Left side is like angels, right side..... :( G-7700

Hello all.

I picked up a G-7700 that's been in storage for some time. Ascetically it's almost mint, just beautiful wood and metal. The one small issue with the exterior is that one of the push buttons has some of the chrome finish worn off. Does anyone know of way to repair this? I'm going to try a craft store, but if someone already has experience with this I would love to hear about it. I have looked on eBay, for a replacement. But only saw the complete set for $40.


Now for the bigger issue. With this beauty hooked up to speakers, the left channel sounds like angels singing. The right channel.... not so much . No sound at all until about 10% to 20%, then it pops in, not as loud as the left, but close. And very heavy static, almost a buzzing with the music. This is the same through the headphones.


I think my skill level is about novice. I've always been interested in electronics, but no real training or schooling. I follow instructions well and I am always willing to learn.

Here's what I've done so far;

read about 4 hours worth of posts here on this model and similar issues. Lots of good info, thanks to all who have contributed.
I have worked DeOxit into all pots and buttons. Repeating it 3 different times on the volume and balance.
I have done the DC offset, and the good channel was around 38mV, and I got it down to 6mV. The bad channel was over 1000mV and bouncing around continually. As soon as the screwdriver touches the pot the value drops significantly, I can get down to bellow 10mV, but it still bounces around a lot, and slowly climbs up into the 50mV to 70mV range, and always bouncing around.

I would like to check the bias, but after a crazy amount of time, I just can't find the points that the service manual wants me to check. Any help would be appreciated. I have added a photo of the board to the bottom of this post, the photo is borrowed from jugulator's thread about his G-7700. Thanks. Mine has no issue with the protection relay.


From the post I've read, and what I've learned on another repair, my best guess at this time is a bad or shorted output transistor. I have a Pioneer SX-3800, that had a lot of static on one channel, not as bad as this, but all the way through the volume. After much reading the most obvious was a shorted output transistors. It seams this model was known for cracked clips that attach the wiring to the transistors. Sure enough mine had cracked clips, replacement clips and a few minutes of time had the receiver tip top again. Many thanks.

The output transistors on the G-7700 seem to be soldered, not clips.
I'm not sure how to test the transistors, can this be done where they are, or do they need to be removed? Or is it obvious that one or more transistors has a short or gone bad, and I just need to replace them?

So, am I on the right path?
Any help would be greatly appreciated, I really want to bring this work of art back to life.

Dave

As good as these amplifiers are, you need to be very careful poking around.....

There are two versions of this amp, the early one has TO-3P output transistors, and the later version has the very large MT200 output transistors.
The heatsink on the early one is die-cast alloy, the later one is extruded alloy.

When these amps fail, boy do they fail and a LOT of the amp will be taken out/fried......Some amplifiers are very forgiving and easy to work on, these are not.
I have seen guys just give up on these amps because of this...
They can be a handful.
However, not always it really depends what is wrong with it....

Now, one culprit for things going very wrong are the DC offset and Bias Trimmers.
The factory trimmer are not very good and they will be oxidized and possibly have dead spots.
If you fiddle with the bias trimmers, there is a chance of hitting a dead spot, and having the bias trimmer go momentarily open will be enough to take out the output transistors....
I have seen all this happen.

Those are the main things to know to start.

They are not the easiest to work on, you have to have the top and bottom off and have it on its side, I find this works best when you are working the driver board....

I do not think you have blown output transistors because if you did, the 10amp fuse on the drive board would blow and keep blowing if you tried to replace it.

The bias test points are not represented well in the SM. One is a metal tag/lug protruding from the component side of the board, to find it, you locate the copper coil, the next to the coil moving to the outside of the board there is a big resistor, next to the resistor moving to the outside of the board is the test point. In mirror fashion, the one for the other channel is on the other side.
Then, next to the coil going towards the center of the board is a gray wire which heads towards the relay, the other channel has a blue wire.
These are your other points.
You can get a mini grabber on those wires near the coil end, just push the insulation back.....
But, before playing with the bias, I would replace those trimmers.

The DC offset does float around a lot on these amps, this is normal, you will never get it to sit exactly on 0mV.....

