Here they are.
First Bias set cold
ColdBias_svt | Flickr - Photo Sharing!
Second, bias pot half way
HotBias_svt | Flickr - Photo Sharing!
EDIT:
I will get a scope tomorrow.
What kind of probes would you suggest to use here?
First Bias set cold
ColdBias_svt | Flickr - Photo Sharing!
Second, bias pot half way
HotBias_svt | Flickr - Photo Sharing!
EDIT:
I will get a scope tomorrow.
What kind of probes would you suggest to use here?
Last edited:
If the negative side is breaking into oscillation it will draw higher current then the positive side. a scope will help you visualize the problem!
Just for conclusion I will post a picture showing parts that were replaced.
ReplacedSVT | Flickr - Photo Sharing!
Oh and I also replaced Bias Trim pot.
ReplacedSVT | Flickr - Photo Sharing!
Oh and I also replaced Bias Trim pot.
If I look at the schematic with your markings of the changed parts; I would suggest the following:
1. Take a clean schematic.
2. Mark al the parts with a fluo pen (eg red) you replaced and are KNOWN to be good.
3. Then start to measure (or remove and measure if you are insure) from the output towards the input of the power amp and mark them green if they are OK or Red if they are bad and need replacement.
Do not try to write it down! Mark them in color!
4. Continue until you encounter a node (where the components touch each other) where all components are green.
There you stop end continue with another part of the schematic.
5. When all redmarked parts are surrounded by green marked parts; the chance that the amp, component wise, is OK is very great.
6. Come back with an annotated schematic with the marked parts in green and red.
Wait for conclusions by severall members before swithing your amp back on.
7. Patience is the message right now...
1. Take a clean schematic.
2. Mark al the parts with a fluo pen (eg red) you replaced and are KNOWN to be good.
3. Then start to measure (or remove and measure if you are insure) from the output towards the input of the power amp and mark them green if they are OK or Red if they are bad and need replacement.
Do not try to write it down! Mark them in color!
4. Continue until you encounter a node (where the components touch each other) where all components are green.
There you stop end continue with another part of the schematic.
5. When all redmarked parts are surrounded by green marked parts; the chance that the amp, component wise, is OK is very great.
6. Come back with an annotated schematic with the marked parts in green and red.
Wait for conclusions by severall members before swithing your amp back on.
7. Patience is the message right now...
Hi.
So I did it.
Found that D6 or D7 was bad, replaced both of them.
There is only Q7 and Q11 in the schematic others are moved to minimize possible MOSFET failure.
After replacing D6 and D7 I got the rail Zero and it alsa was biasable, so if I turned the bias pot I got no voltage on output.
Also checked all the other diodes and caps and resistors to make sure. This was my only find.
Can previously described imbalance be caused by D7 or D6?
So I did it.
Found that D6 or D7 was bad, replaced both of them.
There is only Q7 and Q11 in the schematic others are moved to minimize possible MOSFET failure.
After replacing D6 and D7 I got the rail Zero and it alsa was biasable, so if I turned the bias pot I got no voltage on output.
Also checked all the other diodes and caps and resistors to make sure. This was my only find.
Can previously described imbalance be caused by D7 or D6?
Glad there is some progress.
BUT, BEFORE applying power...
Check the components as asked.
Then come back with the markings on the schematic in red and green .
Other members will comment and if it's safe according to them, apply power with a light bulb in series with the ac and check if there is no short or exsesive current taken.
Only then apply the full ac to the amp.
Off course no load applied...
Patience is half of the repair in your case.
You were lucky that nothing burned up.
So it's not to late to show us that annotated schematic in color.
Cheers
BUT, BEFORE applying power...
Check the components as asked.
Then come back with the markings on the schematic in red and green .
Other members will comment and if it's safe according to them, apply power with a light bulb in series with the ac and check if there is no short or exsesive current taken.
Only then apply the full ac to the amp.
Off course no load applied...
Patience is half of the repair in your case.
You were lucky that nothing burned up.
So it's not to late to show us that annotated schematic in color.
Cheers
If D6/7 are short circuit, the impedance matching circuit will not work on the negative side of the wave form as there will be no/poor drive to the lower half of the complimentary output stage.
There is no trace of any MosFets being "Stressed" on their respective gates. They either work or they don't. They can read short circuit if the gates are not discharged before measuring them. A common error within technicians that have not got the required experience.
Be very careful, the output stage can destroy itself!
There is no trace of any MosFets being "Stressed" on their respective gates. They either work or they don't. They can read short circuit if the gates are not discharged before measuring them. A common error within technicians that have not got the required experience.
Be very careful, the output stage can destroy itself!
I also found that it needs paired transistors for output.
Do you have any idea where to get them?
I need
4xIRFP240
4xIRFP9240
And also it is said that Gate resistors should be matched to MOSFET.
It really starts to grow over my head as there is a lot to keep in mind. I thinks its time to look for someone who can do it.
Do you have any idea where to get them?
