defective sansui AU-7900.. should I buy it?

[spaghetti]

I was offered to buy this defective amplifier, I have not seen it yet but the owner says it works only with tape input, other inputs yield no signal. And only a set of loudspeaker outputs work.

I don't have any sansui amp, I'd love to own one and reviews seem very positive for this model, and if I buy it I will be the one that will try to repair it. I am not skilled enough but in the past I was able to fix some receivers, mostly with the kind help of AK people.

I'm wondering if this could be a complicated repair or not, I know that there're not enough details to make a solid diagnosis, but maybe there's enough to say stay away from that (probably) troublesome amplifier. What do you think?

Thanks
Andrea

 

[Sriskie]

I say go for it. I have one and it's my favourite amp. They're quite easy to work on, the main curcuit board pops right out with a few screws and unplugging some wires. Mine had an issue that the kind folks here walked me through.

http://www.audiokarma.org/forums/showthread.php?t=554758&highlight=au-7900

Lots of love here for this amp. I have a 3900 and 4900 too and recommend replacing all of the 10D1-10D2 diodes for reliability.

 

[spaghetti]

nice story! It seems there're many buyers at this point, so I'm still unsure if I will be able to get it for a reasonable price.
We're still talking about a defective unit, where the pre-amp stage is probably gone. I don't see why people are willing to invest substantial amount of money on it, but if they're, well, there must be some good reason ;-)

 

[rasenthiran]

I will definitely take a risk to get it if it is value for money. One of the best sansui amplifier made.

 

[t.ray]

i have a Sansui AU-7700 which has been to 2 different "reputable" shops for repair of a left channel that fades in and out. Both said they fixed it and of course the problem persists. gets frustrating when you can't find a local tech in Chicago. Anyone?

 

[spaghetti]

Eventually I was able to get a the receiver, I'm quite happy about that. Now the hard part, fixing it. I will probably start tinkering with it the next weekend, I hope I will be able to fix it on my own, otherwise I will come back for help ;-)

Andrea

 

[Binkman]

Greetings;

Do yourself a big favor and start with the power supply.. Got a dim bulb tester?

Free service manual: (pretty good, too!) check out the PS section and diagnostic :thmbsp:

Bink:

 

[spaghetti]

Greetings;

Do yourself a big favor and start with the power supply.. Got a dim bulb tester?
Bink:

Yes I do. And an oscilloscope is coming as well, I will be prepared for the repair

Free service manual: (pretty good, too!) check out the PS section and diagnostic :thmbsp:

Thanks. I downloaded one from hifiengine, quality is ok but I will give this one a try as well, maybe it's better.

Andrea

Bink:

 

[spaghetti]

Got it yesterday. It's not in great shape, unfortunately. There's some oxidation on the top cover, on the knobs and on the top of the faceplate. After turning it on music come out only from the right channel connecting source to the main in inputs.. I expected the left channel to be blown, but after the usual cleaning (dust & deoxit) I found an open 220 ohm resistor that blocked bias dc to the left channel... that's it: my easiest fix so far.

Now, it came with some covers for the RCA inputs: what's the use for the one in the picture? I guess it's for the unused inputs, but it shorts signal with ground... is it really ok? It seems odd.

Thanks!
Andrea

 

[MomirPeh]

That one looks like a cut-off plug from an RCA cable. And, by cutting it off, the wires inside are probably shorted.

 

[Mannegizen]

I guess those shorting plugs are used when setting bias.

 

[petehall347]

shorting plugs generally go into unused phono inputs ..

 

[pete_mac]

shorting plugs generally go into unused phono inputs ..

Yes!

I guess those shorting plugs are used when setting bias.

No...

 

[Mannegizen]

No...

Maybe not on a Sansui, but they are mentioned in the Nakamichi receivers service manuals for bias adjusting.

 

[spaghetti]

shorting plugs generally go into unused phono inputs ..

Thanks for the info! I'll put them to good use.

Andrea

 

[pete_mac]

Maybe not on a Sansui, but they are mentioned in the Nakamichi receivers service manuals for bias adjusting.

No worries, but they are definitely not used for that purpose on Sansui equipment.

 

[spaghetti]

Turns out the repair is not over yet. The amp worked flawlessly for a few hours. Then the relay refused to click in.

I poke around the relay circuit with the DMM and suddenly it clicked. It worked flawlessly for a few hours on and off, then the relay stopped clicking. Turns out there is (now) an open VD1212 diode (D612) that blocks bias to TR603 base. By the way, what's the use of that double diode? To provide a voltage drop of about 1.3V?

I'm attaching some voltages I found, it seems all pretty reasonable to me except for the 44v where it should read 24, but maybe that's because TR603 is not conducting. Am I right?

Thanks
Andrea

 

[bktheking]

Maybe not on a Sansui, but they are mentioned in the Nakamichi receivers service manuals for bias adjusting.

Not the same - rca shorting plugs are for inputs. The shorting plug you mention can be found in other amplifiers, my ka6000 has one- they are removed and an adapter is installed to allow current to be measured in order to set the bias. RCA shorting plugs are to eliminate hum on that selection when a TT isn't installed.

 

[EMark]

Au-7900...

Hi Spaghetticode,
I own a AU-7900 for almost 40 years. It is also the first amp I ever opened up and worked on. Of course, I had help from the good guys at AK. I had a problem with my relay clicking on and off. First, in all Sansuis that have them, you should always replace the tiny dot diodes. Use two 4148N diodes in series as a replacement. Also, remove the infamous Sansui fused resistors and put in regular resistors of the same rating. Those are resistors numbered R31, R32, R41 and R42.
I found the copper traces near the relay were bad causing an intermittent problem with the relay working. Check those traces and also check the transistor R603. I replaced that transistor and I have not had a problem with the relay for a few years already. For more details, see the EMark posts on the AU-7900 in the Sansui forum. Also, if that is the original relay, it couldn't hurt to clean the contacts inside, if not replace the relay altogether.

 

[spaghetti]

Hi Spaghetticode,
First, in all Sansuis that have them, you should always replace the tiny dot diodes. Use two 4148N diodes in series as a replacement.

I will do. Since one is already open I will buy replacements for the other as well. I checked all the nearest transistors (in circuit) and diodes (desoldered them) and they seem fine...let's hope it's just the VD1212. DC offset on both channel is very reasonable at about 3-4mV so the issue is definitely in the protection circuit.

Also, remove the infamous Sansui fused resistors and put in regular resistors of the same rating. Those are resistors numbered R31, R32, R41 and R42.

thanks for the hint, I will. I'd like to keep the amplifier at least for a while, I tested it only briefly but I believe it sounds sweet and I'm planning to keep it.

I found the copper traces near the relay were bad causing an intermittent problem with the relay working.

Relay should be fine now. Yesterday I had it clean because I was having a channel working inconsistently at low volume. I have 5 amps/receivers with relay and I had to open and clean 4 of them.