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LMII Cutting Out

#22
RBasss said:
I'm going to try to pick up an volt/ohm meter before I practice tonight from Radio Shack. What do I touch the leads of the meter too on the 810?
Click to expand...

Basically you want to touch the test probes to the two leads that get juice from the amp. (You do this test without the amp connected to the cab.) Banana-style inputs are easy to probe. A speakon or 1/4" jack won't be quite as convenient but you can still test by inserting a speaker cable in the jack and probing the contacts on the plug at the other end of the cable. (Note the Speakon plug has 4 different contacts - resistance should read "open" or infinite ohms until you find the right 2. You can also dismantle the plug - easy to do - and probe directly on the wires.)

+1 on checking other connections too. 2 different sloppy input jacks gave me a real headache, this last Sunday. (Similar symptoms, too - cutting out when I hit a note hard.)

HTH
 
#24
Well, I tested the cab at rehearsal last night. 1st, I plugged in like normal and played a couple of notes and of course the amp shut down into protect mode. 2nd, Hooked up just the top 4 x 10 section and played and everything was fine. Unfortunately with this cab there is no way to just test the bottom 4 x 10 by itself (it's either on in mono mode or you have to have an amp load in the upper input for the lower to work) but I believe I've narrowed it down that that is where the problem lies. I played through just the top 4 x 10 section the whole rehearsal (about 2 hours) with no problems at all. I remembered that I had bought a multi-meter from Radio Shack a few years ago when I was taking a computer building class so I've got that today and I hope to start testing stuff on my lunch break. My multi-meter has settings for VDC V, VAC V, K Ohms, and speaker CONT. I assume I should use the K Ohm setting? I'm going to check if any wires are loose and then I guess I'll put the meter on the speakers themselves. They're 32 ohm speakers so I'm assuming based on what has been said here that a reading far below this would mean that the speaker is faulty. Thanks for all your help with this, I'll update as I figure out what's going on. BTW, I think I can get through the gig this weekend with just the top 4x10, it just doesn't have the ummph that the lower one has since it's not directly coupled to the ground.
 
#25
Ohms (or Kohms) on the multimeter is correct. Depends on the multimeter display you have though - should be able to measure 32 ohms or so on the Kohm range. Don't use the continuity setting - someone recently did this to try to measure continuity of a pickup coil and it said open when the coil wasn't open. The CONT setting is really looking for essentially 0 ohms.
 
#26
Ok, just got back from lunch and did ohm readings on the lower 4 x 10 speakers. Here's the readings:

Upper Left Speaker - 27.3
Upper Right Speaker - 27.3
Lower Left Speaker - 26.7
Lower Right Speaker - 27.7

So, I'm guessing that the Lower Left speaker is suspect here. It's not off by much, but do you think this is the one causing the problems. All 4 of the speakers' connections were very tight so I don't think that is the problem.
 
#27
mynan said:
The amp has a protection circuit that...protects the amp. It can be set off by any of a number of factors...overheating, overloading caused by a bad cab, fluctuations in the power going into the amp, etc.

It may be a problem with the amp, but probably not...and if it's not the amp, be glad that the protection circuit is working instead of your amp blowing up.
Click to expand...

I played an outdoor show and had less than 100 volts of power going to my rig. This caused the amp to shut down as well.
I got a new feed that was around 120 volts and I was good to go all night with no further shut downs.

Chances are that is happening here. Try a power strip with a meter to see how much you are feeding your amp.

As mentioned by mynan, "be glad that the protection circuit is working instead of your amp blowing up!"

Thats another cool thing about the MB stuff! :bassist:
 
#28
RBasss said:
Ok, just got back from lunch and did ohm readings on the lower 4 x 10 speakers. Here's the readings:

Upper Left Speaker - 27.3
Upper Right Speaker - 27.3
Lower Left Speaker - 26.7
Lower Right Speaker - 27.7

So, I'm guessing that the Lower Left speaker is suspect here. It's not off by much, but do you think this is the one causing the problems. All 4 of the speakers' connections were very tight so I don't think that is the problem.
Click to expand...

Recheck the actual input on the cabinet. It could be that someone rewired your cabinet so that each 4x10 is 4 ohms. If that were the case, you would be running a 2 ohm load, explaining the amp shutting down and why it works fine with only one 4x10 connected.
 
#29
RBasss said:
Ok, just got back from lunch and did ohm readings on the lower 4 x 10 speakers. Here's the readings:

Upper Left Speaker - 27.3
Upper Right Speaker - 27.3
Lower Left Speaker - 26.7
Lower Right Speaker - 27.7

So, I'm guessing that the Lower Left speaker is suspect here. It's not off by much, but do you think this is the one causing the problems. All 4 of the speakers' connections were very tight so I don't think that is the problem.
Click to expand...

