Markbass LMII cuttin out
Played a short set a couple of week ago and my sound vanished just as we started our 5th and final song. I hastily traced my cables to make sure nothing had come unplugged, then turned off the amp and swapped the the speakon speaker cable for a regular TS jack to jack cable, turned back on and had sound for a couple of seconds before it died again. Ended up stopping the song and was able to plug into the desk so we could finish our set.
after we finished I remembered reading about the thermal protection circuit and put it down to that, or maybe dodgy power in the venue. Tried it at home the next day for 2 or 3 hours - albeit at quite low volume - and it was fine.
Fast forward to rehearsals this morning, the first time I had used the amp since. Worked fine for about an hour and cut out. Turned it off for about 10 or 20 seconds and tried again and got about another 20mins out of it before it cut out again. This was using a rented 4x12 cab as opposed to my usual markbass 1x12 traveller cab. So I can rule out the cab causing the problem, or leads as I had tried both the first time it cut out.
When it cut out today I checked to see if the fan was working, which it was and it didn't feel warm to touch at all. So I'm thinking perhaps it's not the thermal protection kicking in - maybe a short of some kind.
I should add that at that first gig, when we we unloading the car, my amp fell out onto the floor when we opened the boot - it's aways in it's markbass padded case and the fact it powered up made me assume it was ok. But as one of my band mates said to me, it may have worked something loose causing one of the protection circuits to kick in.
I have opened it up tonight to see if I could find anything obviously loose, which I couldn't, I also looked for signs of dust and fluff, of which there was a small amount on the huge heatsink near the vents - not enough to cause overheating IMO.
FWIW, although my bass and pedal board are quite high output, I always set the gain appropriately so that it doesn't clip, eq is close to neutral, and I don't use the FX loops (I read on a thread that these could cause a short).
Any ideas of what to try before I take it for repair, or anyone else experience similar issues?
You probably have a cracked solder joint or a broken component lead. It's probably at one of the heavier components like a choke -- something high-mass like that. Most manufacturers silicone heavier components in place, but that doesn't get it done when it comes to protecting against a fall, nor is it designed to. There's just too much force applied to heavy components when the impact happens.
Run the amp with the cover off and, using a chopstick or something else insulated, poke lightly at the heavier components to see if you can make the amp cut out -- or if it's warm, come back to life. You might get lucky and be able to find a broken connection that you can re-solder. If a component lead is broken above the solder joint, though, then the component will have to be repaired or replaced. Unfortunately most of the heavy components that would pop loose or break on impact are not marked, which means finding a replacement would be tough. So you might have to unsolder+remove the component, unsolder+remove the broken leg from the board, and try to fix the leg in a manner that will tolerate the heat of resoldering back in place w/o coming apart again.
If you can't solder, at least you can locate the offending component, or at least the problem area. That will help the repair people if you decide to get the amp fixed. But don't be surprised if they want to throw a board swap at the problem, rather than troubleshoot down to the component level.
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