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Old 6th February 2008, 01:51 AM   #1
Tino is offline Tino  Canada
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Default Soundcraftsmen PM860 transformer buzz

Hi everyone,

I've had a PM860 sitting around for about 3 years. I powered it up today and I'm getting quite a lot of tranformer buzz. Not the mild hum I used to get with it that was very lightly audible. Now I have not tried connecting it to a source and to speakers but I doubt thats why its buzzing (just being powered all by itself)

I'll try connecting it to a source and seeing if it sounds fine otherwise. Are transformers easily replacable by most repair shops and if so, is it worth replacing if the amp is clean and working fine otherwise?
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Old 6th February 2008, 02:10 AM   #2
Tino is offline Tino  Canada
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By the way, I just connected it and listened to it. Sonically its still working flawless, but the hum/buzz from the transformer is louder than it used to be. The only other possibility is that maybe the noise from it was always this loud, but because my listening environment changed from a big open basement to a small dedicated bedroom (which is more quiet), maybe its why I hear it more. Not sure.
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Old 6th February 2008, 01:17 PM   #3
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the caps could be going bad and the transformer is just working harder. also, remove the top cover. does the buzz go away? My SC amps have had this problem as the top cover is steel and the trafo can cause it to vibrate. *** some foam or rubber on top of the trafo and it usually goes away.


Zc
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Old 6th February 2008, 02:50 PM   #4
Tino is offline Tino  Canada
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Hi there. Good to see someone else with this amp. I really like this amp. Surprisingly good sounding. IMO it sounds as good as a Rotel or NAD. My friend dragged his Adcom 555 over one time because he was proud of his purchase and wanted to try it on my system. I was expecting that monster to sound better but the SC amp sounded smoother on the top end and had tighter control of the bass. I was surprised!

Anyway back to my prob With the top cover off, you can still hear the buzz, but its a little less. Its definitly coming from the transformer. I assume replacing the caps would be the first thing I should try right? If I find caps that are the same voltage but slightly higher capacity, would that be fine to slip in there? Thats providing I dont find the identical brand and value.

This amp (and the matching soundcraftsmen preamp) is on my second system. Primary amp now is my Threshold SA3 on a Rotel preamp, but I'd still love to keep the PM860 in service

PS. would you remember what the stock value and voltage is on the PM860 main caps? This is the patient in question
http://powerthings.com/pics/soundcraftsmen/2.jpg
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Old 6th February 2008, 03:44 PM   #5
jaycee is offline jaycee  United Kingdom
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The amp does not have a regular "linear" power supply, but a switchmode supply - the board next to the transformer.

The buzzing could mean theres a problem with the switchmode circuitry - I would not mess with this if you're inexperienced... at best you could toast the amp completely, at worst you could kill yourself.

The buzzing could also simply mean that your AC supply has a DC component and it's saturating the core. A power conditioner might fix this.
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Old 6th February 2008, 05:22 PM   #6
Tino is offline Tino  Canada
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Hmmm thats not good I guess. Maybe the best thing would be to take it to a reputable repair center and let them look at it.
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Old 7th February 2008, 04:10 AM   #7
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this does not have a switchmode power supply. it has a phase controlled power supply. totally different principle of operation.

The stock caps are 11,000uf at 75V

also check that none of the SCR's are latched up. they have a tendency to short out. there 4010 SCR's 400V 10 amp.
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Old 7th February 2008, 06:12 PM   #8
jaycee is offline jaycee  United Kingdom
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Zero Cool, where are the smoothing caps ? I couldn't see them on those photos so I assumed it was an SMPS design.
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Old 7th February 2008, 10:02 PM   #9
Tino is offline Tino  Canada
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What the hell.. I bring the amp to work because we have an inhouse repair technician (he normally works on open reel equipment as well as assorted production level tape players (video). We plug it in and its barely making noise. Very little.

Is it possible that my house wiring has an issue that might cause this amp to make some hum or buzz? I'll take it home again and see if the buzz is louder again.

I read somewhere that certain flourescent lighting can add noise to a circuit in a house. Not sure if thats true or not. That doesnt really make sense to me but maybe I'll try turning off any of those lights.
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