Oh oh... advice needed

pea · « **on:** 15 Sep 2004, 03:29 pm »

Hello all,

Well I have one channel of my 100N+ built and started checking it out last night and I've run into a hitch or two. Here's what I've done so far:

Step 1) Eye-ball the board really well w/magnifying glass one last time, check!

Step 2) Wire main board up to power supply, check!

Step 3) Connect DVM to R26, check!

Step 4) Hold breath, insert plug into AC outlet..., cheeeeckkkkk!

Step 5) Turn around just in time to witness C11 violently going up in smoke!!!!!!!

Unplug everything, turn out lights, go for walk around block breathing fresh air deeply. Get back email Hugh w/pic of freshly smoked board. Read more and figure out C11 was installed backwards. Hugh says all is well, install new C11 and carry on.

Instead I went to bed and slept well knowing I had not utterly distroyed the whole works. Got up this morning, *correctly* installed new C11 and started again with step 1 above.

NO SMOKE!!! However...

Started taking measurements and recorded the following:

R26/R27 = 1.034V
Earth to + Rail = 51.98V
Earth to - Rail = 51.93V
Speaker Out to Earth = 130mV

R6 to Earth = 50.2V ??????!!!!!!
R5 to Earth = 80mV ??????!!!!!!
According to the docs, both of these should be ~24V, I think there is a problem here...

Since Hugh is probably sleeping at the moment I was hoping someone might have some advice.

Thanks for the help -
Bruce

MarinRider · « **Reply #1 on:** 15 Sep 2004, 04:16 pm »

1) Check you have D1 installed correctly (black line to R10) and R10 has 51V on the diode side and about 1V less on the R3 side (if R10 is 100R).

Then measure the dc voltage across R4 (should be about 0.65V).

Let us know which sides of R5 and R6 your measurements are taken.

Dave

pea · « **Reply #2 on:** 15 Sep 2004, 04:33 pm »

Dave,

Thank you so much for your reply! I am, however, confused...

Quote from: MarinRider

1) Check you have D1 installed correctly (black line to R10) and R10 has 51V on the diode side and about 1V less on the R3 side (if R10 is 100R).

D1 is right next to the + rail fuse holder, R10 is directly underneith P1 about two-thirds of the way across the board and R3 is between them directly below C5. Am I looking at the right componets?

Thanks -
Bruce

Rom · « **Reply #3 on:** 15 Sep 2004, 04:48 pm »

Hi Bruce,

you should get around 29 Volts between R5 and R6 reference to ground.

with your reading seems to me that your not measuring between R5 and R6.

I hope these help

Cheers
rom

pea · « **Reply #4 on:** 15 Sep 2004, 05:02 pm »

Hey ROM,

Thanks for the note. I believe the problem is me being dense

This is probably a good time to take a break for awhile and come back with fresh eyes.

Thanks again for the help. I'll let you know how it comes out.

Bruce

MarinRider · « **Reply #5 on:** 15 Sep 2004, 05:06 pm »

Quote

D1 is right next to the + rail fuse holder, R10 is directly underneith P1 about two-thirds of the way across the board and R3 is between them directly below C5. Am I looking at the right componets?

D1 should have the black band at the end furthest from the fuseholder.

R10 is next to D1 and C3 and is mounted vertically (not near P1).

R3 is close to the input.

However I am concerned you have a different revision of the pcb and schematic to mine which is AKSA 174 Oct 2001.

in general I am trying to determine if the front end (T1, T2) is being supplied by the +ve rail.
Then if T1 and T2 are conducting (0.65 volts across base-emitter of each).
Then if T3 is conducting (0.65V across B-E)

The fact that your output is nearly at 0V is good!

pea · « **Reply #6 on:** 15 Sep 2004, 05:11 pm »

Here is how I am measuring R5/R6:

I'm holding my black(-) DVM probe against EARTH and touching the tip of the red(+) probe to either side of R6, repeat same for R5.

Or should I hold one probe on R6 and the other on R5?

Totally confused

Bruce

MarinRider · « **Reply #7 on:** 15 Sep 2004, 05:12 pm »

Quote

R6 to Earth = 50.2V ??????!!!!!!
R5 to Earth = 80mV ??????!!!!!!
According to the docs, both of these should be ~24V, I think there is a problem here...

I just re-read this and if you are measuring the voltage on R5 and R6 where they are both connected together they must be at the same voltage. In other words if they are differnet they are NOT connected together.
The node C4/R5/R6 should be approx 29V.
The node R6/R13 should be around 1.5V
The node R5/R10/C3 should be at approx 50V

MarinRider · « **Reply #8 on:** 15 Sep 2004, 05:16 pm »

Quote

I'm holding my black(-) DVM probe against EARTH and touching the tip of the red(+) probe to either side of R6, repeat same for R5.

Hold the black probe on earth all th time then:
Hold the red probe on one side of R5 - write down the voltage
Hold the red probe on the other side of R5 - write down the voltage
Hold the red probe on one side of R6 - write down the voltage
Hold the red probe on the other side of R6 - write down the voltage

pea · « **Reply #9 on:** 15 Sep 2004, 05:16 pm »

Dave,

I have board v1.75. I'll check the voltages across T1, T2 and T3.

