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Double Bass GK MB150S - I got a Lemon

M

mrjay

Feb 1, 2005
13
0
www.zrabbit.com
#1
A year and a half ago when I was researching what bass amp
to buy I ran across a basic theme about the GK MB150 amps -basically you either get a good one or you get a lemon. Well, I think I got a lemon :bawl: . I bought on E-bay (so maybe it's my fault). It sounded great at first with both EB and URB. However I notice that my Fender Jazz Active would (with attenuation for active turned on) occasionally cut-out - no signal at all. I would have to unplug the cable and plug it back in fiddle with volume and it would be back. I took it to an authorized dealor who could but the problem was intermittant and he couldn't replicate but resoldered some connections and it was fine for about a year. Last night it cut out on my URB (Fishman full circle no preamp) during a solo. I nearly through the amp out the window. I unplugged the cable and plugged it back in and it came back.

I'm still new to URB and still finding my way towards "the sound" I have a PreSonus TubePre that I learned could plug into the receive. I tried it last night and it sounds much better than plugging in direct but I have no EQ and it was warmer but boomy.

I've seen good reviews on the Fishman Pro-EQ Platinum Bass. Does anyone have experience with that going into the receive of this amp. Or should I chuck it and get a new amp (thinking iAMP8000 combo if I can swing it for both EB and URB as I'm starting to player larger venues)? Would the Fishman be a was if I decide on an iAMP down the line. Sorry for the long post :)
 
#2
Man, I know exactly how frustrating that is! The problem with your MB-150S might be that the contacts of the effects send/return jacks need cleaning (a temporary fix is to "exercise" the jacks by pugging and unplugging a cable into them) - they're rather difficult to access to do a proper cleaning - that might be a task left to your electronics tech. The sockets are not of the type which have wiping (self-cleaning) contacts, and they easily become dirty, and intermittent. I had exactly the same symptoms with my MB-150E, and it took my tech about a month of detective work to hunt it down. It didn't depend on temperature, and would occur in a totally random fashion. He would run the amp all day into a dummy load and it wouldn't fail - and then, out of the blue, it would start cutting out. A very difficult problem to diagnose, and it sounds like you have the same symptoms. I'm surprised that you didn't come across my story in previous threads (mine happened about 4 years ago). While he was at it, he did all the ECOs on my amp, and it has worked perfectly ever since. I hope this helps -

FWIW -

- Wil
 
#3
Just to clarify the cutting out has always happened when I plug directly into the input of the AMP with no preamp.

I litterally didn't know about plugging a preamp into the receive until last night when I searching the web for other that might have had this issue. But I'm going to start playing that way so time will tell if the issue is in the pre-amp or post-amp and/or connections.

Glad to hear you got yours working - so it's good to hear a lemon can be transformed into a.... peach :eyebrow:
 
#4
Sorry, but to clarify just a bit more - the effects send and return jacks are normalised to pass straight through - i.e. if they're not being used, they should pass the signal straight through, but if the contacts are dirty (even if the sockets are not being used) you will see the problem… I know, it's confusing, but believe me, if there is a problem with the effects send/return sockets, you will still see it, even if they're not being used… (I should have clarified that earlier - sorry) BTW, try using a patch cord from the effects send to the effects return (thus by-passing the contact switches on both sockets) - that will prove the point once and for all…

Good Luck -

- Wil
 
#6
I had this problem with my '87 400RB. Like Wil said, it will happen even when you're not using the FX loop. I sprayed contact cleaner in the jacks, then had the tech at GK give 'em a once-over when I took it into the factory to get the boost circuit repaired.

I sometimes use a Fishman Platinum Pro Bass EQ going into the FX return of the GK, bypassing the preamp. It sounds great--it's a flatter, more hifi sound with bigger bottom and brighter top. In comparison, the GK preamp has a more pronounced midrange emphasis, which I find useful in loud situations where I have problems hearing my intonation.

The Fishman also has a much higher input impedance (10 MegOhm vs. the GK's 1 MegOhm), making it great for piezo pickups. The variable low cut filter is also very useful for reducing boom/feedback and maximizing volume without wasting amplifier power on subsonic frequencies. The phase switch helps attack feedback and the compressor can help even out pizz/arco levels. It would be great if there was a mute switch and I think a few bands of semi-parametric EQ would be more useful than 5 fixed bands, but it's a very handy tool that works great for DB and BG, live and recording. I'm sure you'd still find the Fishman helpful if you got an iAmp 800 later.

