Blown FETs?

[Robert Cheek]

I think I may have blown FETs.  I was on 20 meters, using the HR500 with Palstar HF-AUTO on a dipole fed with open wire line.  (I have used this antenna with the HR500 for years) 

Got a drop out when I tried to send 50 watts to the tuner to retune to 80 Meters.  The screen on the HR500 showed a FAULT.  From there it is making no power out.

I see drive on the HR500 meter, I see VDD, no IDD, and now output.  Relays click LEDs on the rear panel show 50 V is good, PPT is good and RF Sense flashes with CW input. 

Checking the Drains on the FETs I have 55 Volts but I also have 55 Volts on the gates....don't think that is correct!

Anyone have thoughts?

50 V on the gates make me think the FETs have shorted. 

Regards,

Bob - N4RFC

[jcve...@gmail.com]

Hi Bob,

55V in the drains indicates that the MOSFETs are blown. The protection diodes should have prevented any further damage to the amp PCB.

73,

Jim WA2EUJ

[Robert Cheek]

Jim,

I think you meant to say 55 volts on Gates indicates the MOSFETs are blown...Spec sheet says Gate max voltage is +/- 40 volts so I figured the Gate were shorted to the Drain. 

I saw the diodes in the schematic and was thinking they were there for when things go bad.  And things went bad.  Do you know that the Gate bias typically runs on the HR500?

New MOSFETs on order.

Thanks for the quick reply

Bob - N4RFC

[AC1I]

I just thought I would throw this out there:

A couple of years ago several of us suffered from blown finals; the theory at that time was that temperature cycles were causing the screws securing the MOSFETs to the heatsink to loosen. The suggested cure was to utilize lock washers on those screws (I myself used high temperature Loctite).  I can only say that I have not had a failure since then.

Patrick AC1I

[Robert Cheek]

Just now getting around to changing the FETs in my HR500.  The 10 Ohm resistors are smoked and I am wondering if the Bias Pot and the protection diodes are gong too.  Don't understand why the 10 ohm resistors took a lot of current if the diode is there to protect the bias circuit.  I don't see a resistance value on the schematic for the Bias Pot.

Regards,

Bob N4RFC

 

On Saturday, March 29, 2025 at 9:17:11 PM UTC-4 jcve...@gmail.com wrote:

[Robert Cheek]

Just checked the Bias Pots they look like 500 ohm pots.  Also, is there a parts list for the Amp Board?  Like to get the info for the 10 Ohm resistors.....

[Robert Cheek]

Jim,

I attempted to load Joe Street's V 6.09.  The boot loader got to 97% complete then started time out.  Now the HR500 is Bricked.  The PC can't detect the USB port anymore.  

Can you give me instructions on how to un-brick the HR500.  If I can get it back I will be sticking with your code. 

Regards. 

Bob N4RFC

[jcve...@gmail.com]

Hi Bob,

It's very easy to recover a bricked HR500. The processor is an Arduino Mega 256 if you remove the control board you can plug the Mega's USB port into a PC or MAC and use the Arduino IDE or AVRDude to program the Mega (YouTube instructions abound). In the unlikely case that the Mega itself is bricked, a brand new one is about $20.

You must remove all of the ribbon cables from the control board to use the USB jack on the Mega.

73,

Jim WA2EUJ

[Robert Cheek]

Maybe not so easy....the Bootloader doesn't recognize the board when plugged in directly to the Mega 256.  I have all the cables disconnected from the control board, but the LCD displays are still connected.  The port shows up on Com 7, and the debug loader scans Com 7 but doesn't find the Mega 256.  Does the Mega 256 get power from the USB?

I have the IDE loaded also, but I need a INO file to load into that.  It doesn't seem to like to the HEX files.  Where can I find the source ino files to try the IDE.

I am thinking my cpu is fried.  

Bob N4RFC

[jcve...@gmail.com]

The bootloader only works if the firmware is running because if the FW isn't running it won't respond to the bootloader.

The USB port on the Arduino uses RTS to reset the processor and activate the Arduino's built-in FW update utility.

If you only have the .hex file use xLoader (https://github.com/binaryupdates/xLoader) rather than setting the IDE up with all of the libraries. Which can be quite frustrating.

Here is a video that show how to do it. https://www.youtube.com/watch?v=IMbjURsGs-w

The Arduino and the LCD's can be powered by the USB cable.

73,

Jim WA2EUJ

[Robert Cheek]

Jim,

I am pretty sure this Mega 2560 is toast.  I loaded an example into the IDE to clear the EEPROM space.  It compiled and then timed out on the upload.

