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Author Topic: Testing FET output transistor  (Read 4068 times)
K2OWK
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Posts: 1273




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« on: October 03, 2017, 03:23:50 PM »

Hello Elmer's, I have a TYT-9800 quad band transceiver. I have had this unit for many years (it is the original one). I have had no problems until now; all of a sudden there is no output on any bands. I have re-soldered the output FET. I checked the transmit/receive relays they are working normally. When I key the mike the front panel display indicates as it should. The output meter shows power out at all levels.  The repeater frequency shift shows the proper frequency. The receiver mutes as it should, but my power meter shows zero power out into a dummy load (the UHF/VHF power meter was checked with another transceiver and operates as it should). I checked the power output FET Mitsubishi RD70HVF1” with a voltmeter with the following readings to ground. Key up one side reads 12 V DC (power supply voltage) the other side reads 0 V DC. Key down one side reads 9 V DC the other side reads 3.8 V DC. I would like to know if this is normal operation for this FET. I would also like to know how to test this transistor? This transistor is available for about $20+, but I would hate to buy this transistor replace it and find I still have the same problem. Any help I can receive would be greatly appreciated.
Note: The receiver works normally, with no problems
73s

K2OWK
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KA5IPF
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« Reply #1 on: October 04, 2017, 04:50:33 AM »

Fix the power supply or power cord. The plus 13.8 shouldn't drop over a volt.
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KC4ZGP
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Posts: 1637




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« Reply #2 on: October 04, 2017, 05:41:50 AM »

OWK,

The transistor you mention is a power MOSFET. I'm working on my EB-104 which uses MOSFETs.
During installation, a MOSFET requires static-free environment to prevent static from popping the gate.
Once the FET is installed, the static is no longer a concern.

Looking at the FET, printing right side up, the drain is the top lead, the source are the fat mounting tabs
and the gate is the tab at bottom.

Voltages are read between drain-source and gate-source.

Different voltages exist on the drain and gate so a meter reading between them will be erratic.
Depending on the FET or the meter, the drain voltage could make way through the meter and the high
voltage to the gate could destroy the FET anyway.

But if the 12 volts goes to 9 volts on key down, your power supply is unable to meet the current
demand; it's straining.

Try the power supply on another radio. Maybe a 100 watt transceiver. See if it can provide the current.

Power supply issues cause more grief and make folks suspect other problems.

Kraus
« Last Edit: October 04, 2017, 05:45:49 AM by KC4ZGP » Logged
WB6BYU
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Posts: 17049




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« Reply #3 on: October 04, 2017, 08:10:03 AM »

What are you using to measure the voltages?  Many DC instruments (especially digital
ones) can get confused when there is RF present, and give false readings.  That makes
troubleshooting more difficult, but not impossible.

First, do you have any output at all?  I lost the final amplifier in one of my 2m rigs,
and could hit a local repeater over a particular stretch of highway where it was
line-of-site.  Actual output was 10 - 30 mW or so.  A good test is to put the rig on
SIMPLEX and see if you can hear it on another rig nearby.  If you can't, that would
tend to indicate the problem is in generating the signal rather than amplifying it.

Then check the current draw from the power supply.  Does it seem normal in TX
mode?  Does it change at different power levels?

A very handy tool on the workbench is an RF probe:  a simple device (typically
one diode, one capacitor and one resistor) that plugs into your DC meter and
reads the RF level in the circuit.  With that you can go through and check the
RF level on each amplifier stage, as well as between the output transistor
and the antenna jack (which is a common place for faults, too.)


With regards to the transistor itself:  from the receive measurements it sounds
like an enhancement modedevice.  That means it doesn't draw current
until the gate is driven positive.  If it were shorted, then it would draw excessive
current on receive, since there is voltage on the drain.  So measuring the receive
current would be a good step to confirm this.

The voltages on transmit may be affected by RF.  But we know that either there
is RF drive to the gate, or gate bias being applied (or both), since you get a
positive reading on the gate. (Though that could be RF picked up by your test leads
also.)  I wouldn't expect the transistor to be drawing enough current to cause the
voltage to drop that much, unless you have some added resistance in the circuit.

The most common failure modes for FETs are a short (which the receive voltage
measurements would rule out) or an open, in which the transistor wouldn't draw
any current with drive (though other TX stages would.)  To test for the second
case, you need to drive the transistor with DC so the RF doesn't confuse the
meter readings.

