Help Aleph 3 Problem!

[vsr123]

Hi all
I bought a pait of Aleph 3 boards from Kristijian a while back, put the amp together and everything was working great until today :-( one of the channels just died. The LED does not come on and initially I thought there might be a problem with the connections - I tried wriggling the speaker connections and I saw sparks at the speaker connector!
At this point I disconnected all the inputs and outputs and examined the boards. Nothing seems to be fried, all resistors look ok. Does anyone have any thoughts on where I should start from?
I checked the power supply circuit and that part seems just fine. If I look at the circuit (link below) it looks certain that the 244 device on the top right hand corner closest to the LED has blown, any thoughts?

http://web.vip.hr/pcb-design.vip/aleph-3.html

thanks in advance
vsr

[Cradle22]

Hi!

Although having never built an Aleph so far, I think that most of the time the integrated devices (transistors, diodes) are the cause for a fault, so check first the power transistors, then the drivers.

But a fault in power transistors can also cause the emitter resistors to blow (the 0.47 ohms), so you might check them, as well.

And always check devices unsoldered from the circuit, since connection to other devices changes the values you get while measuring...

And look for the reason why a specific device has blown... too much heat? bad solder joint? Short with probes (unlikely in your case, but my #1 cause for failure )? Or bad luck...?

Bye,

Arndt

[Netlist]

Originally posted by vsr123
If I look at the circuit (link below) it looks certain that the 244 device on the top right hand corner closest to the LED has blown, any thoughts?

Hi vsr
Assuming you have a DMM, you should provide some more details about the voltages you have on the critical points in the circuit.
What do you measure at the output? DC? How much?
What voltage between emitter and collector of the MPSA18 of the current source?
What voltage over the source resistors and so on.
The link you provide is a nice schematic, but for more in dept measurements the PDF at Passlabs is better IMHO.
Keep us informed and (small but important warning) : Discharge the big cans properly before poking around with the soldering iron :nod;

/Hugo

[vsr123]

Hi
thanks much for your posts. I made some measurements. The output is just DC at 27V !! There is no voltage between C and E of MPSA18.

thanks
vsr

[Netlist]

First thing to do is measure the fets.
You will have to take them out to measure accurately.
Here's a good link to know how to measure them:
http://www.eatel.net/~amptech/elecdisc/ampfail.htm
Start with the diff-pair then work yourself trough the outputs.
You can always compare measured values with the good working side.
Check the zeners as well.

/Hugo

[jleaman]

27V on the output... does that mean at the speaker terminals or on the out-put transistor's.?

[vsr123]

Hi Jason
The 27V DC is on the speaker connections on the PCB (measuring between +ve speaker out and ground).

thanks
vsr

[jleaman]

to me that doesn't sound good at all OUCH!! I'm clad that I'm looking at all these aleph post's. as I'm still building the Aleph 2's that i have now.

[janneman]

Originally posted by vsr123
Hi
thanks much for your posts. I made some measurements. The output is just DC at 27V !! There is no voltage between C and E of MPSA18.

thanks
vsr

Which MPS18? There are two in the diagram you posted.
What's the voltage across the 390 Ohms load in the input stage?
If you have drafted this diagram, why don't you number the components? Makes any discussion (and your own work!) much easier.


Jan Didden

[vsr123]

Hi
Sorry, the MPSA 18 I was referring to is the one on the top of the circuit.
I will take the measurement across the 390 ohm later and post it here.

As for why I didnt number the components, I did not draft the figure, its from Kristijan's web site, the Aleph 3 service manual at www.passdiy.com also has the same circuit and the MPSA 18 I was referring to is labelled as Q105.

thanks
vsr