Sansui G-9700 HELP!

[lambord]

Just recently purchased a G-9700 for cheap. Long story short, I was in a massive rush and the price was so low that I didn't even care if it worked. I brought the unit home and was getting intermittent output while turning the balance knob, so took the bottom off cleaned and greased the knobs I could easily get to. HERE IS THE PROBLEM. I only can get an output on the right channel. No matter what knob I turn, button I push, or input I select the left channel will not work. Good part is the right channel sounds and functions perfectly after a little cleaning. Any stereo Guru's out there that wanna give me advice and help me learn?

Side note----I am relatively familiar with a multi-meter and soldiering, but that is it. I do not know very technical terms. I have collected, bought, and sold receivers for about 8 years but it has slowed down since being in college and working.

 

[LBPete]

Start with a good cleaning. If you are getting output from either channel the power amp sections are both working. The channel dropout is most likely in the preamp section. Every, knob, lever or push button on the front panel has a switch or a potentiometer behind it. The audio signal goes through almost every one. They all need to be cleaned and "exercised" before even thinking about a soldering iron.

- Pete

 

[lambord]

I took the front plate off and cleaned every switch and knob, no luck. I know taking off the front plate might not give me the best access for cleaning but I do not think it is worth taking apart the whole amp to clean the switches. Let me know if you disagree.

Thanks

 

[LBPete]

It looks like my response got lost in the server upgrade. Here's a picture of my G-971 (Black trim version of the G-9700) apart for cleaning. It's the only way to get enough access to the switches and pots to spray cleaner inside each of them. Depending on the level of contamination and the difficulty of getting enough of the cleaner on the actual contact areas inside the switch or pot, it can take multiple applications. It's also necessary to "exercise" the control being cleaned to clean the contact surfaces.

I have an on-going issue with the function switch on mine. Even after multiple cleanings it still acts up.



If you are convinced that you have thoroughly cleaned the controls with no change in your symptoms, then the next step is to trace the signal through the pre amp to see where the drop out is occurring. In most cases the drop out will be in the pre amp. These receivers don't have an external connection between the amp and the preamp so there is no easy way to determine where between the two the drop out is occurring.

Signal tracing typically requires an oscilloscope and signal generator. Sansui was very consistent in using a blue and a gray wire to connect the audio signal from board to board. If you have the equipment you can quickly find where the drop out is occurring.

- Pete

 

[lambord]

Wow it looks like a few messages have been lost. I have disassembled the unit just like yours and cleaned every switch.

https://photos.app.goo.gl/UFLkBfmd7vMnvk7y9

Then I tried touching the pin 1 on the driver board (pin 1 is an audio signal input bypassing the preamp from what I understand) and still no noise or hum out of the channel. So now the suspicion is a broke driver board (F3074). I will have to take the board out and do a visual inspection, I will also check the resistor ohms to make sure they are in spec. I could have access to an oscilloscope, signal generator, and basically any electronic equipment through my college. I am not super familiar with all the equipment but I have used it before. Although I do not know if I want to drag these beast receiver into a lab.

Stay tuned for a week possibly, I am starting classes tomorrow and a little busy for this project.

 

[LBPete]

It's highly unlikely it's a driver board. Driver boards typically aren't dedicated to a specific channel so you should be able to swap them left to right. If the same channel stays mute, it's not the driver board.

- Pete

 

[lambord]

I switched the input plugs to the driver boards on the left and right channel and the right channel is still on the only one working

 

[Tom B]

Be very careful troubleshooting this. From what you have tested and done, it appears that
1) the outputs for that channel could be bad along with some emitter resistors
2) the driver board could also have damage from bad outputs
3) speaker switch, or wiring, or speaker protect relay contacts could be an issue.

I always advise the following:
Never swap driver boards from one side to the other or swap outputs from one side to the other as a troubleshooting technique. If outputs are bad, they can cause damage to the driver board (highly likely). If you swap the good driver to the questionable side, you can damage that driver boards components. Likewise a driver board that has been damaged can also fry outputs on the good side after a swap. So you could wind up with bad outputs on both channels, and bad driver boards on both channels.
You need to check the output transistors, emitter resistors on the bad channel, as well as the components on the driver board as well. The 9700 I worked on had open emitter resistors, shorted outputs, and on the driver boards I found 2 bad driver transistors 2 bad pre drivers, bad resistors, and a few other bad components.
If you are not familiar with testing components, I suggest you find someone with experience to help with the testing. I suggest you find someone with an old fashioned analog multimeter. This is the best piece of equipment I have found for testing transistors in circuit. Digital meters work well for resistors, and checking voltage, but I prefer my old Triplett 630 meter for testing transistors.
Feel free to send me a PM and we can discuss how to troubleshoot the components.

 

[LBPete]

Tom, with all due respect, I've never seen a Sansui protection circuit fail to detect shorted outputs. Since this unit isn't in protection, I would assume the output stage is OK.

Lambord, please measure the DC voltage at the speaker terminals with the volume at zero. That will tell us alot

- Pete

 

[KingBubba]

If there is connected/separated switch on that beast, like there is on the 9000, it should be well cycled and cleaned. If the same as the 9000, it will be found on the left side. I have never seen a 9700.

