Loud Hum problem with AKSA 55N+

netaron · « **on:** 16 Sep 2004, 06:20 pm »

Hi all,
This is my first post on this forum so please forgive the blabber. I first want to thank Hugh for all his time and help with trying to solve this hum issue I developed with the AKSA 55N+, and I really don't want to take any more of his time with my questions. I just wanted to bounce this off all the folks here and see what we can come up with.
After building and listening to my AKSA 55N+, one day I got the bright idea of trying to resolder T1/T2/T3 to a more upright position, mistake #1, I forgot to disconnect the power supply form the PCB's. After soldering, both channels produced a very loud hum. I measured output offset which was 32.59Volts and 36.72Volts respectively, bias measured 0.1mV. Hugh kindly sent me a new set of T1/T2/T3 and all output Transistors and the problem still persisted. I have checked all resistors and caps, and all measure within spec. I have tried disconnecting one channel and still no difference. Any ideas would be appreciated since I miss my baby oh so much.

Thanks kindly

Haron

kyrill · « **Reply #1 on:** 16 Sep 2004, 08:08 pm »

Can you check your soldering iron if there is any voltage on it, when hot
?
Should not be, but just to be sure. If there is it is probably AC.

By the way, this is just a hunch from a naïve amateur.

Maybe, if there is it damaged something in the power supply

Carlman · « **Reply #2 on:** 16 Sep 2004, 08:17 pm »

Don't ignore the obvious... you may have inadvertently knocked something into harms path or away from where it should be.. if this is the case, it's something you'd have to see...

Visually inspect everything on the amp and see if every wire goes where it should. Take the fuses out and put them back in.. try some very basic troubleshooting/analysis to be sure nothing's physically wrong.

Just a thought. Good luck!

netaron · « **Reply #3 on:** 16 Sep 2004, 09:56 pm »

Thank you for your replies, I don't know how many times I have traced everything, but I will do it again just to be sure.

AKSA · « **Reply #4 on:** 16 Sep 2004, 11:56 pm »

Haron,

This is becoming something of an epic tragedy..... We really must fix it! You know, AKSA deprivation is a serious condition, often culminating in involutional depression........

Good idea to again read the trouble shooting section carefully. The two single most important measurements on an AKSA are output offset voltage and bias level. If these are OK, then you are 99% assured of a functioning amplifier.

First up, then, switch on, and measure these parameters. If they are OK (offset less than 30mV, bias at 55mV across the output device emitters) then the problem is induced hum OR an earth loop.

If you have the output offset almost at one of the rails, then there is a serious problem and you should NOT connect a speaker. However, you say there is hum, so clearly a speaker is connected, so maybe the offset is OK. If so, then the problem is related to hum induction somehow.

Next step is to remove the input leads to the module. With a speaker connected to the output, is there still hum? If not, then the problem clearly lies with the leads coming into the amplifier, not the amp itself.

This should rev you up for another attempt at solving this problem.......

Cheers,

Hugh

andyr · « **Reply #5 on:** 17 Sep 2004, 07:36 am »

Could you have earthed the "star earth" back to chassis like ginger suggested? Doing this gave me an earth loop in my system & therefore hum.

Regards,

Andy

Grumpy_Git · « **Reply #6 on:** 17 Sep 2004, 08:09 am »

When I first bolted my rca connectors to my case I had a huge amount of hum, this still reappears if i wiggle the connectors around too much. I dont have the case earthed (Whoops!

) so its not a problem from that point of view, and my amp is insulated from the case by a 1" lump of wood.

Check this as it drove me wild when I first set it up as a small movement of the amp flicked it on and off, I keep meaning to sort it out but, damn, metal work is hard and hearing of other's problems who wants to mess with a healthy AKSA?

Nick.

kyrill · « **Reply #7 on:** 17 Sep 2004, 08:44 am »

The AKSA started without the hum problems. They started after resoldering. So you have to disconnect the heatsonk/pcb from the enclosure in a way.

