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Old 22nd March 2016, 12:29 PM   #21
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Depaj

A798 and brothers or cousins might work well outside the circuit and produce all shorts of problems under voltage or operating temperature .

Ceramics and resistor hardly ever produce this type of failure except the ones we already talked about .

Please check a number of screws that support the boards make sure that these are clean and tight .

Remember that this style of amplifier doesn't actually use a star ground system even though it looks like it does ...many of the circuits are grounded from the enclosure locally and all around ..Check that

katieandad

This is bad practice and time consuming procedure that might cause complications ...Electro's are to be removed one way or another with no questions asked ..>Even if you find one that is broken and you replace it who is the one to guarantee that the next one will be broken after 1-6 month of everyday operation ...Think though that we have met before in this issue and its nice to know that you care about this
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 22nd March 2016, 12:46 PM   #22
Depaj is offline Depaj  France
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Hi east electronics

I have had trouble with 2SA979 Dual transistors before, they also tested fine but in circuit they were no good. I will have to order These 2SA798 as they are not available in an every day electronics shop. I also replaced the 4 220R resistors, all the small signal 2sc1313 and 2sa726 transistors in the preamp which calmed the hiss a little but not entirely, and will replace all the electrolytic caps soon. The dc in my first test was due to he fact that the grounding of some parts of the power amp section are connected with the plugs coming from the preamp and I took my reading with that plug disconnected. Now everything works fine except from the pretty loud hiss, independent of volume but definitely coming from the preamp (as the power amp is dead silent when disconnected) and a buzz appearing when volume is half way.
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Old 13th June 2016, 02:04 PM   #23
Depaj is offline Depaj  France
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I replaced the 2 SA798 and now the amp is stable, relay closes, almost no dc offset and was able to set bias without any problem. Still I have a fairly loud hiss coming from the preamp (it's gone when I disconnect pre and power amp. I already replaced all electrolytics and those 2sc1313/2sa726 everywhere. Although usually a ground problem would more likely give a buzz I checked for grounding around the entire amp and everything seems ok.
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Old 17th August 2016, 09:09 PM   #24
Depaj is offline Depaj  France
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I left the amp sitting for a while but am really eager to be able to listen to it. Everything works and the sound is good but even with the volume all the way down I have a quite audible hiss, especially with headphones. It definitely comes from the preamp because it disappears when I disconnect the preamp from the main. Also I'm able to get rid of all the hiss when turning both mid and high all the way to minimum. I replaced all electrolytics and small signal transistors, replaced the zener diode and the two regulator transistors in the secondary power supply but nothing changes anything. Is this amp just not quiet at all or am I missing something here ?
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Old 18th August 2016, 08:27 AM   #25
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That's normal. All old (early to mid 70s) Sansui preamps hiss quite loudly, even when restored. This includes the top of the line models. You can't fix it. Most people don't seem to mind/notice it. I guess they just have bad speakers or hearing. I just use my old Sansuis as power amps. Bypass the junky pre and connect a nice dac with a volume control in its place.
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Old 18th August 2016, 09:13 AM   #26
Depaj is offline Depaj  France
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I did read that a lot but I have an AU-222 and an AU-3900 and both of them are very quiet in comparison to the AU-5900. The 222 has a very slight hum but almost not noticeable and the 3900 is very quiet. How come the 5900 that is from the same series as the 3900 has such a bad preamp ?
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Old 18th August 2016, 12:49 PM   #27
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isolate or hardwire the preamp in a way that less things is connected to it you may start by excluding a complete block of switches S01-02-03

This might point you to a direction ...too much wiring around this preamp

Kind regards
Sakis
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Old 18th August 2016, 09:28 PM   #28
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I didn't do any further testing today but from what I know from my previous tests it isn't the tone control because when I bypass it I still get the hiss, that's why I thought it could only come or from the preamp section on board F-2604 or the secondary power supply that feeds the preamp section. But than again I replaced all the electrolytics, all the small signal transistors (same in power supply plus the regulator transistors and the zener diode). Anyway it's down to those two I would say.
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Old 23rd August 2016, 09:30 PM   #29
Depaj is offline Depaj  France
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Back again after some tests :

-first I injected the signal directly at the input of the preamp (bypassing both F-2602 and 2603) and the hiss was still there.
-Than I took the signal from F-2603 and directly injected it into the tone control board (bypassing the preamp section) and the hiss was gone.
First I suspected the secondary power supply that feeds the preamp (because I can pick up some noise with the oscilloscope on the 22v supply. But the phono equalizer is also supplied by this circuit and that is very quiet.
So the only thing that is left is the preamp section itself but as I said, transistors are new (sa992/sc1845) and capacitors are new.
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Old 7th November 2016, 01:36 PM   #30
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It has been a while since I last touched this amp but I'm back because I've been using it for a while on some B&W Dm4's and although it sounds really good the hiss is still there and very unpleasant.

I used the +23v from the power supply of a sansui au 2900 to feed the preamp of the au 5900 and it gets very quite and sounds perfect so it's definitely the secondary power supply that is faulty. Yet I replaced the 4 transistors, the zener diode and the resistors but the hiss is still there
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