BoSoZ has developed a hum after 4 years... - diyAudio
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Old 19th November 2007, 03:57 AM   #1
HooRide is offline HooRide  United States
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Default BoSoZ has developed a hum after 4 years...

I built my BoSoZ coming up on 4 years ago now, it's been on almost non-stop and it still sounds as good as ever.

However, it's developed a hum.

It's been a long time since I built it and don't remember the circuit all that well. So far i've determined the hum is not volume dependent and does not change with the inputs being shorted. Does anyone had any suggestions on how I should go about troubleshooting?.

thanks.

edit: wow, I thought about it for a while longer, it's actually been 6 years!
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Old 19th November 2007, 04:15 AM   #2
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Is your earth ground tied to your audio ground in the preamp? Could something have changed with your house's earth ground?
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Old 19th November 2007, 04:36 AM   #3
HooRide is offline HooRide  United States
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the earth ground and rest of the circuit are separated by a thermister and nothing in my home wiring...or system has changed in those 6 years

here's a link to the project pdf just in case http://www.passdiy.com/pdf/balzenpre.pdf
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Old 19th November 2007, 05:13 AM   #4
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I'm not familiar with that type of amplifier. But there might be a simple "generic" explanation and fix.

First, see if any significant amount of dust or lint has accumulated, and if so, clean it out and maybe clean any dirty circuit boards etc with some isopropyl alcohol.

If that fails to cure it, and the DC rails measure within specs, and a close inspection doesn't reveal anything sorched or leaking, etc, and nothing is getting hotter than it used to, then think about checking the ESRs (Equivalent Series Resistances) of any electrolytic capacitors, starting with the power supply's main filter caps. They might be starting to fail. If you don't have an ESR meter, maybe you can try this: http://www.fullnet.com/~tomg.esrscope.htm .

Good luck.

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Old 19th November 2007, 06:10 AM   #5
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I popped off the cover again and there isn't much dust or lint inside and everything seems to be pretty clean still

The DC rails are within spec and so are all the other test voltages Mr. Pass put in the pdf.

There's nothing leaking and nothing 'scorched' per say but everything has always gotten very toasty in there. A couple of the power filtering caps have their plastic tops 'popped' though it's just the plastic and nothing leaking. The circuit board is brown around a few resistors but nothing that I hadn't seen years ago.

What semi-worries me are the DC blocking capacitors C1 and C2, they look a bit, well, funky. They are Solen 10uF 85șC, maybe the heat got to them?

I don't have an o-scope to test the ESR...I don't even have my Fluke meter anymore because someone stole it :cries in beer glass: just my old RS meter.

I think i'll order some replacements (the Digikey parts Mr. Pass specified) and see if that takes care of it.

As always, any other input would be appreciated.
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Old 19th November 2007, 07:10 AM   #6
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if hum is same in both channels , and you say that amp was always pretty hot - and caps have caps popped , then these caps are probably culprit
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Old 19th November 2007, 09:17 PM   #7
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'Funky' looking caps are a good place to start, but also check for cold solder joints. They tend to appear over time; not immediately.
If it turns out to be the caps, you might want to take a look at your strategy for dealing with the heat the circuit puts out. The caps should not be giving out after a mere five or six years, regardless of rating. Is there sufficient heatsinking? Is there sufficient ventilation? All the heatsinking in the world is useless if there's not enough air to carry the heat away. If there's a heat problem, it will kill the next caps you put in just as sure as sunrise.

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Old 20th November 2007, 01:49 AM   #8
HooRide is offline HooRide  United States
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if it makes a difference, we're talking about a preamp here

anyways, the hum seems a little louder in the right channel, could just be me though

I reflowed all connections about a month ago when I first heard the hum.

I'd say most of the heat on the audio PCB comes from 6 pairs of 750 ohm 3 watt resistors.

I've purhcased replacement power caps, dc blocking caps and the 3 watt resistors. This time around, i'll elevate the resistors a little ways off the board.

I'll have to try again but i think the hum goes away when the preamp is completely cold and then returns again once it warms up.

...sounds like cap failure doesn't it?
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Old 20th November 2007, 06:09 AM   #9
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by HooRide
if it makes a difference, we're talking about a preamp here
........

not really difference ; hot is hot ..... preamp or amp - who cares ?



anyway - you'll tell us what's catch when you replace all caps ........

what's PSU arrangement?

you can check zenners and caps across them
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Old 20th November 2007, 06:39 AM   #10
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hot is hot....and then there's roasting, i measured one of the MOSFETs on the power supply at 98șC

I will definitely post my findings after replacing the caps, I think i'll start with the 4 'funky' ones first

thanks!
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