I once played a painfully loud indoor rock gig in a smallish club with my New Standard Cleveland DB running through a Schertler Stat-B pickup and the Fishman into the GK and an EA Wizzy Cab. I sent the XLR out of the Fishman into the club's sound system which had 4-18" subs sitting ON the stage. Sounds like a recipe for feedback, but thanks to the Fishman's controls I was able to get a good tone, really loud, with no feedback (unless, of course, I wasn't muting unplayed strings).
 
#8
I had the problem with my MB150E. In my view, it is a problem waiting to happen with any amp that uses those plastic Neutrik 1/4" jacks, unless Neutrik has improved their design. :mad:

If you are ready to throw the amp out the window, make sure it lands in a UPS truck headed for my house :D

Some things I have had to do with my MB150E include: Bypassing the footswitch jack, when the stereo chorus started spontaneously activating itself (It sounded like I had gone out of tune the first time it happened, so I reached for a tuning peg and got even more out of tune!). Next was the effects loop -- somehow I recall adding a 10k resistor on the preamp board to solve that problem, but my memory is dim. You can often temporarily fix this problem by just plugging a patch cord into the effects loop.

Thankfully, the MB150S has fewer jacks to go bad. If it were me, I might be tempted to replace the offending jacks with good old fashioned Switchcraft Type 11 series -- the old open frame type -- and to simply defeat the unused jacks.

I felt that the GK was worth fixing because of its sound and portability. Beware if you take it apart, be careful not to strip the threads on all of those #6-32 screws. Tighten them just enough and no more.

Switchcraft, Switchcraft, Switchcraft
 
#9
Jazzman said:
This isn't just a problem with GK amps...I had the same exact problem with the EBS HD350 head. Has anyone thought about hard wiring a bypassing the effects loop?
Click to expand...

I've done that for dozens of customers over the years. If you do it, try to make it easily reversible, it helps resale value a lot.
 
#11
Didn't read the whole thread, but I think I know what it is. I had a similar problem with an old MB200 (my volume knob would bypass intermittantly and give me '10' -- cool feedback effect on a cocktail gig!). There's likely a hairline fracture through the PC board.

Have someone that understands these things follow through the board with a magnifying glass and he'll likely find it. Another thing to do is to replace the stock jack, which is mounted on the PC board, with a standard switchcraft that you then connect to the PC board with wires and solder. This takes the strain off the board, and if tried first may alleviate the problem.
 
#12
From my experience, it is very likely that the problem is with the effects return jack, which is a switching-type jack. Get some good-quality, no-residue electronic contact cleaner spray (DO NOT use tuner / pot spray with silcone!). Insert a phone plug into the effects return jack to open the switch and thoroughly spray the contacts between the tip connection and the shorting connection.

As someone else mentioned, this problem can happen on almost any make or model of amp. I've seen it happen on several GK's however.

BTW at least on the older GK's I've seen, the rear panel jacks are made by Switchcraft. Only the front-panel input jack is not made by Switchcraft. It would be a good idea to spray the contacts of the input jack too, while you're at it. Just don't over do it. A little contact cleaner spray goes a long way.
 
#14
Maybe this might help...
I had a GK800RB that did the same thing. I had replaced all the jacks with Switchcrafts (I find I have to do that on all my amps and speaker cabinets occasionally, since the humidity in Florida is hell on those things) and it worked for about six months. I then found the problem this way, and you might pass this on to your tech. Tell him to take the BUTT end (not the metal tip side of a screwdriver) and have him lightly tap the foil side of the circuit board while the unit is running. That has worked great for me to track intermittent open circuits, Ive used it to troubleshoot Audio/Video problems in TV's and Stereos. (I used to work as a consumer electronics tech.)
A lot of times the wave soldering machines that are used to build most pc boards will leave a cold solder joint and you have to go in there and resolder it, and yes, you can do this by trial and error (resolder a suspect area) but that tends to be more of a hit and miss kind of thing. If you tap the board with a screwdriver or a wooden dowel while the unit is running, a lot of times you can duplicate the problem by knocking the bad solder joint in and out as the board is tapped/flexed. This will then narrow down the area that you need to resolder. Another trick that we use is to let the unit run on the test bench until it acts up. Take a can of freez-mist (its basically freon, available thru MCM electronics) and spray the foil and component side of the pc boards. When the freon touches the suspect area, it will cool the unit down and rejuvenate the bad connection temporarily, thus narrowing down the suspect area. If the freon cures the problem on the foil side, then its a bad solder joint. If it cures the problem on the component side, my experience is that its usually a capacitor or semiconductor (IC's and transistors). Resistors and coils dont seem to have thermal intermittents as much as the other two. This is of course assuming that youve already cleaned the jacks, but replacing them would definitely rule that part of it out, since cleaning is still kind of hit-or-miss.
For those you that are pretty handy, I would highly recommend MCM electronics for plugs/jacks/cables/sprays etc. (mcmelectronics.com) Their website is worth a look and I would recommend having them send you a catalog. They have EVERYTHING Ive ever needed for electronic repairs.

