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MB150 E issue

S

snowdan

Feb 27, 2006
660
7
#1
my gk mb150 has some kind of illness.

symptoms:

often it won't make any sound. preamp and headphone out are OK and working, silence from power amp. sometimes it comes to work for a second or so after a few minutes. sometimes not. sometimes it works right from engaging the power switch and flawlessly for hours, but the next time it has the same issue again.
same with external speaker, so it is clearly located in the poweramp region.

what could be wrong?

i doubt i want to pay the german distributor for repair, they are expensive.
 
#2
It is possible there is a bad connection inside the amp, as there is a cable between the preamp and power amp, and the headphone driver is on the preamp board. Of course you would have to be extremely careful opening up the amp, because of the high voltage present inside.

Another word of caution about opening the MB combo, is that there are screws threaded directly into sheet metal, and it is easy to strip the threads.
 
#3
if it has an effects loop, or power amp input jack, try
turning amp off & plugging a cord in & out several times at
the effects return/power amp in jack.

unfourtunately, your signal relies on an 'iffy' connection
there as a switch closes when cord is removed.
 
#4
yeah, i have tried that.

but it seems to be a poweramp issue, as i can plug my bass both in input or aux in and always get a fine headphone signal. and aux in is the last thing in the signal chain of the preamp, even after boost. everything except poweramp works as usual.
i will open it up tomorrow.

any other ideas what could be wrong? old/defectice capacitors?
 
#6
Depending on the series, the amp has fairly complex thermal and overcurrent protection. If those circuits are malfunctioning, it could force a shutdown. They would be difficult to diagnose without a schematic and some electronics knowledge. On the other hand, GK has been kind to me in terms of providing schematics on older GK amps.
 
#7
fdeck,

you make me aware of something the former owner told me. he mentioned that the amp someties shuts down when thre is a voltage peak or someting. has in fact happened to me once when i hit a bad switch in my room. the symptoms were the same as with my current problem, except they didn't persist.

what if some protection circuit is broken?

tough repair, right?
 
#8
snowdan said:
fdeck,

you make me aware of something the former owner told me. he mentioned that the amp someties shuts down when thre is a voltage peak or someting. has in fact happened to me once when i hit a bad switch in my room. the symptoms were the same as with my current problem, except they didn't persist.

what if some protection circuit is broken?

tough repair, right?
Click to expand...

It might be a tough repair. However, it is well worth looking for the easy problems like a loose connection on the cable between the preamp and power amp circuit boards.

Also, board-level repair on this thing requires detaching the power transistors from the heat sinks, as is the case on most power amp circuits.
 
#10
not really. right now i have quite a few gigs to come and little interest in funky repairs/modding. however, these modules look interesting. do they need any additional cooling? if repair turns out to be expensive, i would sell the defective amp, cut down the amp-ears from the cab and get one of these markbass 250 amps. in any case i need this tiny cab for bicycle gig/rehearsal access.
 
#11
no obviuos faults inside. i have no time nor cash to pay for a repair job, so i removed the head permanently. the cab will serve me for sessions and home use.

an order for a markbass little mark 250 is placed, chose that one for size and weight and the positive feedback here.

i may happen to get the MB150 repaired sometime in the future, though.
 

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