Should I repair my Sansui Au-7900 myself? Fuzzy left channel

[Normal Steg]

Hey there Audiophiles and Tech Wizards. I am reaching out to this community for some advice. I am a novice at repairing electronics. I can disassemble a device, deox pots, and I think I can even spot bad caps. I have repaired some really basic things, (recapped a pair of JBL L-100s, changed the power cap on an old tube radio), and I can do some basic soldering. My question is, should I attempt to fix the fuzzy left channel on my Sansui AU-7900, or should I take it to a pro to fix it. My issue is I have no one in my town that fixes vintage gear, so I have to travel at least 45 minutes to get to someone that can. Then, I have to wait at least a month to get my gear back. Is this a repair I can do on my own? Will I ruin my prized amp? Here are the symptoms and my troubleshooting.

• When you turn on the amp, there is a fuzzy sound coming from the left channel.
• The sound does not change regardless of what device is plugged in, what the volume is, or what input is selected
• The only thing that even remotely changes the sound is when I adjust the tone and filter selector on the bottom row of switches. (4ths from the left)

I found the repair manual online, and I have a very basic understanding of it. If the issue is a transistor, I am pretty sure I could locate it on the circuit board and replace it. I have heard that can be an issue on this amp. Any help would be GREATLY appreciated. You guys have helped me out a TON in the past with advice/research.

Thanks,

Eric in Normal, IL.

 

[Robisme]

I say give it a try. Clean all the controls if you haven't. Then, there are a couple(at least) boards that most likely have some 2SA726 and 2SC1313 transistors on them.

Replace all of them and that should do it.

Rob

 

[PE9ZZ]

When you turn on the amp, there is a fuzzy sound coming from the left channel

What is a "fuzzy sound"? Buzzing? Scratching? Crackling? The other bullet points indicate an issue in the ouput amp (power stage). Oh wait. There is a lot of processing going on after the volume control. Pull the link between PRE OUT and MAIN IN and report back if that makes a difference. You could even use short phono-phono leads to swap the channels.

I can't recall whether 2SC1313 is on the suspect semiconductor list. Believe so. There are quite a few used on the F-2610 board.

 

[Normal Steg]

It is more of a scratchy sound. Doesn't seem to change at all unless I mess with the tone and filter selector. Other than that, it is just a constant scratchy sound. I'll pull the link this weekend and let you know what I find out.

Yes, there appears to be a lot of the 2SC1313 transistors on that board. I was having trouble locating those to purchase. Are there other transistors I should use, or should I just try to keep it with those? Thanks for your help!

 

[PE9ZZ]

Scratchy is not consistent with bad transistors. Crackling is. That said, apart from bad transistors I cannot think of anything else that would produce a fault like that.

2SC1313 is a general purpose transistor. Any low power type replacement will work. I use 2SC2240/2SA970 because they are low noise and I have a tray full of'em. 2SC1815/A1015 also good. You should verify using a scope but lacking that sensible replacement should solve your issue. Sensible as in test every replacement action to avoid having to troubleshoot an all of a sudden dead amp and having no idea where to start.

 

[infullview]

The "fuzzy" sound can be caused by a leaky coupling or bypass capacitor which changes the bias on a transistor in the audio path. Has the unit ever been re-capped?

 

[percivilla]

Definitely start with cleaning the switches, the tone/filter switch on my 7900 was doing something similar when I got it. They are easy to get to with the cover off, I hit them with De-oxit and then faderlube.

The whole unit has now been rebuilt but that switch was causing some "fuzz" and intermittent volume drop out in one channel.

 

[Normal Steg]

I could also be describing the sound poorly. LOL. I am going to try deoxing the tone/filter switch, then if that doesn't work I'll try replacing the transistors on the F-2610 board and see if that helps. Any tips on where to purchase said transistors? I found some on Amazon and Mouser, but is there a better or cheaper place to order from? As I said, I am a novice, but I would like to start doing my own repairs, so I don't mind having extra transistors laying around if I have to buy in bulk. Thanks again, guys. You are SUPER helpful!

 

[PE9ZZ]

Mouser, Digikey, Newark, there are many. Amazon is not an electronics specialist ("Note: Products with electrical plugs are designed for use in the US. Outlets and voltage differ internationally and this product may require an adapter or converter for use in your destination. Please check compatibility before purchasing." - good grief) and doesn't even have them available. I got mine from Mr. Jack Ma (Aliexpress) but that is considered risky.

Search this forum for more info.

 

[macyjrm]

Eric, be also aware there is a dedicated Sansui forum below where I recommend you hang out. Some very good techs are there offering much help. Ask a moderator to relocate your thread if you are so inclined. By the way I sold my AU7900 years ago to a cousin but recently got it back after she “upgraded”. It’s a permanent resident here now. Good luck to you. These amps are special.

 

[Normal Steg]

One last question. I chatted with someone from DigiKey, and they suggested the following transistors https://www.digikey.com/short/vp4r7wf2. The data on these is a bit gibberish to me. Is there one you would recommend? Thanks again.

I took the cover off tonight and DeOxed that switch and I am going to let it dry completely before I test it in the morning. I'll also try to disconnect the Tone board to see if that get rid of the noise. Thanks again, guys. I could NOT do this without this community.

 

[PE9ZZ]

The list from Digikey is useless as the entries are either not suitable or only bulk (7k pieces order). If all else fails (Mouser and its ilk) try Mr. Ma. I've had good results with these replica transistors. But, as with anything from the middle kingdom, YMMV.