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Old 22nd January 2011, 01:44 PM   #1
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Default Your F5 blew up, now what do you do?

I've helped a lot of folks with F5 issues. To make life easier here are some suggestions in case of difficulties:

First, a pair of current sources is cheaper than a pair of fuses. For setting up you can use your power supply with an LM317/LM337 set up as a current limiter per the National Semi datasheet. If you use 3 ohms between ADJ and OUT you'll limit the current to 400mA which is a good place to start. The voltage regulator will dissipate heat, so should be heat-sinked. Measuring the bias current across the 0.47R resistors is easy, but using a DVM or pair of DVM's on the current settings is easier still.


1) I think I blew up my MOSFETs:
Solution: Unsolder the source pin on each MOSFET. Connect the drain and gate together with a pair of alligator test leads. Connect a voltmeter between the GateDrain and Source nodes. With the current limiter in place, apply Positive 12V to the GateDrain of the N-Channel device (and Neg 12V for the P-Channel), Ground at the Source and measure the voltage. You should measure a voltage in the vicinity of 4 volts. If it's 12V or 0V you've got a blown (problematic) MOSFET. Often only one side of the amplifier will fail. Note that this test reverses the usual connection of the power supply to the amplifier.

2) I think I blew up my JFETs
With the source of the MOSFETs still disconnected, connect the current limited power supply as if for normal operation. Use a 100mV test signal on the input and measure the AC output at each drain. Unless you really solder the heck out of the JFETs they rarely fail. The gain of the JFET stage is approximately 10. Note that on Cviller's boards the drains are clearly marked.

3) I can't control the bias, no matter what I do with the trimpots
Connect the current limited power supply as for normal operation. Connect a voltmeter between the Gate and Source of each MOSFET. Adjust each trimpot until Vgs drops below 3 volts. You'll note that the current drops dramatically. Make a mental note to yourself which direction (CW or CCW) increases the current of the output stage.

Oh, I was contacted by someone who failed to use an insulator between the MOSFETs and heat sinks. That really stinks.

If you are tapping the holes for the screws to the heat sink, make sure that you deburr the hole so that a metal shard doesn't tear into your insulator.

Before you power up the amplifier, always, always,always check to make sure that the flanges of the MOSFETs are insulated from the heat sink.

4) "It oscillates, now what?" 1 If you're testing the amplifier with a lab supply, long test leads will pick up enough RF energy to excite the amplifier into oscillation. Use some local filtering, perhaps 1,000uF on the V+, V- inputs right at the PCB. This is important! You might thing you have a wonderful amplifier with no apparent DC offset, but the thing is singing at 660kHz and knocking WFAN off the air.

5) It still oscillates Try putting a 220pF silver mica cap across the 100K input resistor.

1 Apologies to Jan Didden for re-using his interrogatory.

Last edited by jackinnj; 22nd January 2011 at 01:50 PM.
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Old 22nd January 2011, 02:41 PM   #2
6L6 is offline 6L6  United States
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Great information!!!!!! Thanks so very much for posting!

Moderators - please sticky this!
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Old 22nd January 2011, 02:59 PM   #3
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Absolutley briliant
Jast what I need to know
Jackinnj
have a paint on me.

Maybe you could help with a question.

I was thinking about using A mosfet the smmoting capacitors and the amplifier PCB
to cut off the suply.

What would be the dissipation required (heat load) as those will be used as switches
saturation region?
lets say 4A at 28 V

My thinking is that as the resistance seen across the mosfet is very small also disipation should be smaller.

If not just any mosfet could do as is not in the signall pat

was also planning on a couple of LM339 to detect pulses over a certain level
alowed DC offset as Ac crosses the "ZERO" and set reset a timer if there is to much DC or one the masfet go shorth circuit timer does not reset and either trighers speakers relay or the surmentioned mosfet stop conducting.

Many tanks

Al
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Old 22nd January 2011, 03:07 PM   #4
flg is offline flg  United States
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Quote:
Originally Posted by jackinnj View Post
...Oh, I was contacted by someone who failed to use an insulator between the MOSFETs and heat sinks. That really stinks.

If you are tapping the holes for the screws to the heat sink, make sure that you deburr the hole so that a metal shard doesn't tear into your insulator.

Before you power up the amplifier, always, always,always check to make sure that the flanges of the MOSFETs are insulated from the heat sink...
That really stinks?
Generally, the isolation of the MOSFET drains is very important because they are at the rail voltage of opposite polarities. You would effectively be shorting your power supply to the heatsink. If the heatsink were grounded you'ld have a short to ground. In this case however, with the F5 circuit design, if you do not ground your heatsink, you may never know you have a short because the 2 Drains are connected together and form the output. This is still not recomended as potentially hazardous voltage would be expossed. An accident waiting to happen.
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Old 25th January 2011, 09:15 PM   #5
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I would add just a small thing -

Instead of drilling, tapping, cleaning, etc, that troublesome mounting hole for the Mosfet, I use those large transistor clips and the hole for screwing these down is well away from the Transistor (available everywhere) - my pcbs are often removed for changes and I put a tube cover (bit of heatshrink) over the transistors to keep the "white goo" from painting everything.

I also use an o/p dc protection cct with added "mute" connection - no rail fuses, just the primary line one, and nowadays, an isolation transformer with it's dc trap, filters and dedicated "earth" stake.

Thanks for the Excellent "trouble-shooters" page above, John - very useful indeed.
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Old 25th January 2011, 09:46 PM   #6
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by Bksabath View Post
.......

Al
regarding dissipation - it will be Rds*on x I^2

example for IRFP150

55mR x 4^2 = 880mW

almost no need for heatsinking
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Old 25th January 2011, 11:04 PM   #7
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Tanks Zen
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