Mini Aleph Problem

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happyboy

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Joined 2004
#1
Hi guys,

i just built a mini aleph using BrianGT's board. However when power is applied, the output saw a DC offset of -15V. I am using a +/-16V power supply. Could you guys help me with this problem as i cannot figure out what went wrong.

Thanks!
 
#4
vdi_nenna said:
Sounds like a bad mosfet.

Did you double check your wiring?
Click to expand...

Yup double checked it a few times. I have 2 boards, one is working while the other is not. What i did for checking is that i compare both board's parts to see if they are equal, and they are. So i would assume that its right.

vdi_nenna said:
On the Mosfets, what's the voltage at the Source and Drain reading?
Check the voltages around the .47 ohm power resistors. My schem shows R18 and 19 as .47 Ohm 3w
Click to expand...

Refering to the original Mini A Schematic, for Q6, my Drain voltage is +15.34v while my Source voltage is -14.65v. For Q7, my Drain voltage is -14.95v while my Source voltage is -15.03v.

Thanks!
 
#6
happyboy,

From Grey's original article, "The Mini-A".
The idle voltages will all read about the same as the real Alephs:
--About 4 or 5 volts across R4
--9V across D1
--Roughly +4 volts at the sources of the front end differential, relative to ground
--About 4 or 5 volts across R3
--About 4 or 5 volts across Q4
Click to expand...

I don't have my Mini-A at home or else I'd find out what the output sections should be doing. I think there should be at least a 4v drop across the MOSfets. You're seeing just about the full voltage. That's why I think the FETs are blown. I'm no expert though. Maybe someone else could help with this.

Also, check R18. The Source voltage seems odd.
Q6, my Drain voltage is +15.34v while my Source voltage is -14.65v
Click to expand...
 
Last edited:
#7
vdi_nenna said:
happyboy,

From Grey's original article, "The Mini-A".


I don't have my Mini-A at home or else I'd find out what the output sections should be doing. I think there should be at least a 4v drop across the MOSfets. You're seeing just about the full voltage. That's why I think the FETs are blown. I'm no expert though. Maybe someone else could help with this.

Also, check R18. The Source voltage seems odd.
Click to expand...

Hi,
Thanks for your help. Finally both Mini-A sings already. The reason for the failure was a damaged Q3. Swapped it and it worked. Thanks for your help vdi_nenna! Greatly appreciated.🙂
 
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