Help with Sansui AU-777A, Left Channel half volume???

[Bret Fritch]

Hi, I am finally working on a couple Sansui Stereo pieces. AU-777A, AU-555A and a QS-1.
Starting with the 777A and here is my issue. Left channel is half volume or less to the right. I have cleaned all the controls. On the 1153 board I changed the 4 caps and checked the resistors for weird readings but none found. On the 1183-1 board I changed the caps and looked over. On the 1170 boards I changed the caps. I pulled all the power transistors and checked them, re-installed with fresh heat compound.
I have that the 458's should all be changed as they are known bad transistors so will get those ordered. This is my first Sansui to work on so hoping for some guidance. What else should I be looking at that would cause the lower volume on the left side? Thank you for the help.

 

[DAnny45acp]

Yeah you need to replace all the 458's. You should get yourself at the least a cheap oscilloscope - it would make it much easier- as you can download a sig generator app on your phone and just plug into your aux input and check your signal. What happens when you use the "balance check switch"?

 

[slimecity]

Yea, agree with replacing all the 458's. The troublesome ones look like this:



You also want to seperate the pre and power stages (and test these seperately) of the amplifier to confirm where the problem actually is in the amplifier.

 

[AnalogueBen]

I second what @slimecity has said. I found a couple of 1uf caps that had very high ESR that I believe were giving me low output on one Ch.
I too changed out the 458s with KSC945C, no problems thereafter.

 

[Bret Fritch]

Yea, agree with replacing all the 458's. The troublesome ones look like this:

View attachment 1815497

You also want to seperate the pre and power stages (and test these seperately) of the amplifier to confirm where the problem actually is in the amplifier.

I have the transistors on order now. I did hook it up just using the main output side and sounds was equal on both sides which leaves me with the preamp side to look at. Thanks for the help so far.

 

[slimecity]

Make sure all your switches and controls have been cleaned as well.

 

[TonyFinn]

Hi Brett,

Did you ever find a resolution to the half volume issue? I have the same issue but with the right Channel. I did all of the LeeStereo upgrades, with exception of keeping the mylar green film capacitors. My issue is definitely in the preamp as yours was. All the controls were cleaned. I verified by using the preamp into another amplifier, as well as the preamp from another receiver into the amplifier section of the au-777a, hence the issue is in the preamp on the au-777a. I checked for solder bridges on the tone control board, as well as, the head preamp and could not find any. Perhaps it's a resistor somewhere in the head preamp, or cold solder joint. I also look for any disconnected wires on the head preamp I also checked the resistance on the appropriate portion of the volume pot, which includes the balance pot. Any guidance from you or anyone would be much appreciated. Thanks.

 

[TonyFinn]

In addition, I did stay with polarized electrolytic caps on the foil side of the tone control board but I don't think that is the issue.

 

[TonyFinn]

More info..... I also followed the balance check protocol as shown in the owner's manual, to no real affect. Channels are balanced until the mode select is changed to normal stereo, then the left Channel becomes much louder. In order to get them balanced I must place the balance control to about the 3 o'clock position.

 

[TonyFinn]

For whoever needs this info.
Good News. Issue solved. I was able to construct a circuit tracer, after viewing a Youtube video. Using that, another amp, and the AU-777A schematic, I found the issue, which was a non-fully-functioning mode selector switch. (Reverse Stereo, Normal Stereo, Mono R, Mono L, Mono L/R.) Repeated DeOxit cleanings did nothing to resolve the issue, hence I just bypassed it entirely with 2 jumpers. See the 2 brown wires in the attached pic. I soldered them onto the mode switch thereby bypassing the switch entirely. (In other words, the 2 wires going into the switch are now hard-wired to the 2 wires going out of the switch, R & L, Respectively. Now I get beautiful balanced stereo sound!. Of course, no mono L, mono R, Mono L/R, or Reverse stereo, but only Normal Stereo is important to me, and most other people I would believe. Perhaps in the future, I'll take the mode switch out and see what caused the issue. I would believe that the accumulated oxidation of 50+ years is the culprit. Also, I would think that my modification would not affect any other functions of this fine unit, since there are only 2 wires going into the mode switch (R & L), and 2 wires (R & L) going out of the switch. The basic R/L circuit path is from the RCA inputs on the back, through to the Function Selector switch, and then the Mode Switch. The phono RCA's of course goes through the phono preamp, after the Function Selector switch, then through the Mode switch. (Well, there are other switches in the mix too. i.e. Tape, Loudness, etc....which are not affected by my modification)
This mod is kind of cheating a bit I know, but I may find in the future that this mod switch may not be repairable, and a replacement mostly unobtainium.
I had initially thought that I botched my LeeStereo Mods somewhere, but not the case....well, this time anyway! In the end, I'm happy. I hope this can help someone in the future.

