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Old 10th March 2012, 01:35 PM   #31
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I've got some 300W (yes 300Watt) power resistors just for this purpose - a boxful of various values.

We used them at work to fault find a degaussing fit where the amps were outputting 30A in error.

I'll lend you the 2 x 10R if you want to pay the postage.
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Last edited by KatieandDad; 10th March 2012 at 01:49 PM.
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Old 10th March 2012, 01:36 PM   #32
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Quote:
Originally Posted by Lucas_G View Post
Is this within limits?

I als checked Q3 and I get there 41,3 V between D and S...
The input stage appears OK.

You've got 4V to GND where it should be.
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Old 10th March 2012, 01:39 PM   #33
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I really would suspect the Electrolytics.

My A4 runs at 50 Degrees which will kill electrolytics eventually. The A4 does not have so many just one.
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Old 10th March 2012, 01:54 PM   #34
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If you are itching to carry on fault finding you dont neccessarily need 220uFs to replace the suspect ones. Anything from 100uF to 470uF will prove if this is where the problem lies.

We are not after an amplifier that sounds good at this stage, we are just trying to stop it blowing fuses.
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Old 10th March 2012, 02:15 PM   #35
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If this does prove to be the Electrolytics I would change the caps in the working mono-block too as they must have had the same treatment and heat experience.
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Old 10th March 2012, 03:39 PM   #36
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Quote:
Originally Posted by Lucas_G View Post
For the 220 uF caps I used Panasonic FC

For the small C8 (10pF) I used this one (styroflex?):
Whatever you used is largely immaterial. Something has failed.
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Old 10th March 2012, 03:45 PM   #37
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In my Aleph 4 my brother in law used Elma Silmic IIs for the one and only 220uF.
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Old 10th March 2012, 09:43 PM   #38
Lucas_G is offline Lucas_G  Netherlands
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Today, I checked the voltages on the three 220 uF caps.

On C10 I got 325 mV,
on C9 15 Volt,
and on C5 70 mV.

When I made contact with the poles of my DMM on C10 I noticed that some vibration within the amplifier stopped. I thuoght it came from the tranny, but apparently it has to do something with imbalance / oscillation.

It seems that C10 is indeed the culpit!

I will order a new one in the coming days.

Thanks to you all for pointing me in this direction
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Old 11th March 2012, 02:31 PM   #39
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Quote:
Originally Posted by Lucas_G View Post
For the small C8 (10pF) I used this one (styroflex?)
Lucas,

exchange those, and for other types.

Polystyrene capacitors should be soldered with extreme care, when the core reaches 85C they're a goner.
Means that 1 of 3 DIY folks who pick these for a project terminate them during the build-up (2nd one was lucky, 3d got wiser after an earlier mishap)

The styroflex cap is sitting right next to the heatsink of the input IRF9610, the 5-finger does 24-25 C/W thermal resistance.
With a T1 dissipation of +1/2W, that makes more than 12C above ambient temperature inside your Aleph 2 case.
With all the wiring around/above it, slim chance of decent air circulation, the heatsink temperature will likely be above 60C.
A foil cap that handles 85C max, placed next to a +60C heatsink, is what you'd call a sitting duck.

Secondly, if your memory fails you on the cap type (styroflex ?), good chance you picked a junker.
9 out of 10 axial polystyrene caps are garbage, only a handfull good axial types were manufactured in the styroflex era, of which most don't even look like half an inch of transparant hose with a silver wire (aka similar appearance as the 'buisjes' used to fix chronic inner ear infections )

C8 should either be ceramic or mica, better as a cap and more fail-safe.
In Cheesica you also have the luxury to pick PTFE for peanuts, or polypropylene versions, should be possible to squeeZe in between T1 and C4.
Examples :
Paradise Computers en Electronics
Trimmers -hf-electronics
Sometimes go for 50 cents on eplague.

(325mV across C10 implies it's not short circuit. C10 is not there for stability purposes, but to enable dynamic current supply by blocking DC. Did you measure more than 325mV across R21+C10 ? )
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Old 27th March 2012, 06:49 PM   #40
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I've just got back from a course. How did the repair go ?
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