SAE Mark XXXIB blowing fuse: help! - diyAudio
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Old 14th August 2014, 09:07 PM   #1
KanedaK is offline KanedaK  Belgium
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Join Date: Feb 2011
Unhappy SAE Mark XXXIB blowing fuse: help!

Hello all,

I need your advice as I don't know much in electronics.

I bought a few days ago an SAE XXXIB power amp wich arrived by mail.

Connected it in my system (SAE Mk 1B preamp, klipsch LaScala modded with ALK engineering constant impedance crossovers, 8ohms flat).

Played it at very moderate level for about 30 minutes, sounded fantastic, but I could hear a slight mechanical (transformer?) buzz with my ear close to the amp.

Then suddenly, paf, the amp went "dead". No "bam" in the speakers, no smoke, no smell. Burnt fuse.

Took the fuse out, it was a 2A slow blow instead of the 1,25A slow blow recommended by the service manual (i live in europe, that's a 220V amp)

opened up the amp, couldn't see anything abnormal but I noticed the two big filter caps wich originally were 6000uF 50v had been replaced with 10000uF 63V. All the rest is original as far as I can tell. Except maybe the output transistors as the original are supposed to be branded as "SAE" and the ones I have are Motorola branded.

I know that increasing the value of the filter caps increases the inrush current.
I guess that's why the fuse was a 2A instead of the recommended 1,25A.

Could it be that 2A was just "limit" and that the fuse gave up after possibly a few years?

or is it more likely that there's a short in a filter cap or a transistor, or worse is my transformer dead (and the buzzing was a sign it was nearing the end of its life)?

I have 3A fast blow fuses at hand, what would I risk if I'd try with one of those just to check if only the fuse was responsible?

The seller told me that the amp had been playing every day without problem eevr since he decided to put it on sale, and that the buzz had always been present (he bought the amp 10 years ago from a second-hand store).

PSU delivers +40 and -40V to the full complementary amplifier. Output power is 50watt/8ohm.

Any advice on what I can / cannot try, or what could be broken, is very welcome...

thanks in advance!
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Old 14th August 2014, 10:00 PM   #2
diyAudio Member
 
Join Date: May 2005
Location: toronto
Try the 3 amp fuse, I have a few of these amps and increased the caps to 27,000 uf and changed the power supply diodes to 25 amp hex fets works fine. Could be just inrush current, or a bad filter cap. Or diode , or bad transformer , or blown output devices...
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