Possible faults....
You could have an intermittent protection relay (contacts pitted, needs replacing)
Tape monitor switches are notorious on these, I still have to clean mine form time to time....it is harder than you think to get the deoxit in there....
The source selector switch is also difficult to clean....
The tone bypass switch is another source of this problem....
The -20db muting is another one to re-check.
The worst of all is the balance pot, ALL the audio goes through this pot, they need to be clean clean clean......

Let us know how you get on......

Thanks for the response, I can see from other posts that you have invested some time into this model. I'm really liking the sound this receiver puts out, at least on the left channel.

I had let it sit for a couple of days, doing other stuff. Now it's front and centre on my bench. Your response gave me confidence to run it longer and put some volume to it. Here is what I've found, It now has sound all through the volume on the bad channel, not kicking in at around 15%. Still bad static. Could the DeOxit have worked in a little more from sitting?

I have also noticed that with the balance all the way to the bad channel the protection kicks in at about half volume after a few seconds. On the good channel I went as high as about 60% for a bit and no protection. Didn't want to push any more yet. The good channel has smooth quiet volume all the way.

You mentioned oxidization on the trimmer pots. Could these be cleaned with DeOxit, or is replacement the only safe fix?

I will go at all pots and switches again with DeOxit, as suggested.

Just to double check the bias points, here's a marked photo. Though in the photo I can see the "89". that I couldn't see with my eyes. The brown and blue wires are very close to the coil.




Also here is a photo of my heat sink. Is that enough to tell which model I have?



Thanks again, much appreciated.

Dave

You have an early version. You have the older JFETs too......
The heatsink I was referring to was the one on the outside of the amplifier...

You can try and get the deoxit into the trimmers, it will help.
multiturn 10:1 (ten turns to sweep through the trimmer range, instead of one turn, great for fine adjusting) trimmers are good for the DC offset as it is a bit fiddly to adjust otherwise....

Those are the bias points, if you want, you can pick up that gray wire on the other side of the circuit board if you want......

I am suspecting you have a faulty relay with the symptoms you describe, however dont give up deoxiting the switches and such, seems the problem has improved, could be that the deoxit has taken a while to seep in there, so maybe if you are lucky its not the relay...

Well, new info.
I went at it hard with the cleaner last night, including the trimmers. Then worked it in and left it to sit.

After finishing, I left the bias pots all the way counter clockwise as the manual states that this is the starting position for bias setting.

This morning I started with the bias. With the probes hook on, I could not get any reading on the bad channel, nothing. I ran the trimmer quit a bit, but never all the way around, but "00.0" all the time. I checked and rechecked the connections, but nothing.

I thought I would try the good channel, if it was the same, I must be doing something wrong.

With the good channel I had no problem getting the SM recommended "3.3mV" very smoothly and quickly. Locked right in and stayed there.

So I went back to bad channel, and tried again. Still "00.0", and many attempts to "jiggle" the connections, and run the trimmer around.

I then re-set the DC offset. It is sill the same, the good channel is now slightly bouncing under 5mv, the bad channel is bouncing wildly between 30mV and 80mV.

Also the bad channel is back to no sound until about 20% volume, then kicking in with harsh static. :thumbsdn:

Thanks again for the help.

Dave

Sound like you might have some resistors drifted out of spec in the output section.....
Check,
R36 150Ω
R40 150Ω
R52 220Ω
R54 220Ω
R60 4.7Ω
R86 4.7Ω
R66 4.7Ω
R88 4.7Ω

You can test these in place without removing them......

Report the readings.

Sansui G-7700

Thanks, I'm at work right now, but I'll test those when I get home.

Again some new info, this has me excited and might really help.

After this mornings tests, and no readings on the right side bias, I was just letting it play on the good channel, responding to this thread, and reading more posts on similar issues. All the while working dials and buttons on my G-7700, hoping something might work in. But still bad on the right side.

