AlephJ Mimi! (Mini)

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lykkedk

lykkedk

Member
Joined 2002
Evening gentlemen :)

So i stuffed up my second channel, and gave it power.

Everything seems at first to be okay, so i set bias and offset to near the same as the other working channel. -No problem so far.
Then i tried to attach my cheap pc-speakers; and my source and gave it some music (excatly as i did on first channel).
What i noticed right away, when i powered it up, was some humming (no music) and right after that the source resister R19 at Q8 gave some "magic" smoke :eek:
I ofcause powered it off imidiatly, and after that it cooled a little of, i shorted the input again and powered it on. -I think everything is okay at this point, i am not sure but maybee the Q8 heats up faster than it did before? Bias + offset is allright (allthrough offset is a bit jumpy)

So it looks as the amp. fails, only when a source is connected. If i connect speakers and still have input shorted, everything seems okay still.

Well, i can ofcause plugin the working channel and take some meassurements there. But before i do that i really would like to understand what is happening here? -Can anyone here help with some meassurement? -Or maybee explain how the circuit works, making me understand better what happens when source is connected and music applied! -I feel i could be better prepared for this fault clearance if i now better how circuit is working.

thanks in advance.

Jesper.
 

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Zen Mod

Zen Mod

Official Court Jester
Joined 2003
zenmod.in.rs/
Paid Member
power up with inputs shorted , no load

check Iq and offset , in temp equilibrium

power of

connect one input cable , power on , recheck values , connect speaker , check functionality

power of

connect second input , power on , recheck values , connect speaker , check functionality

if some problem persists , post detailed pictures and measured values
 
lykkedk

lykkedk

Member
Joined 2002
Zen Mod said:
power up with inputs shorted , no load

check Iq and offset , in temp equilibrium

power of

connect one input cable , power on , recheck values , connect speaker , check functionality

power of

connect second input , power on , recheck values , connect speaker , check functionality

if some problem persists , post detailed pictures and measured values
Click to expand...

I will do sir. Zm. 😄

Is it possible that you write a little explanatio, what Q8 does in this circuit? I cant figure it all out...

Jesper
 
lykkedk

lykkedk

Member
Joined 2002
well ......... sort of .......... but not exactly

remember - in A class , where both halves are active , they're working together all the time
Click to expand...

Salas said:
A scenario

Oscillation burst when an input interconnect is present. At power on until the bias settles gives R19 a fast massage and then goes away.
Click to expand...

If i just think, i would replace the two bjt's (bc550+bc560), and the Q8 mosfet + its source resistor (R19) .. I hope not the 2sj109 is gone (this one has been working in another circuit i know, but could have been damaged desoldering?)

Salas about the oscillation, do you think it's a damaged part?

Thanks for helping both!

Jesper.
 
lykkedk

lykkedk

Member
Joined 2002
Zen Mod said:
recheck all solder joints , all connections , mosfets mounting (both tightness and non-connection with heatsink)

all resistor values

replace those jumpers at gates with 470R-1K resistors

that,as first step
Click to expand...

Cool (or hot)

Resistorvalues checked, mosfet connection checked

I unfortunately haven't much time this evening, but i tried firing it up again, and set the bias at 0.5vdc. The Meassurement is the same on both R19 and R16. R19 and R16 are sourceresistors for Q8/Q5.

I then clipped on my DMM on sourceresistor R19(Q8), and plugged in my Ipad, which i use for source on testbench. Now i meassure 0,5vdc here. When powering up, the voltage imidiatly raised to 1,8-1,9vdc (3,8A/72watt :flame: call this burning amp.) -Voltage on R16(Q5) is still 0,5vdc(Bias)

When unplugging the source, the voltage goes back to 0,5vdc.

So long story short.

Q8 and R19 goes high as soon source is connected, and therefore i saw some smoke from that sourceresistor yesterday.

Jesper.
 

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lykkedk

lykkedk

Member
Joined 2002
FAULT Clearance for dummies aka' me

:frosty:

I solved it...

There was no fault at pcb, the fault was the cable connected from the ipad :)
The first channel, which worked right away, i used another cable for testing. So while i was scratching my head i throught it could be no harm trying the "old" cable.

Everything is working now, and it plays music like the first channel.

I take this cable to work friday, to figure out what the problem could be with this cable???

Anyway thanks alot for helping me out ZenMod and Salas; wish we was closer, i would then buy a "stupidity :clown: cake

So next important thing today is picking up our daughter, which have been for a highschool trip to Hamburg, Germany! -We miss her a little i guess

Jesper.
 

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