 

[lambord]

The DC voltage of the left channel after the amp was on for a few minutes is 8 mv and the right was mid to high 90's. This was a little weird but the left channel was giving me a constant 8 mv while the right was constantly moving no matter how I moved the connectors or scraped a little with the probe. Could be my $40 multimeter but it has not failed me in the past. Could the bias on the right channel have a dirty adjustment pot?

KingBubba, I am not sure what you mean by connected/separated switch, are you referring to the relay that clicks on when the unit is powered up?

Thanks for everyones input

 

[kevzep]

The DC voltage of the left channel after the amp was on for a few minutes is 8 mv and the right was mid to high 90's. This was a little weird but the left channel was giving me a constant 8 mv while the right was constantly moving no matter how I moved the connectors or scraped a little with the probe. Could be my $40 multimeter but it has not failed me in the past. Could the bias on the right channel have a dirty adjustment pot?

KingBubba, I am not sure what you mean by connected/separated switch, are you referring to the relay that clicks on when the unit is powered up?

Thanks for everyones input

It was the previous G models which have that switch, the X700 series don't have them, so don't worry about that.

 

[Sansuiman]

Connected/separated switch is a feature present on some Sansui higher end amp and receiver models starting with AU-717, and including the G-9000 amongst a number of others. I don't believe the G-9700 has one.

For models that do have one, it is an inexpensive slide switch and is prone to causing signal cutout as the switch contacts oxidize with age and lack of use. Usually the first thing I check on models so equipped when signal drop outs are evident.

Edit: Kevzep was posting the same as I typed this.

 

[LBPete]

The X700 series doesn't have pre outs so no connected/separated switch either.

The DC voltage of the left channel after the amp was on for a few minutes is 8 mv and the right was mid to high 90's. This was a little weird but the left channel was giving me a constant 8 mv while the right was constantly moving no matter how I moved the connectors or scraped a little with the probe. Could be my $40 multimeter but it has not failed me in the past. Could the bias on the right channel have a dirty adjustment pot?

Remind me, it's the left channel that is working and the right channel that is mute? Have you reconnected the inputs to the driver boards to the correct boards?

It's not unusual for the DC voltage at the speaker terminals to fluctuate a few mV. High 90s is above spec but you should be able to adjust it down with the trimmers on the driver board. Be very, very careful to use those trimmers correctly. Follow the service manual procedure for DC offset adjustment.

8mV is a near perfect value but a value that low could also indicate an open circuit. Does your meter reading change when you probe that channel? Do you have speakers connected? Does the voltage increase a little if you increase volume?

Although it's out of spec, it's not what's causing your channel dropout. We needed to confirm that the outputs were good. Your channel dropout could be anywhere. Other than guessing there is really no good way to find it without an oscilloscope.

- Pete

 

[kevzep]

You need to isolate the issue, I would leave the power-amp alone right now.
I would inject a signal into the amplifier, then check at the input of the power amp modules with my scope to see if the signal is making it there, if it is not, then I would work backwards from there.

As Pete says, it could be anything.....
Culprits for this type of fault are Tape/Source, Tape MON, Balance, various looseness of connectors etc etc...

 

[lambord]

The left channel was the non-working channel but it has the about 8 mv on the output. The inputs were reconnected. I have adjusted the right channel to start around -4mv and once it warmed up its sitting at 5 mv . I tried adjusting the left channel and it is not adjusting.....no movement on the dc output of the left channel with movement of the adjustment pot. So i assume its a open circuit somewhere.
No speakers connected
The right channel (working) goes up when volume increased but the left channel (non-working) stays the same.
I also checked the bias's on both side and they are basically 0
Kevzep- I think we kinda isolated the issue to the driver board? Previously I switched the left and right channel inputs on the driver board but still only the right channel worked. Also to try to slightly replicate what you are asking, I took the connector off the driver boards again and measured the a/c voltage as i turned the volume up and down to random points, the ac voltage at both side when playing a tone off my phone where the same on both the left and right channel. I would interpret this as functional preamps and something past that connection is broke on the left side. Let me know if you disagree.

 

[petehall347]

did you check dc offsets at pins 7 of driver boards ?

 

[lambord]

Do I measured from the speaker outputs. The service manual suggests that....

 

[petehall347]

best measuring at signal outputs of driver boards .

 

[kevzep]

Kevzep- I think we kinda isolated the issue to the driver board? Previously I switched the left and right channel inputs on the driver board but still only the right channel worked. Also to try to slightly replicate what you are asking, I took the connector off the driver boards again and measured the a/c voltage as i turned the volume up and down to random points, the ac voltage at both side when playing a tone off my phone where the same on both the left and right channel. I would interpret this as functional preamps and something past that connection is broke on the left side. Let me know if you disagree.

Nope, if the problem did not move when you swapped the driver boards over, this means both driver boards are working and there is no audio making it out of the left channel of the pre-amp.
If it was the driver board, then the problem would have moved with the driver board to the other side.

I hope you have tested the output transistors and emitter resistors and base resistors, because past the driver stage that is the only thing that can fail or be a problem, but to have no audio, ALL the emitter resistors would have to be open.

Have you made a connection to the speakers before the relay??

Make sure the offset is OK before you do that....