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Old 9th March 2011, 05:39 PM   #441
Mooly is offline Mooly  United Kingdom
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Think that's all you can do.

From my days as a service tech, dealing with noisy line output stages and PSU's were a major problem. We used to soak (immerse) LOPTX's (line output transformers) in varnish, use epoxy on noisy wound components such as coils etc etc.
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Old 9th March 2011, 06:43 PM   #442
johnm is offline johnm  United Kingdom
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It was ever so quiet and very high pitched. Wasn't even sure if it was the amp until I switched it off and it stopped.

Anyways fingers crossed it was just 'one of those things' as they say. However the sooner Farnell complete my order the sooner I can get every single old cap replaced.
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Old 9th March 2011, 06:51 PM   #443
Mooly is offline Mooly  United Kingdom
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I suspect it's going to be just one of those things...

The PSU runs at around 20khz but it will be a "messy" 20k with lots of harmonics/ringing etc and it's that, that excites the ferrite components and they start to sing/whistle.

If that is the reason there is no easy answer.
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Old 14th June 2011, 02:29 PM   #444
johnm is offline johnm  United Kingdom
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Thought an update was in order after having used and thoroughly enjoyed the amp for a few months now - finally going to recap the main power-amp board first, followed by the other sections. PSU already done, of course.

I still get that occasional very high pitched whistle from the SMPS, and it goes if I tap the case so it's obviously a vibrating component. Noticed on the SMPS board that there's a coil that isn't covered in any kind of resin like some of the other parts are, so I'll probably slip some heatshrink over that and see if that doesn't stop the occasional noise. I've noticed many modern SMPS's have heatshrink over the smaller coils as well so hopefully that should be a safe and simple fix.

Other than that it's been working absolutely fine everyday and sounds great. I still need to fix that stuck VU meter though but that's at the bottom of things to do.

Not sure what to do about removing wires to the power-amp board to allow easier access to desoldering caps etc. All the wires are wire-wrapped in place. Do these pull off, or do they still need to be desoldered? Or does one apply heat to the 'wrap' section whilst sliding it off the anchor point?

Cheers,

- John
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Old 14th June 2011, 03:09 PM   #445
tvi is offline tvi  Australia
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I usually find slowly unwinding them works best.
They aren't soldered to the pins, its meant to be a cold weld if gold pins and silver plated wire are used.

Rgds
James
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Old 14th June 2011, 05:18 PM   #446
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It is probably the SMPS transformer vibrating depending on the mains harmonics. The cure would be to dip it in polyurethane or maybe try the heatsink trick mentioned above. It shouldn't be anything to worry about though
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Old 14th June 2011, 05:42 PM   #447
johnm is offline johnm  United Kingdom
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Quote:
Originally Posted by tvi View Post
I usually find slowly unwinding them works best.
They aren't soldered to the pins, its meant to be a cold weld if gold pins and silver plated wire are used.

Rgds
James
Thanks for your reply - Luckily it turns out the wires connected to the power amp board are long enough so that it can easily be flipped over to de-solder the old caps. Saves a bit of hassle
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Old 14th June 2011, 05:45 PM   #448
johnm is offline johnm  United Kingdom
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Quote:
Originally Posted by jaycee View Post
It is probably the SMPS transformer vibrating depending on the mains harmonics. The cure would be to dip it in polyurethane or maybe try the heatsink trick mentioned above. It shouldn't be anything to worry about though
I'll try the heatshrink and see what happens - fingers crossed!

Whilst going through my parts bin for a couple of 1000uF 25V caps for the power amp PCB board (C310 & C360), I found a pair of Panasonic FMs, Rubycon ZLHs and Rubycon ZLJs.

Does anybody know which would be the best choice I'm out of my depth here:

http://www.rubycon.co.jp/en/catalog/...RIES-CHART.pdf

http://www.panasonic.com/industrial/...A0000CE108.pdf

Cheers,

- John

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Old 14th June 2011, 06:20 PM   #449
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Old 15th June 2011, 07:00 AM   #450
Mooly is offline Mooly  United Kingdom
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Hi John, really pleased the amp is still going strong... great job.

The caps. Its not a "demanding" application at that point in the circuit, certainly not with regard to ripple current and low E.S.R. etc. Usually the larger a cap is physically (for the same V and C) and the better. I would always fit 105C temp rated parts these days.

Its not demanding because its fed via 4.7k resistor from the main rails which severely limits any "rate of change" of voltage across the cap. The cap then goes on to feed an internal regulator in the IC.

The varnish trick Jaycee mentions is something I used to do a lot with line output transformer on TV's. You have to literally submerge the tranny for several minutes, then just hang it up to drip and dry. Sometimes successful, sometimes not but nothing lost.
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