Need help troubleshooting AU-999 integrated amp LOOONG.

[240sx4u]

Heres the scenario. This may be rehash for some people, but its a background (I had posted some info in another thread). I aquired this unit with a low output channel. One of the fuses for the power supply to the driver board was blown. Just for giggles I put a new one and It did not blow on me.

I then went to set the bias and offset of this unit. The service manual is wrong (as pointed out by a member) and indicated that I had to turn screws clockwise to lower the bias fully (blatantly wrong, I cant belive I actually did it). Basically at this point in the game the unit would work with normal channel levels but make a DC pop out of one speaker after the amp warmed up.
I was not able to get the dc offset and bias set properly on one channel. I also treated the sockets for the boards with deoxit, which actually did change the voltage I was able to obtain (but it was still way too low). I also checked all the voltage regulation DC numbers from the schematic and they metered out just fine.

Fast forward to now.

It had never occurred to me to check the output of the headphones when I was tinkering. I found out later that the headphones had only one side working. Who knows if thats my work or not. I found a few cooked resistors on the suspect driver board. I am trying to figure out what burnt the resistors.

I have checked every resistor on the driver board, tests fine, same deal with transistors (bought a tester). I also metered across the caps. Everything seems fine there as well. A tech at some point had replaced two transistors and a DC offset pot.

I have desoldered the bad resistors that I found on the board and ordered new ones. That is where I stand right now. Is there any other testing that I can do with this thing before I get my new parts to install (i ordered 2 of each in case they cook again). I also plan on replacing the pots after I get this situation figured out. My shaky hands have trouble tweaking single turn pots.

Oh, and I did swap the driver boards back and fourth and the bad channel did not change. I know that means the crux of the problem may not be on the driver board (but I still need to figure what smoked the resistors).

Thanks so much guys - Evan

 

[240sx4u]

Update;

I found a bad transistor on the power supply board for the suspect channel. I think I may have burned this when I was attempting to set bias etc...

Maybe this is the root of the issue.

Thanks - Evan

Any suggestions are welcome! Im about ready to buy an ocilliscope (as if I know how to use it) to trace signal.

 

[rulerboyz]

I have desoldered the bad resistors that I found on the board and ordered new ones. That is where I stand right now.

When you tested the resistors in question did you make note of what the value was? Quite often you will get a resistor test lower than its true value while it is in the circuit. When you desolder it and pull it out it may now read the proper value (even a burnt resistor may not lose much of its resistance...some resistors are more resilient than others). When this happens it could indicate that the problem is elsewhere often in some component adjacent to the resistor in question. I had a case where some resistors were reading low, but were fine when I took them out of the circuit. I changed a bad power transistor and after that those same resistors were reading the right value.

In fact, you will often find yourself trying to convince yourself that you've found a bad part. Upon closer inspection you might be disappointed when you realize the part is still good.

 

[rulerboyz]

I found a bad transistor on the power supply board for the suspect channel. I think I may have burned this when I was attempting to set bias etc...

You should make a mental note everytime something you do makes something go bang and does something like blow a fuse. What side were you working on when it happened? What part of the board were you touching. These things can give you clues where the bad transistor, diode etc.. is.

Regarding testing transistors. It is very useful to test the DC current gain on them as well as doing the bipolar junction test. By testing the current gain you get a definite picture of the lead configuration. It is possible to put the transistor in incorrectly in such a way that the emitter and collector have switched places.

 

[240sx4u]

Well the plus side of the sansui boards is that they are clearly marked C B E so they are pretty easy to test. Im sure this one is bad.

As for the other resistors, they were either VERY dark in the center, or split open. Definitely cooked.

I bought the amp with the fuse blown btw, and it has not happened since.

Another concern. The transistor that tests bad is a cdc9002-1c. This same transistor is also used elsewhere on the driver board. It has been replaced by a tech with a toshiba 2sa510x. I cross-ref both of these numbers, and get two different suggestions from the NTE cross reference guide. Did the tech that worked on this before actually swap in the wrong transistor? It is located on the problem side of the amplifier. Oh, the way I know that this number is used on the driver board is because there are two of them (I used them to compare between each other).

