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Sansui AU-666, looking to replace 2SD223 Transistor

nintendoeats

nintendoeats

New Member
Hello! I think I know what to do here, but I'm a noob at proper stereo repair and I know transistor subbing is one of those things that is tricky. I was hoping for a bit of handholding and confirmation that I am not Doing It Wrong.

I have a new-to-me Sansui AU-666 ($100 CAD shipped, but not working and I need to make a new top cover for it which is within my capabilities).

All testing done with a 1KHz tone on the aux input.

External Symptoms:
- Power light comes on
- No audio comes out
- An oscope on the pre-out shows nothing, which indicates to me that the preamp stage is bad (with the speaker switch in the A+B position)
- With a cable connected to the main in, if I tap the cable I get audio out of the speaker which indicates to me that the main amp is at least basically functional

Internal Symptoms:
- All fuses read +40V or -40V. The service manual does not provide any voltages, but that seems at least sane.
- Oscope shows signal on the input to the EQ/Preamp board, but nothing on any of the output wires.
- Voltmeter shows nothing on the power wire from the ripple filter board to the EQ board.
- Ripple filter board input power connection shows 40V, and the ground connection to the chassis is good.

From this I infer that the ripple filter board has a fault.

Investigation of ripple filter board:
I pulled all of the capacitors and they measured good, actually read ~20% higher than their rated capacitance but that might just be my cheapo meter.

I pulled the transistor (2SD223) and attempted to test it. If I am doing that correctly, then it is faulted (which is really what I expected from the beginning). I get ~70Kohm resistance from base -> collector, but infinite resistance from base to emitter.

I cannot find a datasheet for this transistor, but alltransistor has values that make sense to me, so I'm assuming they are correct .

So then to replacing it. There are a few on eBay, but I am disinclined to spend $50 CAD replacing a 55 year old transistor with another 55 year old transistor.

I found some people suggesting the KSC5603D . This part is discontinued, but there are plenty on eBay and they are pretty cheap. I also note it doesn't seem to be a perfect match, but I'm not sure how much the differences matter. I did find a few other transistors available new that might work, the closest of which was the FJP13009. However, the minimum base current rating on that is much lower than for the original (8-40A as opposed to the original's 30A min). I don't know how much that matters in this case. There could also be other things I don't know to check that would render it unacceptable.

So the questions
1. Is my reasoning sound?
2. My inclination is to get some of the used KSC5603D from eBay, since it seems like people have had success with those. Good move?
3. Since the service manual doesn't provide voltages, and I am not sophisticated enough in circuit analysis to know if the rippled filter board (F-1268A) drops the voltage as well as providing filtering, I'm not sure if it would be safe to simply bypass it in order to test the rest of the unit. My intuition is that it is supposed to be forwarding a nominal 40V, but I'm not connecting the power supply directly to another board based on intuition! I've included a picture of the board schematic in case that helps, though note that D001 is depopulated on my unit (and I saw another online which was the same way, so I think that's normal). Seems to me that it's a bridge rectifier, so I guess this board can also be fed by AC in some devices.

Thanks!

1791739760402.png
 
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It’s just a pass regulator. Just put in a TIP31C with a heatsink and be done.
You did make sure that D001 is putting out voltage, right?

As an aside, I would be replacing all the caps here, and be sure to get the flying saucers in the unit. The AU-666 and AU-888 are incredible units, and I replace as much as I can.
 
ivandezande said:
It’s just a pass regulator. Just put in a TIP31C with a heatsink and be done.
You did make sure that D001 is putting out voltage, right?

As an aside, I would be replacing all the caps here, and be sure to get the flying saucers in the unit. The AU-666 and AU-888 are incredible units, and I replace as much as I can.
Click to expand...
Thanks, if that's all I need in this case that would be great!

I just had to do a bit of investigation and realized some confusion about D001. D001 the bridge rectifier is producing the +40V/-40V that I measured on the fuses. There is also a D001 which is NOT PRESENT on my F-1268A board, but which is silkscreened on and shown on the drawing in the service manual, but not shown in the full schematic (which I found after posting OP).

Re caps, I'd like to see the thing working before I think about that as it adds expense and a possibility to break something. I really was surprised that all the caps I measured seemed to be within their rating, though I know they could also generate noise or who knows what.

What are flying saucers? The big transistors?
 
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nintendoeats said:
Thanks, if that's all I need in this case that would be great!

I just had to do a bit of investigation and realized some confusion about D001. D001 the bridge rectifier is producing the +40V/-40V that I measured on the fuses. There is also a D001 which is NOT PRESENT on my F-1268A board, but which is silkscreened on and shown on the drawing in the service manual, but not shown in the full schematic (which I found after posting OP).

Re caps, I'd like to see the thing working before I think about that as it adds expense and a possibility to break something. I really was surprised that all the caps I measured seemed to be within their rating, though I know they could also generate noise or who knows what.

What are flying saucers? The big transistors?
Click to expand...
Then I suppose my question is are you sure TR001 is getting power to the collector?

Unless you are measuring at working voltage, and also measuring leakage, I wouldn't take that measurement to mean anything. The caps are 60 years old.

White bodies with black tops that are round:
CDC 8002 → TTC004B
CDC 9002 → TTA004B
 
I do have some replacement caps on hand I could put on that board, but this is kind of supposed to be an exercise in learning diagnosis so I'm trying to avoid just parts hanging until it works, make sense?
 
I am happy to report that the new transistor has brought life! This is the first time I have ever fixed a piece of eletronics using a signal generator, an oscilloscope, and the schematics. To this point it has all been intuition and fiddling. Very exciting moment for me.

So far everything seems to be working, but there is a pretty serious channel imbalance. It's fine at high frequencies, but as you go down past 1KHz one of the channels goes very low and out of phase (these are both channels of the pre-out). This one I'm guessing is down to a bad cap or two.

1792450267052.png

Here's the new guy doing his job. I will probably resolder him so he sits a bit more proud and I can put on a heatsink.

IMG_1389.JPG

Now that I've got it working, let's demonstrate why this amp has thusfar cost me a little over $100 CAD.

First off, here's the picture from the listing:
1792450398601.png

Unfortunately that rackmount bracket, while cool, required the integrator to permanently modify the faceplate. As you can see, both the bottom and sides have been machined down and the there are a few holes now. The holes I could tolerate, but the machining makes me sad.

IMG_1386.JPG

You will also observe that there is no top cover, but that's an opportunity to make something custom and develop my skills doing finishing work.

The tone knobs work, but they all feel different and one of them has a slightly bent peg.

Here's a picture of the insides in all their glory:

IMG_1388.JPG

So, did I get a good deal? Yes, great learning opportunity and a nice amp. Am I going to put a lot of time and effort in to wind up with an amp that isn't worth much more than I paid due to its physical imperfections? Uh...possible.
 
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