Active PA Speakers Mackie SRM350 whistling after warming up

Thread Starter

Saltech

Joined Mar 2, 2020
4
Hi All, I'm trying to troubleshoot a set of 4 Mackie SRM350 speakers that have been installed in a dance studio. They were mounted high up near the ceiling and were left on permanently. After a couple of years they have started making a loud high pitched whistling noise all the time. When they were taken down for repair and switched on to test (after being off for several days) they sounded fine - clean clear audio - but after being on for about 20 minutes a static noise / fizzing / high pitched tone starts which slowly increases in frequency over several minutes until it settles to the high pitched whistling noise. The noise will increase in level as the input gain is turned up, but does not disappear with the input gain all the way off. The speakers still reproduce music as before but the noise over the top makes it unlistenable.
Has anyone come across this or similar before? It appears to be a heat related issue - if the speakers are switched off and restarted soon after, the noise is back straight away, but if left a while the noise takes time to appear - and it's the same for all 4 speakers, which have had identical usage from new - they have always been used together in the same installation. Any ideas where to look first would be most welcome.
 

abrsvc

Joined Jun 16, 2018
92
You can download the service manual from the link below. From a quick glance a the schematic, it looks like you may have a defective IC (NJM4580) since this is one of the few components before the gain control. Try replacing this. It is likely a surface mounted IC and is inexpensive. I find it interesting that all 4 are having the same problem, but anything is possible. You should also check the electrolytic caps in that area while you are in there.

Link: https://elektrotanya.com/mackie_srm-350.pdf/download.html
 

Thread Starter

Saltech

Joined Mar 2, 2020
4
Thanks for the swift replies. I won't be able to get into the speakers until next week now, I'll report back early next week. Electrolytic caps was one of my first thoughts as I know they can degrade over time, but I couldn't figure why they would be OK to start with and then go bad after warming up.
 

Thread Starter

Saltech

Joined Mar 2, 2020
4
Nothing obvious inside. Lots of the components have been covered with a cream coloured hot glue type plastic, to hold the parts solid - it looks like it was dripped / poured on and then set solid. This makes it hard to inspect - it's hard to tell if an electrolytic is bulging with this stuff on it. They are SRM350 V3 so that schematic is not the right one unfortunately.
 

bassbindevil

Joined Jan 23, 2014
171
That might be a job for freeze spray; cool suspicious things down and see if that makes the problem go away.
Bad caps that I've encountered nearly always split and leak at the top; the merely bulging ones that I removed still tested OK. Heat was clearly a factor in those, because they were in an industrial PC with no fan cooling. Long-term heat might cause different failure modes where the cap just gradually dries out or something, like the caps in a couple of Jerrold TV converters I fixed.
 

Thread Starter

Saltech

Joined Mar 2, 2020
4
Progress! I think I've traced it. (skip to just before the diagram if you don't want..) The long story :

There are 2 main sections inside, the power supply / power amp section which is full of large caps and big amp transistors on heatsinks and inside it's own shielding metal box, and a smaller input / preamp section inside a plastic box, connected to the power section with a ribbon cable.

I started off by disconnecting the ribbon cable and running the speaker for a while to see if the squeal noise came (it didn't, just regular static input noise - pretty loud - I'm hoping it was OK to run it like this...) then I reconnected the ribbon to the preamp to the 'hot' / warmed up power section to see if the noise originated in the power section but needed the preamp connected to be heard e.g. a fault in the power supply to the preamp. [Hot power section, cool preamp section] There was no squeal, so I left it running to see if it would start squealing after the preamp warmed up. It didn't, but this was because it was resting upside down on the back of the face-down speaker, so not warming up as much. I put it back in the speaker box and it soon warmed enough to start squealing. Leaving it powered on, I carefully lifted it out and rested it back on top of the speaker and after a short while the noise stopped.

Next step was to fully expose the preamp circuitry by disconnecting it from the panel and use a hot air (soldering) blower with a fine tip to direct heat to different parts of the preamp circuitry with the speaker powered on. After the first pass at 100 degrees, nothing, so I upped the temp to 150 and tried again and managed to induce the squealing. With more precice heating followed by cooling with my finger tip (to speed up the recovery to no squealing), I'm 90% sure I've identified the faulty component, I couldn't read any markings on it, but got the details from the pcb markings (U14) and after checking the service manual, it's a NJM4580M dual op amp. The 2 halves of the chip are the first amp for the 2 input channels: SRM350 V3 preamp partial.jpg
So kudos to abrsvc - you nailed it!
bassbindevil, I couldn't use freeze spray as the natural state (on the bench) was not hot enough to cause the fault, so I did the same thing (?) but in reverse, using heat.

I've ordered 10x NJM4580M which will take a few weeks to arrive, enough for all 4 speakers, with some spares.
But now I'm now concerned that the preamps use quite a few NJM4580Ms and if one is faulty under these conditions, then maybe several are, or near the edge...
Any thoughts are most welcome, and can anyone explain why it makes a squealing noise?
 
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