Thanks alot rulerboyz for your help! - Evan

 

[rulerboyz]

I like to use Audio Lab of Georgia's website to cross reference the NTE numbers to see what else is available. As you already know, CDC9002-1c crossreferences as NTE128. When I put down NTE128 in the parts search at Audio Lab of Georgia and click on the cross reference link I get the following:

http://www.audiolabga.com/mal_cart/mal_cart.php?nte=128

You will notice that in this case only the first 3 types are in stock, but you may be able to find the others elsewhere.

The correct way of doing it of course is to get a proper Semiconductor cross reference book. Perhaps Echo Wars will chime in and let you know what a good substitute is in the toshiba line.

If what you are saying about the tech replacing CDC9002-1c with that particular Toshiba part is correct, then there is a major error present beyond it being a different cross-reference number.

CDC9002-1c is an NPN transistor while 2SA510 is a PNP transistor = apples and oranges.

 

[240sx4u]

Alrighty, I guess the consensus is that the toshiba is the WRONG part number for this application. It pretty much explains all the problems that I had.

That was what was confusing the crap outta me, I thought I must have been reading it wrong when the numbers crossed wrong.

Evan

 

[rulerboyz]

To be sure, do you have the parts list written on the schematic? That is the first place to look to know which kind of transistor is supposed to go where. From your description I gathered that you thought that the tech replaced an NPN transistor with a PNP transistor...but I might be mistaken since I can't see it for myself.

 

[240sx4u]

Sure, I have a parts list. I will list it out to clairify.

This is for location Tr813 on board number F-1159, there are two of these boards of course for a stereo application both have the same part numbers and location numbers.

cdc9002-1c according to the service manual is required, and is what was located one of the boards.

The other board has obviously had some work done. The tech substituted a toshiba 2sa510x for location TR813 instead of the cdc9002-1c.

Odd thing is that with my transistor tester, they both test as PNP's despite what all the lit says on the toshiba. Could be my tester is wacky too though.

Evan

 

[rulerboyz]

The "sa" part of the transistor name tells you it is a PNP transistor. "sc" for NPN. I wouldn't worry too much about how the 2sa510x tests if it has been inserted in the wrong place it needs to be taken out of the equation (either throw it out or keep it as a curiosity).

Your diode test should give you forward voltage drop values generally from 0.5v to 1v depending on what type of transistors or diode you are testing. 0.6v seems to be a very common value for small signal transistors. I had a multimeter before that was giving me readings that were too high. It's important that you have a multimeter that gives you the right ranges, since you often want to compare the values between transistors of the same type (a value lower by about 50mv or more could indicate a damaged transistor)

 

[240sx4u]

Rulerboyz, thanks so much for all your help! Im going to order parts in the morning!!

Evan

BTW, after looking at the way this thing is pinned out (npn) the base is in a different position. I cannot believe that this amp actually worked.

 

[rulerboyz]

Well like I said, when you do a transistor test with the emitter and collector in switched positions you may still get a very small gain (such as 4 instead of something like 250). So perhaps the transistor can be forced to work in that position but it no longer is performing it's job of amplifying signal at the proper gain value. Another thing you need to watch out for is to make sure that the leads of the transistor don't touch each other when you install the replacement. Often you will find a little plastic sleeve protecting the base lead from the other leads. It depends on the case style of the transistor. Larger transistors tend to have very sturdy leads that go straight in (and are no subject to bending in such a way that they could touch one another).

When you put the new transistor in you should put a small crocodile clip on each of the three leads, as they act as a kind of heatsink. Transistors (especially the small ones) are very heat sensitive and can only withstand brief touches with the solder iron. If you are really good with the solder iron you can make the solder joint in less than a second and expose the transistor to very little heat.

Once you have finished installing the transistor don't forget to take of the crocodile clips off before you turn the unit back on, or else you will have another bad transistor. You should also use the crocodile clips when removing a part you suspect is bad, just like gathering and protecting evidence at a crime scene. In the process of removing the transistor you could easily damage it and then end up wrongly accusing it of being the bad part you were looking for.

 

[240sx4u]

Craplola

Well, I got all my new components installed and fired it up, literally. I had a couple of other transistors smoke on the driver boards. I obviously did not actually fix the problem.

BOTH driver boards had this happen to them, is it safe to assume that the source is common to both boards? I did notice the voltage coming out of the voltage regulators is too high by about 4vdc I dont know if thats enough to make smooke. I did order extra parts from mouser incase I burnt a transistor or something.

