Help...Smoke- AKSA 55W fuse & safety resistor FRIED!!!

Fredly · « **on:** 20 Sep 2005, 01:26 pm »

Hugh/Anyone, had a strange thing happen on Sunday.

After playing my AKSA 55W for about 4-6 hours, the right channel just gave up.

Close inspection the following day showed I had fried both the fuse and safety resistor! YOWSA!

Link to Pix

http://ca.pg.photos.yahoo.com/ph/fred_kabelitz/detail?.dir=4d37&.dnm=7658.jpg&.src=ph
http://ca.pg.photos.yahoo.com/ph/fred_kabelitz/detail?.dir=4d37&.dnm=f2b2.jpg&.src=ph

I have had this AKSA up and running without a problem, for about 4 months, and didn’t do anything (to my knowledge) to instigate a short.

Anyways, more importantly is the steps in getting my baby up as fast as possible.

I have a planned listening sessions with the boys for Saturday, and without my AKSA running…. it really won’t be much fun.

Above and beyond confirming no ground shorts can be found, will simply replacing both fuse/resistor breathe life back into the amp?

Equally important is understanding WHY this happened to begin with.

Did it over-heat? I did have it running pretty loud for a few hours there, but not when it gave up.

Any comments/advice would be greatly appreciated.

Fred in Canada.

Rom · « **Reply #1 on:** 20 Sep 2005, 02:48 pm »

Hi Fred,

There are few that I could think is the scenario that you experienced.

1. Without you knowing, the amp may have reach the critical load limit which is 3.6 ohms , IIRC, this normally come when deep bass on material and your speaker went to this area of lowest impedance.

Or 2. The Transitor on top of the Output transistor went a bit loose, and you will have the same result.

You must check your output transistor and the two driver and also the Vbe
just to make sure. I was lucky one time that I only have a burn resistor and Fuse to replaced like what happen to me on scenario 1 but scenario 2 will most likely you need to replaced the output transistors, so checking them first is what I would do.

I hope this help.
Cheers
rom

AKSA · « **Reply #2 on:** 20 Sep 2005, 11:46 pm »

Fred,

Rom gives great advice, thank you Rom!!

Check all transistors are firmly mounted, screwed down tight. Thermal bonding is important.

Replace the fuse, try it again, see what happens. If it blows again, you have a damaged output stage; one or both devices blown. I do not know why this has happened; my own amps never blow their output stages, so it could only be a speaker or thermal problem.

If you replace the fuse to no effect, remove the output devices, and test for continuity from base to emitter and collector to emitter. If either/both is a short, the device is blown, and you will need new ones.

If it's a 100W (I suspect it is from memory) then you will need a matched pair. I have some in stock, of course.

Good luck, Fred!

Cheers,

Hugh

rabbitz · « **Reply #3 on:** 21 Sep 2005, 11:58 am »

Had a fried resistor and blown fuses once and was a shorted speaker terminal on the case.... the insulator had a split. Just grab a DMM and test for a short from the hot speaker terminal to the case.

Worth a check as I went nuts looking for other stuff before I found it.... didn't quite get to the stage of cursing Hugh

Fredly · « **Reply #4 on:** 22 Sep 2005, 02:02 pm »

I PM’d Hugh on this last night, but thought I would post as well.

Upon closer inspection last night it appears ALL 100ohm safety resistors, on both channels, are a little bit or a lot charred.

Also on the working channel, one of the fuses had also been blown.

I plan on replacing all resistors and fuses, verify I have no grounding problems throughout and sparking her up to see if she’s alive.

Also, as a precautionary measure, I added addition venting to both top and bottom of my case to ensure things don’t get too hot inside.

Anyways, again thanx for all you valuable feedback and wish me luck.

Cheers, Fred in Toronto, Canada.

AKSA · « **Reply #5 on:** 24 Sep 2005, 07:50 am »

Hi Fred,

How are you getting on with the repair?

Cheers,

Hugh

avahifi · « **Reply #6 on:** 24 Sep 2005, 12:53 pm »

"I plan on replacing all resistors and fuses, verify I have no grounding problems throughout and sparking her up to see if she’s alive"

Bad idea. Resistors do not fail all by themselves. They fail because something else is dumping way way way too much current through them. The most likely suspect is the output transistors shorting, putting the whole power supply current directly through them. A reasonable guess at the failure scenaro is that an output device failed frying the resistor and fuse on one half of the amp, causing the amp to go full DC offset to the other side, and that caused the other fuse and resistor to fail too.

I suggest you replace the output pair first, and check for speaker wire shorts or connection problems before trying again. Note that it may seem to test good out of the system after repaired when not connected to speakers, but could blow again instantly when connected back to speakers (the output circuit is open without the speakers in the loop), so running it up with an AC variac, signal generator, and scope on a dummy load bank is a good idea before putting your speakers at risk.

Note that in general, a failure caused by external reasons has a good chance of just killing output devices, a failure caused from an internal problem likely caused a lot more circuit damage and will require skilled troubleshooting.

Note that the above thoughts are not directed about AKSA products specifically, but apply to any bipolar power amplifier in general.

Frank Van Alstine

Fredly · « **Reply #7 on:** 28 Sep 2005, 01:35 pm »

Again, Hugh/Frank thanx for your concern and suggestions.

I’ve been laid up with a cold the past few days, but had a spare hour yesterday, to resolder the output devices (after checking both on the DMM) back into the module, to begin measuring the voltages from the beginning.

Well I quickly found out what the problem was. (duh)

It appears I am no longer getting the +36V from the Power Supply to the right module. The -36V is OK, but no +36V.

So…….a few questions on how to proceed.

Can I assume it’s the rectifier(s) that went, and that the caps are OK?

What’s a quickest test to confirm which diode it is? Can I assume it’s the closest to the +36V tab?

Will ANY BYQ28E-200/T022AB rectifier substitute do?

And while I’m in there unsoldering things, is there any sonic benefit to replace ALL diodes with a better one, such as Schottky (If I can get my hands on them).

And one final question, is there a quick method of draining the PS caps. (In the past I have used a light bulb, which seemed to work pretty good, but am open to alternatives methods)

Looking forward to all suggestions/input.

Fred in Canada.

AKSA · « **Reply #8 on:** 28 Sep 2005, 10:36 pm »

Hi Fred,

Ah, power supply!!

It looks like a short all right.......

You must replace all three diodes for the faulty channel. If both channels are affected, all six should be replaced.

You can replace just one or two, but you may have damaged all of them through overcurrent. Then you have to pull it all down again to replace the bad apple; better to replace all three on each channel.

I have plenty, email me privately to get the show on the road.....!!

Are you sure that the outputs are OK, Fred? They took a beating, and they too might be blown or damaged. You should check continuity from collector to emitter and base to emitter in circuit with power OFF. This should give a reasonable indication. Anything less than 100R is a damaged/shorted device.

Cheers,

Hugh