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Old 28th August 2014, 12:38 AM   #1
Fedess is offline Fedess  Argentina
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Default Can't find the problem!! :(

Hi everybody,

I made my amplifier years ago, it was my first one and worked fine for many years. Built two units to make it stereo.

Now I am modifying the system, making it bi-amped.

The thing is that I accidently burned the power transistors of the stage (re adjusting bias... my bad...) so I replaced them.

I connect the amp with 22ohm 5 W series resistors, adjust DC offset, adjust bias current; remove the series resistors, re-adjust DC offset, re-adjust bias.

I set bias current to about 120mA; and leave the amp connected for few minutes and it's stable.

Then I connect the speakers and input, and after a few minutes of testing it, the power transistors blow...

When this happens the other channel keeps working fine, with the same configuration...

The power supply is +/- 35V

The amp makes no strange noises when it stops working, just the same "pop" noise as if I disconnected it, but now the 2A fuses and IRF540 are blown.

I re-checked all the circuit, replaced the power transistors like 4 or 5 times, and also replaced all the other transistors, some diodes and capacitors, and the 0,27ohm resistors,

It stills blowing and I cannot find the problem!!

Any suggestion?

edit: this is the amp circuit,

http://www.pcpaudio.com/pcpfiles/pro...ic/publics.pdf
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Old 28th August 2014, 02:07 AM   #2
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1. Check every resistor with a DMM something may be off in value.
2. Check/replace every diode.
3. Where did you get your power transistors, there are fakes running around. Make sure it's a reputable source.
4. Is there a bad/poor connection on a part pad after all the soldering de-soldering it might be a poor connection.
5. Do you have an oscilloscope? It's possible parts have changed over time you should check for high frequency oscillation.
6. Run the amp with a variac and a light bulb (incandescent, 100W in series..) that should help you ramp up voltage and limit current preventing destroyed devices.
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Old 28th August 2014, 03:27 AM   #3
djoffe is offline djoffe  United States
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One part of the design is a little scary, especially if the bias pot is old. If the wiper opens up intermittently, perhaps due to contamination and age of the pot, then the bias spreader voltage gets high enough to pop the output transistors and fuses.

In many cases, the bias pot is always configured so that if the wiper opens, the amp is under-biased, not over biased. You should probably put in a new bias pot AND make that change in your circuit.

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Old 28th August 2014, 04:16 AM   #4
Fedess is offline Fedess  Argentina
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Quote:
Originally Posted by SpeakerScott View Post
1. Check every resistor with a DMM something may be off in value.
2. Check/replace every diode.
3. Where did you get your power transistors, there are fakes running around. Make sure it's a reputable source.
4. Is there a bad/poor connection on a part pad after all the soldering de-soldering it might be a poor connection.
5. Do you have an oscilloscope? It's possible parts have changed over time you should check for high frequency oscillation.
6. Run the amp with a variac and a light bulb (incandescent, 100W in series..) that should help you ramp up voltage and limit current preventing destroyed devices.
1. Checked, will do it again.
2. replaced the 2 that go next to the power transistors
3. Two different sources, both failed...
4. Checked that but it's always good to re-check
5. I dont have an oscilloscope, but replaced all transistors and some caps (almost the entire amp is new!)
6. Good tip!, I'm tired of destroying devices...

thanks!
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Old 28th August 2014, 04:20 AM   #5
Fedess is offline Fedess  Argentina
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Quote:
Originally Posted by djoffe View Post
One part of the design is a little scary, especially if the bias pot is old. If the wiper opens up intermittently, perhaps due to contamination and age of the pot, then the bias spreader voltage gets high enough to pop the output transistors and fuses.

In many cases, the bias pot is always configured so that if the wiper opens, the amp is under-biased, not over biased. You should probably put in a new bias pot AND make that change in your circuit.

Update My Dynaco

Akitika GT-101 Audio Power Amplifier Kit

That's really interesting... I'm not sure how to do that, will do further reading then, and consider it for next building!

By the moment I can try replacing the pot... this gave me hope!

thank you
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Old 28th August 2014, 05:19 AM   #6
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Quote:
Originally Posted by Fedess View Post

Now I am modifying the system, making it bi-amped.
What exactly did you change? Did you burn out the originals right after the modification? Are any other transistors or other components getting damaged before the fuse pops and if so, which ones?

Last edited by shredhead; 28th August 2014 at 05:23 AM.
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Old 28th August 2014, 06:38 PM   #7
sbrads is offline sbrads  United Kingdom
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Quote:
Originally Posted by Fedess View Post
That's really interesting... I'm not sure how to do that, will do further reading then, and consider it for next building!

By the moment I can try replacing the pot... this gave me hope!

thank you
Use a fixed resistor from base to collector, and the pot used as a variable resistor from base to emitter.
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Old 28th August 2014, 11:16 PM   #8
Fedess is offline Fedess  Argentina
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Quote:
Originally Posted by shredhead View Post
What exactly did you change? Did you burn out the originals right after the modification? Are any other transistors or other components getting damaged before the fuse pops and if so, which ones?
No, I just wanted to re-adjust bias and DC offset because I didn't touch it since I finished the amp few years ago. I accidentally turned too much the bias trimpot and burned the transistors (the fuses blowed immediatly but only protected the rest of the circuit, because power transistors got burnt...)

System modification is not done yet, I still have to re wire the speakers. And I couldn't finish it because this amp keep blowing...

Today I'm changing the trimpot and power transistors again, and hope this time it gets working fine
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Old 28th August 2014, 11:25 PM   #9
Fedess is offline Fedess  Argentina
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Quote:
Originally Posted by sbrads View Post
Use a fixed resistor from base to collector, and the pot used as a variable resistor from base to emitter.
Thank you, seems I can do that easily using the same PCB...

The pot in this case is 1K; should I replace it with a lower value and compensate with the fixed resistor (so both get 1K), or just add a fixed resistor as you say? (this will get more than 1k ohms resistance... I think it wont be a significative change, I am right?)
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Old 28th August 2014, 11:39 PM   #10
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I would suspect oscillation blowing output transistors.
There is usually B-C capacitor on the VAS on most amplifiers.
Without this the VAS is too fast and the output transistors oscillate.
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