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 Post subject: Filter cap question
PostPosted: Apr Mon 30, 2012 3:24 am 
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Hello. I have a question about replacing filter caps in a solid state receiver. The set in question is a Sony STR-AV480, probably from the 1980's? Anyway, there is a 'buzz' in the audio even with the volume turned all the way down. I'm familiar with the 'hum' you get in old radios when the filter caps need to be replaced. Is this 'buzz' the same thing, indicating a need for filter caps? Thanks for any advice.

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 Post subject: Re: Filter cap question
PostPosted: Apr Mon 30, 2012 8:54 pm 
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Vinton, the buzz you hear is not indicative of bad filter caps. First thing to do is tighten all the grounding screws on the boards and all the screws on the rear panel.

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 Post subject: Re: Filter cap question
PostPosted: May Tue 01, 2012 3:33 pm 
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Thanks, Jim. I'll check that out and see if that makes a difference. Looks like it may be time to get out the signal tracer and maybe the 'scope.

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 Post subject: Re: Filter cap question
PostPosted: May Wed 02, 2012 6:25 pm 
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I would agree if it were a harsh buzz but if is a hum then just try adding or jumping with clip leads in another filter capacitor of equal or greater value onto the suspect bad cap. Be careful and ground it right where the ground is on the bad cap (all grounds are not necessarily the chassis ) . I would take my time doing this ,leave the radio off for a couple minutes (NOT SECONDS ! ) then read the values on the caps MFD ,and VOLTAGE are both very important. Then locate a FRESH made in the last 5 years same value or greater . Jumper it in then plug it in ( i try to have the switch already in the ON mode so you don't have to touch anything) be ready to unplug it rapidly ,ware safety goggles be ready if the new cap pops ! Fire extinguisher near by ! If it works just solder it in place after you shut it down and wait 2 minutes . If it still hums then try another cap ,usually there are up to 4 caps that would cause this . And it is possible more than one is bad !
If you had a harsh buzz its possible that the rectifier might be opening or starting to short located between the power transformer and the capacitors for filtering . They are usually the first thing to get damaged from lightning or surges.
You can replace them unless you know how to test them . If that's not helping then signal tracing with a scope is best.


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 Post subject: Re: Filter cap question
PostPosted: May Thu 03, 2012 12:49 am 
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Those Sony A/V receivers are notorious for having poor solder connections on the heatsinked regulators, and ANYthing mounted on a heatsink, including the output transistors.
Also the interconnect plugs and jumpers are prone to the same thing... soldering cracks.

Careful going-over the bottom of the PC board with a magnifying glass and good lighting is needed.

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 Post subject: Re: Filter cap question
PostPosted: May Thu 03, 2012 2:38 pm 
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Thanks for the additional information. The sound is more of a harsh buzz than the hum I'm familiar with from filter caps, so I would believe there could be connection problems. I think this receiver sat disused for some years before finding its way to me but that's really all I know about it. It looks like it won't be much fun removing the circuit board since several of the interconnects are soldered in place rather than unplugging. I hope to spend some time on it this weekend, and I want to start by checking some voltages before I start taking it apart.

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 Post subject: Re: Filter cap question
PostPosted: May Thu 03, 2012 4:10 pm 
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voroush wrote:
Thanks for the additional information. The sound is more of a harsh buzz than the hum I'm familiar with from filter caps, so I would believe there could be connection problems. I think this receiver sat disused for some years before finding its way to me but that's really all I know about it. It looks like it won't be much fun removing the circuit board since several of the interconnects are soldered in place rather than unplugging. I hope to spend some time on it this weekend, and I want to start by checking some voltages before I start taking it apart.


It should have a removable access/service panel on the bottom.

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 Post subject: Re: Filter cap question
PostPosted: May Thu 03, 2012 5:48 pm 
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Thanks for the heads up. Stupid me, I haven't even turned the unit over yet, just took off the cover and started looking things over inside. I'll definitely check out the underside.

