Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: Jack_Hester on February 22, 2016, 08:35:50 am
-
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.
-
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
-
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.
-
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