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Old 1st April 2004, 11:33 AM   #1
otaku is offline otaku  New Zealand
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Default a dead Musical Fidelity A100

Hi, I have a musical fidelity A100 which runs like a wok. To my regret, I happened to forget to turn it off last night, and when I was back from work tonight it was as hot as a volcano.

I quickly turned it off and scrubbed it with semi-wet cloth to cool it down.

After an hour or so, the amp was powered up for check. Power was ok, but no sound. I'm sure it's not the thermal fuse, as the thermal fuse failure doesn't turn it on at all.

Any idea is appreciated. Thanks,
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Old 1st April 2004, 01:39 PM   #2
hjelm is offline hjelm  Sweden
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If you have all the voltages denoted in the diagram i would guess that it is either the 1u electrolytes.
Is there no protection circuitry on this amplifier?
Have you checked the speakers and all other connections?
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Old 1st April 2004, 01:47 PM   #3
otaku is offline otaku  New Zealand
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Thanks for your reply, Hjelm
I did not check the voltage due to the funny design of this amplifier. The enclosure itself is the heatsink. So when I open the case to see the circuit, I can't power it up and measure the voltage. (Perhaps, I need very good fan to start with)

I hooked my other amp up and the speakers and all others are ok.
Electrolytics for this amps are rated 85degrees where it runs in over 90degrees environment. Everyone suggested me to replace them with 105degrees. "It ain't broke, do not change it " philosophy prevented me from doing that. Perhaps it's now a good reason to replace them...
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Old 1st April 2004, 01:53 PM   #4
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First question.

Why was it so hot? Perhaps a short took out one side of the power supply. If that's the case, you may have cooked your speakers.
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Old 1st April 2004, 01:55 PM   #5
hjelm is offline hjelm  Sweden
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I assumed that they were class A.
The voltages suggests that the output transistors would have to dissipate close to 50w.

Edit: to be exact some 32w each.
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Old 1st April 2004, 02:00 PM   #6
otaku is offline otaku  New Zealand
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I do not have the inside look of this amp, but there is an excellent site shows everything of A1, which is a little brother of A100.
http://www.mhennessy.f9.co.uk/mf_a1/

Yes, it is class-A claiming amplifier--excessive heat is then justified
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Old 1st April 2004, 02:16 PM   #7
Netlist is offline Netlist  Belgium
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Did you look at this thread?
Wierd behaviour-> Musical fidelity A100
There's some good info there.
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Old 1st April 2004, 02:25 PM   #8
otaku is offline otaku  New Zealand
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Quote:
Originally posted by Netlist
Did you look at this thread?
Wierd behaviour-> Musical fidelity A100
Yes, I did, thanks.
Everything I know about this amp has been obtained from this thread.

They however, don't give me a straightforward answer towards my problem..

In summary, I think what has been discussed in the thread is:

1. Check any physical defects, intermittent soldering etc.
2. replace all electro caps
3. replace all transistors
4. replace some resistors

Of course, it's very good practice to do so. But I just like to know where will mostly likely to be the problem, before I spend excessive labour, time and money.

Thanks for quick replies!!
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Old 1st April 2004, 02:36 PM   #9
hjelm is offline hjelm  Sweden
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Well you can measure, almost, all of the suggested components while the amplifier is off.

The amplifier is stone dead according to your description so the broken device should be possible to find.

Do you have a ohm meter then all caps should look like shortcuts, or increase in resistance as you measure them.

A diod tester should tell if the zeners are broken.

Measure if possible from leg to leg on all components so possible faulty traces are found.

Start from the input and back.

Good luck, it is fun to search for errors if you have the time!
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Old 1st April 2004, 02:46 PM   #10
otaku is offline otaku  New Zealand
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Quote:
Originally posted by hjelm
Well you can measure, almost, all of the suggested components while the amplifier is off.
That's true. I can do the measurement
Quote:
The amplifier is stone dead according to your description so the broken device should be possible to find.
At least power goes on. But it doesn't play any music

Quote:
Good luck, it is fun to search for errors if you have the time!
It's very kind of you. Your encouragement and advice make me feeling like to start work straightaway!! Woops..it's 4am here need some sleep. Good night, you guys!
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