I would go back where you started when soldering. Nick might be right, try to decouple a channel from the whole enclosure of the amp, and without input, or better even with the input shorted
in yr first post you wrote the output offset to be over 30 Volts? Instead of 30 millivolts?

Noel T. · « **Reply #8 on:** 17 Sep 2004, 10:39 am »

I have seen a similar symptom to this in other amplifiers when the ground lift resistor goes open circuit. Perhaps you could try double checking R2.

andyr · « **Reply #9 on:** 17 Sep 2004, 11:04 am »

Quote from: Grumpy_Git

When I first bolted my rca connectors to my case I had a huge amount of hum, this still reappears if i wiggle the connectors around too much. ... Nick.

Nick,

Could this be bcoz your RCA holes are not big enough and, by "wiggling" the RCA sockets, you're shorting the RCA earth to chassis earth?

BTW, i wouldn't describe yours as a "healthy" AKSA ... delicate, yes!!

Regards,

Andy

Grumpy_Git · « **Reply #10 on:** 17 Sep 2004, 12:30 pm »

Andy

I know that is the reason I was getting the hum, I just cant do anything about it, anyway my casse isnt grounded, I just isolated the entire amp,heatsink toroid setup from the case with mahogany (for that rich red sound). I moved from my parents a year ago and I no longer have any powertools to play with

.

I had managed to royally screw up the case anyway so at some point I shall create a more suitable enclosure.

For now, I dont move the amp and it works great, maybe a touch too much hiss from about a foot with no source on, but thats fine by me.

My Aksa is more healthy than I am, something to do with the brewskies and good food and brickwall aussie reds, thump goes my head!

Nick (been off the sauce longer than I'd like)

andyr · « **Reply #11 on:** 17 Sep 2004, 12:44 pm »

Mate, I think you need a glass or three of fine Aussie Cab Sav ...!!

Regards,

Andy

netaron · « **Reply #12 on:** 17 Sep 2004, 03:44 pm »

One of the first things I tried was disconnecting the inputs and check for shorts between star earth and ground. Also R2 has been removed, checked and reinserted. I have also disconnected one channel completely and placed the pcb's on my bench to make sure nothing was shorting out, and still no go. All speaker conncetions check out ok as well. The offset is still high, although I am using a variac so not to damage the new parts Hugh sent me, it would be around 30Volts when at full AC power.

Thanks you all for your help.

Haron

T-Bone · « **Reply #13 on:** 17 Sep 2004, 04:50 pm »

You could put the 55 on a scope to electrically look at the hum. If you live here in the states and you see 50/60hz it's a ground problem ie; a speaker connecter or possibly a RCA jack has gone to ground. Have you checked your interconnects? If the shield wire breaks internally the amp will hum. Getting back to the scope, if you see around 120hz you have a rectifier problem. I trouble shoot starting at the signal with the interconnects and work back to the power supply. Some guys start at the power supply and work out. To each his own. The point is to develop some type of system that will allow you to isolate the problem.

With the amp unplugged!
1.) Check interconnects for continuity (both shield and hot) I've had brand new interconnects defective right from the factory!
2.) Check speaker and RCA jacks. If you have resoldered the RCA's more than three or four times the possibilty exists that the internal plastic insulator has melted and went to ground. With your meter set on "continuity" clip one probe on the amp enclosure and leave it there. Take the other probe or lead and touch the shield and hot connections (individually) of the RCA. There should be NO continuity on either one. Now move to the speaker connection. Make the same test. One lead clipped to the enclosure. Touch the other (individually) to the positive and then the negative speaker posts of the speaker connecter. Again there should be NO continuity.
3.) Now check your internal speaker and RCA connections for continuity from the boards to the connecters.
4.) Disconnect the power supply leads from the boards.
5.) Using your meter clip the neg. lead to star earth on power supply with pos. lead clipped to pos. rail
Plug in the amp without your speakers connected. Switch ON. Observe meter reading. Switch OFF. Unplug. Check neg. power supply rail using the same procedure.

