Hey, how did the repair job go? Is it already working?
It looks a lot like the older AB1100, there are a lot of electronics packed inside that case 😀
It looks a lot like the older AB1100, there are a lot of electronics packed inside that case 😀
Actually, I'm working on it right now. So far, so good. Nothing's shorting out. With the bias being set so low, the clip light is on; but as soon as I up the bias, it goes out. I believe that it's a good sign.
Dumb question time...
The TO3's are the main outputs i assume, what are the 8 TO-264 transistors doing? I assume the ones on the far left and right of the angle are the drivers....
The TO3's are the main outputs i assume, what are the 8 TO-264 transistors doing? I assume the ones on the far left and right of the angle are the drivers....
The amp powers up just fine. I have set the bias and zero'd the dc offset. I do get clean, undistorted sound from the channel, but resistor 80 (which connects the collectors of the NPN and PNP outputs, gets extremely hot. The voltage across it is 195-200, even with no input. The voltage rails (if my measurements are correct) are 65 and 130.
The NPN side collectors sit at 130v, while the PNP side sits at -65v, with no input.
I have no idea where to look now; please help.
The NPN side collectors sit at 130v, while the PNP side sits at -65v, with no input.
I have no idea where to look now; please help.
R62 or D16 open, or one of the four transistors in the high rail step circuit shorted would be my guess. Any of these faults will make it turn on continously.
Before trying with a load again verify that the rails switch as they should with an oscilloscope and fall down to 0V when the output is swinging the other direction. There should be three levels visible - high, low and 0V. If a step switch is still not working and doesn't turn on the driver transistors will try to drive the output by themselves and may blow up if the output is loaded.
Before trying with a load again verify that the rails switch as they should with an oscilloscope and fall down to 0V when the output is swinging the other direction. There should be three levels visible - high, low and 0V. If a step switch is still not working and doesn't turn on the driver transistors will try to drive the output by themselves and may blow up if the output is loaded.
I wouldn't put a speaker load on it until you can verify that there is NO DC on the output!!!! There should no more than .05 Volts DC on the output. I would leave the offset pot where it was set at the factory.
Did you email for the schematic?
Also I thought you said only one channel was blown. You should be able to pull the other channel and make comparative resistance measurements to get you started.
Did you email for the schematic?
Also I thought you said only one channel was blown. You should be able to pull the other channel and make comparative resistance measurements to get you started.
I did check to make sure there was no DC on the output. I have played with the pot to get the dc down to 0-1mV.
I did check R62 and D16, and both check out good. I could also not find any shorts in the rail switch transistors.
I did check R62 and D16, and both check out good. I could also not find any shorts in the rail switch transistors.
You can make a real nice dummy load out of an old electric oil type heater. You sometimes see them discarded for the rubbish man. They look like the old steam radiators that used to be in old homes except they have wheels.
Usually they start leaking oil but the element is still good.
The element is what you want- they usually read about 10 to 11 ohms and they're about 1000 Watts.
You'll need a big wrench to remove it from the heater. Mount it up with a BNC connector so you can monitor the signal with a scope.
And you won't disturb the neighbors when you're testing.
Usually they start leaking oil but the element is still good.
The element is what you want- they usually read about 10 to 11 ohms and they're about 1000 Watts.
You'll need a big wrench to remove it from the heater. Mount it up with a BNC connector so you can monitor the signal with a scope.
And you won't disturb the neighbors when you're testing.
What is the voltage on both ends of R59? Measuring that will help narrow down the problem to either the circuit around the upper three transistors or the lower transistor and its associated components. One of the small caps of x00pF caps could be shorted causing the same symptom.
The supply rails should run +140, +70, -140, -70.
On my amp I had to replace all the .43 ohm 2W resistors.
Also I had to replace several of the .27 5W resistors.
Its normal for R80 to get a little warm. If its burning hot check for a shorted Q42,Q43 or Q23,Q26.
Also, additionally, if the MJL21193s/94s got very hot, you could have a shorted rubber insulator.
On my amp I had to replace all the .43 ohm 2W resistors.
Also I had to replace several of the .27 5W resistors.
Its normal for R80 to get a little warm. If its burning hot check for a shorted Q42,Q43 or Q23,Q26.
Also, additionally, if the MJL21193s/94s got very hot, you could have a shorted rubber insulator.
R59 sits at ~140-141v. The drop across it is 0. Nothing else gets hot other than R80 due to ~120v across it.
This may sound quite obvious, but you should find out what rail switches are triggered, if any, by measuring its corresponding Vbe.
I measured a small drop across R59 (2K). The left side measures 138.7 and the right side measures 139.0. So there is a .3 drop across it.
Something else to think about is that it works into Q21. Even though Q21 (and Q22 on the other side) is not represented as a such on the schematic, it IS a darlington. And darlington transistors can be a little difficult to check since you can't get at all the elements of the two transistors that are inside.
I had to replace both darlingtons on my amp but then it was pretty blown up!
Something else to think about is that it works into Q21. Even though Q21 (and Q22 on the other side) is not represented as a such on the schematic, it IS a darlington. And darlington transistors can be a little difficult to check since you can't get at all the elements of the two transistors that are inside.
I had to replace both darlingtons on my amp but then it was pretty blown up!
.3V over the resistor means Q24 leaks 150uA - which sounds ok. But not if R54 is open. If it is then this (or even lower) current will be enough to keep the rail switch on at least for low current draws. For higher output currents the switched rail will be pulled down with excessive dissipation in the half-on switch, possibly blowing it up.
So make sure the emitter-base resistors (both for the darlingtons and the high-current transistors, 220 ohms and 5.1 ohms in schematic) in all rail switches are OK and not open.
So make sure the emitter-base resistors (both for the darlingtons and the high-current transistors, 220 ohms and 5.1 ohms in schematic) in all rail switches are OK and not open.
I checked the 220 and 5.1 resistors and they are OK. Also I checked R59 &R60 on the other module and the drop there is also about .3V. So I think that drop is correct.
LAJ, isn't your amp one that IS working correctly? The 150uA leakage current will not be a problem if the 220 ohm resistors are OK.
If the 220 ohm resistor is open though very very little leakage current through the comparator transistor will turn the rail switch on. Maybe this is what is happening to xplod's amp.
If the 220 ohm resistor is open though very very little leakage current through the comparator transistor will turn the rail switch on. Maybe this is what is happening to xplod's amp.
Yes, it is working correctly. The 220s read about 208 - 210 in circuit.
Well, I thought my input would be helpful to xplod, so I am offering the information as a comparison on this forum.
Well, I thought my input would be helpful to xplod, so I am offering the information as a comparison on this forum.
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