GK MB150E Problem: Speaker cuts out

jedgargroover · #1 12-21-2010, 07:44 PM

So I got this amp a few years back and at first it was ok, no problems. But after a very short time the amp would cut out on me. At gigs. Just quit. No sound. And it would stay dead for a random period of time and then come back on.

Once it had cut out once it would then cut out again with increasing degrees of frequency...and I'd look really dumb. The guys in the band would get more and more pissed off.

I took the amp in for repair to my upright tech (although I don't play upright anymore...it's still a great portable amp...when it works) and he said that he had his buddy in the repair shop across the street (in Midtown Manhattan) look at it and it didn't appear to have any problems.

As it turns out the tech was an a**hole and screwed me over on a consignment sale and some other transactions...but that's a different story. But I'm thinking one of three things:

1) My upright repair guy never actually brought the amp to his friend. He was too lazy. He just told me he did and gave the amp back to me.

2) The amp WAS looked at and the tech really didn't see anything wrong...in which case...what's up with the amp?

3) The amp is crap and I should put on eBay for cheap as a parts amp or for a project for someone more skilled with electronics than I am.

So...anybody ever own this model amp and have a problem like this? What should I do? Is there something I should try before dropping $$ on a repair shop tech?

Guide me bassists of the world.

Bassman822 · #2 12-21-2010, 07:57 PM

Ive had a MB150s for 18 years and havent had any isues like that. The first thing Id try is swapping the speaker out

jedgargroover · #3 12-21-2010, 08:01 PM

Actually, I forgot: I did swap out the speaker. Didn't fix the problem.

BillyIVbass · #4 12-21-2010, 08:05 PM

I had trouble with mine. Similar thing. It would stay powered on but the sound would phase in and out.

I internal speaker mute button could have something to do with it. I bought mine used, used it HARD for 2-3 years then sold it for parts.

billfitzmaurice · #5 12-21-2010, 08:12 PM

If it has a thermal protection circuit that's the likely source of the problem.

jedgargroover · #6 12-21-2010, 08:19 PM

Quote:

Originally Posted by billfitzmaurice

If it has a thermal protection circuit that's the likely source of the problem.

I knew a girl who had that same problem. Needless to say, she went alone to prom.

Thanks bass people! I will go to a tech and have them check out the thermal protection circuit. Would the issue be that it's fried and needs to be replaced?

fdeck · #7 12-21-2010, 08:22 PM

I have a MB150E. My advice: When it cuts out, try all of the jacks. See if you get sound when plugging into the stereo aux input, for instance, or the effects return. You could have some dodgy "switching" jacks, particularly in the effects loop. Try a patch cord in the effects loop. Plug in an external speaker. In short, try all of the possibilities, to isolate the problem to a specific portion of the amp.

manbass · #8 12-22-2010, 12:17 PM

Weird...I just had my 20 plus year old MB150E (I bought it new in 1990) do the same thing only no sound...just last night. Not cutting in and out...just no sound at all while the light stays on. Internal spkr button on or off doesnt change it. I used it the night before worked perfect as previous and until now, it never had a twitch, ever.

It shows power, green light is on. I get a small pop...very small, when powering on without a jack in the Bass Input. I get no sound from the headphone jack.

The thermal protection sounds possible as I moved from forever in Florida to Maryland, and the amp hasnt seen below 68 degrees until this week. How do I test or find this circuit? Open the top?

I love this thing, been 1000% reliable til yesterday....I will try all jacks to isolate and report back if I find anything. I have an extension Schroeder cab, will try that as well.

thejumpcat · #9 12-22-2010, 01:24 PM

I had the MB150s and it did the same thing (before I sold it).

I lent it to Ronnie James with the Fab T-Birds for a small club gig, along with with my Bag End S-15. The sound from the amp speaker AND the ext. cab cut in out during the gig --- but the sound from the XLR to the mains never did. Pretty frustrating, to say the least.

My drummer has a newer MB150s and he says he's had a similar problem, although not consistently. Mine wasn't either. Only happened one other time.

jedgargroover · #10 12-22-2010, 03:01 PM

Quote:

Originally Posted by manbass

Weird...I just had my 20 plus year old MB150E (I bought it new in 1990) do the same thing only no sound...just last night. Not cutting in and out...just no sound at all while the light stays on. Internal spkr button on or off doesnt change it. I used it the night before worked perfect as previous and until now, it never had a twitch, ever.

