Need help with Sansui AU-7900 amp
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New member Username: StstPost Number: 1 Registered: Mar-09 | I have a sansui au-7900, When it was cold, it didnt work, but after 10 - 15 minutes it works fine ... But this week he stop working... he do the same thing but when he open i need to put on full volume to hear a little sound from my inputs ... I'm new with that and dont know what to do ... Capacitor ? after 30 years ? |
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Platinum Member Username: Jan_b_vigneDallas, TX Post Number: 13434 Registered: May-04 | . We can't diagnose such a problem in an amplifier over the forum. If you're interested in keeping a 30 year old amp going, take it to a repair shop. You're unlikely to find anyone in a typical shop willing to take responsibility for an amplifier this old. Place "vintage audio repair" in a search engine. http://www.atlasaudiorepair.com/ . |
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Platinum Member Username: NuckPost Number: 11912 Registered: Dec-04 | Power supply chain is leaking badly. Off to the shop. Dont plug it in again, as the supply may hook up during the test and go full output. |
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New member Username: StstPost Number: 2 Registered: Mar-09 | Thanks alot, I will bring it to a repair shop this weekend, If some one interested about what it have i will give up news ... I never think about the power supply ... some people around me said change capacitor first ... |
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Platinum Member Username: NuckPost Number: 11920 Registered: Dec-04 | The power caps are in the supply chain, but would likely cause other things if it was just a cap...but never know. Let us know? |
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Platinum Member Username: NuckPost Number: 11921 Registered: Dec-04 | And welcome to the forum sd. |
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New member Username: StstPost Number: 3 Registered: Mar-09 | Thanks, I'm happy i got a really fast answer .... We have that amp over 25 years at home and we want it back in working order :P |
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Platinum Member Username: Jan_b_vigneDallas, TX Post Number: 13439 Registered: May-04 | . If the electrolytic power supply caps were "leaking", there would be a familiar and constant 60Hz hum when the amp was operating. Electrolytics don't generally warm up and then make contact to pass signal, they are either good or bad and little in between. In an amplifier of this age, the electrolytics used in this amp would probably have leaked a brown, gooey substance from their base which could be seen after removing the top cover. (Don't poke around in any amplifier, the p.s. caps store high voltage and can give a nasty shock even when the amplifier is unplugged.) If this can be spotted, then the cap needs to be replaced. Not all elctrolytics of this age did leak anything however, they just went bad. While most amps this old are due for some capacitors being replaced I doubt this is actually the cause of the problem here. Numerous amplifiers from this time period employed relays in the power supply or in the speaker switching, not allowing full voltage to outputs until the amp had reached a certain operating temperature. These relays are often times the cause of the sort of problems described by the op. They stick and won't make contact until the amp has been powered up for longer and longer periods, eventually they don't work at all. These relays aren't easy to source nowdays and this could present problems in the repair of this amplifier. Additionally, a solder joint might have eventually opened or a circuit trace on a board might have cracked or lifted due to 30 years of moving the amplfier around. Smaller caps or resistors in one of the tone control, muting, loudness, etc, circuits might be defective which would keep the amp from working until the bad component has warmed up sufficiently to make partial contact. At that point driving full voltage through the open circuit might cause just enough to pass to hear the minimal volume described. The only real clue we have is the problem is common to both channels which typically starts at the power supply or parts common to both channels such as relays. So, yes, there are probably some caps that could benefit from replacement but I doubt the p.s. caps are leaking and causing the amp to not power up for 10-15 minutes, noise is the common signal of leaking caps. Since the caps are specific to each channel the noise would typically start in only one channel. A technician cannot predict when a cap will need replacing any more than you can predict when a light bulb will burn out. So replacing all the old caps in this very complicated integrated amplifier would become a major task and a major cost. Usually you either bite the bullet and do all the caps at once - a major cost if you're paying someone to do the work - or you wait for a cap to fail and then replace the caps piece meal as they fail. Unfortunately, one cap failing is all too often an indication more caps are going to fail. But I suspect there are other things going on with this amplifier and the op will have to decide what it is worth versus the cost of repair. Today's better components do offer better sound quality over this amplifier. The alternative argument would be that this amp has lots of features not found on modern equipment - which unfortunately makes it doubly hard to find a problem due to the complexity of the boards and the existence of numerous points of contact in switches and controls. As I said, it is impossible to diagnose this problem over a forum due to the multiplicity of possible causes. "Recapping" as an answer to vintage amplifier problems is too simple in most cases even though caps are often among the first components to fail due to age. The others and often more likely suspects are any components that can oxidize, gum up or mechanically wear out, of which this integrated amplifier has many. I would suggest the op ask for an estimate before authorizing any repairs on this amplifier - assuming the op can find someone to do the work on a 30 year old amplifier from a company that doesn't exist any longer. With a diagnosis and a potential for cost vs. likleyhood for longevity - keep in mind parts continue to age and wear in a 30 year old amplifier, capacitor and relay repairs are often just the beginning of a long road to more repairs - the op can decide whether this amp gets another lease on life. . |
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New member Username: StstPost Number: 4 Registered: Mar-09 | Thanks Jan for all this help ... And you are right the problem started only with the right channel ... Dont know if that can help but when i plug a power amp at the pre amp output of my amp it work all the time but when i play with the volume some time we can hear a scratching nose but it work fine so i think the problem is in the power amp section and what are you talking about relays ? |
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Platinum Member Username: NuckPost Number: 11932 Registered: Dec-04 | Items of this age use electro-mechanical relays to channel power and signals. the relays are usually powered in relay logic as well. It is not unusual to find a series of breaker-point relays, too. Any or all of these devices are subject to point oxidization, coil leakage and poor connectivity. These relays could be pulled by any voltage, as any designer decides, so there is no standard and often no replacement available off the shelf. using various voltages, often bu convenience of circuit layouts also means that a voltage sucking defective coil could affect various parts of the unit. These relays are often surface soldered in place, meaning that the board has to be removed to service the relay, thus disturbing all of the other 'vintage' componants and wiring as well. It can be a bundle of snakes in there! |
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New member Username: StstPost Number: 5 Registered: Mar-09 | Yeah many wire and they are all fixed one on top of the other so i will wait until a tech look inside it before touch anything and he say its my wrong ... maybe just a solder joint... cause the amp alearty did it and he came back off in working order and now i moved it and it died again ... |
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Platinum Member Username: Jan_b_vigneDallas, TX Post Number: 13448 Registered: May-04 | . Take it to the shop, steve, and don't use it until a tech has looked at it. If you're just the slight bit careless or someone else has access to the unit, it could start playing at full volume and damage your speakers. Then your problems and cost of repair are compounded. . |
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New member Username: StstPost Number: 6 Registered: Mar-09 | Finally, I dont get it to the tech, I tryed to fix it myself, This morning I try it, it dont work, and I play with the switch separeted/combinated, and it finally works, lets see how long it going to work, thx for your help every body |
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Platinum Member Username: NuckPost Number: 12196 Registered: Dec-04 |  Babes and Fools, steve, like most of my work, try it and find out. Nice work man! Chase down the switching now you have Karma! Oooohhhhmmmmm |
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