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Old 26th January 2013, 09:12 PM   #71
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JonSnell Electronic's Avatar
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
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!
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Old 28th January 2013, 10:50 AM   #72
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I also found that it needs paired transistors for output.

Do you have any idea where to get them?

I need
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.
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Old 28th January 2013, 11:15 AM   #73
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JonSnell Electronic's Avatar
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Location: The Jurassic Coast, England. GB
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
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Old 28th January 2013, 04:10 PM   #74
llwhtt is offline llwhtt  United States
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Location: SoCal
Check with "TechDIY", I bought those exact parts from him matched for SUMO Andromeda II I repaired.

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Old 4th March 2013, 11:36 PM   #75
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Location: MN
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!

It's not about what you can BUY...It's about what you can BUILD!
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Old 17th September 2013, 04:11 PM   #76
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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
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Old 18th September 2013, 06:27 AM   #77
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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.
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Old 18th September 2013, 09:04 AM   #78
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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.

Click the image to open in full size.Uploaded with

Last edited by fuzzman72; 18th September 2013 at 09:31 AM.
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Old 18th September 2013, 09:44 AM   #79
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Did you check R29 to R31 and R40 to R43?

Show us voltages between D4-D7
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Old 18th September 2013, 10:13 AM   #80
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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.
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