Starting my Au-999 Recap
- Thread starter tobyj17
- Start date
kevzep
Its all about the Music
Okay, good.
Lets just have one driver board in for the next step, and put the fuses in, then power it up.
What you could do is to clip you meter across one of the emitter resistors with it switched to mV and monitor the current.....If it shoots up very high into volts past mV you will probably see the bulb light up which would mean we have a problem......
With all things being good you might see a couple of mV here once the supply stabilizes.....
kevzep
Its all about the Music
They are the 3W 0.5Ω (square white ceramic) on the 1277 and 1231 board, just make sure you clip on to which ever one the driver board will be plugged into.....
You connect it to each side of the resistor, we are measuring the voltage drop across the resistor to calculate the current (Ohms Law)
Those boards are the supply board as well as housing the emitter resistors for the output transistors in that particular location.....
kevzep
Its all about the Music
Okay, If you put your red probe on the body of the output transistor, and black probe to ground, you should be getting +44-46VDC is that correct?
It should NOT be a negative voltage.....
Dont have this thing fired up for long like this....you might fry something.....
kevzep
Its all about the Music
Yep good call, with the shutting off after a few seconds......
Not sure what you mean by the front and rear.......I will explain in more detail....
The TO-3 output transistor body, the part which is insulated from the heatsink is the collector, the collector is connected to the socket by the two screws which go through the heatsink and onto the socket.
All you need to do is put the red probe on the transistor body, and the black probe to the chassis...
Im looking for the correct voltage and polarity here, most importantly the polarity.....
If that is correct, get your screwdriver on that bias trimmer and wind it up while looking at the voltage across one of the emitter resistors.....let us know whether it goes up or down.......
tobyj17
Comfortably Numb
What I meant was the transistor closest to the front of the unit, and the one to the back of the unit. And the readings I got are the 1.5 volts and -.5volts
Do you still want me to see if i can adjust bias confirming the practically 0 volt readings?
Edit: I got brave and checked it.. and adjusted it both ways as far as it would go.. and it stayed at the 474 MV reading..so no change
kevzep
Its all about the Music
Oh right, front and back...haha...Im a dumbass
Okay, thats not good......
Measure the voltage on the collector of the 2SC680 driver, that should be +46VDC as well......
Lets take the output transistors out, and then turn it on and see if it lights up the dim bulb tester......
There is two possibilities here, now is that the thermal bias transistor is shorted (I have my doubts about that) or there is a dead short somewhere, as one of those outputs should have 46VDC on the collector.....given it has 1.5, and there is a huge amount of current going to ground, Im thinking there is a short somewhere which is eluding us....
I get home form work in about 4.5 hours, what i can do is get my one on the bench and make some measurements, and I will be sitting in front of my one so I can visualize where you are at.....It will make it easier......
tobyj17
Comfortably Numb
Okay, I got 1.5 volts on the collector of the driver transistor..before taking the outputs out.
I took the outputs out.. and turned it on. I got a dim light.. but I heard a light pop from the driver board.. and some magic smoke. Funny thing though I can't see anything burnt or discolored..and it wasn't enough smoke where I could see exactly where it was coming from.
I've got the other board here, I'll do some checking to see if anything varies from one board to the next.
I took the outputs out.. and turned it on. I got a dim light.. but I heard a light pop from the driver board.. and some magic smoke. Funny thing though I can't see anything burnt or discolored..and it wasn't enough smoke where I could see exactly where it was coming from.
I've got the other board here, I'll do some checking to see if anything varies from one board to the next.
kevzep
Its all about the Music
Okay, I will pull the pants off my AU-999 when I get home Toby and get some measurements which we can do with the power off, so we dont have another malfunction.....
Meanwhile, check for short circuits in the output section....go over your work, make sure those output transistors and driver transistors have their mica washers and there is no chance they could be shorting out anywhere.....
Whatever has happened has been done while the work happened, this is why I dont think anything has failed......
Im convinced there is a short circuit in the main DC Supply........and its going to ground through those emitter resistors which makes me think there is something wrong with the mounting of the output transistors or the transistor sockets......
tobyj17
Comfortably Numb
I did a quick check without desoldering anything and all the values were within several ohms of one another..so maybe i flipped it off soon enough :scratch2:
I checked the driver transistors as well.. and those are okay.
I didn't realize you were at work.. i forget the huge time difference. If you need to get back to work that's totally fine with me.. probably help the both of us if you were in front of yours.
I'll check the outputs again, and see if there's any wires that are touching..etc. It's there.. i just gotta see it!
Thanks again for all your help Kevin!
I checked the driver transistors as well.. and those are okay.
I didn't realize you were at work.. i forget the huge time difference. If you need to get back to work that's totally fine with me.. probably help the both of us if you were in front of yours.
I'll check the outputs again, and see if there's any wires that are touching..etc. It's there.. i just gotta see it!
Thanks again for all your help Kevin!
tobyj17
Comfortably Numb
Well Kev, this is what I've gotten so far. I found a dead short to ground on the collector of 2sa566.. but not on the channel we were testing. I found that the holes that the screws went through were off a tad.. so i took a drillbit to them 
.. being careful with the shavings of course. Still had a short, and found a couple cracks in the mica insulator.. I used electrical tape doubled up with the mica in the meantime..took care of it for now....not a permanent solution.
Also, with the channel we were testing I found a really funny reading on the emitter of one of the 2sc893 outputs..With neg. on ground, I get -1,5xx ohms..and it jumps all over the place..seems to actually climb over time. This is with or without the transistor in place. If you're looking at the front of the unit, it's the right side..the front one..a blue wire. It goes to the emitter resistor on the F-1277 board.
.. being careful with the shavings of course. Still had a short, and found a couple cracks in the mica insulator.. I used electrical tape doubled up with the mica in the meantime..took care of it for now....not a permanent solution. Also, with the channel we were testing I found a really funny reading on the emitter of one of the 2sc893 outputs..With neg. on ground, I get -1,5xx ohms..and it jumps all over the place..seems to actually climb over time. This is with or without the transistor in place. If you're looking at the front of the unit, it's the right side..the front one..a blue wire. It goes to the emitter resistor on the F-1277 board.
kevzep
Its all about the Music
This is from the Right Channel. Its important to note it will be opposite on the left channel.
Right, the rear mounted output transistor Collector has +45VDC. Both measurements here Referenced to the chassis...
The front output transistor Collector has 0VDC (this is how the Quasi complimentary design behaves.)
Now its the F-1277 board under the right channel outputs and you should be finding the front emitter resistor has -45VDC referenced to the chassis.....
Now given you have 1.5 VDC where there should be 45-46VDC you have a major problem.....
Now, what happens when you power it up with the driver board OUT...?
Heres a few shots from in my 999..........see if you can spot something different that I might have missed.....
kevzep
Its all about the Music
I found a dead short to ground on the collector of 2sa566.. but not on the channel we were testing. I found that the holes that the screws went through were off a tad.. so i took a drillbit to them
Good work, the more I think about this, the more I think you just have a short to ground, and with this amp it wont matter what channel it is because the supply is common to both channels......
Its just commonsense and detective work.......
Its because you are measuring the resistance in the main filter capacitor, and the meter is charging it up the longer you leave the probe on there......
Okay, yes leave the fuses in and lets fire this thing up......