Wierd behaviour-> Musical fidelity A100 - diyAudio
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Old 9th September 2003, 01:08 PM   #1
A 8 is offline A 8  Sweden
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Default Wierd behaviour-> Musical fidelity A100

I am trying to fix an old (1985)Musical Fidelity A100 integrated class A amp.

The symptom is as follows, for the first minutes or so (up to 5min) one channel distorts heaviliy and then there is a small click and it starts to sound ok.

It also seems to get better if I do not switch it on for a few weeks, it will then be ok for a few start ups and then get back to how it was.

I just have a voltmeter and measuring the voltage over the output resistors R31 and R30 it seems that one (or both?) of the output transistors do not draw current until after the click.
I initially figured there was a bad solderjoint somewhere and re-soldered the whole board but no change.

Any ideas are much appriciated!
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Old 9th September 2003, 01:10 PM   #2
A 8 is offline A 8  Sweden
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Here are the schematics
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File Type: jpg mf_a100v1.jpg (49.4 KB, 2752 views)
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Old 9th September 2003, 01:44 PM   #3
sajti is offline sajti  Hungary
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Check the electrolytic capacitors, and replace them.

Sajti
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Old 9th September 2003, 01:44 PM   #4
DRC is offline DRC  United Kingdom
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A8,
Check the input electrolytics. Looks to me like if the leakage is
high it could defeat the biasing - but I could be wrong (very small diagram)

I would expect to see a small dc difference between TR1/3 bases
and a ratio of this accross R30/32.

Dave /looks like Sajti beat me to it !!
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Old 9th September 2003, 01:55 PM   #5
A 8 is offline A 8  Sweden
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Thx guys, I agree the other option would be the caps but can they really pass DC when faulty?

EDIT Just remembered the input caps C6 and C7 have actually been changed for film types.

And a larger pic
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Old 9th September 2003, 02:32 PM   #6
sajti is offline sajti  Hungary
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In this case C13 C14 can be faulty too.

Sajti
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Old 9th September 2003, 03:38 PM   #7
DRC is offline DRC  United Kingdom
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Also C10

It's not unusual for an ageing electro to have a high leakage current.

Dave
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Old 9th September 2003, 03:49 PM   #8
HDTVman is offline HDTVman  United States
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A 8, Change the electrolitics, after all they are almost 20 years old and one of the top two failure modes is high leakage (passing DC current). Its a good excuse to upgrade the caps any way. Remember to do both channels while your at it and think about the power supply caps too.

Good luck
BZ
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Old 9th September 2003, 11:03 PM   #9
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Quote:
Originally posted by HDTVman
A 8, Change the electrolitics, after all they are almost 20 years old and one of the top two failure modes is high leakage (passing DC current). Its a good excuse to upgrade the caps any way. Remember to do both channels while your at it and think about the power supply caps too.

Good luck
BZ

I agree with that! change all the caps!!
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Old 10th September 2003, 08:11 AM   #10
A 8 is offline A 8  Sweden
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Thx again,

The plan was to change the most of the caps but not until I found what was wrong.
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