This difference doesn't sound very significant to me. At least not shorting problems which is what happened to me to cause the LMII to go into protection.

Edit - what shakerattleroll said is another possibility. I was assuming this was an un-modified cab.
 
#30
shakerattleroll said:
Recheck the actual input on the cabinet. It could be that someone rewired your cabinet so that each 4x10 is 4 ohms. If that were the case, you would be running a 2 ohm load, explaining the amp shutting down and why it works fine with only one 4x10 connected.
Click to expand...

The cabinet was brand new when I got it (straight from Ampeg, had a minor artist endorsement at the time). I've had it 3 years now, this just started happening...Wondering if when my Traynor YBA200 blew up at the beginning of the year if it shorted one of the speakers...

Are the ohm readings normal?
 
#32
slyjoe said:
Just noticed something - if All your speakers are 27 ohms, you are effectively running at 3.375 ohms. May be on the hairy edge of the LMII, and will be worse with low voltage.
Click to expand...

That's a good point. I did have a bunch of stuff plugged in to my power supply when we were rehearsing (2 powered monitors, other guitar players' amp, etc.) We've also been practicing in a warehouse so I've got about a 50' extension cable running from the outlet to the power supply. There's a power supply closer that our lead guitarist has been using, I'll try plugging just my rig into that and see if I get the same results. I really don't want to get rid of my LMII if this is the case...
 
#33
RBasss said:
That's a good point. I did have a bunch of stuff plugged in to my power supply when we were rehearsing (2 powered monitors, other guitar players' amp, etc.) We've also been practicing in a warehouse so I've got about a 50' extension cable running from the outlet to the power supply. There's a power supply closer that our lead guitarist has been using, I'll try plugging just my rig into that and see if I get the same results. I really don't want to get rid of my LMII if this is the case...
Click to expand...

I don't think you need to get rid of the LMII. But, double check the actual jacks on the cabinet. If it reads less than 4, that could be enough to engage the protection circuit. At that point, unless you are competent with speaker repair, I say take the cab to a reputable repair guy and get it fixed so that you're LMII is seeing a 4 ohm load at the input of the cab.
 
#34
That's good you're narrowing it down!

In my case, the bad driver read 0.03 ohms. That was definitely a smoking gun! I don't see that in your case here.

My understanding is when you measure a speaker's resistance, the reading is somewhat lower than the official rating of the speaker. So 27 sounds appropriate to me for a 32 ohm driver. Four of those would make an 8 ohm section so I don't think the bottom section you tested was modified to be 4 ohms. Unless someone modded only the top half for some really weird reason, the overall cab is probably 4.

Maybe one speaker is shorting only while it's being driven. I'm not a tech so I don't know how to test for that. (At least not scientifically. :D)

Maybe the overall cab offers just slightly too little resistance for the LMII? Apparently the actual impedance can vary quite a bit among different cabs that supposedly have the same rating.
 
#35
As someone already said, intermittent problems are FUN!

True, speakers tend to read a low DC resistance. I'm wondering now if the Traynor blow up did something to one or more speakers that only occurs when they are driven hard. Very hard to isolate/test for.
 
#36
Joeykun said:
I played an outdoor show and had less than 100 volts of power going to my rig. This caused the amp to shut down as well.
I got a new feed that was around 120 volts and I was good to go all night with no further shut downs.

Chances are that is happening here. Try a power strip with a meter to see how much you are feeding your amp.
Click to expand...


Where is a good place to get a powerstrip with a meter? I want something small I can measure voltage with at venues.

Thanks!
 
#40
Well boys and girls, I believe I have narrowed down the root of the problem. And the winner is... Voltage. As mentioned in a previous post I was running a 50' extension cord to my power conditioner which was powering not only my rig (amp, pedalboard, tuner, etc.) but 2 powered monitors, a small mixer, and our lead singers' combo amp. I plugged just my rig into a shorter 20' extension cable into a closer outlet and viola, no amp protection shut-offs. I played as hard as I could and could not get the amp to budge. So it looks like either I was drawing too much voltage or there just wasn't enough going down that long pipeline to power everything efficiently. I'm guessing that I was okay running just the top 4x10 because I wasn't drawing as much power to run that. I think I'm definitely going to look into purchasing one of the Furman PL-Plusses after this episode. We play a lot of clubs that don't have great power so it would be good to have that kind of protection. Thank you all for your comments and suggestions and for helping me out with this situation.
 

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