Thanks -
Bruce

pea · « **Reply #10 on:** 15 Sep 2004, 05:33 pm »

Ok, more info...

T1 (B+E) = 60V
T2 (B+E) = 60V
T3 (B+E) = 63V

R5 Left side = .787V
R5 Right side = .782V

R6 Left side = 50.23V
R6 Right side = 50.85V

MarinRider · « **Reply #11 on:** 15 Sep 2004, 05:55 pm »

These measurements are very concerning (understatement)!

I think it's best if Hugh sorts this since I'm sure he has a lot more experience with fault finding by email than me.

He should be finishing his breakfast by now.

Regards,

Dave

Actually, don't panic. The more I think about this the more I am convinced I am looking at a different schematic.
The fact that you have nearly but not exactly 0V on the output is good. It is hard to get this unless this amp works, especially as the voltages across the 100R current limit resistors (R26, R27) seem correct.

pea · « **Reply #12 on:** 15 Sep 2004, 05:56 pm »

Dave,

Thank you for your time and effort I appreciate it very much. I'll let you know how it turns out.

Thanks again,
Bruce

Rom · « **Reply #13 on:** 15 Sep 2004, 06:06 pm »

Hi Bruce,
check T5 and T6 the driver make sure they are on the right place,
If they are its possible that T5 has a shorted Base collector, if I were you
I will remove all the transistor and measure them to find out but before doing so, check C4, remove it and short the lead and measure, if Ok check T5 for base collector resistance , if OK refit C4 and remeasure
black lead on the ground and positive between R5 and R6 for 29 Volts.
if these don't work, I will wait for the AKSA guru's advice ( Hugh ).
Hope these help.
Rom

AKSA · « **Reply #14 on:** 15 Sep 2004, 09:34 pm »

Thank you very much Rom, David,

Bruce, it ain't all bad. Part of the problem is, as David identifies, the fact that you have the very latest 100W pcb, version 1.75, and he has 1.74, so although the circuit is identical the physical descriptions do not coincide.

Now, here's the steps:

1. Red probe on output (don't slip, put heatshrink on all but the last 2mm of the probe) and black probe on module earth. Should be under 200mV with fuses OUT and power ON.

2. Next, put probes ACROSS R7, now the 2K2 bootstrap feed resistor. You should measure 12-16 volts. Divide this current by 2.2 to get mA of VAS current. Should be 7.5-8.5mA.

3. Place probes ACROSS R8, now the 3K3 boostrap lower resistor. You should measure 18-24 volts. Divide this current by 3.3 to get mA of VAS current. Should be identical to 2 above. If not, C5, the bootstrap, is faulty (this is incredibly unlikely but it's a powerful test).

4. If all above tests are OK, as I think they are, switch off, put in the fuses, switch on again, and measure output offset again. Place RED probe on output, and BLACK probe on module earth.

5. This offset should now be less than 100mV. With the probes still measuring offset, turn P2 CW to increase offset voltage (make more positive), and CCW to decrease it (make more negative). Set it to less than 10mV.

6. Now set up the bias to the 55mV as specified in the instructions.

Bruce, I believe your amp is functioning. These above tests will be show us definitively if this is indeed the case. Believe me, your baby is perfectly healthy, it may have a slight cold, but nothing more!

Cheers,

Hugh

pea · « **Reply #15 on:** 15 Sep 2004, 11:46 pm »

Bravo Hugh!!!!!!!!!!!!!!!!!

Here's what I got:

Output = 118mV
R7 = 19.7V - 8.95mA
R8 = 29.65V - 8.98mA

I didn't go any further because R7 and R8 were a little on the high side and I wanted to check with you first before I put the fuses in.

Thank you -
Bruce

AKSA · « **Reply #16 on:** 16 Sep 2004, 12:28 am »

Bruce,

This is right on the money, go ahead, put in the fuses, and fire it up for testing/biasing.

When biasing and offset are both done, you can connect speakers and listen to beautiful music. You've just successfully built an AKSA!

Cheers,

Hugh

pea · « **Reply #17 on:** 19 Sep 2004, 03:58 pm »

I just wanted to post a follow-up to let everyone know how it all came out.

The problem, as I anticipated (builder error

), turned out to be a couple of resistors I had installed incorrectly

Hugh, a genuine gentleman if there ever was one, provided excellent support as he guided me through the troubleshooting process. Always reassuring and infinitly patient, Hugh had my new AKSA 100N+ up and running in no time

Thanks one more time Hugh for all the patient help! And I apologize again for my sudden bout of color-blindness but I swear I measured all those resistors before I installed them... really!

AKSA · « **Reply #18 on:** 19 Sep 2004, 10:59 pm »

HI Bruce,

Thank you for your post; comments like yours make it all MORE than worthwhile, much appreciated,

Cheers,

Hugh