How to Fix Sansui Sansui G-6700 G-7500 G-7700 e G-8500 Oscilation Problems Damage Output

Aleksei77

Active Member
The Sansui G-6700, G-7500, G-7700, and G-8500 can be used to solve the extremely high oscillation problem of the mega-house.
These devices usually from one hour to another overheat for no reason whatsoever, which takes in a few minutes when the oscillation is strong and damages several components of the output.









This board is from the G-7500 but they are all alike, just put 25pf or 47pf capacitors in these places for the oscillation to stop.










Aqui é a placa do G-6700









 
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I'll have to check, but that looks like in parallel with C17 & C18, 5pƒ, so this makes them 30pƒ or ~50pƒ.

It definitely deserves a check out next time I can get one of them to the bench.

Thanks - - -
 
I'm sure that I'm gonna jinx myself here, but I've worked on a number of those receivers, and have never seen a problem with oscillation. Could the problem be with counterfeit parts?
 
Counterfeit parts are often a possibility, but I have been careful, and try to always check my parts. OP TRs come from reputable sources. Components are checked with meters and I use a curve tracer for the other transistors.

IMHO, there is definitely a gremlin lurking in the design of the F-2980 board. My G-6700 thread is ~350 posts over nearly five years and my memory is fuzzy (some things you try to forget) but I remember there being three (or more) events. After replacing parts and thinking "I got it this time" I would power it up and watch the meters for a few minutes, and then something would jerk the smile off my face and take out the OP TRs. Once it actually played for a few hours.

Some knowledgeable people here have suggested oscillations as a possible/probable cause, but I have never observed that at the right instance of time. I think you'd have to be very fortunate to time it just right, spend a lot of time watching a scope, or figure out some way to record it.

For now, adding a couple of caps is an easy way to take a precaution, so what the hell.
 
I've never seen a G7700 in Oscillation either, and I have spent quite some time working on them over the years.....
But not to say they don't do it under certain conditions, like fake transistors and the likes....

Deserves more investigation though...and wouldn't hurt to put those caps in....
 
So long as it doesn't negatively effect the high frequency response in the audible range, it won't hurt anything.
 
Pf value caps are waay above hearing limits. Would be nice if scope trace included frequency. Had Ghazzer's unit on my bench several years ago and no trace of oscillation yet a few hours down the road it gobbled itself up in a hurry. As Kevzep stated its worth a try :)
 
In the right (wrong) place, a 47pF cap can absolutely effect high frequency response in a negative way.
 
I think you'd have to be very fortunate to time it just right, spend a lot of time watching a scope, or figure out some way to record it.

This is where the digital oscilloscopes come in handy. They can be set up to trigger on just about any event type of event imaginable and record the event for later play back.

Of course if the event leads to the self destruction of the device under test, one has to come up with a work around.
 
Unless I'm off a decimal point, 47 pF with 20 KOhms comes out at 16.940 KHz.
 
Unless I'm off a decimal point, 47 pF with 20 KOhms comes out at 16.940 KHz.
When aligning tuners, I couple my RF gen to the front end with a 10.7mhz sweep through a 100pf cap, as per the service manual instructions?

I have not yet looked closely at this, but I doubt if this cap in actually in the audio path....
 
Don't get me wrong. I'm not saying that it will effect frequency response. I'm just saying that when we go around second-guessing the engineers at Sansui, we should at least check the math.
 
Don't get me wrong. I'm not saying that it will effect frequency response. I'm just saying that when we go around second-guessing the engineers at Sansui, we should at least check the math.

Oh yes, I absolutely agree. Full spectrum analysis needed after a mod like this...
 
Trying to recall all we tested on Ghazzer's G7700, I do not think we tried forcing a really high frequency square wave through it looking for ringing or overshoot although I do recall sine waves at 10k looking clean. Seeing how Steve just happens to have one perhaps he would put it back on the bench. If nothing else the original post certainly has brought up an interesting point about why a few of these units blow modern outputs for no apparent reason at all. To Tarrior's point, I will admit r-c filter networks are a real pita when one tries to get exact values but as the effective value of 'r' decreases the high f point rapidly climbs thus my observation a few posts back that 25-50pf should not affect response in the audible range. Sansui was pushing the limit with these dc-dc amps, tampering with the ft values of replaced parts might well indeed induce oscillation. EW had to add a small pf cap in the feedback network of his awsome sx1980 output module replacement modification to bring ringing under control so others have been down this path before.
 
I think if I was doing this I would use a C0G/NP0 Ceramic rather than what looks in the pictures like a 'bog standard' Ceramic. And I would remove existing capacitors to avoid having two in parallel, replacing with a single one of appropriate value.
 
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It's a good idea to check the stability so you know what you are "fixing." Look at a 20Khz square wave at 1V p-p out and 8 ohm load. Look for ringing on the transitions. A small amount is ok.
Then put in a 1 KHz sine wave at 10V p-p out, 8 ohm load. Measure the input to the amp and write it down. Monitor the output, and increase the frequency till it drops to .707V p-p. ZThat's -3dB.Keep going until the output drops to the input level. That's unity gain. Keep going and make sure it doesn't come back up above unity. If it does, it's unstable and you have to increase capacitance somewhere to roll it off Before it reaches that frequency. It's tricky because you are adding phase shift and you have to add it in the right stage or you will cause oscillation.
 
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