You might or might not be able to do that with the transistor in place, depending
on the circuit.  If the gate is grounded (for example, through a transformer winding)
then it is very difficult.  But if the gate voltage is supplied through a resistor, you
might be able to apply some DC bias from another source on receive and see if
the transistor draws current.  I'd do this very carefully - you want to bring it up
slowly so the transistor doesn't draw too much current.  (A current-limited supply
might be a good idea.)  If you can increase the receive current by 100mA or so
using this method, then you can reasonably say that the transistor is responding
and should be working properly.



Some other thoughts:

1) keep the power setting low while debugging.  If the fault is in the output
path, driving the output transistor to full power isn't a good thing.

2) Did the transistor need to be resoldered?  If the solder had melted, that
is a sign that the transistor is overheating, which could be either because it is
drawing too much current or that there is a fault between it and the antenna.
(Or that it doesn't have adequate ventilation to run high power.)
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KC4ZGP
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Posts: 1637




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« Reply #4 on: October 05, 2017, 11:24:27 AM »


Good question. Why did he resolder the FET.

Heat will destroy the transistor before the solder could melt.

While the FET is still installed, check resistance between the gate and drain. You should get millions of ohms.

It slowly decrease as the gate conducts. FETs are funny guys. If you measure a few hundred ohms, and the
resistance doesn't change, the FET is gone.

Replace and make sure you, tools, bench, soldering iron, etc stay grounded, stay at the same potential.

Kraus
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K2OWK
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Posts: 1273




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« Reply #5 on: October 05, 2017, 08:06:47 PM »

Thanks,for the information so far. I will try to answer the questions one at a time. First the reason I soldered the FET was because according to the Yahoo TYT- 9800 group this was a problem with the first transceivers sold (poor solder joints on the final) Mine looked OK, but I saw no problem with soldering it. I did heat sink it before soldering it with a grounded soldering station. This did not solve the problem.

As for the power supply it is a 35 amp 13. 8 V DC and is now powering my extra 2 meter unit at 50 watts out with no problems.

The volt meter I am using is a digital unit.

I checked for RF output as per WB6BYU recommendation. With the transceiver terminated into a dummy load and set to simplex. I was able to transmit a strong signal with good audio to a very close by transceiver (HT) and receive a strong signal from the HT. My Wattmeter which was checked and working with my spare transceiver showed no outut on any of the power settings  low 5W med low 10W med 20W and high 50W. The current draw was 4 amps from the power supply with no noticeable change on any of the power settings.

The receiver showed a current draw maximum of about 300 MA.

FET measurement in circuit. The source is grounded to the chassis(soldered and screwed). The measurement gate to drain measures open( 2000K scale)  one way, and if I reverse the leads it seams to indicate a charging capacitor.

I can replace the FET if that sounds like the problem. I have a grounded soldering station and have replaced FETS before. I just want to be pretty sure that this is the problem. The FET Mitsubishi RD70HVF1 is about $20.00. Not a great amount of money, but very annoying that after replacing, I still have the same problem.

Just to note I have checked all three relays and all seam to be working as they should.

Thank you,

73s

K2OWK
 
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WB6BYU
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« Reply #6 on: October 05, 2017, 08:36:47 PM »

Does the voltage reading on the gate of the transistor vary with the output power?

What is the resistance from the gate of the FET to ground?

If the gate is grounded (as may well be the case in an FM rig) then the voltage you are
reading is due to the RF.  If you have RF on the gate but no output, when either the
transistor is not conducting, or there is an open or short between the transistor drain
and the T/R relay (assuming that the receiver works properly on receive.

This is where the RF voltmeter probes comes in handy:  you could measure whether
the voltage was RF or DC.  If the voltage you are reading on the gate is ONLY DC,
then the problem is in a previous stage, because that would mean that no RF drive
is reaching the transistor.  (You might be able to verify this by reversing the leads on
the meter and seeing whether it reads positive or negative, but that makes some
assumptions about how the RF affects your voltmeter that might not be correct.)

Or you could pull the FET from the rig and test it on the bench before ordering a
replacement.  A simple test jig would have the source grounded, the gate connected
to ground through a 1k to 10k resistor, and the drain grounded through maybe 10k
to 47k ohms.  The resistors prevent static build-up.  Apply 12V to the drain and
measure the current:  due to the gate resistor, the transistor should be off.  (A small
light bulb in series with the drain will limit the current and give you a visual indication
of current flow.)  Now apply some voltage to the gate, maybe through a resistive divider
and pot connected across the drain voltage.  (You want to make sure you don't apply
too much voltage to the gate.)  Start with the pot at zero, and slowly increase the
voltage while watching for drain current.  If you get to max and don't see any current,
check the gate voltage at that point.  If the gate voltage is significantly lower than with
the gate disconnected, you have a gate-source short (unless the input voltage to your
gate voltage divider also drops.)  If you can't get the transistor to draw current, then
you have an open drain or source.

