terrible humming in one channel of the 55

zachslc · « **on:** 30 Oct 2004, 02:40 am »

one channel of my 55 is humming terribly. When I disconnect the interconnect (either from the am or the ream itis diminished, but I have tried it with different interconnects and different reams and it doesn't seem to matter. I have tried all sorts of grounding schemes, resoldering many of the joints, blocking the DC on the mains, and everything else I can think of. The humming comes and goes cyclically every few minutes. The other channel is dead silent. Is it possible that there is a problematic component? I am absolutely stumped. Can anoyone shed any light? I wouldn't post about a noise/grounding issue if I were not at my wits' end. Thanks.

AKSA · « **Reply #1 on:** 30 Oct 2004, 03:37 am »

Hi Zach,

Check that R2, the 10R ground lift near the earth connection on the module, is good. Switch off, check with a DMM.

Sometimes this component will fail, giving the symptoms you describe.

Failing that, it's tricky. Does it still play music? What is the output offset? What is the quiescent current? These are the important measurements.

Cheers,

Hugh

AKSA · « **Reply #2 on:** 9 Nov 2004, 09:29 pm »

Hi Zach,

Did you solve the problem?

Cheers,

Hugh

zachslc · « **Reply #3 on:** 14 Nov 2004, 10:53 pm »

Thanks for the input, Hugh. It seems I have solved the problem, simply by rewiring yet again--apparently there was a poor connection or shield somewhere, but it seems I have a MUCH bigger problem with the other channel at this point.

When I put everything back together it was fine in both channels for a couple of days. Then yesterday the mains fuse blew. I disconnected the amps and replaced the fuse, powered up and it blew again. It turns out there was a bad diode in one side of the power supply--it has now been replaced and the supply is fine, but the amp that was connected to the side with the bad diode has a terrible and rather loud hum. Naturally I replaced cables, checked all solder joints, etc. I set the bias at minimum and began the original testing procedure. I get a very low voltage reading across the rails, about .1v. At this point I came here. Did I damage the board with the power supply problem? How would you suggest I proceed? Thanks much, again.

AKSA · « **Reply #4 on:** 14 Nov 2004, 11:08 pm »

Hi Zach,

Check R2, the 10R earth float resistor near the ground spade on the pcb module. Sometimes it will take a fair whack of voltage in the circumstances you describe and go high.

Replace it with a half watt 10R and this hum should go away.....

Of course I'm guestimating, difficult from afar, but this could be it.

Cheers,

Hugh

zachslc · « **Reply #5 on:** 15 Nov 2004, 02:22 am »

Hugh,

I replaced r2, still no significant voltage across the power rails. The trimpot is turned all the way ccw, so I didn't measure further. What do you think? Thanks.

AKSA · « **Reply #6 on:** 15 Nov 2004, 02:34 am »

Hi Zach,

I think now we confirm that the dud supply, now repaired, has not damaged the filter caps. A shorted diode will subject these big caps to AC, which can overheat and damage them. So, power up the module from the other supply, and see if the hum is still present. If not, you will need new filter caps, and if still present, the problem lies with the module.

Is the bias correct? Is the offset correct? Does it play music properly despite the hum?

Cheers,

Hugh

Greg Erskine · « **Reply #7 on:** 15 Nov 2004, 04:54 am »

JFYI.

By accident, I connected a set of 4 4,700uF Nippon Chemicon caps (as used in the standard AKSA power supply) the wrong way in a 35-0-35 PSU. The 4 four caps got hot enough to push the tops out a few millimeters before the fuse blew. I don't know how long it took for the fuse to blow, probably only a few seconds. They were hot enough to be unpleasant to touch. I think I did it twice before I realised my error.

To my surprise they still appear to function OK, although I haven't done a A/B test to prove this.

Thanks
Greg

andyr · « **Reply #8 on:** 15 Nov 2004, 08:34 am »

Quote from: Greg Erskine

JFYI.

To my surprise they still appear to function OK, although I haven't done a A/B test to prove this.

Thanks
Greg

Greg, I suggest you should replace them. No point in compromising your fabulous AKSA ... just put the expense down to "experience"!

Regards,

Andy

Greg Erskine · « **Reply #9 on:** 15 Nov 2004, 09:39 am »

Hi Andy,

Sorry I didn't make it clear, these caps weren't used on my AKSA, but a SC480, so it doesn't matter.

I love my AKSA so I've promised myself not to waste any more time, or money on my "other" amps, so the SC480 is staying the way it is, stuck in the back of a cupboard.

Thanks
Greg

andyr · « **Reply #10 on:** 15 Nov 2004, 09:54 am »

Quote from: Greg Erskine

Hi Andy,

Sorry I didn't make it clear, these caps weren't used on my AKSA, but a SC480, so it doesn't matter.

I love my AKSA so I've promised myself not to waste any more time, or money on my "other" amps, so the SC480 is staying the way it is, stuck in the back of a cupboard.

Thanks
Greg

Hi, Greg,

With respect ... doing nothing is a waste! Why not get some more caps from Hugh, replace the ones in the SC480 and then sell it on eBay!!?? With the money you get, you can then buy another AKSA amp ... or upgrade what you've got to N+ status.

Regads,

Andy

Greg Erskine · « **Reply #11 on:** 15 Nov 2004, 11:19 am »

Quote from: andyr

[With respect ... doing nothing is a waste! Why not get some more caps from Hugh, replace the ones in the SC480 and then sell it on eBay!!?? With the money you get, you can then buy another AKSA amp ... or upgrade what you've got to N+ status.

Hey Andy, who (or hugh) you been talking to.

I all ready have another AKSA/N+, a TLP/N and a GK-1 on the way. I have a collection of non-AKSA amps, but they are in the cupboard, where they belong.

I'm not sure, but I don't think the caps have gone bad. The SC480 compares OK with the non-AKSA amps, although it does lack a bit of bass, I suppose it could be the caps. I should change them to complete the learning exercise. I probably should sell the stuff in the cupboard, one day.

andyr · « **Reply #12 on:** 15 Nov 2004, 11:39 am »

Quote from: Greg Erskine

Hey Andy, who (or hugh) you been talking to. I all ready have another AKSA/N+, a TLP/N and a GK-1 on the way. I have a collection of non-AKSA amps, but they are in the cupboard, where they belong. ....

Sorry, Greg,

I was unaware you are one of Hugh's top customers!

Regards,

Andy

zachslc · « **Reply #13 on:** 15 Nov 2004, 04:18 pm »

Hugh,

I am actually using the power supply from my other AKSA, so I know it is not the caps. Is it safe to play music through it with the trim pot turned all the way CCW? I probably should not have done that. I got so frustrated last night that I had to sleep on it. When I get home this evening I will post some measurements. Thanks again.

AKSA · « **Reply #14 on:** 8 Dec 2004, 10:40 pm »

Zach,

How goes your AKSA repair?

Cheers,

Hugh

zachslc · « **Reply #15 on:** 9 Dec 2004, 06:39 pm »

Hugh,

Remember we sent a few PMs back and forth? Anyway, the amp should be arriving (at your place) any day. Thanks.

AKSA · « **Reply #16 on:** 9 Dec 2004, 08:09 pm »

Ah, OK Zach,

I will expect them!

Cheers,

Hugh

zachslc · « **Reply #17 on:** 9 Dec 2004, 09:21 pm »

Don't worry, Hugh. It's only one. Would you let me know if it arrives ok? No rush. Thanks.