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Old 14th Aug 2018, 2:50 am   #33
Radio Wrangler
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Quote:
Originally Posted by 'LIVEWIRE?' View Post
All I can say is that the fuse in the amp I'm in process of repairing was a 2A(T) one, and in the circuit diagram I downloaded it iis shown as being 2 Amp. Maybe it was changed during the production run, which, if I understand correctly lasted almost 30 years. I didn't time it precisely, but the rectifier flashed over, blowing the fuse in no more than a minute
That clears the fuse of any culpability. If the rectifier flashes over, you want the fuse to blow to save the mains transformer.

The question switches to why did the rectifier flash over?

1) Fault in the rectifier valve

2) Too much current demanded of it, EG HT line short or demand from amp beyond its capabilities.

3) Too much voltage on rectifier - especially with HT switch off.

4) Mains transient. Transformer saturation acts to limit these, either way the fuse gets it.

David
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