PA Fets blown by connecting USB?

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peterjbrennan
 

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Joined: 08 May 2022, 02:18

PA Fets blown by connecting USB?

Post by peterjbrennan »

I have an issue that is similar to others trying to use CAT control, and also to Bruce G4ABX where the PA Fets appear to be blown after connecting USB.

My newly finished kit (US group buy) was working well until I tried to get the USB to work with wsjtx on a Mac, and now it appears the PA Fets are blown.

I was not trying to transmit - just to get CAT control working. I had the rig attached to a dummy load / RF meter at all times, and did not see any RF output on the meter.

I could not get CAT control to work - I could hear the relays clicking, but then got an error message and the rig locked up.

I tried this both with and without 12VDC, and after stopping the test, found that the rig no longer transmits, although receive still works fine.

I'll see if replacing the PA Fets fixes this once I get some new ones, but I'll have to give up on CAT control, which is really disappointing.

Does anyone have any idea why attaching the USB would blow the Fets without transmitting?

73 Pete VA7YBW
CT7ARQ
 

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Joined: 31 Dec 2021, 00:51
Location: Esposende - Portugal

Re: PA Fets blown by connecting USB?

Post by CT7ARQ »

I had on outher project something similar and the fets I got had the same footprint of the 2n7000.
The bs170 of my trusdx group had the same situation and the group buy organizer identityed that the footprint was inverted.
I inverted the finals and they work fine.
This could be one reason.
"There are three sorts of people: those who are alive, those who are dead, and those who are at sea"
G0UJA
 

Posts: 31
Joined: 14 May 2022, 09:22

Re: PA Fets blown by connecting USB?

Post by G0UJA »

Pete,

Although I can't confirm it, I think mine may have died the same way.

Kit up and running FB for 2 days, good SWR and then they died.

USB/CAT was connected all the time and working well but other than component early failure the USB connection had crossed my mind as a possible cause.

The good news is that new FETs should solve your issue - I went for genuine Fairchild BS170s so the pinout matches the PCB and I'm back up to full RF output.

I'm not sure if we could ever prove it but I am NOT connecting mine to USB for a few weeks to test and then I will and see how it performs.
73 de James G0UJA
peterjbrennan
 

Posts: 8
Joined: 08 May 2022, 02:18

Re: PA Fets blown by connecting USB?

Post by peterjbrennan »

I don’t think I had this issue since the Fets seemed fine until they died. Power out on 80 was a little high (~7w) despite tweaking the coil, but 40 was about 5w and 80%. I had the rig set on 40 while I was trying to get CAT working, and a dummy load attached.

Unfortunately with parts shortages, I can’t be sure of the replacements I’ve ordered, so I may hit this problem next.
Currently though, I’m pretty sure that connecting the USB killed the Fets without transmitting.
73, Pete VA7YBW
CT7ARQ wrote: 18 May 2022, 18:49 I had on outher project something similar and the fets I got had the same footprint of the 2n7000.
The bs170 of my trusdx group had the same situation and the group buy organizer identityed that the footprint was inverted.
I inverted the finals and they work fine.
This could be one reason.
John
 

Posts: 49
Joined: 19 Feb 2022, 08:56

Re: PA Fets blown by connecting USB?

Post by John »

I have a couple of ideas, based simply on my R&D and repair experience, since I've not yet hooked up both a PC and a 12V supply to my (tr)uSDX, and have run into no problems as yet. But first, 2 caveats: 1.) I'm not an EE. I don't even have an Associates degree. 2.) I have a good deal of trouble reading schematics that just show devices and labels of signals and don't show actual traces going between devices so I can envision the operation of the circuitry. I'm old. I wasn't raised on CAD drawings. In fact, I'm still working on fully understanding the ramifications of D3 and D4 as protection diodes.

Having said that, the only 2 possibilities I can think of are the fact that (A.) by having a PC and a 12V power supply both connected to a (tr)uSDX possibly introduces 2 different ground references, so there is the possibility of ground loops or odd momentary reference fluctuations depending on your USB and 12V power sources. (You might want to run your CAT control PC on battery power instead of plugged in to AC, or run the (tr)uSDX on a 12V battery to eliminate this possibility.) (B.) the second possibility is that L15 is creating an inductive spike during the transitions between 5V USB power and 12V external power, (I'm still trying to picture exactly what D3, D4 will do at the drain of the FET's when that happens,) so the order in which you hook things up *may* induce the problem. I mention these things because I note that the Absolute Max Gate/Source voltages allowed on the FET's are +/-20V, the FET's are always attached to the 12V power source whether transmitting or not, and you have already eliminated high VSWR and such other possibilities that I can think of. Again, please take my comments with a grain of salt as I am not an RF expert, just a repairman, and it would be better left to Manuel to resolve. But if you are seeing 'mystery' transistor deaths and he is busy elsewhere at the moment, those possibilities might help point you in the right direction.
peterjbrennan
 

Posts: 8
Joined: 08 May 2022, 02:18

Re: PA Fets blown by connecting USB?

Post by peterjbrennan »

Thanks for the suggestions John. My electronics education was back in the 70s, so I know what you mean about the schematics.

The mac was running on batteries, and my 12v power to the trusdx is also a battery, so no ground loops, but I like your suggestion about the order of the connections. I had the radio running off the USB while trying to get CAT to work, and then plugged in the 12v supply. In future I’ll stick to doing it the other way around.

On a positive note, I got CAT working! As noted by others, setting DTR high and RTS low helps, and I found that I had to select TS450 rather than TS480. I thought I may as well try that with the dead Fets, since I will be very cautious once I replace them.
DL2MAN
 

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Re: PA Fets blown by connecting USB?