On the other hand, if you can adjust the drain current up to 100mA or 1A, or something
that lights the lamp, and you can control the current by adjusting the pot, then the
transistor is working properly.
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KC4ZGP
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Posts: 1637




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« Reply #7 on: October 06, 2017, 05:12:59 AM »


I see the same resistance characteristic between gate and drain on my EB-104. I say the FET is fine.

Have you a schematic? Maybe I can find one.

In reference to gate current. Yes as BYU said, if you can find way to adjust gate current and you can
affect change to the gate current, the gate is not popped.

You might well have a problem of getting drive signal to the gate. And BUY said it as well get an RF
probe. I use one on my EB-104. Wow. I feel like a little man inside the components watching electrons
fly past me.

They are simple to make. A capacitor, resistor and diode. But they don't like a lot of power. One needn't have
10 watts applied to trace paths. A half volt peak-to-peak signal will suffice.

Kraus
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KC4ZGP
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« Reply #8 on: October 06, 2017, 06:14:54 AM »


Your power supply 13.8 volts, 35 amperes.

My Astron 35 uses four pass transistors. The manufacturer states 27 amperes
continuous service.

If one pass transistor is faulty, the other three must carry the load. If two are
bad...etcetera. It wouldn't hurt to check the pass transistors as well as their
pass resistors.

I found yesterday the very same.

My Alinco at full key down one-hundred watts. Three of four transistors were
getting warm. One stayed ice cold. I discovered a pass resistor was open.

Replaced and viola! A fully functioning power supply. All four pass transistors
get warm.

Getting closer to bringing the EB-104 back to traffic.

Kraus
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K2OWK
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Posts: 1273




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« Reply #9 on: October 09, 2017, 01:02:40 AM »

Hello Elmer's, I'm back, I just want to let the Elmer's, that are helping me with this problem, I appreciate it. I have not solved it yet. I am in the process of building an N5ESE RF ball point pen probe. It looks easy and according to his article it is pretty accurate. My power supply is working as it should. It is rated at 30 amps continuous. Just to eliminate all power supply problems (if the are any) I switched to a 12 volt 35 amp hour lead acid battery (fully charged). I should have time to finish up the probe in a few days. I have all the parts needed. I will get back to you with the final measurements when completed. Thank you for your patience. If all the readings indicate that the RF is getting to the FET, but not being amplified, I will replace it and hope that solves the problem.

73s

K2OWK     
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KC4ZGP
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Posts: 1637




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« Reply #10 on: October 09, 2017, 05:01:59 AM »


Alrighty. We'll be right here.

Kraus
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AC2EU
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« Reply #11 on: October 09, 2017, 06:34:11 AM »

For some reason, I'm still not sold on the bad FET idea.
The 4 amps indicates that SOMETHING is alive in the tx section, probably it's mostly due to the FET being biased on with PTT, but has no drive.
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KC4ZGP
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Posts: 1637




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« Reply #12 on: October 09, 2017, 06:45:11 AM »


EU,

I wish I could find the schematic and with probe in tow, I'll find the problem.

It's an amplifier. Signal goes from here to there to there. How complicated can it be.

Oh and by the way, there is an AL-811H on e-Bay, $275.00. He says it powers up
but no output when an exciter is connected. I don't think the seller is a ham.

...no exciter connected...not a ham term.

It's inside is immaculate. He has only two tubes however.

Suspicious.

Kraus
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AC2EU
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« Reply #13 on: October 09, 2017, 07:02:50 AM »


EU,

I wish I could find the schematic and with probe in tow, I'll find the problem.

It's an amplifier. Signal goes from here to there to there. How complicated can it be.

Oh and by the way, there is an AL-811H on e-Bay, $275.00. He says it powers up
but no output when an exciter is connected. I don't think the seller is a ham.

...no exciter connected...not a ham term.

It's inside is immaculate. He has only two tubes however.

Suspicious.

Kraus

Remote repair by committee is always makes it more difficult!  Grin
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KC4ZGP
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Posts: 1637




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« Reply #14 on: October 09, 2017, 07:12:11 AM »


Yup.

Too many cooks spoils the stew.

Kraus
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