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Old 12th January 2008, 11:26 PM   #1
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Default P101 Problem. Bad MOSFETs???

I really would appreciate any advice or thoughts on the following:
I purchased two P101 PCBs many months ago. I've built them exactly to the guidelines (High power version) and have used all the correct components but I'm having problems.
When testing the P101s I'm using a bipolar 17V PSU through two 100R resistors but nearly the full supply is being dropped across the resistors leaving -0.6V on the -ve rail and +1.2V on the +ve rail of the PCB. I have the bias set to minimum resistance. A strange thing is there is about +7V present on the cathode side of D3. Both boards have exactly the same symptoms!

I'm using 2SK1058 & 2SJ162 MOSFETs. I desoldered the MOSFETs from one board and checked each with an Atlas Component Analyzer. The 2SK1058s seem fine as the Analyzer is detecting them as "Enhancement Mode" MOSFETs. However, when I analyze the 2SJ162s, one of them appears as a "Depletion Mode" MOSFET, the other an "Enhancement Mode" MOSFET. I checked two spare 2SJ162 devices I have and these are also seen as "Depletion Mode" MOSFETs. I haven't much experience with troubleshooting MOSFETs but I do know that Depletion Mode MOSFETs are quite rare. Also, I'm not sure whether the Atlas is 100% reliable with these devices. It says in the manual that it distinguishes Enhancement Mode MOSFETs by the protection diode between the Source and Drain.

Does anyone have experience with the Atlas Component Analyzer????? It's a blue thing with three test leads. I don't know the model no. offhand.

One thing that has crossed my mind is that the MOSFETs could be fakes. They have no Manufacturer on them, just a date code, but I got them from Donberg Electronics here in Ireland who are a reliable Component Warehouse.

If any of you have any thoughts about any of the above I'd really appreciate your reply as I've been puzzled for quite some time about this.
Thanks!

Vinny.
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Old 13th January 2008, 12:47 AM   #2
jnb is offline jnb  Australia
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Check the power ground lift resistors (something like 2 ohms). They may need to be higher power than expected to prevent nuisance burning out.

MOSFETs are very sensitive to measurement. Merely measuring the gate circuit is enough to turn them on as the DMM will charge the gates. They will stay that way until discharged after which time the device will behave oppositely.
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Old 13th January 2008, 01:04 AM   #3
Kinnja is offline Kinnja  United States
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Default Re: P101 Problem. Bad MOSFETs???

Quote:
Originally posted by vinyvamos
...I really would appreciate any advice or thoughts on the following:
I purchased two P101 PCBs many months ago. I've built them exactly to the guidelines (High power version) and have used all the correct components but I'm having problems....
Vinny.
Just for clarification, your talking about this project right?
http://sound.westhost.com/project101.htm

If so, have you tried ESP's forum?
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Old 13th January 2008, 12:37 PM   #4
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Yea its the ESP High Power MOSFET Amplifier project. Sorry I should have made it clearer what design I'm building.

Yea there's a 10R 1W resistor for the power GND lift and that's remained intact on both boards. I see what you're saying about the MOSFET testing. They were being turned on when I tested them with my DMM and I had to short terminals to get proper readings but I'd say the Component Analyzer does that before every test, otherwise I'd be getting very strange results. It can measure the gate threshold and one or two other characteristics of the device under test. Really handy as long as it's telling the truth!

I've emailed Rod and I'm awaiting his reply. I know the main manufaturer for the devices I'm using is RENESAS, who bought the technology off HITACHI who were making them previously. However, there is no manufacturer logo or anything on my devices...

I've not found anything in the ESP forums so I'll post this problem there too...

Thanks!
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Old 13th January 2008, 01:46 PM   #5
Did it Himself
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Well they certainly are not genuine parts, they will be clones made by a third party, or fakes.

The bias should be set to minimum resistance for the minimum voltage across it, that is probably your problem with the supply being dropped as the MOSFETs are both turning on a lot.
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