Q2, AP1, R10 and R11 form an adjustable zener diode with AP1 controlling the conduction of Q2 and therefore the voltage between the gates of the output MOSFETs. The closer together the gates are, voltage wise, the colder the bias and vice versa. Since Q2 is an NPN the more positive the base the more it conducts and vice versa. With the wiper of AP1 in the full CW position, as you look at it on the schematic, basically the wiper is connected to the base of Q2 the conduction is low and the further apart the gates are, and the hotter the bias. With the wiper in the fully CCW position the base is more positive , the more it conducts, the closer the gates are together and the colder the bias. So you need to start with AP1 in the highest resistance position.
As far as the proper setting Ampeg is usually pretty good about putting all of that info on the schematic somewhere as there were no service manuals that I know of. I was a warranty station for SLM long ago. If you need factory help/parts contact LOUD Technologies these days.
Craig
As far as the proper setting Ampeg is usually pretty good about putting all of that info on the schematic somewhere as there were no service manuals that I know of. I was a warranty station for SLM long ago. If you need factory help/parts contact LOUD Technologies these days.
Craig
Correct.
And just to make it 200% certain: START WITH THE BIAS PRESET AT MAXIMUM RESISTANCE which means MINIMUM bias. Funny, huh?
Check it with the amp unplugged and your multimeter.
Even safer: start with the amp without speaker and plugged into a lamp bulb limiter with a 100W lamp.
If it blinks and dims, fine; if it stays somewhat bright , check bias.
Unfortunately, the Schematic page posted does no explain the bias setup.
If somebody posts it, fine.
If not, you'll need 3 or 4 hands, or clip your multimeter with red to the top of either R32/34/36/38 0.47 ohm ballast resistors, and black to the bottom of either R33/35/37 or R39
Set it on the 200mV DC scale, it should show 0 mV or a *very* low value.
Then start moving the trimmer, slowly because it "jumps", until you read around 10mV .
That means that each MosFet is passing 10mA , which in absence of other info, is a reasonable value.
Good luck
And just to make it 200% certain: START WITH THE BIAS PRESET AT MAXIMUM RESISTANCE which means MINIMUM bias. Funny, huh?
Check it with the amp unplugged and your multimeter.
Even safer: start with the amp without speaker and plugged into a lamp bulb limiter with a 100W lamp.
If it blinks and dims, fine; if it stays somewhat bright , check bias.
Unfortunately, the Schematic page posted does no explain the bias setup.
If somebody posts it, fine.
If not, you'll need 3 or 4 hands, or clip your multimeter with red to the top of either R32/34/36/38 0.47 ohm ballast resistors, and black to the bottom of either R33/35/37 or R39
Set it on the 200mV DC scale, it should show 0 mV or a *very* low value.
Then start moving the trimmer, slowly because it "jumps", until you read around 10mV .
That means that each MosFet is passing 10mA , which in absence of other info, is a reasonable value.
Good luck
Hi.
Thank you for clearing this one up.
Max value on the trim is where I start.
I have the initial schematic for ampeg SVTIII poweramp. and it has some voltage values. I will upload it when I am at my computer. It turned out that I could not get thouse 0.47 resistors today so I ordered them and they will probably arrive next year (like waiting the whole year) So I will be back with results.
Though on question.
Once I start to look for that sweet spot. What exactly am I after? Bias is needed to remove crossover distortion.
But what indicates the sweetspot?
Thank you for clearing this one up.
Max value on the trim is where I start.
I have the initial schematic for ampeg SVTIII poweramp. and it has some voltage values. I will upload it when I am at my computer. It turned out that I could not get thouse 0.47 resistors today so I ordered them and they will probably arrive next year (like waiting the whole year) So I will be back with results.
Though on question.
Once I start to look for that sweet spot. What exactly am I after? Bias is needed to remove crossover distortion.
But what indicates the sweetspot?
Your ears are not acurate enough (mine neither 
) for a precise adjustment without help.
*Best* would be to drive some sinewave (say 1000 or 400 Hz ) into a 4 ohm load, connect an oscilloscope, and you will clearly see crossover distortion as a "kink" or bend at the base of the sinewave.
Quite unmistakeable.
Then slowly raise the bias pot setting (in this case lower the resistance value until you *start* to see the distortion "melt".
A little more and it disappears.
That would be the "sweet spot", high enough to dissapear; still not *that* high that it overheats the transistors.
But setting them to a measurable yet low bias current (10 or 20 mA per transistor) is a very reasonable value.
Previously you must have set it to, who knows? , maybe 200 to 500 mA, way too much.
Oh well, now it's repaired, congratulations :thumbsup:
) for a precise adjustment without help.*Best* would be to drive some sinewave (say 1000 or 400 Hz ) into a 4 ohm load, connect an oscilloscope, and you will clearly see crossover distortion as a "kink" or bend at the base of the sinewave.
Quite unmistakeable.
Then slowly raise the bias pot setting (in this case lower the resistance value until you *start* to see the distortion "melt".
