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Old 21st December 2012, 08:16 AM   #11
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Hi.


So I solved that bias issue.
But the problem doesn't end here.
Sound cleaned up, but now the amp burned down.

It was probably because it was too cold for use. We have a winter time here and its really cold outside and after the amp was transported it was directly powered on. ( can't stop blameing myself for letting this to happen) But I can't do anything about it now.

So what I found was that R27 and Q6 was burned.
What is the function of them and what could be the result or cause?
Have a look at the schematicClick the image to open in full size.
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Old 21st December 2012, 04:42 PM   #12
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Cold weather could make old dried up electrolytic caps act up, but you said you already tested those on the rail voltages. Cold could also make a big transistor contract on the heat sink and touch the loose mount screw or find the bad part of the heat sink insulator. Cold could cause any bad solder joint to open up.
Looks to me like Q6 and R27 are part of the DC bias current network. AP1 and Q2 set the bias voltage, then Q5 and Q6 amplify it in current I think. Or maybe limit how big bias current gets. Are you sure you don't have DC voltage coming out on the speaker terminals? if an output transistor blows, It may put +-65 VDC on these low voltage rated interior parts through the blown gate and the little parts blow up. Doen't mean the big transistors aren't hosed. You check a FET in high humidity (or with a grounded wrist strap) by alligator clip lead connecting gate to source, then seeing if the disabled fet will hold of a significant voltage of the right polarity for it's position. Like Q7-10 want +voltage on the drain. For OT test I use 17 VDC out of a capacitor on a car battery charger, then through a 47k resistor so I don't blow up anything, then through the 200 ma scale of my DVM. You want the minus of the battery charger on the FET source on Q7-10. Opposite on the opposite polarity ones. . Not much current should flow on a good FET. The 2 VDC out of the ohms test of the meter was not enough to predict OT's that were going to explode when I put rail voltage on them.
If any OT is bad, all the parts on this board should be individually tested. At least that is what I did on my amp.Diodes were blown or shorted across, transistors were blown or shorted across, resistors were vaporized or opened up, a couple of PCB traces were vaporized. Capacitor checking was a bit of a trial. I had 50V rated ceramic capacitors on the driver board that were opened up by the 85 v rail voltage. If it had vaporized them they would have been easier to spot. My PV1.3k amp had a bad solder joint on the input to an op amp that would cause the amp to occasionally whang the B channel output to +170 VDC. The previous repair man handled it by pulling all the power supply connections to the B channel, leaving the output transistors bad, and putting a label on it "do not use B channel". My repair wasn't economic, took me a long time, but it is permanent.
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Last edited by indianajo; 21st December 2012 at 04:57 PM.
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Old 21st December 2012, 05:10 PM   #13
llwhtt is offline llwhtt  United States
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Q6 and R27 are part of the VI limiter circuit along with the ones on the positive side. They monitor the voltage across the source resistors and limit the drive to the output gates. You can remove them for troubleshooting purposes and the amp should run fine without them. Don't forget to replace them after you have the other problems figured out.

Craig
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Old 23rd December 2012, 08:57 AM   #14
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Thank you! I will digg into this after the christmas time!
So I will test the FET and investigate what is going around it there.

Merry Christmas to you guys! I will come back after I have oppened all my presents.
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Old 23rd December 2012, 11:29 AM   #15
JMFahey is offline JMFahey  Argentina
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Quote:
So I solved that bias issue.
But the problem doesn't end here.
Sound cleaned up, but now the amp burned down.
I wouldn't call it "I solved" anything.
You turned a $1000 amplifier into a doorstop.
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Old 23rd December 2012, 01:04 PM   #16
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Well you dont have to call it at all, Its my amp after all.

And there is nothing that couldnt be fixed.
I didnt turn it into a doorstop because I am going to use it as an amp after I have fixed it.
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Old 23rd December 2012, 04:06 PM   #17
JMFahey is offline JMFahey  Argentina
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Fine with me, but now you'll have to troubleshoot it yourself or have somebody do it.
Start by building a lamp bulb current limiter, you'll need it.
Then you'll have to start measuring , visual clues do not help much.
Here's the schematic, converted to .gif to avoid downloading time and again from 2 shared or whatever.
PS: unfortunately the bias circuit design is not foolproof, Q2 *can* be shorted base to emitter (they should have added a resistor in series with the pot) biasing output transistors into death.
Attached Images
File Type: gif Ampeg-SVT3Pro-BW.gif (81.8 KB, 165 views)

Last edited by JMFahey; 23rd December 2012 at 04:10 PM.
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Old 29th December 2012, 03:12 PM   #18
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Hi.

So I digged into this.
All the Mosfets exept one IRFP240 in the Power Amp section are gone.
I built a test circuit for them and run them with 12V PSU. All of them were shorted exept one. I guess this one just survived for no reason.


Also all the current limiting 5W 0.47 ohm power resistors exept R32 (for the MOsfet that survived) are gone.

So I will buy new Mosfets and at first run the amp only with one pair of mosfets for testing.

Is there a reason that makes sense why this could have happened? Can extreme cold startup cause that (-15 Celsius)?

The Bias I did for this amp was only AP1 Trim, this only evens the PP sholders and not the power. So I guess that if there is something seriosly bad with this amp or its just bad luck.

Last edited by served; 29th December 2012 at 03:20 PM.
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Old 29th December 2012, 05:03 PM   #19
JMFahey is offline JMFahey  Argentina
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Ouch !!!
No, I wouldn't blame cold for it.
You simply biased it too hot, (literally), maybe didn't notice it because you did not have it on long enough (there is a lot of thermal mass inertia, after all) but under actual use it overheated to death.
I would replace *also* the surviving MosFet and ballast resistor, they have been certainly overheated and can't be trusted.
Consider all semiconductors to the right of C6/C7 suspect, including the bias transistor.
Not necessarily replace them "just in case", I try to mess as little as possible with PCBs, but have a couple spares handy.
They are cheap parts anyway.
The trimpot is also suspect, I'd replace it (and start by setting it to max value).
And ... good luck, the amp is repairable, just go slow and steady
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Old 30th December 2012, 02:32 PM   #20
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Hi.

I will change them all in this case.

Could you give me a little explanation of how this Bias trimpot actually works?
I think I have mis understood the function of this Trimpot.

I thought that what it does is regulates the input signal to keep it between the limits for Mosfets. But there is something else that I dont understand. For example. What impact it has on the current that all the mosfets are conducting?

I will try to simulate the schematic and read some stuff to educate myself.

EDIT: Something I already found out is that the distortion that I heard before was crossover distortion not Clipping distortion.

And what probably happened was that my biasing left all the MOSFETS on. Even if there was no signal.
So just to say it out lout. Biasing is needed to add Gate voltage just so that MOSFET is exactly on the point of turing on. That way there is no crossover distortion (read: crossover Dist is reduced).

So I ruined it, a lesson learned.

Now the main question.
How to get the bias set correctly? As I understand correctly then starting point should be TRIM set to 0 ohms.
Should I find out the point where all the Fets open up (without any signal) and then turn it back a little?
I understand that it would be more precise if I had a signal generator and a scope, but I don't have a scope so I can't see what is going on there. But I have a good DMM.

Last edited by served; 30th December 2012 at 02:51 PM.
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