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Old 16th March 2007, 04:02 AM   #1
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Default Help a noob fix a ZV9

Hi its me again...
The story thus far. One channel works great, which emboldened me to build the second.
The second one not so good. The history of the not so good channel is this:
1. With the chassis open, and everything in place I tested it on an old pair of speakers and a beautiful melodious sound eminated.
2. Put the two halves of the chassis together and a serious hum appeared.
3. After much poking and disassembling i discovered that the JFET was grounding through a washer (damn those fets are small) onto the chassis. When the two halves were not joined the voltage didn't go anywhere, upen closing the voltage was draining out through the speaker terminal, by way of the star ground... I think
4. Great i thought, just to make sure i pulled the FET off and with it hanging in space i tried the amp again, and it all seemed fine, reatatched the FET and all looked good.
5..... or so it seemed. After an hour "POP" and no sound.

So know i am sure something has blown. I am new to this and this is about as much as i have managed to determine.

1. At the DC i measure a potential difference of 52V which is higher then the 46V that i was getting before and the 46V on the still working channel.
2. Where i used to measure the voltage at R18 and R7 and set it at 23V (half of 46V) I get almost nothing, 2 or 3 volts.
3. At the speaker terminals there appears to be a very minor extremely unstable something in the millivolts and a minor pop st the speaker on power off.

I have order more JFET's because i assume the JFET has gone and possible the MOSFET's.

How can i trouble shoot this, maybe a resitor went or something else? No burn marks on board so no serious short.

Ant and all help would be greatly appreciated

Thanks
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Old 17th March 2007, 01:23 AM   #2
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Help anyone please
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Old 17th March 2007, 06:07 AM   #3
trango is offline trango  Sweden
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Hi,

I'm sorry to hear that you are having trouble with your Zv9.

Did you attach the JFET to the heatsink properly when you did the second test? Since you had it up an running for an hour, it sounds like some temperature related issue ?

/Trango
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Old 17th March 2007, 01:49 PM   #4
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Thanks Trango.
Would fault would a tempertature reltated issue likelt cause. I am trying to figure out what would have blown. I imagine one of the FETS?
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Old 17th March 2007, 01:55 PM   #5
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On a related issues. How have you managed to attach the JFET to teh heatsinks as there are no mounting holes???
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Old 17th March 2007, 02:27 PM   #6
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Quote:
Originally posted by neil_kaye
On a related issues. How have you managed to attach the JFET to teh heatsinks as there are no mounting holes???
Hi Neil

Click the image to open in full size.

this is how Steen managed it.

Please, post some pictures of the opened chassis (both parts) so somebody can realize better what's going on when you close it.

Are you sure all mosfets are well insulated? (Measured mosfet tabs and chassis ground for shorts?)
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Old 17th March 2007, 04:24 PM   #7
trango is offline trango  Sweden
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I'm using clips bought from Mouser in combination with AluOX insulators and thermal grease.
They are not avaible in the right format for the JFET but I used the smallest size they sell at mouser and it seems to be working. I've acutally tested my Zv9 for the first time today, everything seems to work - only final assembly left.

The picture below may give you the idea even though the JFET is not visible...
Attached Images
File Type: jpg zv9-13-small.jpg (36.2 KB, 763 views)
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Old 17th March 2007, 05:06 PM   #8
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trango,

the picture is too small. Have you a part number for Mouser insulators?

Congratulations for another Zen playing
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Old 17th March 2007, 05:25 PM   #9
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thomaseliot, the mosfet may indeed be a problem as well as a heatsink issue (i think it is a combination of both). How do i measure mosfet tabs and chassis grounds for shorts?
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Old 17th March 2007, 06:41 PM   #10
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Quote:
Originally posted by neil_kaye
How do i measure mosfet tabs and chassis grounds for shorts?
With Ohm meter. One probe to heatsink (in a screw hole) the other to tab.

If resistance is zero, then there is a short (probes are connected), otherwise resistance is infinite, that is, there is no connection between probes.
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