Help Please, Blown AKSA 55

vsr123 · « **on:** 18 Jan 2005, 07:31 pm »

Hi Folks
I have been using an AKSA 55W that I bought from someone else for the last couple of months and everything was working just fine. This version is using two transformers. I have some of the diagnostics/manual from the original owner but I am afraid he lost some of the pages so I dont have it all...anyway here is the problem -

My usual setup is Marantz CD->GK-1->AKSA

I got a Shanling tube CD player recently and wanted to try that, so I went from CD->AKSA.

Everything was playing great as of yesterday night (new setup). This morning I switched on the shanling and the AKSA. I heard crackling sounds (along with some music) for about 2 seconds and then saw smoke coming out of the aksa. It seemed to be coming out of only the left channel.
I opened up the case and found that all 5A fuses are blown (except for F1 on left channel but that looks like it was going to blow too).

1. What could have caused fuses on both channels to blow?

2. A visible examination of the components on the top side of the PCB showed no visible blown components

3. I checked for the power supply voltages between +ve and -ve and the voltage is 69V (b/w +ve and -ve) which seems fine.

What should my next step be? Should I try and examine the bottom part of the PCB ? (I went through some thread which talked about the 100R mounted on the bottom?).
OR
Should I just get new fuses and install them, measure the amp output voltage and leave it on for a few minutes and if it seems fine, plug the speakers in?

I have not checked the speakers with any music as yet, I measured the DC R across speaker terminals and they dont seem to be blown.

Could the CD out have caused this in anyway? especially if there was some problem with the tubes within the Shanling (tube-output). This also has a variable volume built in. I dont know what else to think as I tried going from the CD to an old receiver that I have and the CD player seems fine...no problem.

any help would be greatly appreciated

thanks
Sridhar

PSP · « **Reply #1 on:** 18 Jan 2005, 08:20 pm »

Hi Sridhar,
If there was an "out of the blue" catastrophic part failure Hugh will be the best person to diagnose it.

Is there any chance that the amp outputs to the speakers where shorted (either at the amp or at the speakers)? That is instant death.

You should measure +/- 36 volts between power supply earth and the rails... 69 v from + to - seems OK.

I would suggest that you NOT put some fresh fuses in and risk cooking any more parts, making the problem harder to diagnose.

Good luck,
Peter

vsr123 · « **Reply #2 on:** 18 Jan 2005, 08:32 pm »

Peter
thanks for your reply. The outputs were not shorted, that I am sure of. what I am surprised about is that fuses on both channels are blown, but there does not seem to be any problem with the power supply.

does anyone know whether having a bad input signal (more than 2V) to the amp can cause something like this?

thanks
Sridhar

AKSA · « **Reply #3 on:** 18 Jan 2005, 09:35 pm »

Hi Sridhar,

Thank you for your post. This is unfortunate.

I have seen failures of the 'raised earth' circuit on the AKSA 55W with poorly earthed sources. I do not know the Shanling, and recognize it as very high quality, but likely double insulated. Some Chinese equipment has unusual earthing arrangements.

Anyway, here's what I'd do:

1. Replace R2 on the AKSA. This is a half watt 10R resistor right next to the center earthing terminal; brown black black gold are the bands. Usually in these circumstances it cooks.

2. Remove the speaker leads and the input leads to the amp.

3. Replace all four 5A fuses on the two modules.

4. Switch on, and measure first offset at the speaker output wrt earth - should be less than +/-30mV. Then measure quiescent from one output device emitter to the other output device emitter. Should be 55mV +/-4mV.

If this checks out, switch off, replace all the 100R fuse resistors, connect speakers and interconnect, run a wire from the chassis of the Shanling to the chassis of the AKSA, and switch on.

All SHOULD be sweetness and light. If not, get in touch with me privately, you might have to start replacing output devices and the diff input pair.

It's all fixable. No question. But it will take patience and some time.

Cheers,

Hugh

vsr123 · « **Reply #4 on:** 18 Jan 2005, 10:19 pm »

Hugh
Thanks so much for the prompt reply. I will surely try that out - A couple of clarifications -

1. The output device emitters that I shoud be measuring between is T7 and T8 correct (left lead of the output device)?

2. Is the 100R resistor of any particular W rating? pardon my ignorance, is the fuse resistor different from any plain old resistor?
When you say replace all 100R fuse resistors, I assume you mean R21 and R22 on both channels (4 resistors in total)?
The 100R are on the bottom side of the PCB...?

thanks again!
Sridhar

AKSA · « **Reply #5 on:** 19 Jan 2005, 01:55 am »

Hi Sridhar,

You wrote:

1. The output device emitters that I shoud be measuring between is T7 and T8 correct (left lead of the output device)?

Yes, left leads of T7 and T8. WARNING: Don't slip with the probe!!

