Zen V4 smoke....

[rebojorge]

Hi everyone....

I have a Zen V4 that I have been using for over a year now. I worked on the amp for quite some time until I had it sounding... then I left it that way with the guts all over and a few days ago something shorted on one channel and R0, and R1 went up in smoke.

So I replaced them, connected the channel to the variac and slowly turned it up... but I did not get very far because when the output of the variac is around 30V ac the voltage across R0 is already .7 Volts .... I replaced Q3 since it gives the .66 Volt junction to R0 and R1, but I got the same result.

What could be causing this?

Also I need to check if Q1 and Q2 were damaged but I dont know how to do so.... any suggestions?

All help will be apreciated... thanks

 

[GRollins]

Sounds like you lost a MOSFET. The same procedure that Nelson gives to match devices will work quite well as a test of whether they're functional. He put essentially the same thing in several papers: A-75 and the one about testing MOSFETs (although you don't need to do the full MOSFET test circuit to see if it still works) come to mind, but I believe it's in a couple of others as well.

Grey

 

[rebojorge]

thanks Gray.... I will check that and report back.

 

[Andersonix]

Be careful to electrically isolate the transistors from the heatsink. What happened to you sounds just like what happened once to my Z1 when I was negligent. I guarantee you that will never happen again, as I now measure the resistance between middle pin and heatsink EVERY time.

 

[apassgear]

Hi Jorge,

Sad to hear you had this problem. I think the advise you have received already will solve the amp trouble.

Are you using a fuse on the trafo primary?

Hope you deal soon with this and let us know if we can be of further help.

 

[Nelson Pass]

So I replaced them, connected the channel to the variac and slowly turned it up... but I did not get very far because when the output of the variac is around 30V ac the voltage across R0 is already .7 Volts .... I replaced Q3 since it gives the .66 Volt junction to R0 and R1, but I got the same result.

What could be causing this?

The value across R0 is regulated and will come up to .7 V or so
quite quickly, even at low AC line. Does it go beyond .7V?

 

[Blues]

If Q1 is gone, so is Q2 and mostly likely Q3. Without power to the amp check continuity of Drain to Source...a short (~0 Ohm) means they've gone to Zen heaven...they've served their purpose as 3-legged fuses

 

[rebojorge]

Thanks everyone for the help....

I replaced Q1 and Q2, and tried again.... this time it behaved more the way I expected. I was able to turn the variac up, and calibrate the voltages.

Now I need to plug it in to check the sound. It seems that the problem is solved... but I am not sure yet. Before doing that I want to put everything in place and make sure things do not get shorted when moving the amp from place to place.

Tony... good to see you are still around. Yes I had a fuse to the primaries of the trafo, but I am using a separate transformer for each channel and I made a mistake and wired both fuses in parallel from the ac mains to the turn on switch so all current was divided between them and..... you know the rest.

Mr. Pass... Yes, It went past the .7 Volt figure, but I did not let it go to high, so I would not burn R0 and R1, again.... does this means that Q3 is gone too?

Thanks everyone for the help!!

I owe everyone some pics of my zen that I have not uploded since back then.... I will do it soon!

 

[Blues]

...and mostly likely Q3. ...they've served their purpose as 3-legged fuses

I meant Q5, as very high current will certainly mean lights out. In the ZenV4 if one power transistor fails, the high current will fuse together the D-S junctions and will also take out the other 2 power transistors. If your transformer primary is not protected by a correct fuse it will burn R0 and R1 like what happened in your case.