Thoughts, suggestions? I am thinking its something power supply related. Who knows wtf I did because this started as a bad channel. Maybe swapping driver boards for troubleshooting was not in my best interest. Or maybe it was a matter of time! Who knows.

I double checked everything before I did the repairs as far as pinouts of the transistors and resistor values.

Evan

 

[rulerboyz]

Whenever you get in that kind of situation it's important to have a dim bulb set up to plug your unit into. Only consider plugging it directly into the wall once the bulb doesn't light up.

http://audiokarma.org/forums/showthread.php?t=51853

 

[240sx4u]

I feel like a dumbass now. I read that entire thread awhile back and forgot about it. I have a ton of junk extension cords that Im going to rig up for a test circut.

Im starting to feel like the sansui is gonna win LOL.

Thanks bud - Evan

 

[240sx4u]

Only dim bulb here seems to be myself :banana2:

I rigged up a bulb setup, pretty cool. I assume when the light stays lit that there is a short (or too much current flow) that lights the lamp?

I pulled both driver boards and powered it up with the bulb, and the bulb did not stay lit. Is it safe to think there is still something funky with the driver boards? Or is removing the driver boards actually breaking the circut that is the culprit... I am not sure yet. What has me scratching my head is that both boards started smoking and identical parts failed. I am not sure if this is due to the fact that I swapped drivers back and fourth a couple times (and damaged the same part on both boards). Or if its a power supply issue that is supplying both boards from the same circut.

Sorry for posting so many times, and making this so long. I am hoping this thread might help some other newb at some point that needs assistance!

Thanks again audiokarma - Evan

I think a donation is in order after I figure this out!!! :thmbsp:

 

[rulerboyz]

Just because the dim bulb does not stay lit does not mean that everything is fine (in terms of sound performance). It simply tells you there isn't a major electrical problem. Given the right set of problems can plug an amp in that doesn't give you any sound at all, in your case with missing components and yet it will pass the dim bulb test.

Now you can troubleshoot using the dim bulb without needing to worry about those specific parts smoking. The troubleshooting consists of trying to find the weakness by testing parts (transistors, resistors etc...) using your multimeter (resistance and diode tests with the power off, DCV with the power on). If you have the schematic you can compare the voltage readings on it to what you are seeing at various points on your unit, especially near or on the problem area. You can figure out the voltage at a given point by setting your multimeter to DCV and then connect the black lead to 0V or Ground (usually the negative terminall of the largest filter cap) and then the red lead to the point you want to test. Then compare that value to what you see on the schematic.

 

[240sx4u]

My schematics kind of suck, they dont show very many voltage test points. One of the large transistors that is a voltage regulator I think took a dump. There are two, one is showing about 4x more voltage out than the other one. I am going to desolder and give them a quick test.

Okay, tested the transistors in the power supply. I belive one is kaput. The instructions for my transistor tester indicates that a change in tone may indicate a defective unit. I need to check it with a meter. I have the pleasure of working the night shift so thats all for the night!

Another odd thing, it looks as if a cap is backwards on the power supply. Both big caps have a black dot (gnd im assuming) one cap has the black dot grounded and the other one has the dot on the power side. Im going to check into that a bit further.

I really wish I knew of all the service this has gotten in the past to help iron out some details.

Thanks - Evan

 

[rulerboyz]

If you really want to know if the capacitor is in the wrong way follow the path on the schematic and then compare it to what's on your circuit board. Be aware, that putting a capacitor of that size the wrong way will make it explode with a very very big bang and lots of smoke, so don't do anything, such as switch one 90 degrees, unless you are absolutely sure.

Remember, the negative side of a polarized electrolytic capacitor is referred to as negative and not the ground side. The + side is the Positive side. I only think of a part as grounded when I see it soldered directly to the chassis or ground wire.

 

[240sx4u]

Trust me man, ive seen caps go KABOOM. Just seemed odd to me.

And one side of the cap is grounded, there is a unshielded wire going from chassis ground to the cap on both caps. But not on the same terminal on each one. Funky. Ill likely leave it alone for now because it obviously has not popped and im sure it was done like that (if its in fact incorrect) along time ago.

Thanks again for the safety tip though

Evan