Crate MXB50 SS Bass Amp.........

[Jack_Hester]

Yesterday at Church, our sound man approached me with a problem, concerning his SS Bass amp.  He loans it to another Church who leases ours for night services.  The amp made a loud pop, and went silent, one evening last week.  He's also the sound man for them. 

Fortunately, I leave some form of a Bass amp (tube) at the Church, in hopes that we will someday have a Bass player to use it.  This one is a Gibson Thor Bass amp, that they pressed into service.  It's not very loud, even though it has EL34's in the Power Amp.  The Pre-amp is very tame.  I'm planning on swapping it out with an Ampeg B-15-N that has seen no usage since I revived it, maybe a couple years ago.  I'll bring it up on the limiter, to be sure all is well.  I have a thermistor on the line-in supply, so it will always have a soft-start.

I brought his SS amp home to take a peak inside.  I have been unable to find anything more than an owners manual.  Does anyone have a source for a service manual or schematic?  I'm thinking of contacting Crate, to see if they will provide a PDF file of a service manual.  I'm not counting on it. 

My first suspicion is a fuse soldered to a circuit board, that I can replace with a fuse socket.  Or, mount one in the chassis, for access from the outside.  He's good with that, if this is indeed the case.  And, something had to blow it. 

I would appreciate any help with this.  I'm not excited about solid state, but want to help him out.  Thanks. 

Jack

Meant to say MXB50. 

[Jack_Hester]

The kind folks at Loud Technologies sent a link to the schematic: 

https://supportloudtech.netx.net/loud-public/#category/13090

I have it downloaded and will begin troubleshooting.  I'll report my progress.  Hopefully, it won't overwhelm me. 

Jack

[PRR]

Simple (cheap) chip-amp.

Usual process. If possible, disconnect the audio power stage. If RC1, 12-pin ribbon cable, can be disconnected, do it.

Check power supply voltages. There should be +22V and -22V _DC_ on the two big caps C26 C27. If not, go back to the transformer which should have two ~~15V _AC_ windings to rectifier D9-D12. If that no good, backtrack to PT primary: 117V AC? Switch? Fuse?

If you have +/-22V DC at main DC power, re-connect power output stage. Look for "zero" (<1V) DC *across* speaker wires. Note that both sides are "hot", don't ground one. If >1V DC here, it will probably be 20V DC, and the speaker may be toast. However such dead-short DC-on-speaker failure is rarer in chip-amps than some discrete amps.

[Jack_Hester]

Thanks for the troubleshooting roadmap.  I'll probably start on it Saturday.  I popped a standard 2 amp (non-slo-blow).  I now have the correct slo-blow on hand.  Will put one in, Saturday morning, and report back with my findings.  I made a quick check for board replacements, and haven't found them.  Just a Google search and eBay search. 

Jack