jmdh Posted September 7, 2012 Share Posted September 7, 2012 Hi, I have a poorly Stageline PMX-400 mixer (belonging to a local amateur company). Aside from a few noisy pots which we can probably live with, the left channel of the main mix bus appears to randomly cut out. It'll work for over an hour (during set-up) and then fail for the show. Evident in both the amplified output, headphones and main outs. It seems to be a simple loose connection, as I can sometimes trigger it cutting out by flexing the chassis slightly. But where to go from here? I'd assume that I need a scope of some sort to probe various points of the board (and that building a probe lead and plugging it into other audio gear would be silly idea). If so can anyone recommend something suitable for general audio testing? There seem to be a large number of affordable devices on Amazon (eg http://www.amazon.co...47027565&sr=8-1) , but it'd be good to get some advice specific to audio gear. Don't mind spending a bit more for something which will come in handy for more complex audio test requirements. Also, of course, if anyone has any hints about where to probe first on this particular device that'd be great. I have a schematic from the manual but it doesn't help figure out the physical layout, and there doesn't seem to be much in the way of labelling. Thanks!Dominic Link to comment Share on other sites More sharing options...
Brian Posted September 7, 2012 Share Posted September 7, 2012 It's either... a) a dry joint orb) a broken track on the PCB Time to get a bright light and a magnifier out. Link to comment Share on other sites More sharing options...
Dmills Posted September 7, 2012 Share Posted September 7, 2012 A can of freezer spray is also useful for fault finding this sort of thing. Be warned that there is a reason EVERY engineer hates intermittent faults, you are never quite sure you have if fixed. The fact it gets both the cans and the main outputs is a clue that the fault is probably not in the power section, give the soldering to the switches and pots (including the faders) in the master section a careful eyeball (or just re do the lot). Oh, and if there is an insert jack in the signal path, try running a plug into and out of it a few times, the switching contacts sometimes give trouble on A gauge sockets, working them a bit sometimes helps. An old school crystal earphone is actually a remarkably good signal tracer for audio faults of this kind. Regards, Dan. Link to comment Share on other sites More sharing options...
Sound Man Posted September 7, 2012 Share Posted September 7, 2012 Have you tried plugging a stereo jack plug with the Tip and Ring terminals joined together into the relevent Insert Socket. Sometimes the insert sockets can cause loss of output. David Link to comment Share on other sites More sharing options...
Oldradiohand Posted September 7, 2012 Share Posted September 7, 2012 One of the most useful fault chasing tools I've got is a single headphone earpiece to croc clips - use a 100microfarad cap in series to block any DC and trace your audio through the circuit board using it. A signal injector is useful, too - I've got a 1KHz 0dBu battery powered oscillator built into a small grey box but you can always plug in a CD player.... Link to comment Share on other sites More sharing options...
KevinE Posted September 7, 2012 Share Posted September 7, 2012 Ah this is essentially a Studiomaster / Prosound unit. Popular faults for intermittent (everything) are the tiny solder joints on the ribbon cable connectors. Resolder them all. Have the master section out and resolder that too especially around the pots and connectors. And as soundman says, check/replace/clean any insert sockets. They will be Rean types with an extended unthreaded nose...not easy to get new. The clue there is that during a fault, the bargraphs will show proper output though the amp will dip. Most mixer section faults will cause the bargraph to react to the fault. Link to comment Share on other sites More sharing options...
jmdh Posted September 7, 2012 Author Share Posted September 7, 2012 Ah this is essentially a Studiomaster / Prosound unit. Popular faults for intermittent (everything) are the tiny solder joints on the ribbon cable connectors. Resolder them all. Have the master section out and resolder that too especially around the pots and connectors. And as soundman says, check/replace/clean any insert sockets. They will be Rean types with an extended unthreaded nose...not easy to get new. The clue there is that during a fault, the bargraphs will show proper output though the amp will dip. Most mixer section faults will cause the bargraph to react to the fault. I don't follow your comment here about the bargraphs? In the schematic I have, the inserts come before the mix faders, so will affect the bargraphs as normal (and the bargraph does drop during this fault). I think I've ruled out the insert socket, as the problem manifests in the same way with and without a shorted jack inserted (but when the fault doesn't show, the shorted jack doesn't affect the sound). I'm reluctant to do a wholesale resoldering job, simply because I'm not very good as soldering, but will pay particular attention to the ribbon cable connectors - and will pick up a crystal earpiece.. Thanks all - will report back... Link to comment Share on other sites More sharing options...
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