I had some stuff to do so I powered down and put the wood cover back on. Then just before I went outside, I turned it on again, (I'm a little obsessive), one more try.
And..... Perfection! Both sides sounded great. No explanation?? I listened for a while, cranked it up to around 75%, it sounded great on both sides. Not quite as loud on the bad side, but I never did get the off set or bias done.
I turned it back down to a normal level and left it. I was gone about a half an hour, came back, and still running great.
Now I thought I should run through all of the dials and switches to see if anything specific would mess it up again. Then I could isolate the issue.
Nothing specific did. It just slowly started to get static on the bad side. About 5 minutes after it was first noticeable the bad static was back.
I started working through the switches again, and accidentally hit the power off. I turned it back on again, and the sound was good again!
I have now discovered that if I turn it on and the channel is bad, turning it off then back on a number of times will give me a good sound after one or two tries.
It will go out again after some time, but a restart will bring it back.

I am assuming this points strongly at the relay.

Thanks
Dave

kevzep said:

Sound like you might have some resistors drifted out of spec in the output section.....
Check,
R36 150Ω
R40 150Ω
R52 220Ω
R54 220Ω
R60 4.7Ω
R86 4.7Ω
R66 4.7Ω
R88 4.7Ω

You can test these in place without removing them......

Report the readings.

Click to expand...

I got:
R39 150
R40 151
R52 220
R54 220
R60 5.0
R86 5.0
R66 5.0
R88 5.1

Those look good.

Dave

Well, after more time it seems to power on with the bad channel being smooth at going up in volume every time now. No more kick in at 20%, with crazy static.

So, not too sure if there is a relay problem or not now.

The real obvious problem now is the rough sound on the "bad" channel. I would assume it's the DC offset, as I sill can't get it to stabilize. It seams to want to sit around 550mV. I can adjust it down to around 10mV, but with in 2 minutes it will climb back up. All the while bouncing around radically.

I'm wondering if the progression of this issue is making sense, this unit was in storage for a while I was told.

Thanks
Dave

Sounds like the relay could use cleaning/replacement to me.

Boucing around erratically is a symptom of bad differential transistors on the driver board- just went throught this with a marantz 2270. Not sure on this unit, i'd have to pull the manual.

bktheking said:

Boucing around erratically is a symptom of bad differential transistors on the driver board- just went throught this with a marantz 2270. Not sure on this unit, i'd have to pull the manual.

Click to expand...

Differential JFET's on this one.....

This is the channel which you cant adjust the bias on right? You have resistors wich have drifted out of spec you need to check those resistors I listed several posts back...
Once you have the bias problem sorted out, then this other problem with the DC offset will probably be solved.....
Fix one problem at a time.....:thmbsp:

Yup- didn't see the bias part but yes fix that first.

kevzep said:

Differential JFET's on this one.....

This is the channel which you cant adjust the bias on right? You have resistors wich have drifted out of spec you need to check those resistors I listed several posts back...

Click to expand...

I have my results in post #8, all seemed Ok. Unless the bottom 4 being 0.3Ω over is an issue.

Dave

Keep working on the bias string and check voltages in the pre-driver stage. Do you have any cold-mist spray, an oscilloscope?

Don't worry about the relay at this point; it is not relevant to the bias current measurement, although they can mess with your head if you're setting DC offset via the speaker terminals.

tlcasper said:

I have my results in post #8, all seemed Ok. Unless the bottom 4 being 0.3Ω over is an issue.

Dave

Click to expand...

Sorry, missed that......very surprised none of those resistors have gone high....

But indeed as the last poster says, you need to address the no-bias problem first, this is a more critical situation....Once you have the amplifier operating correctly, then you can re-asses the DC offset issue....

Print the schematic out and start measuring voltages as per the schematic, post the results, then we can see where you are at....

Thanks guys.
I don't have an oscilloscope, or know anything about cold-mist spray, I'll look into it though. I'm learning, and find this stuff fascinating.

I have the service manual, but am unsure of where to measure voltages, or the bias path. Am I on the right path with this board:



And this is the schematic for the 7700:



I may be over my head at this time, but if someone can point me to some of the things I should be looking at and checking I would appreciate it.

Thanks
Dave

bumping this if Dave ever got around to replacing the relay.

This one is sitting for now, but thanks for the reminder. I should look into it again.

Hi there just to ask you Kevzep or members here if you can, can you tell me if these are an early or later version at all?
reason is i have one that is in terrible condition and bought a donor The last one is the one i own that someone has made a big mess of .The first one is the one i have bought for a donor.. although to be honest the donor will be repaired as its in far much better condition that mine.