I need
4xIRFP240
4xIRFP9240
And also it is said that Gate resistors should be matched to MOSFET.
It really starts to grow over my head as there is a lot to keep in mind. I thinks its time to look for someone who can do it.
Hi, it really depends where you live as to the availability of these MosFet transistors.
If you are in the UK, they are readily available through many wholesalers. if you have an account and are fairly reasonably priced.
I can supply them but will not offer any warranty unless they are fitted and set up by an Ampeg Trained Technician, as the power stage is tricky to set up and easily damaged.
I repair these amplifiers and will be willing to take a look for you if required.
See my web site
Welcome to my Web site
If you are in the UK, they are readily available through many wholesalers. if you have an account and are fairly reasonably priced.
I can supply them but will not offer any warranty unless they are fitted and set up by an Ampeg Trained Technician, as the power stage is tricky to set up and easily damaged.
I repair these amplifiers and will be willing to take a look for you if required.
See my web site
Welcome to my Web site
For anyone that has a bad SVT 3pro transformer I _MAY_ have a solution for all of us!!!! I have been talking to Antek about building a custom transformer for this. They have a stock model that has the correct High current winding but no High Voltage winding. They can build a special version of this for a very very reasonable price! a rough guestimate of price is about $70ea but they would need to make 50 of them for it to work out. I might be able to convince them to do it if i can get enough interest to get close to half right off the bat. I would probably buy 2 so that leaves 23 more I would need to find buyers for. If anyone would be interested let me know!! speak now!
Zc
Zc
Hello
I had a problem on my SVT and after I've read this whole thread, I've changed all the mosfets with new and matched one, substituted all the ballast resistors.
Now the fuse doesn't blow no more, but turning the bias trimmer (AP1) doesn't change anything.
The DVM (on the 200mv scale) across the 0,47 resistors remain at zero.
I've checked all the resistors and transistors involved in the bias circuit and they all tested good.
Do you have any idea that explain this behavior?
Greetings From Italy
Stefano
I had a problem on my SVT and after I've read this whole thread, I've changed all the mosfets with new and matched one, substituted all the ballast resistors.
Now the fuse doesn't blow no more, but turning the bias trimmer (AP1) doesn't change anything.
The DVM (on the 200mv scale) across the 0,47 resistors remain at zero.
I've checked all the resistors and transistors involved in the bias circuit and they all tested good.
Do you have any idea that explain this behavior?
Greetings From Italy
Stefano
Hi Stefano.
Can you post and mark the voltages like I have done?
svtStep3 | Flickr - Photo Sharing!
That way you can see if the supply voltage is in balance and we can spot a problem for you.
I am now pretty good at this by the way
I could fix 10 of thouse things now.
After I fixed mine I also fixed my friends amp.
You just have to be very careful and make alot of measurements and mark things down. From there it gets easy.
Can you post and mark the voltages like I have done?
svtStep3 | Flickr - Photo Sharing!
That way you can see if the supply voltage is in balance and we can spot a problem for you.
I am now pretty good at this by the way
I could fix 10 of thouse things now.
After I fixed mine I also fixed my friends amp.
You just have to be very careful and make alot of measurements and mark things down. From there it gets easy.
Thank You very much for your help.
I've marked down the voltages on the schematic.
Measurements were taken at the following condition:
DVM at ground reference with no load and no current (lamp) limiter, bias trimmer (AP1) turned fully CCW.
I've marked down the voltages on the schematic.
Measurements were taken at the following condition:
DVM at ground reference with no load and no current (lamp) limiter, bias trimmer (AP1) turned fully CCW.
An externally hosted image should be here but it was not working when we last tested it.
Uploaded with ImageShack.com
Last edited:
Hello Served
I think I've solved the problem.
I feel stupid now, but I was frightened about the description of that sensible trimmer and I was not turning it far enough in the CW direction.
Now I've been able to set all the mosfet at right bias.
I will post another schematic with all voltages and mosfet at bias for future reference.
In the meantime here are all the voltage across 0,47 ballast resistors:
IRFP240 IRFP9240
24,3mV 25,2mV
26,3mV 25,1mV
24,3mV 25,4mV
23,0mV 23,6mV
What do you think about this? I guess they're in the ballpark, aren't they?
However that trimpot is a nightmare, I'm thinking to replace it with a 20 turns pot for more precise biasing.
I think I've solved the problem.
I feel stupid now, but I was frightened about the description of that sensible trimmer and I was not turning it far enough in the CW direction.
Now I've been able to set all the mosfet at right bias.
I will post another schematic with all voltages and mosfet at bias for future reference.
In the meantime here are all the voltage across 0,47 ballast resistors:
IRFP240 IRFP9240
24,3mV 25,2mV
26,3mV 25,1mV
24,3mV 25,4mV
23,0mV 23,6mV
What do you think about this? I guess they're in the ballpark, aren't they?
However that trimpot is a nightmare, I'm thinking to replace it with a 20 turns pot for more precise biasing.
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