Power amp Problem

[Khaled011]

Hi All,

This is my first post. Since I am new to make/ maintain amplifiers, I need your help.

I bought a Sunn Alpha 112R guitar amp (late 70's era) and it worked fine for a few months. Later sometime, it made a strange noise for a second and finally went dead.
I got the amp schematic via the internet and tried to figure what went wrong. I found that power transistors (2N3055/ MJ2955) were fried due to a dry solder point at TIP30.

First problem: I checked and changed all suspected transistors but the problem persisted.

second problem: I used TINA (TI version) to simulate the amp since I was interested in understanding the tone section. Also to check the bais setting instructions mentioned in schematic but all readings from simulation did not even come close.

I am thinking even of modifying the power amp section to simplify thing a bit.

Can any one help me with these problems.

Greating and thanks.

[singa]

Hi,We are not psychics please provide the link to your schematic or post a file of it so we can start a conversation or else it's a guessing game.The fact that you have TINA
implies that you have electronic knowledge ,most people do not state their
level of knowledge so we give a general view point but no specifics.

Do not modify the circuit to "simplify" it if in the first place if you don't
understand how it works.You learn nothing from it.

From your description even after replacing suspected transistors the problem
is still there means a critical part or connection has not been found.Try tracing the wires or pcb traces that will give a sense of where the things connected.It's like learning topography or map reading.

[indianajo]

I'm repairing my first SS amp with more than 2 TO3 output transistors per channel. It came with a sticker that said "Channel B dead, use channel A" so I did the double diode test on all Chan B transistors. Many were shorted. There were burned parts on the driver PCB, I checked all those transistors, many failed, some were blown apart. Crowbar triac had melted a pcb trace protecting the speaker. Then I put 400 ohm resistors across the inputs, and 10 ohm 400 watt resistors on the output, and turned it on. No excess voltage on the input - safe to hook a transistor radio up to it. But the channel A 10 ohm resistor started smoking. Turns out Chan a was putting out 90 VDC, despite the crowbar not looking blown up. Also a blown O.T on Chan A, turns out. Only paid $55, not angry, just working the puzzle. You should follow a similar path, from the outputs back. Are the power supplies okay? Is the amp sitting at 0 v DC output like it should? If you can get that far, then inject some music (tsistor radio) and watch the signals through the schematic with a scope.
I made the 10 ohm resistors by hooking up 2 each 5 ohm 225 watt resistors in series, and mounting them on computer case metal with tabs turned up so the resistors wouldn't burn the coffee table. Not a non-inductive load, but then why test amps with a non-inductive load, when the speaker is an inductive load.
After the amp is fine and makes nice waveforms on the scope coming out, then hook it up to the speaker.

[Khaled011]

[QUOTE=singa;2570279]Hi,We are not psychics please provide the link to your schematic or post a file of it so we can start a conversation or else it's a guessing game.

I am sorry. but I made sure I've Included the schematic.

Link: Sunn 112PR

Thanks.

[Khaled011]

Dear indianajo,

The problem started with changing the power transistors, It kept blowing fuses so I took the power transistors off the board and turn the amp on and checked the power supply and it was fine. Also the preamp section, and it all seemed good.
Putting the power transistors back in, shorted the power amp section as in the schematic, put a dummy load 8 Ohm resistors and measured output, almost burnt my fingers, approx. 2.5 Volts were there. I measured the voltage across the last two 0R33 Ohms (I hope you've got the schem. link: http://music-electronics-forum.com/a...pha-12revb.gif), and found 2.5 Volts across the 3055 emitter resistor, and around 3 mvolts across the 2955 emitter resistor.
The adjust. trimpot has almost no or a little effect changing that voltage, though I've checked the rest of the transistors.

Any suggestions are highly appreciated.

Thanks.

[indianajo]

Originally Posted by Khaled011

approx. 2.5 Volts were there. I measured the voltage across the last two 0R33 Ohms (I hope you've got the schem. link: http://music-electronics-forum.com/a...pha-12revb.gif), and found 2.5 Volts across the 3055 emitter resistor, and around 3 mvolts across the 2955 emitter resistor.

Well there are two likely possibilities of 2.5 V across emmiter resistor. Transistor is bad, or Vbe junction is greater than .65 Vdc, turning on the transistor. I presume you have done the double diode test on the transistor. This test doesn't prove the transistor good, but will prove it bad. I'm scrapping 4 transistors that read half the value of the b-e junction (430 ohm) on the c-b junction (190 ohm). Even though they do read infinity backwards. Your meter may read different on good forward junction. To check the Vbe voltage you just need a couple of clip leads and a multimeter on 20 VDC scale. If the previous circuitry is turning the transistor on DC, then some of it is blown up too. All my driver parts were blown on the B channel, through 2 levels of transistor, back to and including the input op amp. Weird, the last driver transistor is rated 250 V and should have held off a shorted O.T. unless the heat of driving a short cooked it. You may have something similar. The bias circuitry is supposed to supply just enough current to the O.T. to produce 20 ma or so bias current, or just barely on. You may have to disconnect the bias or fiddle with the pot to get Vbe below .6vdc. Some bias circuits, if the pot wiper loses contact, they force the output transistor to full on. These are the kind that divert the bias voltage to ground with the wiper of the pot- a fail stupid design.
I think your bias is the 4250 transistor with the R168 trimpot. I would first short across R167 to see if the problem goes away. If that doesn't work, pull out Q109. If that doesn't work try to find out why Q103 is turned on, or maybe it is shorted.

[digits]

lemme guess they have way too little heatsinking on.

[tauro0221]

Hi,
I have a suggestion. Make sure that the input of the amplifier is shorted. Check the base voltage of Q103. If it is read 2.5volts then that is why you see 2.5 v in the output of Q104. With Q104 and Q105 remove check the voltage at the base of Q103 while adjusting the R168. If everything is Okay you should be able to see the voltage change from pos to neg. If not voltage change then check to make sure that you have a positive and negative voltage across the capacitor C125. It is possible that Q109 is open. I think if you short the cap C125 you should be able to see a zero volts at the base of Q103.

[sregor]

Need some clarification. First - 2.5 volts across one emitter resistor and 3mV across the other - ohms law says current has to go somewhere - my guess would be that you have large DC on the output, and current is going through your load resistor. (also there seems to be no protection, so could fry speakers if you connect them) Also, besides the discrete transistors, there is an opamp input IC107 which could cause high DC. This is also directly couple to the previous stage, so check to see that no DC on the input side of R148. Also, please be careful of plus and minus voltage signs - some cases it will make a difference in trouble shooting. I don't think you can really check your bias until you get DC voltages under control. Besides checking the semiconductors, I would check the +-15 on the basis of q108 and q101, amd probably change IC 107 if all the other stuff looks okay. And don't connect up the dummy load until the DC is taken care of. Good luck with it.

[Khaled011]

Thanks all for your posts.

I did some checking on the circuite a after I earthed pin 6 on IC107 and found that the bais trimpot has a very little or no effect, so I suspect that Q109 is damaged (replaced all suspeted transistors, execpt Q108).

I could find all the transistors locally exept the PN /2N4250, I did some search for an alternative and found 2N5087 and 2SA970. I have the spice model for the first But not for the second for simulating the circuit before acual replacement (I tried to replace Q108 with 2N5087 in model but the readings was not right).

Any ideas regarding these replacement alternatives? what should I look for in the datasheet to get as close as possible.

Thanks.