If everything checks out to here and a problem still exsists you have narrowed it down to one or more of the amplifier boards. With the amp unplugged re-connect the boards and power supply. Without speakers connected switch ON the amp. Using your AKSA documentation perfom all tests as described in the troubleshooting guide. Write down all observed meter readings.

Hope this helps

Brad

Martin · « **Reply #14 on:** 17 Sep 2004, 05:24 pm »

All hum Ive experienced
has been from my pre amp. My 2 cents worth
Once I got that resolved, any remnant
is from cables. I'm working on that.
Martin

kyrill · « **Reply #15 on:** 17 Sep 2004, 06:57 pm »

Quote from: netaron

One of the first things . . . All speaker conncetions check out ok as well. The offset is still high, although I am using a variac so not to damage the new parts Hugh sent me, it would be around 30Volts when at full AC power.

Thanks you all for your help.

Haron

The output offset you say is over 30 volts for both channels. is this the same offset that should be under 32 millivolts?

If yes, your offset is 1000 times too big and you cannot get it lower?
If so, than hum is yr secondary problem not yr first

AKSA · « **Reply #16 on:** 17 Sep 2004, 11:18 pm »

Haron,

Your amp is, in fact, mortally wounded. Please don't connect it to a loudspeaker as it will destroy the woofer! An offset of this magnitude indicates grave injuries. Often this means a damaged output device, but there may be issues with the input devices and even T3, the voltage amplifier. As I recall, we've replaced most of these now, yet still the problem persists. This indicates some of the good parts we've put in may have been damaged again, and until the gross fault is found this might happen again, a sobering thought.

You use of a variac is a good move. However, you need to immobilize the output stage and loop back the feedback from the NPN driver's emitter. This will enable you to examine the earlier parts of the circuit, and isolate the outputs, which could be faulty.

Problems of this nature require quite a lot of experience in analog electronics to identify and fix. Even as the designer, it took me a couple of years of repair experience to really learn the symptoms and their various causes, and even now the medium of email is so imperfect that to and fro descriptions are only partially successful. I've spent hundreds of hours over the last five years helping people with construction difficulties and fault finding, and it seems to me looking back that unless extreme rigour and discipline is applied from the start (and the educational aspect is downplayed because that REALLY hogs the time) the quest is likely to blow out for 10-20 emails, which is a very poor use of everyone's time. I'm beginning to realize that extending the merchant obligation to 10-20 emails threatens the business model and certainly denies me time with my family; I'm obliged to work seven days a week because people want access within a day or so at most.

The best option is to send it to me; I'll fix it in about two hours usually, and then give you a full description of how and why it all happened. I'm sorry I can't offer anything better; but you really need someone experienced in amp repair and thoroughly familiar with the AKSA to get it going.

Cheers,

Hugh

netaron · « **Reply #17 on:** 18 Sep 2004, 01:37 am »

Hi Hugh,
Thanks again for all of your time, eventhough I was trying to save you some of the headaches by posting my problem here, you still come through. You are a true gentleman, and I have enjoyed dealing with you very much. As for my baby, well I'll have her crated and sent downunder faster than a New York minute.

Haron

AKSA · « **Reply #18 on:** 18 Sep 2004, 02:03 am »

Haron,

Here's hoping we beat that NYC minute; my golly, not much chance of that from Downunder.......

Take the pcb off the heatsink, and just send the board, nothing else. This is, after all, the offending bit, and I have LOTS of heatsinks here for test purposes!! Save postage!!

Cheers,

Hugh

Martin · « **Reply #19 on:** 18 Sep 2004, 04:25 am »

Haron
Don't feel bad...one of my modules have been in the operating room at "Hughes Hospital". The procedure was a success. Both the module and I have recovered nicely.
Thanks again Hugh!
Martin