It shows power, green light is on. I get a small pop...very small, when powering on without a jack in the Bass Input. I get no sound from the headphone jack.

The thermal protection sounds possible as I moved from forever in Florida to Maryland, and the amp hasnt seen below 68 degrees until this week. How do I test or find this circuit? Open the top?

I love this thing, been 1000% reliable til yesterday....I will try all jacks to isolate and report back if I find anything. I have an extension Schroeder cab, will try that as well.

Yeah...that's basically what mine does. When I say "cutting in and out" I mean "going totally dead even though the power light is on and then staying that way for several minutes, then working for a few minutes and then going dead again for increasingly longer periods of time until I want to put it through a wall".

I've heard that these amps sometimes have this issue...and I too will follow up on the excellent advice from posters.

BassmanPaul · #11 12-22-2010, 03:52 PM

If the amp is fan cooled take a look and see if the cooling path is blocked by a build up of crud. If it is then vacuum the amp out.

manbass · #12 12-22-2010, 03:53 PM

Nope no fan...heat sink only on top.

craig.p · #13 12-23-2010, 08:41 AM

Let me pick up where BassmanPaul left off: Go buy a cheap personal fan from Wal Mart and point it at the vents on either side of the chassis. It could keep internal temps low enough for the problem never to surface again.

You could also open up the amp and look for cold/detached solder joints, loose/flaky plug-in PCB connectors, etc. You'll need a good magnifying glass and a strong light source for that.

manbass · #14 01-09-2011, 12:53 PM

Update - Tried to open the amp top, removed all screws but all the boards are adhered upside down to the heat sink top...wiring is keeping the top from being pulled fully open enough to look for problems. Closed it. Checked the fuse...all good there. Tried plugging in the extension cab....no sound. Its going to service now. Reading the manual it must be a protection circuit fault...just guessing. I have a few times long ago, left the thing on by accident overnight. Could have heated up some connections enough to be brittle then now they have broken/sheared with temp changes. Will advise what Service says.

manbass · #15 02-13-2011, 06:59 PM

Update again....took the MB150E amp to Jivesound, checked the jacks are ok, opened it up, cant find the problem..suspect its in the preamp. Benchtime costs on this may exceed the value. Not sure if I have a totalled MB150E here. Likely will cost $80 to find out. Advice?

domdec314 · #16 02-13-2011, 09:46 PM

Did you try putting a patch cable between the effects loop jacks? I had what sounds to be the exact same issue and the problem ended up being dirty effects loop jacks.

RobJ · #17 02-13-2011, 10:29 PM

I've had my little GK for well over 12 years now and did have that problem as well and it about drove me nuts. I'd done some research and there were certain serial numbered amps that had a known problem with similar symptoms so I called the factory. Mine wasn't one of them but working with the factory tech over the phone we figured out it was the input pot. As strange as it sounds cleaning the pot seems to have fixed the problem as it hasn't reappeared and that was probably close to 7 years ago. Great little amp imo and worthy of repair.

ygdartis · #18 02-14-2011, 10:22 AM

Hi, i just got the last model of MB150E delivered after an 8 hours trip by plane,

I had a extreme hard time to set it up at his supposed level and to have it 'sounding' without getting an umbearable distortion.

Active -> third of the Knob -> distortion
Passive -> Half Knob -> distortion

Not more than the Half for the EQ volume knobs, before to distort the concerned freq band.

I tried all configs to isolate preamp from Speaker in order to know if problem was mecanic or elec(tron)ic, easing the speaker with another one plugged in the 'ext speak' Left, right, Local speaker ON or OF, sent the preamp throught lines out on a mix board symetrically or asym (line out) plus DI on the mix board,tried with two bases, Fender and Peavey, changed jacks (there been a bit of an improvement here)

Signal saturates at more than half each gain knob. Im quite far from the pleasure a lot of people share bout this amp, so i think there may be a problem. (Input could be a thing for me and Thermal protection could have been damaged in the plane)

I got a couple of cuts as well, the reason i come on this post, so im gonna look forward and try some of your suggestions. Im in Cambodia and electros are not having meetings here.

Some feedbacks will also be welcome, i will give some if i find something which can be helpful.

Thanks to you guys, for making such a clean nice post.

manbass · #19 02-14-2011, 03:46 PM

I have decided to spend the jack for a Tech (a GK Authorized) on finding out what the deal is. Will report back.

RobJ · #20 02-14-2011, 06:20 PM

Cool. I'm curious to know, thanks.