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Old 2nd February 2013, 11:36 PM   #1
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Exclamation Problem Sansui AU505

Hi everybody, I have a problem with a Sansui au505 I bought on eBay... I got it arrived last Saturday: first thing I notice is that the light is not working; also, the mic and phone plug do not click when a jack is inserted. So I decide to open it and I find out that both of the female plugs for the jack are broken and the lamp is broken too. Nevermind, I go to RadioShack and on Sunday I replace the parts. Everything works fine, and I listen to the amp for almost a couple of hours, sounds really good. Today, after a week of work, I switch it on again and it works wonderfullyfor almost half an hour, but then it starts making a very annoying white noise even at volume zero and very little sound comes out of the speaker even at 10 volume... What happened? what can I do to fix it?

Thanks for your help,
Andrea
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Old 3rd February 2013, 01:23 AM   #2
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Hi Andrea.
This amp is very old. Perhaps 1971. It is full of electrolytic capacitors, a lot more than modern amplifiers and these are not everlasting. Over time and the heat inside the case, they dry out and lose their quality completely or even short out, explode etc.

When you buy old electronic equipment, this is what is lurking and causes failure and collateral damage to other parts such as transistors, resistors, even small fires, some period after powering it up again after so many years of neglect. A technician would not simply apply power, they would use a current limited power source or simply replace all capacitors before even testing it.

If there is no sign that much has been replaced, you need to download the schematic, find the BOM and make a shopping list for the electrolytic capacitors. It won't matter that you have to buy higher voltage parts than shown or even higher capacitance up to 60% more rating, as new parts are smaller. However, never use less than original voltage or capacitance ratings. Don't buy cheap, unknown brands just because that's all you can buy locally. You may regret that and the sound may not be quite what you were hoping for. There are regular quality, industry standard and boutique parts but industry standards like Panasonic, Rubycon, Samwha, Nichicon, Nippon Chemicon. etc in appropriate grades will be fine.

When ordering, check the outside dimensions and type (axial or radial leads) that will fit the space and the lead spacing matches closely or you won't have much success.
Note, these are polarized components, don't confuse, lose, or mix up the correct orientation of positive and negative leads. Mark as (+) or (-) what you see on the cap onto the PCB, if there is no printed layout on it. Do this before removing the cap. Only remove the next 1 or 2 caps as you replace them, to avoid losing your way.

You need to check how to do many things here, beyond one answer and this is not a quick fix like changing a fuse. It's restoration work and when you have finished, don't simply power up until you can test that it is safe by making up your own current limiter; an incandescent 60W lamp in series with the active mains connection to the amplifier. It's call a bulb tester and you can find details by searching this forum.

This is important wiring, critical to your safety, so if you can't wire this up using even a table lamp with a suitable mains socket and wiring, don't proceed.

Hopefully, the amplifier will still be OK after this but recheck for obvious burnt components, signs of heating etc. as you proceed. You are at least at a point where measurements could be meaningful and you can troubleshhoot the amplifier, if necessary.
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Old 5th February 2013, 04:42 AM   #3
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Ok, thanks for your great answer!... I have more info about the problem: I tried with other speakers an also with earphones, and the problem is still there. Anyways, when I turn the amp on without any input, I notice that the right channel has no hum, whether the left one is really noisy; however, sound is coming out extremely low and distorted from both channels.
Do you think there's a way to spot the damaged capacitor? Also, how much will it cost me to replace all the capacitors? Sorry for these silly questions but I'm new in the field...

Thanks for your help,
Andrea
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Old 5th February 2013, 09:58 AM   #4
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Join Date: Apr 2010
Location: Coffs Harbour, on the east coast
There could be many problems with your amplifier, apart from the capacitors. Using it in that condition, as I explained, is sure to cause more damage that will not only increase cost to you but also make repairs very difficult to carry out, even for a technician. Don't attempt to use it!

It is not a case of which capacitor is damaged when, if they are original parts, all will be damaged to some degree. It is not worth anybody's time to argue what may or may not be faulty. Replace them all if you want a usable amplifier with some chance of it lasting for a few more years.

I would like to add to the first reply that you should really increase the values of the large output capacitors, C819,820 to 3,300-4700 uF for cleaner bass with a little more depth.

When that is done, you can look at the condition of the transistors but technicians would probably have tested them too, perhaps whilst waiting for the capacitors to a arrive, in case the output stages have already been fried by trying to use the amplifier when the capacitors have failed. Normally, they would also be removed to test properly if there was some doubt there.

Be prepared to spend a lot of time and money here as it could cost about $50 US on caps alone, plus you will need good electronic soldering equipment with a fine tip of 1-1.5 mm, a reasonable quality digital multimeter, small tools like screwdrivers with Philips and plain blades, long-nose pliers and side cutters. You may also need sundry materials like PVC sleeving or heat-shrink tubing to insulate axial leaded components, if any.

Transistors may not be expensive but the cost of postage, etc. can be high. Fakes are everywhere but there is no point buying fakes if you go to the trouble of repairing the amplifier properly. Use guaranteed sources and just because you see nice pictures on Ebay, doesn't mean anything. Parts may "work" but not properly if the chips inside the cases are nothing like the correct part, and that is a common experience of unwary Ebay clients.

So far, I think you will have to agree there.
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