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 Post subject: Re: Filter cap question
PostPosted: May Fri 04, 2012 2:58 pm 
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Well, apparently, Jim called this one. I loosened and re-tightened all the screws on the rear panel and one grounding screw on the circuit board, then turned the unit on and there's no buzz at all. I turned the volume up all the way with input set to CD (but nothing attached to the CD input) and it's virtually silent. I played with all the tone and balance settings and all seems to work fine. B+ was at 44.8v and B- at -44.8v so that seemed to be good, as well. I do still intend to inspect the solder joints on the circuit board, there was an acess panel on the bottom. This should make a decent set to listen to in the garage. Thanks for all the advice.

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 Post subject: Re: Filter cap question
PostPosted: May Fri 04, 2012 4:33 pm 
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voroush wrote:
Well, apparently, Jim called this one. I loosened and re-tightened all the screws on the rear panel and one grounding screw on the circuit board, then turned the unit on and there's no buzz at all. I turned the volume up all the way with input set to CD (but nothing attached to the CD input) and it's virtually silent. I played with all the tone and balance settings and all seems to work fine. B+ was at 44.8v and B- at -44.8v so that seemed to be good, as well. I do still intend to inspect the solder joints on the circuit board, there was an acess panel on the bottom. This should make a decent set to listen to in the garage. Thanks for all the advice.


If you look at the 3-pin regulators on the heatsinks, with a strong magnifying glass, you'll see tiny cracks forming around the pins in the solder pads.
Left alone, these pins/connections cause intermittent faults.
I'd go over the whole board with a magnifier - particularly inspecting those heatsinked parts, and re-flow a bit of fresh solder carefully on those pins.
But discharge the main filter caps with a 100 ohm resistor before that.
Then check various caps for residual voltage left in them.

Any spark created can fry a processer chip. :shock:


I've done dozens of them to know.

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 Post subject: Re: Filter cap question
PostPosted: May Fri 04, 2012 5:40 pm 
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Thanks for the tip, I'll examine those closely. Do you think the primary weak point of these sets in around the main power supply circuits?

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 Post subject: Re: Filter cap question
PostPosted: May Sat 05, 2012 12:44 am 
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voroush wrote:
Thanks for the tip, I'll examine those closely. Do you think the primary weak point of these sets in around the main power supply circuits?


The whole main board and its connectors are suspect.
They run hot - the expansion/contraction does the rest.

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 Post subject: Re: Filter cap question
PostPosted: May Thu 10, 2012 8:23 am 
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Repair Tech is right.

Sony receivers are known for aging solder connections. A typical Sony will require that several dozen connections be resoldered. Typically, you'll find the bad connections --

(1) In the driver portions of the power amps.
(2) In the portions of the regulated power supply where components (semi's and resistors) get hot.
(3) At the rear panel -- specifically, the RCA jacks.

Also, the switches and controls should be cleaned (deox'ed) with Caig DeoxIT D5. In most instances, the protection relay needs to come apart and have the contacts burnished with #1000 or finer sandpaper. In 2012 we're replacing protection relays all the time. We use relays that have a higher current rating.

* * * * *

Tightening all screws to restore ground plane integrity is a must. You've already done that and solved your primary problem

* * * * *

Enjoy your machine!

Fred
Classic Audio Repair

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 Post subject: Re: Filter cap question
PostPosted: May Thu 10, 2012 4:26 pm 
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Thanks, Fred and Repair Tech, for the great advise and input. It's interesting what you pointed out about the rear-panel RCA jacks. I've been using the Sony as I troubleshoot an Elac turntable and have noticed apparently intermittent problems with the phono input RCA jacks on the Sony. I thought the trouble was in the turntable, but last night I started believing the trouble was more in the Sony rather than the TT. Your post seems to confirm that suspicion. I better set the Sony aside until I finish the TT, then come back to it.

BTW - I'm ignorant with modern electronics. What is a protection relay and how do I service it?

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