[Fredly]

I’m a relative newbie to DIY kits, and this is certainly the most advance kit I’ve ever built, so having said that I’d very much like some input/feedback on the problems I’m experiencing on the final testing stage of my AKSA 55W kit.

So lets begin at step 3 (or 23) – Power on & adjust initial bias

With the fuses yet not in a quick voltage read across R21 measures 1.2V (Excellent)
Now reading across R22 we get exactly the same measurement 1.2V (Excellent)

Do the same thing for the other PCB

1.8V for R21, and again, 1.8V for R22 (still within the 1.2V-3.2V guidelines - Excellent)

Verifying the voltages across the positive and negative rail for PCB1 we get +39V & -39V (Excellent)
Verify the other PCB voltage, again +39V & –39V (Excellent)

Next we proceed to step 4 (or 24) – Measure & Trim Output offset Voltages.

Checking the voltage we get for PCB1 between speaker output and earth we get –8.13V (NOT GOOD)
Checking the same voltage on the other PCB we get 53mv (Not UNDER the 50mv suggested, but pretty close)

Lets continue testing the “good” PCB ONLY and go to step 5 (Step 25) – Fit 5A Fuses and adjust Bias

With the Probes across TP1 and TP2 we adjust the bias pot screw until we get 50mv (Done)

Next we verify the voltage between output and earth is less than 30mv (Crap – I get 72.5mV – NOT GOOD)

So my friends, this is where the party stopped for me and some serious troubleshooting on each PCB begins.

For the first PCB with the –8.13V output to earth reading, I’m going to follow the Situation 2 of the Diagnostics.  With a negative reading, it looks like I MAY have a shorted PNP transistor.

I’m I proceeding correctly here?

For the second PCB, with the 72.vmV reading at Step 25, I guess I need to start tinkering with R3 and find a good value. Once I get the voltage to less than 30mV, I can proceed with the testing phase.

Again, is this my best/only method of recourse?

Needless to say, I’m a bit frustrated that neither of the PCBs I built is working correctly, but heh that’s life!

The reason for this post is to verify I’m proceeding correctly, or perhaps someone out there can give me alternate methods of quickly identifying what’s wrong with my PCBs.

Thank you in advance for any information you can send my way.

Looking VERY forward to hearing this amp, when it finally works.

Cheers, Fred in Toronto, Canada

[AKSA]

Hi Fred,

Sorry you are having difficulties.  Let's see what we can do.......

First, on the 'good' pcb, measure the offset wrt earth without the fuses in you get 53mV, and with the fuses in and bias adjusted, 72mV.  This indicates you may need more current into the diff pair.  However, be sure that you have cleared away flux from the soldering under T1/T2, and R7, R8 and R9 and that they are quite dry.

Now, measure the voltage across R4, the resistor across the base/emitter of T3.  It should be between 590-630mV.  If outside these limits, there may be a problem with the transistor.

Now, measure the voltage across R1 (47K) and from the top of R9 (82K), both with respect to earth.  They should be 50-60mV (stock) and 100-120mV (Nirvana) closely match, and if not, we may have a problem with the output stage.

Next, measure the voltage across R15 (215R).  It should be between 1.15 and 1.25V.  If not, there may be a problem with the drivers.

Lastly, measure the voltage between the two outside terminals of each output device.  These should be 0.5-0.58 volts, and if not, one or other is not switching on properly.

In closing, R2, the 10R ground lift resistor, will give these symptoms IF it is not properly soldered to the pcb.  Give it a prod with a hot iron just to be sure.

Get back to me when you are ready with the results, and we'll talk more.  You have two amps very close to operating;  there is no traumatic bias levels, so troubleshooting is a snap because you can leave it on indefinitely!

Cheers,

Hugh

[Fredly]

Hi Hugh, thanx VERY much for the quick turn-around support.

Last night I measured each of the steps you suggested TWICE, to ensure I had proper readings.


First, on the 'good' pcb, measure the offset wrt earth without the fuses in you get 53mV, and with the fuses in and bias adjusted, 72mV. This indicates you may need more current into the diff pair. However, be sure that you have cleared away flux from the soldering under T1/T2, and R7, R8 and R9 and that they are quite dry.


Done. For that matter I sat done and gave both boards the once over to ensure all flux was cleared away, with special attention to the areas above.


Now, measure the voltage across R4, the resistor across the base/emitter of T3. It should be between 590-630mV. If outside these limits, there may be a problem with the transistor.


Good. I read a measurment of 624mV.


Now, measure the voltage across R1 (47K) and from the top of R9 (82K), both with respect to earth. They should be 50-60mV (stock) and 100-120mV (Nirvana) closely match, and if not, we may have a problem with the output stage.


Bad. The readings across R1 and R9 read 120mV and 125mV respectively (I'm a stock 55W AKSA)


Next, measure the voltage across R15 (215R). It should be between 1.15 and 1.25V. If not, there may be a problem with the drivers.


Bad. The reading across R15 read 386mV.


Lastly, measure the voltage between the two outside terminals of each output device. These should be 0.5-0.58 volts, and if not, one or other is not switching on properly.


Good. I got readings of .59V for each outut device. I little higher than the range, but probably acceptable.

So two "good" readings and two "not-so good" readings. hmmmmmm.

Hugh, I'm VERY comfortable in desoldering and using components from my other board to confirm any theories you may have on which component(s) are suspect.

Looking forward to your response,

Fred in Toronto, Canada

[AKSA]

Fred,

Check R3.  It should be 68K.  I'm guessing that you have something closer to 40K than 68K;  check the actual resistor, and check it's soldering.

This could be one problem.

The other problem is that T4, BD139, may not be inserted properly into the circuit, OR that one or both of the driver transistors is either wrong way round, OR improperly soldered.

Email me privately with your response, and good luck!

Cheers,

Hugh