not pass thru it as such. However, the MIC/LINE switch on these consoles might be questionable.

Cheers
Tim.