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AU-919 FET problems

kohngallery

kohngallery

New Member
I have a Sansui AU-919 that goes into protection mode so I asked Northridge Electronics to take a look. Here is the response:
Your right channel power amp intermittently produces DC at the output and that's why the protection is not turning on. I suspect the problem is the dual differential FET in the right channel front end on top of possible other circuit issues with the right channel power amp. The FET that is in your unit is labeled 2SK97 and I do not have a source for one. Therefore, I cannot repair your unit.
Does anyone have an idea of how this can be repaired in the LA area? I'd love to save this amp!
 
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I doubt it's the FET. Try separating the preamp from the amp with the switch and see if you are still getting DC at the output.


Rob
 
Tom at Northridge Electronics knows his stuff. If he says it's the FET, its the FET. A quick look on the web didn't turn up much so the only other option is to salvage one out of a donor unit. The G 8000, G 9000, G 8700 and G 9700 use them on the driver boards. Try posting in Barter Town for someone that has a smoked driver board from one of these models.

- Pete
 
LBPete said:
Tom at Northridge Electronics knows his stuff. If he says it's the FET, its the FET. A quick look on the web didn't turn up much so the only other option is to salvage one out of a donor unit. The G 8000, G 9000, G 8700 and G 9700 use them on the driver boards. Try posting in Barter Town for someone that has a smoked driver board from one of these models.

- Pete
Click to expand...
ind
 
Tom does indeed know his stuff! I am hoping we can find an FET just like what you suggested, in another model being parted out. I know many of you are quite expert with Sansui. Many thanks all.
 
I used to send guys to Tom, and even talked to him on the phone a couple of times. But a year or two ago, an AK member I sent that way was treated in a rather shabby manner when one of Tom's repairs did not go well. I've hesitated to recommend him since, and I'd be taking any advice coming from that quarter with a grain of salt.

If the problem seems to be in one channel only, swapping the FET from side to side ought to remove all doubt.
 
This FET problem is something that needs to be worked out. Justin and I have built some circuitry for testing FETs for another circuit we build. They vary tremendously one to the next, even from the same batch.

Supposedly, you can just put a couple of similar FETs together in a 717/919, and from what I've heard it doesn't work, you'll get thermal runaway, maybe not runaway, but disparity between FETs. Since we've already developed a way to test FETs for gain, etc. I was thinking about using some thermal epoxy, which I use in an LED lighting project I have worked on, to essentially lock two matched FETs together, thermally short circuited to each other.

So, they would be matched and at the same temp. What do you think? Could that be the reason for the Dual FET IC to begin with, to thermally short them? Does anyone have any idea of an appropriate single FET to use? I'm no expert on FET design and have basically done trial and error to get them to work in the past, trying many different types, and eventually hand selecting them for gain. I have quite a few of 2n5457, 2n5458, J201, and J112 in my parts bins. I think the SK97 is N channel, right?

Everything I have studied points to not using FETs for a differential pair. However Sansui has done this with the 717/919 series and we end up fixing them. This ought to be a solvable problem.
 
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LBPete said:
Tom at Northridge Electronics knows his stuff. If he says it's the FET, its the FET.
- Pete
Click to expand...

Well, he said he "suspects" the FET, on top of other issues. It just may be the FET, but we don't know how far he dug into it.
That statement makes me think, not too far. That's all.

Plus, I never heard of the place or the guy that worked on it, so I didn't take that into consideration when I posted my reply.


Rob
 
Many thanks all. I have forwarded your comments (except the cranky stuff) to Tom to see if they will help him. He already wrote me to say he will separate the amp/preamp and check the DC. Will report back soon.
 
Robisme said:
Well, he said he "suspects" the FET, on top of other issues. It just may be the FET, but we don't know how far he dug into it.
That statement makes me think, not too far. That's all.

Plus, I never heard of the place or the guy that worked on it, so I didn't take that into consideration when I posted my reply.


Rob
Click to expand...

Sometimes you can't tell because of the design. And that was not a way for the manufacturer to screw you, sometimes it is just the way it is.

Here's an analogy, we used to flip cars. So if we bought a car with the engine not running, we had no idea if the transmission was good or not. There is simply no way to tell without spinning that torque convertor and pump at at least 500 RPM. And that doesn't even tell you if it shifts right.

I have looked for ways to fake out the front end of power amps for testing but come up with nothing viable. You can ask my ex-employers who still exist - if I can't do it, nobody can. Well, I wouldn't go so far as to say that, but it is close. I would love to be able to completely test the output stage of an amp with the front end of it all FUBARed up. The only thing that might work for a DC coupled amp is maybe a transformer.

Anyway, depletion mode FETs are still available. Just find the best ones. If that company didn't want to do that fine. Some places are like that, can't get the right part, can't get the part, so toss it. Most of the newer depletion mode FETs are MOSFETS and have the intrinsic diode, but that does not matter in this application because the voltage never reverses. Sometimes it does matter, but not here.
 
Jurb,
So is the 2sk97 made up of 2 depletion mode N channel MOSFET's? I thought they were simple JFET's. But then you could write all my knowledge of FET's on the back of a matchbook. I want to figure out how to replace the 2SK97, any suggestions would be appreciated.

The reason that you can't replace the front end of a modern amplifier for testing, is that the front end, ie. the differential pair, is the amplifier. Such an amplifier is really a high current operational amplifier just like a 4558 or 5532. The differential pair controls the entire rest of the amp and its characteristics determine slew rate, distortion, and frequency response.
 
Ehh...
2SK97 is two regular JFETs, 99.9% of all the worlds small signal JFETs are depletion mode. There is really no mystery here.
There will be no runaway if the FETs are not matched, just a large propensity for offset and generating second harmonic distortion. There are MANY JFETs that could replace the 2SK97 provided a reasonably matched pair is found. As for the basics, find a JFET which has approximately the same Idss (current at Vgs=0) and Goss (forward admittance). And when I said approximate, I really mean it - 30-50% tolerance is acceptable because this is normal tolerance within two units of the same kind of JFET. One way to do it is to build a simple current source out of the JFET (single resistor between G and S) and measure current with a known resistor at a known Vdg - and keep this around 10V or so, because for lower voltages the JFET will be more of a variable resistor than a current source. Select JFETs with similar resulting current when connected like this. Maximum Vds of 30V and up will do.
Statistically, the JFET would be somewhere close to the end of the list of possible faults with those symptoms, to be considered only when the classical problems present in this amp (multijunction diodes, black flag capacitors, zeners, fusible resistors, solder joints...) are sorted out.
 
So, ilimzn, thanks. That is exactly the tester I have built up on a bread board to match FETs, for something else.

I'll just take and match up a pair of J112's, glue them together at the flat part with some heat conductive epoxy (completely encapsulate it) and try it in a 717 or 919. It would be cool if it would work.

It's always nice to get advice from someone who actually knows something about what you're working on.
 
Select J112 to have a higher or equal Idss to the original FETs under the same conditions. Of course, keep the pin-out in mind! On most JFETs (save for some high powered RF ones) the D and S are interchangeable, which helps, BUT be certain to test them this way as there can be a small difference in current depending how you turn them around. No need to glue them, just dab a bit of heat transfer grese between the flat parts on the case and then heat-shrink them together.
 
So, now the problem has disappeared for the time being. Tom suggests taking it home, using the amp until it goes into protection mode then uncouple the preamp/amp. I'm supposed to report back to him as soon as I do this. This way we will find out whether the problem is in the pre or the amp.
 
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