AMPEG SVT PRO 7 switches OFF after 2 seconds - why ?

[Tilman]

Hi,

the SVT Pro 7 is a class D (digital) amplifier with a switch mode power supply.
To be honest : I have no idea how a class D amplifier works.
The amp behaves like this :

- switch power ON ; 5 seconds nothing happens
- the green ON LED lights ; the fan starts for two seconds
- OFF

I checked the fan supply voltage - 5V - OK (so the SMPS seems to work).
I checked the power MOSFETs FDP61N20 (very rare...) and they seem to be OK.

What else could I do or maybe someone had a similar problem with this amp before and knows where to look ?

Thanks!
Tilman

[Enzo]

The fan supply remains when it turns off?

SWitching power supplies usually have boocoo protection, and will shut down for most any problem. If you are saying the fan gets proper voltage for the short time teh SMPS is runnig, that is fine, but if the SMPS shuts down the amp won;t work.

I don't see the SVT7pro in my service disc or on my support web site.

Disconnect th powr amp or amps from the power supply. Now will the thing fire up and stay on? If so, you have a blown channel. If it still shuts down the SMPS, then I would check each output voltage it makes and see where that goes - or at least unplug them. In fact if the fan has its own plug to the SMPS, then I might disconnect all the loads and see if the fan stays alive that way.

If the SMPS shuts down no matter what, then I start looking for shorted rectifiers on the secondary side.

[Tilman]

I get your point, but unfortunately the SMPS and the power amp are on one big PCB. The only thing I can disconnect from the supply is the fan.
On top of all that - 60% of the components are SMD. Is that our future.......SMD components and class D amplifiers run with switch mode power supplies.....?
They will need no repair techs anymore. They just ship it back to China, throw in a new PCB and up it goes.....
Tilman

[Enzo]

I wouldn;t be surprised if the power rails pass through some jumper wires or even a convenient cutable trace on their way from SMPS to PA.

Where have you been? SMD is not the future, it is the now. Everything uses that. Oh it hasn't invaded a Hot Rod DeVille yet, but it is no longer up and coming technology, it is here. You want to be in the repair game these days, you need to be prepared to deal with the tiny stuff.

I bet the output transistors are not SMD, look on the heat sink. ANyone there come up shorted?

As a pro repair tech for most of the major brands, I can tell you quite honestly that NONE of them are telling us to swap out boards. We are expected to do component level repair on everything, right here in the field. Most makers these days do have a DO NOT REPAIR list, but that is about the cost of the item, not the way it is made. They would rather send out a new amp that cost them $20 than pay me $50 to fix it. To bring that concept home, how many people would pay you $10 to fix a $10 guitar cord?

Digital amps with SMPS is very efficient. That is how something like the PV IPR1600 can put out 1600 watts and still weigh less that 7 pounds.

[Tilman]

Hi Enzo,

"Where have you been?" I was born, raised and still live in east Germany (I travelled the world for several years though)!

"You want to be in the repair game these days, you need to be prepared to deal with the tiny stuff." You are right.

I checked the output MOSFETs. Although my component tester does not recognize them and starts to blink very strangely...I did tested their function as a switch and they work as they are supposed to.
There are no jumpers to disconnect the SMPS from the main amp. Since I will recommend the customer to send the thing back to the AMPEG distributor, I wont cut traces on the board.
In the future I will not take class D amplifiers in for repair since I have no clue what is going on inside them. As a not so high flyer in electronics - I should stay with conventional tube or solid state equipment. This one came in a box labeled "bass amplifier" - there was no way of knowing it would be a class D.

BTW: I wonder why AMPEG seem to have stopped to produce such amps ?! As far as I know, all the newer models are conventional amplifiers. Maybe it is because many of the old fashioned guys don't understand them ? Or is it a reliability matter ?

Thanks for your help!
Tilman

[Enzo]

If the outputs are not shorted, it can't hurt to check the secondary rectifiers in the SMPS. Unlike in a linear supply, a shorted rectifier in an SMPS usually just shuts it down.

[Tilman]

Mhm....if there is a shorted secondary rectifier...how could I get a 5V supply at the fan (even though is stays there just for two seconds before shut down) ?
However, I will check that on Monday.

[Enzo]

Because the power supply has the brute force to push current out. In fact even in your Fender Twin with a shorted power tube or B+ cap, in the second it takes for the main fuse to blow, the pilot light will blink on.

Those main power amp rails are capable of a LOT of current, so if a diode shorts across one, it may load the SMPS down and it will detect the overcurrent, but it can't utterly zero out the power supply. And come to think of it, a shorted rectifier doesn't ground out the supply, it usually either send what passes for AC in a SMPS to the filters and circuitry if it is half wave, or if it is full wave, it leaves its brother rectifier acrowss the winding.

[Tilman]

Hi,

I have checked the secondary diodes - they are OK. I also checked the primary caps, rectifier spike protector diodes and switching MOSFETs - all of them measure OK.
So this will be my second "give up" repair this year. The first on was the FENDER Passport 250 SMPS.

Enzo, thanks for your help!
Tilman