Post by DL2MAN »

What I´ve observed is: When any program is initiating a CAT connection, it puts the Radio on TX in some cases. Mostly it´s short pulses, but there might be a failure somewhere, and it goes into permanent TX.
Other than by TXing I could not imagine another way of destroying the PA Fets.
But that´s an interesting topic, and I will discuss it with Guido, to get his opinion, as noone has more insight in the Software, as he has.

73 Manuel; DL2MAN
G0UJA
 

Posts: 31
Joined: 14 May 2022, 09:22

Re: PA Fets blown by connecting USB?

Post by G0UJA »

Manuel makes a valid point - I noticed my FET's had died soon after playing around with CAT settings and reloading firmware without an antenna connected.

It may be worth putting a note in the firmware upload instructions to ensure a dummy load is connected whenever the (tr)uSDX is connected to a computer via USB?
DL2MAN wrote: 19 May 2022, 07:38 What I´ve observed is: When any program is initiating a CAT connection, it puts the Radio on TX in some cases. Mostly it´s short pulses, but there might be a failure somewhere, and it goes into permanent TX.
Other than by TXing I could not imagine another way of destroying the PA Fets.
But that´s an interesting topic, and I will discuss it with Guido, to get his opinion, as noone has more insight in the Software, as he has.

73 Manuel; DL2MAN
73 de James G0UJA
John
 

Posts: 49
Joined: 19 Feb 2022, 08:56

Re: PA Fets blown by connecting USB?

Post by John »

I can easily see people hooking up their 12V and USB cables for CAT operation without first attaching an antenna or dummy load, and not realizing that the CAT software drivers may be pulsing the transmitter before the system is all set for 'transmit' modes, so that could be it. 'Random' power-up and connection states are something that commercial equipment companies analyze and spend good time and money to eliminate from their products, but they could easily bite us experimenters in the butt when we're trying to get software up and running that wasn't specifically designed and debugged in order to work with the (tr)uSdx, and certainly when dealing with any software that wasn't written or at least verified by the (tr)uSDX team. So I guess we'd better have a good antenna or dummy load hooked to our rigs before we ever plug in the USB and 12V cables, and keep it hooked up while there is the possibility of power being applied to the rig. But we are experimenters, and this is experimenting. :D I bought a dozen extra FET's at the time I ordered the Xceiver kit. The FET's are dirt cheap and easy to change, but only if you have them.
peterjbrennan
 

Posts: 8
Joined: 08 May 2022, 02:18

Re: PA Fets blown by connecting USB?

Post by peterjbrennan »

A quick question: on the display, there is an indicator that shows ‘R’ when receiving, and now when I try to transmit, it shows ‘P’.

I thought it showed ‘T’ when my rig was transmitting (though I might be wrong), so I’m wondering if ‘P’ tell anything about what is wrong?

I’m still waiting for replacement BS170s, so I can see if the problem is the PA-Fets as suspected.

73, Pete VA7YBW

DL2MAN wrote: 19 May 2022, 07:38 What I´ve observed is: When any program is initiating a CAT connection, it puts the Radio on TX in some cases. Mostly it´s short pulses, but there might be a failure somewhere, and it goes into permanent TX.
Other than by TXing I could not imagine another way of destroying the PA Fets.
But that´s an interesting topic, and I will discuss it with Guido, to get his opinion, as noone has more insight in the Software, as he has.

73 Manuel; DL2MAN
DL2RMM
 

Posts: 21
Joined: 07 Jan 2022, 21:15

Re: PA Fets blown by connecting USB?

Post by DL2RMM »

peterjbrennan wrote: 24 May 2022, 19:13 A quick question: on the display, there is an indicator that shows ‘R’ when receiving, and now when I try to transmit, it shows ‘P’.

I thought it showed ‘T’ when my rig was transmitting (though I might be wrong), so I’m wondering if ‘P’ tell anything about what is wrong?

I’m still waiting for replacement BS170s, so I can see if the problem is the PA-Fets as suspected.

73, Pete VA7YBW
Pete, the 'P' means you enabled the CW Practice option in menu item 2.8
Set it back to "off" and the 'P' will become a 'T' again when you try to transmit.

73 de Jens, DL2RMM
DL2MAN
 

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Re: PA Fets blown by connecting USB?

Post by DL2MAN »

Exactly ! Thank you Jens !
peterjbrennan
 

Posts: 8
Joined: 08 May 2022, 02:18

Re: PA Fets blown by connecting USB?

Post by peterjbrennan »

Ah, thank you. Silly me - I turned on practice mode to play with a new paddle and had entirely forgotten about that! Good thing I asked, otherwise when I get the new Fets, I’d still think it was broken.
DL2RMM wrote: 24 May 2022, 19:31
peterjbrennan wrote: 24 May 2022, 19:13 A quick question: on the display, there is an indicator that shows ‘R’ when receiving, and now when I try to transmit, it shows ‘P’.

I thought it showed ‘T’ when my rig was transmitting (though I might be wrong), so I’m wondering if ‘P’ tell anything about what is wrong?

I’m still waiting for replacement BS170s, so I can see if the problem is the PA-Fets as suspected.

73, Pete VA7YBW
Pete, the 'P' means you enabled the CW Practice option in menu item 2.8
Set it back to "off" and the 'P' will become a 'T' again when you try to transmit.

73 de Jens, DL2RMM
peterjbrennan
 

Posts: 8
Joined: 08 May 2022, 02:18

Re: PA Fets blown by connecting USB?

Post by peterjbrennan »

My replacement Fets arrived today, and after soldering them in and turning off practice mode, it’s all looking good again.

For any Canadians looking for BS170s, I can recommend Next Gen Guitars (nextgenguitars.ca). It sounds like an odd place to get electronic parts, but they were out of stock at Mouser etc. Next Gen had them in stock, shipped quickly, and they worked!
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