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Nelson Pass F3 amplifier help wanted
Nelson Pass F3 amplifier help wanted
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Old 2nd September 2017, 05:44 AM   #51
Gyuri is offline Gyuri  Hungary
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FETs and JFETs can be tricky, because electric charge of your body can switch them. Mostly out of circuit of course.
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Old 2nd September 2017, 06:05 AM   #52
Gyuri is offline Gyuri  Hungary
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Quote:
Originally Posted by JeffYoung View Post
What's the voltage across the resistors that are getting hot? (They're all in parallel, so you can just measure any one of them.)

Also, what's the voltage across the one at the bottom which is also discoloured? (You should read about 21V here.)

You could also check the voltages on the JFET (the small transistor hanging off the board just below the smoking resistors). You should get about 3.5V at the D, and about 1.1V at the S.

Report back with results.

(BTW, Nelson posted the F3 schematic and some other good stuff here: Pictures of a >>GENUINE<< Lovoltech LU1014D Wanted!)
Thanks JeffYoung,
I somehow missed it, there are important information about F3.
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Old 2nd September 2017, 06:32 AM   #53
russell66 is offline russell66  Australia
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I will get some hi res shots of the circuit board later. I have studied it under magnification and to my eyes i can not see anything glaringly wrong.
I can see the center pin on one Jfet may need more solder but the contact still looks ok.

Now 2 things have occurred to me

1. I have 4 of these boards in total, one or two had this issue previously, now two more have the issue one could be the same board but the other is not. Which means at least three out of the four boards has the fault. To me it would be unlikely that it is a solder joint unless of course you could get a faulty batch of solder. It more points to a component fault or a design fault. What do you think?

2. The green green black resistor i was pointing out earlier. Looking closely at the black one it is 3.75w exactly the same as the ones that are over heating. It is measuring 0.8 not 1.2 is there an obvious reason for this. Perhaps the others are in Parallel and that one is in series if that is the opposite correct term.

Carefully look at the small to92 bipolar transistor... Is this the smaller one that attaches to the heat sink that was referred to earlier as the Jfet?

The Bias looks very similar on all channels.
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Old 2nd September 2017, 08:59 AM   #54
2 picoDumbs is offline 2 picoDumbs  Australia
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The bipolar transistor is the smallest transistor on the board not connected to any heatsink.
This transistor is critical for correct circuit operation.
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Old 2nd September 2017, 09:49 AM   #55
JeffYoung is offline JeffYoung  Ireland
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Nelson Pass F3 amplifier help wanted
If the back tab on your JFET (the small black thing under the little finned heat-sink, otherwise known as a Loveltech LU1014D) touches ground, it will pull a bunch of current through its cascode MOSFET, and heat up those resistors. So it's worth checking.

The blue pad (you are correct that these transfer heat but not electricity) under it should isolate it from the main heatsink. But the little heatsink that's bolted on top of the JFET could, if angled too steeply, make contact with the top edge of the tab.

(This might also explain why some of the channels have gone from good to bad. Perhaps when re-mounting them the little finned heat sinks were tightened more or less.)

It might be best to wrap the blue pad around the top tab so that it isolates both the back of the JFET and the metal tab sticking out the top of it.

Cheers,
Jeff.
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File Type: jpg possible shorting.jpg (25.0 KB, 234 views)
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Old 16th October 2017, 01:40 AM   #56
russell66 is offline russell66  Australia
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Default Help with Pass F3

Quote:
Originally Posted by 2 picoDumbs View Post
If you live in Melbourne I can come and look at it for you.
Hi I cant PM you your in box is full. Hope you get this. Thanks
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Old 16th October 2017, 01:49 AM   #57
2 picoDumbs is offline 2 picoDumbs  Australia
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I will clear it out.
Been busy with a few projects.
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Old 18th October 2017, 11:35 AM   #58
2 picoDumbs is offline 2 picoDumbs  Australia
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Confirm your pcb has all these components.
Record any differences to this circuit.
Circle which resistors are smoking on circuit.
Circle any other components you are concerned about.
Attached Images
File Type: jpg F3.jpg (69.3 KB, 125 views)
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Old 8th November 2017, 08:42 PM   #59
russell66 is offline russell66  Australia
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Quote:
Originally Posted by 2 picoDumbs View Post
Confirm your pcb has all these components.
Record any differences to this circuit.
Circle which resistors are smoking on circuit.
Circle any other components you are concerned about.
HI Thanks NO sorry i hadn't seen it, i will get on to it and get back to you.
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