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My amp (McCormack DNA-1 Deluxe) has been producing a hum lately. Though I'm a total ignoramus about electronics, in the last 2-3 weeks I've twice taken the top off the amp to see if there's any visible problems like caps leaking.There isn't anything wrong that I can see. But today - like the previous time a couple weeks ago - I turned the amp off and unhooked the input ic's, opened the amp up and checked to make sure connections were secure (they were), and lightly tapped each of the big caps with the rubber end of a screwdriver. I then replaced the cover, put the ic's from the preamp back in and turned the whole system on.
Both times I did this resulted in the hum disappearing. What could explain this?
Edits: 03/14/15Follow Ups:
I just looked at some google'd photos of the DNA-1 Deluxe.
It has 8x4700ufd caps PER SIDE in a distributed arrangement.
That is a pretty robust 75k TOTAL…….and a real PIA to replace!
I couldn't tell from the pics if they were 85c OR 105c but if possible, I'd replace with the higher temp.
Too much is never enough
a well respected techie I took the amp to when this problem first started a couple months ago told me its now hard to find the type of caps mounted in the manner the DNA-1 employs. He basically told me I need a new amp.
I'm gonna call SMc today and see what the cost and wait time would be for upgrades, then decide what to do.
Either cold solder joint(s) or the cap(s) are going. They've exceeded their lifespan. I would change all the big lytics.
Check the traces as well.
Cheers,
W
could be the caps?
The DNA-1 is about 18 years old and perhaps it's time to audition some later model amps.
From 2001 - 2003 I owned the highly acclaimed DNA-2 Limited Anniversary Edition, an excellent performer and only to be significantly outdone when I purchased a DNA-2 Rev. A model which to me was the cat's meow at the time.
But at the request of a distributor who was already familiar with my McCormack amps, I auditioned a pair of nuforce Rev 9 SE models that were clearly more musical and even more dynamic even though the nuforce's only had half the power rating of my DNA-2 Rev A. Then went with a BMC C1 int. amp and now I'm using some phenomenal 575wpc Class D amps by Wyred 4 Sound retailing for $2400 the pair..
The point being that technology moves forward and considering the problem you're now encountering maybe this is a good time to see what else is available.
I've had the amp for 24 years, so its past time to replace caps. I've heard nothing but praise for Steve's upgrades, and doubt I'd find anything better for what it'll cost me.
The drag is the time it takes.
/
I understand SMc is the guy but you can get service at McCormack of Virginia and possibly drop off your amp if you get to the DC area. McCormack of VA is also Conrad Johnson and since they are McCormack...their work should make your amp right. But I understand if you want Steve's crew doing the mods, upgrades and such.
I didn't think the DNA amps had big power supply caps, just the smaller ones at each of the output transistors on the side boards. Nothing big in the middle of the amp.
Since you say you tapped the caps, I bet a solder joint has decided it's time to play havoc. An old, cold solder joint can oxidize or come apart from expansion and contraction. If you are capable, a solder iron cleaning fluid, flux and a good solder might do the trick with little pain. Of course, since your in there with tools in hand, the suggestion to replace the electrolytic's is spot on.
do the pencil test with the amp playing. tap the filter caps and see if you can cause the hum to start and stop.
if so you have a bad solder joint. anything intermittent by mechanical means is usually a bad solder joint or wire rap connection. but I would replace those caps anyway.
And if you _hadn't_ tapped anything? What about just turning it off for twenty minutes (or however long it was off) and turning it back on?
nt
A hum can be a transformer if it's heard at the amp and not the speakers. A transformer's core lamination can extend and contract twice during a normal voltage or current cycle. This means that the transformer is vibrating at twice the frequency of the supply, i.e. for 60 cycles per second supply frequency, the noise or vibration is moving at 120 cycles per second. With older equipment that can begin to happen if the core lamination has begun to dry out on the transformer.
Edits: 03/14/15
nt
If it's at the speakers it's electrical most likely a ground issue. If you haven't already done it try lifting the ground. It might not be the amp you may have to try lifting the ground on the preamp or one of the source components.
The amp is around 20 years old by now.
ANY chance the PS caps are in the starting stages of simply 'going south'?
They may be 'drying' up (out?) and getting near the time of failure.
Changing them NOW is a low risk proposition which ANY tech should be able to pull off without problem.
heck, if I could get the right or even upgraded parts (IF 85c, than 105c), I could do the job.
Too much is never enough
why has the hum gone away after I just lightly tapped all the caps? This happened twice, weeks apart.
Perhaps you shifted the internal foils when you knocked them? My guess is that with use, and with an aged or impoverished insulating medium, EMF forces may cause them to work themselves back into looser arrangements. This might cause the hum you hear.
Big J
"... only a very few individuals understand as yet that personal salvation is a contradiction in terms."
One explanation or another?
IF new caps do the trick, than the explanation can wait for a dissection of the OLD caps.
Yeah, caps of the PS type DO dry out. The media is an Aluminum carrying 'goo'. And sure, why not? The Dry Dielectric 'flopping' in the field. Tap Tap and it setttles down for a week.
Make SURE you use 105c rated caps. Even IF 85c had been spec'd.
Too much is never enough
in the ICs perhaps
"Man is the only animal that blushes - or needs to" Mark Twain
The amp was producing a hum thru the speakers with everything else off and the ic's from the pre disconnected from the amp. The spkr. cables are pretty new, and anyway I did zilch regarding the spkr. cables.
Edits: 03/14/15
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