See pics of the mod, as well as, the inside of my AU-777A. The sound of this beast did not disappoint. Many people have ranted and raved about the sound of this unit, and I definitely concur. Smooth, Tube Like, good detail, not clinical, not harsh, and a bunch more good stuff, but in the end, a joy to listen to, and well worth the time spent in restoring it.

Cheers!

 

[Monicker1]

I agree that some controls are almost never used. I think balance is also overrated and does not deserve its own knob on a control face. Many Sansui products have the balance control ring behind the volume control which is where it belongs IMHO. I used an AU 777a for many years, and almost never had the mode turned away from the normal setting. If I can ever get my old 777a back, I might part it out.

 

[Bret Fritch]

For whoever needs this info.
Good News. Issue solved. I was able to construct a circuit tracer, after viewing a Youtube video. Using that, another amp, and the AU-777A schematic, I found the issue, which was a non-fully-functioning mode selector switch. (Reverse Stereo, Normal Stereo, Mono R, Mono L, Mono L/R.) Repeated DeOxit cleanings did nothing to resolve the issue, hence I just bypassed it entirely with 2 jumpers. See the 2 brown wires in the attached pic. I soldered them onto the mode switch thereby bypassing the switch entirely. (In other words, the 2 wires going into the switch are now hard-wired to the 2 wires going out of the switch, R & L, Respectively. Now I get beautiful balanced stereo sound!. Of course, no mono L, mono R, Mono L/R, or Reverse stereo, but only Normal Stereo is important to me, and most other people I would believe. Perhaps in the future, I'll take the mode switch out and see what caused the issue. I would believe that the accumulated oxidation of 50+ years is the culprit. Also, I would think that my modification would not affect any other functions of this fine unit, since there are only 2 wires going into the mode switch (R & L), and 2 wires (R & L) going out of the switch. The basic R/L circuit path is from the RCA inputs on the back, through to the Function Selector switch, and then the Mode Switch. The phono RCA's of course goes through the phono preamp, after the Function Selector switch, then through the Mode switch. (Well, there are other switches in the mix too. i.e. Tape, Loudness, etc....which are not affected by my modification)
This mod is kind of cheating a bit I know, but I may find in the future that this mod switch may not be repairable, and a replacement mostly unobtainium.
I had initially thought that I botched my LeeStereo Mods somewhere, but not the case....well, this time anyway! In the end, I'm happy. I hope this can help someone in the future.

See pics of the mod, as well as, the inside of my AU-777A. The sound of this beast did not disappoint. Many people have ranted and raved about the sound of this unit, and I definitely concur. Smooth, Tube Like, good detail, not clinical, not harsh, and a bunch more good stuff, but in the end, a joy to listen to, and well worth the time spent in restoring it.

Cheers!

Hey there, sorry for the delay in writing back off your original question. Glad to read that you figured out where the problem was. I did get mine going with new transistors and then went through everything else. I ended up selling it due to so many other pieces just sitting here. Great troubleshooting skill on this matter and will keep these in mind. Enjoy the music you will be listening to from this unit.

 

[TonyFinn]

Thanks for the responses guys. Much appreciated. See my correction update below to close out this issue.

After listening for a bit, the unit sounded nice, but something still didn't seem right, so I reanalyzed the issue. I re-configured the Mode switch back to stock, and listened closer with my circuit tracer, another amp, and various test points on the Tone control board, mode switch, and head preamp board, and determined that the root cause was not the mode switch, but the fact that I installed a transistor backwards on the tone control board! Geez! Now the unit's is correct, balanced, and spectacular to my ears. I also replaced the shielded wires going from the mode switch to the to the Tone control board with an old RCA cable that I modded, as I had damaged the original wires shorting one of them to the grounded shielding wire.

Lessons Learned:
If you have to do acrobatics to recap a board, chances are that you should try to desolder it entirely or partially to gain the best access when installing new caps/transistors/etc..

Cheers.