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pioneer sa708..humming! help!

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fulltilt

New Member
Hi.
Recieved my "new" pioneer sa708 yesterday in the post...looks in great condition..however when i connected speakers to it, a loud constant humming noise comes through the speakers! The volume does not affect the hum, and it is in both channels.Touching the case does not alter the hum and neither does moving the amp around while plugged in.I am no expert in these things and was wondering if anyone could give me some advice as to likely causes of the problem and possible solutions.Amp seems to be fully functional apart from the loud hum which makes music impossible to listen to at reasonable levels.
Also there is a kind of pop noise when turning the amp on, before the blue meters light up...is this normal? some kind of protection kicking in?
Any advice very very much appreciated.cheers
 
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There may be another, smaller cap connected to the voltage rails that has failed.
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I had symptoms like yours in my Technics SU-8080. This cap was shot.
 
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There are some real experts here (I'm NOT amongst them!) but 'hum' is usually the filter caps failing .... identify (from the service manual) ... and replace and that might be all there is to it.

Keep us posted!

Best
 
That cap is BEYOND shot! lol!

I would start with the power supply caps. Uh, one question- does it hum with absolutely nothing connected to it besides speakers?

I've seen cases where folks connect other gear that via grounds is connected to the cable TV system- and that's a recipe for hum. A lot of times, there's a substantial amount of AC energy on the cable that does not belong there.
 
thanks for the replies....i am not an expert by any stretch of the imagination and have no idea which caps i would need to replace.
The amp does hum loudly with no sources plugged into inputs.
I am unsure whether to return the item or have a go at fixing it...i do like the sound of the amp and wonder if i was to buy another of the pioneer sa range, whether or not this problem would occur again at some point in the future due to these being old amps and caps needing replacing is inevitable?.
I paid £90 for the amp which seems fairly reasonable to me.
Also, with regards the pop a couple of seconds after the power switch is flicked on and the blue leds light up.. this a normal thing due to a protection circuit?
Is there any other likely causes and if it is just a cap replacement presumably it is a cheap repair- im a man of limited budget and lvery limited electrical knowledge...cheers again
 
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just downloaded a schematic..not sure even where on the circuit to look..i presume the mains filtering caps would be somewhere close after the transformer? mmmmm...maybe i should get someone who knows what they are doing to fix it!
 
fulltilt said:
just downloaded a schematic..not sure even where on the circuit to look..i presume the mains filtering caps would be somewhere close after the transformer? mmmmm...maybe i should get someone who knows what they are doing to fix it!
Click to expand...

Yeah, maybe you should, if your anything like me than you know your limits......better safe than sorry eh ?
 
Did your schematic have a parts list? You can narrow down the troublesome caps by looking for the high voltage caps. Make sure you use a power resistor (~50 ohm, 5 watt) to discharge your main filter caps (the big caps).
 
I have no experience in recapping but willing to give it a go if at all realistically feasable for a novice..no parts list on my shematic..will try and find another...is the sa708 same inside as a sa7800?As I understand it,the sa708 is the european version of the sa7800.....?
how much would i expect to pay an engineer to recap this amp?
what about the popping sound shortly after turning power switch on...protection circuit or transister problem???
thanks again...even if i have to return the amp to the ebay seller im learning loads.
one last question to prove my naivety.....is the current discharged from the power filter caps dangerous or relatively low level? To discharge it i put a high value resistor accross the +ve and -ve terminals of the cap?
 
You need to make sure there is no voltage in the main filter caps before you start poking around in the amp. There are dangerous voltages inside. Yes, you connect a hi watt 50 ohm resistor to the capacitor leads to drain the voltage.
The popping sound is most likely related to your hum problem: a bad cap.
 
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