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Old 10th July 2002, 04:08 PM   #1
fcel is offline fcel  United States
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Default Aleph 2: Shorted Main Board? Help!

Current Condition:
All the following measurement has been done with a 100W light bulb connected in series with the AC power line for sign of shorts.
Without the main board connected, I got +45v and -45v at the power supply. The light bulb does not light up.
With the main board connected to the power supply, I got only +6v and -6v. The light bulb light up brightly.
With one power terminal (either V+ or V-) disconnected on the main board, the power supply voltage goes back up to +45v and -45v. The light bulb does not light up.

Situation:
I accidently connected the V+ and V- on the main board in reverse. I'm pretty sure this did all the damages.

Solution:
Some of you may already know what's the problem. Please advice. Thanks.
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Old 10th July 2002, 05:51 PM   #2
nar is offline nar
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I simply believe there is a short circuit in your board,seems like all current available is sucked from the toroid into the problems place...
Should be some heat somewhere,no?
I believe your PSU is OK,must be another place that sucks.
*Maybe a huge ground-earth loop?...

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Old 10th July 2002, 06:30 PM   #3
Apogee is offline Apogee  United States
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Fcel,

First question:

The amp when working correctly draws lots of current from the wall... Are you certain that it's not operating correctly and just lighting up the bulb due to the fact that it's a class A amp and sucking lots of current all of the time? What size of light bulb are you using?

If in fact something is wrong, it sounds like you might have a large short and my guess is that you may have smoked one or more outputs...

I would start by disconnecting the output banks and hooking up just the board to see if the short still exists...

If it does, then you know your problem is on the driver board... If not, then I would measure each bank of output fets and see which measures as a short and narrow from there...

Have you double checked all of your wiring? Are you absolutely certain that you hooked it up backwards the first time?

Hope this helps,
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Old 10th July 2002, 06:33 PM   #4
fcel is offline fcel  United States
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After I posted my question and while driving to work, it occurs to me that:

1. This is a Constant Current Source circuit (which makes it so special) which draws approximately 2.5A from the 120V AC line and that it dissipiate 200W at idle.

2. Since there is a voltage drop of 39V (45V power source minus 6V measured) and the constant current is 2.5A at idle ... that gives 39V X 2.5A = 97.5W for 6 current source transistors. Using the same method of calculation, the 6 gain source transistors should gives 97.5W too. So, the total wattage dissipitate at idle is 97.5W X 2 = 195W which is close to 200W.

Somebody please tell me that my line of thought here is correct and that what I've measured (6V)is normal. Please tell me that I've posted a silly questions ..... and then I'll remove the light bulb ... and then I'll be able to feel the 200W heat from the heatsinks!
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Old 10th July 2002, 08:26 PM   #5
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Your line of thught might be correct after all. Why not use fuse instead of light bulb. If it doesn't blow, there is probably no short in your circuit (or better Variac).
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Old 11th July 2002, 01:54 PM   #6
fcel is offline fcel  United States
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Peter and Steve,

For your information ..... and thank you for your feedback.

I've double check with the Aleph 2 service manual and it did indeed says 2.5A (rms) current draw from the wall and since the path of least resistance is still the light bulb (because of such high current draw) .... that is why the bulb light up brightly.

I removed the light bulb and yes, I do have a operating Aleph 2 amplifier! Works beautifully.

I did take some measurement. I'm using a 1KVA, 35-0-35 transformer. With load, the power supply voltage are +42.2V, -42.2V and the speaker ouput DC voltage reading is 10mv which I think is low enough. There is no hum at all even with my ear 6" away from the speaker and there isn't any weird turn-on thump either.
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Old 11th July 2002, 01:56 PM   #7
nar is offline nar
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Glad to hear it,and have fun with this wonderful amp!!!

Regards
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Old 11th July 2002, 02:01 PM   #8
BrianGT is offline BrianGT  United States
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Great! Now post some pictures

--
Brian
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Old 11th July 2002, 06:12 PM   #9
Apogee is offline Apogee  United States
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Excellent!!!!

Congrats on getting it up and running....

I too would love to see pics!!!
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Old 11th July 2002, 11:12 PM   #10
fcel is offline fcel  United States
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Brian,
As soon as I get hold of a digital camera, I'll post some pictures.
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