[DT]

Anyone,

some time ago I bought an AKSA 55W and completed it about 80% before other responsibilities took it's place.  I had the boards stuffed but never got around to the power supply. But today while things were slow at work I decided to play a little.  I temporarily put together the power supply and hooked it to the assembled boards.  I slowly increased the variac to about 60V, then the positive side rail fuse blew.  I then disconnected power and checked all my connections and replaced the fuse.  This time the fuse didn't blow so I decided to check DC offset on the speaker output next.  On the speaker wire I read essentially the same as my supply voltage, which is about 39volts.

It's obvious I did something wrong, but I can't figure it out.... Any help will be much appreciated.

DT

[AKSA]

Hi DT,

You live in a city steeped in history;  Samuel Barber, the University, Oak Ridge, with it's links to the Manhattan project, and C&W music - even our own Keith Urban - soon to wed the Kidman girl, has a big property at Knoxville!!

Yes, if the offset is positive 39V wrt ground, you have a fully on positive npn output device.  First check that drivers and outputs are correct and properly orientated.

Most of these faults can be traced to the input stage, which controls offset.

Check that the voltage amp is on, by measuring voltage across the base/emitter.  It should be 0.6-0.64 volts.  If not we have insufficient current flowing through R3 (35K7), which supplies the diff pair with current.  Check soldering on this component, and also that you have the input devices, T1/2, in the correct orientation.  My suspicion is that you have no stage current in the input diff pair.

Hope this is helpful,

Cheers,

Hugh

[DT]

Dean,

thank you for your helpful insight, I'll check it out tomorrow when I have time at work.

One thing that did puzzle me today, was the silk screening on the board for T1 and T2.  The way the silk screen looks, T1/2 should go one way but the board is labeled the opposite of the way the small diagram is drawn on the board.

Not sure if that makes sense or I'm just imaging it.

Thanks again for your support.

DT

[AKSA]

Hi DT,

How are you getting on?

The two emitters of the input devices are joined together;  the instructions have a diagram of the TO92 transistor pinout.

The emitters appear on the left side of the board;  collectors on the right.

Hope this is helpful,

Cheers,

Hugh