Let's Get Technical > Power Amp Tech
B200S behaving badly - blown fuses
MikeB:
Hi all,
I lost a channel on my B200S a couple of days ago. Yesterday I popped it open and found that one of the PCB mounted fuses had blown. It was an 8A 32V fuse. I replaced it today and when I fired it up, with both volume pots on zero, it made a loud hum and then another fuse blew.
Bit of history: This unit was originally owned by @rnolan. Back when it was his, something went wrong (richard can fill in these details) and he had a friend rebuild it for him. While he was doing that he filled it full of fuses.
There is one in the wire that goes from the transformer to a thing that is screwed to the chassis. Sorry I can't be more technical than that. It is the second one that blew. It is on the same channel as the original fuse.
Bit more history: In 1997, just before I was about to head overseas for what turned out to be six years, this same unit went dead on me. I left it with a friend and wrote out a big long explanation of what had happened and asked him to take it to a tech to be repaired. Anyway, so when I returned six years later, he gave me back the amp and the two pages of notes I had written for the tech. He had done nothing. I took it to be repaired and the tech replaced the transformer. It has been fine since then. All I have done since then is replace all the jacks and the pots (within the last year or so).
Can anyone give me an idea of where I might start looking for problems. There is a guy nearby that has just set up shop repairing analogue gear so I'll probably take it to him if no one has any suggestions.
Many thanks.
Mike.
MarshallJMP:
That 8A fuse is between the out of the amp and the speaker out. So if that blows it's usually that you have DC on the output which means you have blown one of the output transistors (or a cascade of one of the driver transistors).
Can you take a pic of that other fuse?
rnolan:
The original "something went wrong" way back was a speaker died and took out one channel (the cone was ripped from a guitar peg head and I used some cotton and sowed it up but over time the coil bobbin got a bit off centre :facepalm: ). Replaced the output transistors (can't remember what else, driver transistors sounds right ??) and, as Mike said, tech put a bunch more fuses in it so it wouldn't happen again (ie blow the transistors due to speaker short). It's also a very early B200s, my later one is allegedly improved :dunno: . However, this time the speakers are fine (thankfully as it's my spare split stack wired stereo Mike was using). BTW in case it matters, the split stack has 75w 16 ohm celestions and was connected in stereo when it stopped working, so the channel was into a 16 ohm load.
MikeB:
Here are the pics. The inline fuse that blew is actually from the "thing" on the chassis to the pcb - on the dark wire. The other photo is of the original fuse that went. I replaced it with a 7.5A 150V fuse. I hope thats alright. The inline one that went is a 2D250V5A. Its tricky to get a decent angle on any of this stuff. Let me know if you want me to redo.
Thanks
MarshallJMP:
Ok I see they put an extra fuse between the secondary's of the tranny and the bridge rectifier. Good idea.But still I think the problem is a blown output transitor (one of those big blocks with 3 pins,2SA1494, or 2SC3858).
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