Sketch uses 1374 bytes (0%) of program storage space. Maximum is 253952 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 8183 bytes for local variables. Maximum is 8192 bytes.
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_getsync(): timeout communicating with programmer
Failed uploading: uploading error: exit status 1

Obviously there is micro code in the Arduino that allows downloading and it has been overwritten by the boot loaded code.   I suspect there is no way to re-initialize the Arduino from this point.  

I see Mega 2560 Rev 3 all over Amazon, from $23 up.  Will a Rev 3 board work in the HR 500? 

Regards,

Bob N4RFC

[jcve...@gmail.com]

Hi Bob,

You might try rebooting your PC before giving up on the Arduino, sometimes when the programs fail, they don't close the serial port and other programs can't use it.

It is possible to reprogram the Arduino bootloader but it requires some additional hardware; either another Arduino or an ICSP programmer.

ASAIK, any Mega 256 will work in the HR500.

73,

Jim WA2EUJ

[Robert Cheek]

Jim,

I have rebooted, powered everything down, changed the USB cable, and changed USB port on the computer, Xloader didn't do anything different.  No joy.  All I see is the TX Led flash every 15 seconds on the Mega 2560.  I tried reloading the firmware from the Tools menu on the IDE, it times out.  So I will order a new board and see what happens then.  I have a clip on programmer that may work, but I have had very mixed results with that on other things.  I have programmed Nano's before and never had a problem.  

Thanks very much for your help. 

Bob N4RFC

[Robert Cheek]

Jim,

Hopefully this is the last issue I have.  I found I had a USB Tiny ISP programmer.  I think I used that where I was working on a Arduino Nano in a home made StepIR controller.  A friend upgraded his StepIR and gave me the driven element from his old version.  I built a simple controller to run the elements in and out.  Made a nice fairly portable 5 band dipole....Anyway....

I refreshed the Bootloader on the Mega via the ICSP port.  Then I was able to install my original the V3.3 program in the Mega 2560.   I put the HR500 back together and all worked fine.  But then I see there is a V 3.4, so I thought I would install that.  But, the Xloader or the HR 500 Autoloader doesn't see the HR 500 from the USB port on the back of the Amp.  I pulled the top off and didn't see anything disconnected.   So, I pulled the front panel off the amp and loaded V3.4 directly to the Mega 2560.  Seems to be working fine except for the rear panel USB. 

Thoughts on the rear port not communicating?

[jcve...@gmail.com]

Hi Bob,

If you can issue commands and get responses through the USB port on the back then all of the hardward is working just fine. The timing on the bootloader can be tricky if there are other programs running it can get out of whack.

73,

Jim WA2EUJ

On Friday, November 14, 2025 at 10:50:58 AM UTC-5 wb4...@gmail.com wrote:

Jim,

[Robert Cheek]

Two days later....I tried some commands to the HR500 on USB....seems to work.  I tried the control panel download on the google site but Malwarebytes REALLY didn't like that app.  Just kept putting it in the quarantine file.  It wasn't happy with it.

So I tried the AutoLoader and it detected the HR500 port and seemed to be good.  Not sure what changed...but seems to be working OK now.  I ASS_U_ME that the HR500 firmware has a boot loader that runs that alternate port where the rear panel USB is connected.  Or, does the "arduino " boot loader check the other ports besides the one on the 2560 board?

But anyway, all good here now on 3.4B version firmware.

Also, I did load Joe's 6.09 version.  It had a long delay after the transceiver dropped the PTT line before the HR500 switched back to receive (bypass) mode.  That was a constant aggravation, not an occasional one.  About 250 - 350 mS before the transceiver got the antenna back after dropping from transmit.  On a fast exchange (I chase a lot of POTA contacts) I was missing some characters.  

 

As always, thanks Jim for your great support on the HR500.  

Regards,

Bob N4RFC

[jcve...@gmail.com]

Hi Bob,

The "normal bootloader" (NB) for the Arduino uses the on-board USB port which has the RTS line connected to the reset pin on the Arduino. The NB asserts thr RTS line resetting the Arduino processor. After reset the Arduino listens for data from the NB and if it gets it, the Arduino doesn't run the program stored in its flash but uploads a new program.

Since the USB port on the back of the HR500 can't reset the Arduino, I wrapped the entire process in a PureBasic program that issues tells the Arduino to reset, then starts AVRDude. A lot can go wrong because the time window for the Arduino to connect to the NB is very small and the PC has to issue the reset command and start AVRdude within that window. Depending on what else is happening on the USB bus, that the disk drive is doing and whatever Windows is doing in the background, it's easy for the timing to get screwed up and work sometimes but not others.

Normally, I wouldn't release a product with such as buggy process but since you can always pull-out the Arduino and directly connect it to the PC, I figured that it would be OK. If you do need to connect directly to the Arduino, remember to disconnect the rear panel board because they share a com port on the Arduino.

73,

Jim WA2EUJ