A little more and it disappears.
That would be the "sweet spot", high enough to dissapear; still not *that* high that it overheats the transistors.
But setting them to a measurable yet low bias current (10 or 20 mA per transistor) is a very reasonable value.
Previously you must have set it to, who knows? , maybe 200 to 500 mA, way too much.
Oh well, now it's repaired, congratulations :thumbsup:
sounds like what I have seen way too many times with these amps. you had stressed gates on some of the mosfets and adjusting the bias changed it just enough to make it go away, then when cold too much bias and it broke into oscillation and poof.
These amps are very unstable. which they shouldn't be. when I first power one of these up i do it a bit differently then most amps. i turn the bias pot to the middle instead of all the way down. then watch the scope and the line current drawn and bring the variac up slow. once above 90v quickly adjust the bias down if needed keeping an eye on the scope at all times.
Once running, I will let the amp warm up for an hour and recheck the bias. then shut the amp off and let it cool at least 2 hours and power it up and watch the bias and the output for oscillations. while still cold I will lower the variac down to about 100v and watch for oscillation. then run it at 1/3rd power for 5-10 minutes and watch the heatsink temp. again watching the scope all the time. then when at 1/3rd power I will lower the variac again to 100v and watch the scope. it's overkill but that's the only way to be sure.
Unlike a home audio amp, PA and bass guitar amps get the crap beat out of em! especially small or underpowered amps. they get pushed past their limits and fail. bass guitar amps really need a lot of power to play onstage. and these little 200 watt @8 ohm amps just take a pounding! the weak little VI limiters just don't stand a chance. inevitably a peak or 10 makes it past the limiter and the gates get stressed then fail.
Zc
P.S I need a power transformer for a SVT3 if anyone has one.
These amps are very unstable. which they shouldn't be. when I first power one of these up i do it a bit differently then most amps. i turn the bias pot to the middle instead of all the way down. then watch the scope and the line current drawn and bring the variac up slow. once above 90v quickly adjust the bias down if needed keeping an eye on the scope at all times.
Once running, I will let the amp warm up for an hour and recheck the bias. then shut the amp off and let it cool at least 2 hours and power it up and watch the bias and the output for oscillations. while still cold I will lower the variac down to about 100v and watch for oscillation. then run it at 1/3rd power for 5-10 minutes and watch the heatsink temp. again watching the scope all the time. then when at 1/3rd power I will lower the variac again to 100v and watch the scope. it's overkill but that's the only way to be sure.
Unlike a home audio amp, PA and bass guitar amps get the crap beat out of em! especially small or underpowered amps. they get pushed past their limits and fail. bass guitar amps really need a lot of power to play onstage. and these little 200 watt @8 ohm amps just take a pounding! the weak little VI limiters just don't stand a chance. inevitably a peak or 10 makes it past the limiter and the gates get stressed then fail.
Zc
P.S I need a power transformer for a SVT3 if anyone has one.
Hi.
So the lightbulb tester is a keeper.
But what bothers me more is that I see -38 DCV on the output (DVM connected to signal(red) and ground(black)) without speaker connected.
And if I turn the Bias pot, it will go lower.
The Bias pot works as it should. If resistance is max, LightBulb goes darker, if I turn the pot smaller it will go brighter.
So no issues on how the mosfets react.
But DCV is leaking into output. And its coming from the IRFP9240 side.
So the lightbulb tester is a keeper.
But what bothers me more is that I see -38 DCV on the output (DVM connected to signal(red) and ground(black)) without speaker connected.
And if I turn the Bias pot, it will go lower.
The Bias pot works as it should. If resistance is max, LightBulb goes darker, if I turn the pot smaller it will go brighter.
So no issues on how the mosfets react.
But DCV is leaking into output. And its coming from the IRFP9240 side.
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Some resistors that were measured and were good.
This transistor Q4 is the only unknown link. I will take more readings.
Image:
Ampeg SVTIII | Flickr - Photo Sharing!
This transistor Q4 is the only unknown link. I will take more readings.
Image:
Ampeg SVTIII | Flickr - Photo Sharing!
I also found this
ampage > Music Electronics > Guitar Amps > Repair & Maintenance : 1 : Ampeg SVT 3 Pro output has DC
Its a webcache, but there is a very similar problem with the same amp.
All the parts he already checked didn't lead to victory. I will see what he has done.
I am having problems to find the correct forum, my browser says no connection for this link. So I can't see if it continues or not.
So there he looked over all the Zener diodes. C6 C7; Removed AutoBias circuit.Removed Q3 and Q4 function. But maybe it would not give a result if Q4 would be broken. I will get a new Q4 tomorrow to see what happens.
This voltage is not coming from Feedback. Because between R13 and TL074 the voltage is -15VDC
I am starting to think that thouse isolators between MOSFETS and heater are conducting. I will change them tomorrow.
ampage > Music Electronics > Guitar Amps > Repair & Maintenance : 1 : Ampeg SVT 3 Pro output has DC
Its a webcache, but there is a very similar problem with the same amp.