The problem with my 800RB was isolated by tapping on the board that is visible when you take the top plate off the amp. GK wouldnt sell me a schematic, but Im guessing its part of the preamp/EQ circuit, since the power output side is located more towards the rear of the amp and under that board. Once I resoldered it, that was it. Ive used it for ten years after that and havent had a problem. FWIW, I had the same problem on a Peavey bass amp, and after tapping around I found the loose connection, resoldered it, and that took care of it.

Hope this helps.
 
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#15
Oops, almost forgot....
Ray is right about having the guy look at the board thru a magnifier, there's a lot you can see with those gizmos that you will miss with the naked eye. Your tech should have an apparatus on his bench that looks like a flourescent light bulb formed into the shape of a circle with a big magnifying glass in the center of the bulb. This thing is mounted to a steel arm that sits about 3 feet high and can be flexed/turned to hold the magnifier in position while he inspects the board. If there is a crack in the board, your tech should be able to repair it this way.
When you are looking at the foil side of the board (where the solder connections are visible) you will see the solder connections are joined together by a thin copper foil called a trace. If the board is cracked, then one or more traces will also be broken. First, he should epoxy the board where the crack is. After that dries, he can hardwire the broken traces by locating the nearest solder pads on either side of the broken trace, and solder a wire connecting those two pads together. Some shops will probably try to sell you a new board, but Ive been able to repair 1000's of boards this way, thus making the customer happy, and not getting a declined estimate (i.e. giving them such a high price that they decided it wasnt worth fixing.)

As far as Mikes question goes, yes, theoretically you can bypass it this way, but the problem is, if you have cruddy input/output jacks, or something else in the circuit between them is causing an intermittent open circuit, then you will still be stuck with the same problem. I dont know anything about this amp, but if the effects loop is known for having this malady, I would replace the jacks first, then go from there, since jacks and potentiometers (i.e. volume/tone controls) seem to be the most notorius things for failure.

One last thing about cleaning jacks. If you can get the cover off and have access to the jacks, spraying cleaner onto a Q-tip and hitting the metal tab thats shaped like a fish hook will clean the inner conductor better than just spraying cleaner thru the hole via the outside of the amp. Also, run the Q-tip into the input jack and clean the inside of the barrel (outer conductor) as this will clean the jack better than merely spraying it.

Good luck
 
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#16
Well I got the Fishman bass ProEQ and connected it to the receive and it sounds great. I really like the compression on that and the phase inverse does really cut feed back. I found it works well for doubling as I can have the EQ of the standard input for EB and the fishman for the URB via the receive. Just unplug the Fishman and I'm all set for EB and vice versa.

I also cleaned up the jacks and swapped cables and no cut-outs since then but if I do I'll try switchcraft and checking the board for cracks.

Thanks to everyone for the tips this site is really great!
 
#17
Just my opinion, in these days of epoxy fiberglass circuit board material, cracked boards are less likely than solder joints working loose from stress, or even cracked traces. And a magnifier is a great idea. I use a small magnifying glass and a flashlight in my toolbox for this purpose. My eyesight is just not good enough any more.
 
#18
Passinwind said:
I've done that for dozens of customers over the years. If you do it, try to make it easily reversible, it helps resale value a lot.
Click to expand...
It sounds like You are a Tech.I have a worse problem with my Previously Mint GK MB150-S
My amp was mint at the time a few weeks ago when some moron sent me 220 volts . I live in Taiwan. My techs were able to get it up and running by replacing the two main power supply caps which exploded, along with several other parts silicon devices... The amp performs beautifully, except when You shut it down. If the limiter is engaged, when You shut down the main power switch, it goes Pop, the limiter light goes out, and then pop-pop and the main power light goes out.
If the limiter is not engaged on powerdown... The exit sound is normal. What do you suppose is happening?
 

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