High-Mid frequency distortion on 55N+

netaron · « **on:** 11 Dec 2005, 10:06 pm »

Hi all,
I just installed an attenuator on the 55N+ and shorted the right pcb input, I ran it this way for a minute or so and there was no sound from the right channel. After investigating, I realized I had shorted the inputs, so I fixed it, but now I get a lot of distortion on what seems above 800-900Hz (guessing here) but music still comes through. I have been probing and comparing both channels, so far nothing stands out, but this has been all in circuit testing, so I wanted to see if anyone else had experienced this before I become intimate with the soldering iron.

Thanks

Haron

AKSA · « **Reply #1 on:** 11 Dec 2005, 11:21 pm »

Hi Haron,

Take it back to what it was before, that is, remove the pot wiring, and see if this condition is still apparent. If not, move forward, step by step, reinstalling the attenuator. Oh, and measure both bias and output offset just to be sure it's within limits (55mV and <+/-30mV respectively).

Cheers,

Hugh

netaron · « **Reply #2 on:** 12 Dec 2005, 02:21 am »

Hi Hugh,
As always thank you for your kind response. I wish you and your family a merry Christmas.

Now to business:

I removed the attenuator as you suggested, no difference, still distortion on the right channel!

Bias Left channel (Good channel) .592mv
Offset Left channel .57mv

Bias Right channel (Bad) 1.14Volts!!!!
Offset Right channel .31mv

Sounds like both channels are having problems. I have been seeing a decline on bass lately, I wonder if these readings have something to do with that.

Thanks Hugh
Haron

AKSA · « **Reply #3 on:** 13 Dec 2005, 08:48 pm »

Haron,

These values are way off!! On the good channel, offset should be less than 30mV, plus or minus, while bias should be 55mV. Be sure to read off the right scale; this is a common error because the decimal point is not always clear.

Now, on the other channel, if your measurements are correct, you would have 1.25 amps of bias!! This would be around 90W of dissipation on the outputs which would fry them instantly. I suggest this measurement might be 114mV, which would be 125mA of bias, which is certainly much more than it should be.

Suggestion: Remove both output devices, test each with a ohmmeter, looking for anything less than about 200R between collector and emitter and base and emitter.

Also check that the emitter resistors, the sand cast 5W square ones, are not open circuit. Sometimes excessive current bursts will take out these resistors.

Lastly, check that the binding posts for the speakers are not contacting the frame and shorting. This will destroy output stages in microseconds.

Hope this is helpful, and sorry about delays.

Cheers,

Hugh

netaron · « **Reply #4 on:** 15 Dec 2005, 02:35 am »

Hi Hugh,
I replaced both output devices and walla, sounds great all around. Only issue is the bias still reads .60 Volts and .59 Volts for both channels. I can not get a reading with the mV setting on my DMM.

Thanks

Haron

netaron · « **Reply #5 on:** 15 Dec 2005, 03:32 am »

Hi again Hugh,
I figured out the bias setting, it's now at 55mv on both channels. The only issue is the offset, one channel is 27mv and the other is 71mv.

Thanks

Haron

AKSA · « **Reply #6 on:** 15 Dec 2005, 08:33 am »

Haron,

You can move the offset up and down by varying R3.

Check out the instructions; they give details on this. Essentially, increase R3 to raise offset; decrease R3 to lower offset. You should not need to trim the value by more than about 2K2.

Hope this is helpful,

Hugh

AKSA · « **Reply #7 on:** 20 Dec 2005, 06:18 am »

Hi Haron,

Any problems? All fixed, ready for the yuletide?

Cheers,

Hugh

netaron · « **Reply #8 on:** 20 Dec 2005, 04:14 pm »

Hi Hugh,
Almost reday, I am trying to locate some 3k0, 2K7 resistors so I can set the offset to around 30mV since it is a little high for both channels(60mV), other than that everything is good. Your support is second to none.

Thank you

Haron

netaron · « **Reply #9 on:** 28 Dec 2005, 09:11 pm »

Hi Hugh,
I replaced R3 with some 3K4 resistors and offset is aroun 0 mV from .50mV. This is good right?

Thanks

Haron

AKSA · « **Reply #10 on:** 28 Dec 2005, 10:17 pm »

Hi Haron,

If your 3K4 resistor is ADDED to the existing R3 in series to get this very low offset, everything is peachy, no doubt about it!!

Happy New Year!!

Cheers,

Hugh

netaron · « **Reply #11 on:** 29 Dec 2005, 05:24 am »

No Hugh,
The new 34K ohm replaced the original 35.9K ohm. So the new resistor has 1.9 K ohm less resistance.

AKSA · « **Reply #12 on:** 29 Dec 2005, 05:31 am »

Haron,

My bad; didn't realise..... thanks for the explanation.

I'm delighted offset is so nicely controlled now!

Happy listening again, Haron!

Cheers,

Hugh