2. Is the 100R resistor of any particular W rating? pardon my ignorance, is the fuse resistor different from any plain old resistor?

Half watt, or even one watt if you wish......... Once the fuse is in place, it passes no current anyway.

When you say replace all 100R fuse resistors, I assume you mean R21 and R22 on both channels (4 resistors in total)?
The 100R are on the bottom side of the PCB...?

Yes, on both counts!

Cheers,

Hugh

vsr123 · « **Reply #6 on:** 19 Jan 2005, 06:29 am »

Hugh
One fuse resistor on each board is blown. Both of them are under F1 on each board. R2 was not blown so I used the same R2 on each board.
After replacing the fuses, this is what I measured -

1. DC output between +ve and earth on both channels varies between 10mv to 20 mv. I get this value after about 2 seconds of powering on, once I switch off the value goes very high.

2. emitter to emitter dc voltage is 48mv on one channel and 49 mv on the other. Would you consider that to be in range? From your post I should be looking for 51mv to 59mv?

If you think this is ok, I am ready to replace the blown fuse resistors and try switching it on.

thanks
Sridhar

vsr123 · « **Reply #7 on:** 19 Jan 2005, 07:32 am »

Hugh
I also wanted to add that both the transformers are rated at 22V, 3.6 amps rather than the 25V that the kit is supposed to be using. I am just mentioning this in case the emitter to emitter voltage of 48 to 49 mv is alright or whether you suggest that I slowly adjust the pot to raise it to 55mv?

thanks
Sridhar

AKSA · « **Reply #8 on:** 19 Jan 2005, 10:14 am »

Sridhar,

All seems well, outputs are intact, bias could be raised to 55mV after 15 minutes idling, you can put in the fuses, switch on and connect up speakers.

It should be OK now.

Cheers,

Hugh

vsr123 · « **Reply #9 on:** 19 Jan 2005, 07:47 pm »

Hugh
thanks a lot for all the help. Its up and running. I ran it for about 15 minutes in the morning and everything seems ok. I assume I need to check for the 55mv when there is no input signal? because when I measure this voltage with music playing it varies a lot.

I have yet to run it with the Shanling, perhaps I will try that this evening, but I will check that the earths are connected before switching them on.

cheers
Sridhar

AKSA · « **Reply #10 on:** 19 Jan 2005, 09:29 pm »

Sridhar,

My pleasure! I hope it gives years of good service and listening pleasure!

Adjust the idle current with no signal to 55mV; check it's the same after 15 minutes idling, and if not, reset it to be sure.

The lower voltage is no problem at all.

Yes, connect the two chassis before connecting the signal from the Shanling. You've been lucky this time; don't risk it next time!

Cheers,

Hugh

Felipe · « **Reply #11 on:** 20 Jan 2005, 10:32 am »

Hi,

I didnt quite understand the source of the problem here, if the amp blew because of the fact that the Shanling has strange earthing, how is it solved just by connecting the 2 chassis?
If the amp would had a wooden box and star earth not connected to mains earth.... how would one solve the problem ?

I had a similar problem with my aksa100 last year, and it happened when i connected my amp to a Arcam cd player that has earth connecttion.
I fixed the amp with Hugh's help, but never since connected the amp to the Arcam again. Or other equipment with earth connections.

But this seems a fault, and the amp MUST be able to run earthed CD players like Shanling. How do you connect the 2 chassis?

Filipe

vsr123 · « **Reply #12 on:** 20 Jan 2005, 04:19 pm »

Hi Felipe
I feel the same way here - confused

I still have not hooked up the Shanling to the AKSA.
doesnt star earth always require connection to the mains earth? I ran the shanling on a pass labs Aleph 3 that I have and had no problems before connecting it to the aksa (perhaps I did not run it long enough on the Aleph to notice any problems).

cheers

AKSA · « **Reply #13 on:** 20 Jan 2005, 07:44 pm »

You could try connecting both star earths together; this would remedy any fault in the source earthing.

Cheers,

Hugh