All the parts he already checked didn't lead to victory. I will see what he has done.
I am having problems to find the correct forum, my browser says no connection for this link. So I can't see if it continues or not.
So there he looked over all the Zener diodes. C6 C7; Removed AutoBias circuit.Removed Q3 and Q4 function. But maybe it would not give a result if Q4 would be broken. I will get a new Q4 tomorrow to see what happens.
This voltage is not coming from Feedback. Because between R13 and TL074 the voltage is -15VDC
I am starting to think that thouse isolators between MOSFETS and heater are conducting. I will change them tomorrow.
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Please don't shotgun it, introducing more unknowns.
Back to the bulb limiter for now, no speaker, no audio in.
Measure and post:
1) Output rail voltage.
2) + and -V rails.
3) bias voltage, measured from Q2 emitter to top of R10.
Give me 2 values: with trimmer AP1 on max. (10K) and when set so bulb limiter starts becoming shinier (meaning output MOS are starting to turn on).
Thanks.
Back to the bulb limiter for now, no speaker, no audio in.
Measure and post:
1) Output rail voltage.
2) + and -V rails.
3) bias voltage, measured from Q2 emitter to top of R10.
Give me 2 values: with trimmer AP1 on max. (10K) and when set so bulb limiter starts becoming shinier (meaning output MOS are starting to turn on).
Thanks.
Hi.
So here is what i found.
Measured with DVM. Black to ground. Red to meas.point
There is -42,3VDC on the neg side of D7.
-41,3 on the pos side of D7
-39,7 on the pos side of D5
-37,5 on the neg side of D5
I will measure the points you asked in an hour. This was what I got yesterday evening.
So here is what i found.
Measured with DVM. Black to ground. Red to meas.point
There is -42,3VDC on the neg side of D7.
-41,3 on the pos side of D7
-39,7 on the pos side of D5
-37,5 on the neg side of D5
I will measure the points you asked in an hour. This was what I got yesterday evening.
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So I don't know if this is what you asked for. (this doesnt replay do your final post but previous)
Bias pot set to 0 resistance: low bias
1)-38,3V
2)-95,6V
3)3,4V
Bias pot set to as high as it allowed (amp tuns off if it is turned pass this point, the bulb will glow a bit more.
1)-33,6
2)90,3
3)3,8v
Bias pot set to 0 resistance: low bias
1)-38,3V
2)-95,6V
3)3,4V
Bias pot set to as high as it allowed (amp tuns off if it is turned pass this point, the bulb will glow a bit more.
1)-33,6
2)90,3
3)3,8v
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1) Output rail voltage. 1)-38,3V to -33,6 Fine, about what you posted earlier.
2) + and -V rails. 2)-95,6V to 90,3 Don't understand. +/-V rail schematic values are +65V and - 65V , I expected somewhat less because of the bulb limiter. You provide a single value, too high, the first time with a negative value, then a positive one. I'm baffled.
3) bias voltage, measured from Q2 emitter to top of R10. 3)3,4V to 3,8v Well, that's reasonable.
Now, I have a problem with:
Please check it.
2) + and -V rails. 2)-95,6V to 90,3 Don't understand. +/-V rail schematic values are +65V and - 65V , I expected somewhat less because of the bulb limiter. You provide a single value, too high, the first time with a negative value, then a positive one. I'm baffled.
3) bias voltage, measured from Q2 emitter to top of R10. 3)3,4V to 3,8v Well, that's reasonable.
Now, I have a problem with:
because it's the exact opposite .
Please check it.
Hi.
Yes you are correct. My mistake.
I did the measuring like this:
Bias pot set to max resistance: low bias
1)-38,3V
2)-95,6V (it was measured from -65V to +65V)
3)3,4V
Bias pot set as low as it allowed (amp tuns off if it is turned pass this point, the bulb started to glow a bit more.
1)-33,6
2)90,3 (it was measured from +65 to -65)
3)3,8v
Step2
R18 and R12 lifted.
Changed Q4, Q2; Q5
And measured again (it can be that bias pot is not set exactly like the previous measurement. I will try to get as close as I can.
Max Resistance
1)-23.1
2) -39,2 +39,6
3)3,4
Bias.
1)-14.0
2)-32,0 +32,0
3)3,5
Yes you are correct. My mistake.
I did the measuring like this:
Bias pot set to max resistance: low bias
1)-38,3V
2)-95,6V (it was measured from -65V to +65V)
3)3,4V
Bias pot set as low as it allowed (amp tuns off if it is turned pass this point, the bulb started to glow a bit more.
1)-33,6
2)90,3 (it was measured from +65 to -65)
3)3,8v
Step2
R18 and R12 lifted.
Changed Q4, Q2; Q5
And measured again (it can be that bias pot is not set exactly like the previous measurement. I will try to get as close as I can.
Max Resistance
1)-23.1
2) -39,2 +39,6
3)3,4
Bias.
1)-14.0
2)-32,0 +32,0
3)3,5
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