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After a month of perfect operation in FT8 mode, my sBitx v3 has stopped producing the output RF signal. I hoped it might be a software issue, like described here: .
Unfortunately, no correction in the software configuration restores the correct operation.
The status of my radio is as follows:
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Reception works perfectly. The malfunction didn’t affect the receiver.
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When I start transmitting in CW mode, the current consumption increases from 0.35A to 1.05A at DRIVE=100 or 0.84A at DRIVE=50. The peak-to-peak voltage at 14.068 MHz is ca. 11V for DRIVE=100 and 272 mV for DRIVE=50 (measured with 50 Ohm 40dB attenuator as a dummy load, and multiplied by 100)
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The sBitx meter displays 0W and SWR=1.0
When working, the radio was used with the antenna matched via ATU-10-0A tuner with SWR up to 2.
The symptoms suggest that I have fried the transistors in the final stage due to too intensive usage in an FT8 mode.? Am I right?
Should I pair the replacement IRF510N transistors according to their threshold voltage when replacing them?
Is it possible to use IRF520 (with higher maximum ID) instead of IRF510?
Thank you,
Wojtek - SP5DAA
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Wojtek,
Are you facing the same problem on other bands as well? Can you check the power out on 80,40, and 10 M? If the problem is persistent on all bands, you might have to have a look at the driver and the PA section.
IRF510's are quite rugged and you can use two unmatched IRFs from the same batch if matching them isn't an option.
73,
Ragav
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Hello Ragav,
Yes, the problem is the same on all bands. I think that the problem resulted from overheating (the heatsink was quite hot after long operation in FT8 on low bands). I think that after replacing the IRF510 (I have one pair delivered together with sBitx, and I have ordered a batch of them manufactured by Vishay), I'll attach the 12V fan to the heatsink.
73, Wojtek
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I had a similar faiure once (no short, just loss of power) with still tiny bit of output (approx 20db down).? ?Problm was plastic movement of SOLDER which ended up with poor connection for one of the final MOSFETs? ? Re soldered, worked fine!
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If you are in there, and can get access to Aluminum nitride insulators, I would consider replacing the mica with the alum nitride which have much better thermal conductivity.? ?Might keep the junction considerable cooler by making the heat sink considerably HOOTTER.??
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Of course, could have fried the mosfet.? ? The latter has only happened to me once and it resulted in a shorted mosfet -- comletely shorted
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Good luck!
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While in there, it is up to you whether to add back in the TVS diodes.? ?They are incredibly tiny.....? Ashhar thinks they are unnecessary with the V3 ciruitry.
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I do not think that you can put in the IRF520 beause the V3 circuitry is not designed to drive the much higher input capacitance of the IRF520
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73
Gordon KX4Z
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In case of shorted MOSFET, wouldn't it impair the reception?
In fact, indeed it may be a solder problem, because once I have lost power and stopped using my sBitx.? Next day I switched it one again, and the normal operation returned.
This time the normal operation was not reestablished even after long cooling of the rig.
Well, I have to disassemble the device, check MOSFETs and the solder. Anyway, adding a fan for active cooling of the heatsink seems to be a reasonable extension for longer operation in digital modes.
73, Wojtek SP5DAA
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If one of the MOSFETS? ?**shorts**? the entire system fails to operate because it shorts out the power supply, may blow fuses, and you cannot establish the needed voltage on the system due to the short circuit from?+v to ground through the shorted mosfet.
The problem you are having is very different and Im quite?curious what you find as the cause!
Gordon KX4Z
In case of shorted MOSFET, wouldn't it impair the reception?
In fact, indeed it may be a solder problem, because once I have lost power and stopped using my sBitx.? Next day I switched it one again, and the normal operation returned.
This time the normal operation was not reestablished even after long cooling of the rig.
Well, I have to disassemble the device, check MOSFETs and the solder. Anyway, adding a fan for active cooling of the heatsink seems to be a reasonable extension for longer operation in digital modes.
73, Wojtek SP5DAA
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Hello Wojtek,
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I had similar problems with my DE.? That turned out to be overheating drivers in one instance and a failure in the Q9 and Q101 equivalent circuits (the DE documentation did not have Q101 listed).? Verify that there are 12 volts on the drain of Q9 (the tab) when the PTT is pressed.? Also, verify that Q12 to Q15 are on with a 0.6-volt drop from Base to Emitter.? This may be difficult to determine if one has failed since the two are connected in parallel.? The other test is to verify the voltage on the emitter resistors to check for the transistor being on.? They should all be close to the same.
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Another possibility is that the PA_Bias1 circuit has failed. Verify that D6 has 5.6 volts on the banded side when the PTT is pressed.
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Here is the relevant circuitry:
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I agree with Gordon that a shorted IRF510 is unlikely. The first step is to check for proper soldering on the devices.??
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I also strongly agree with Gordon about installing the AlN insulators instead of the mica.? Do this while you add the external fan. As Gordon said, the AlN will make the heat sink even hotter.
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The sbitx uses a bar to hold down the transistor, so you do not need a hole and screw insulator.? If you have other applications, buying the insulators with the hole may be a better choice.
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Post on 开云体育:
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Note: Ashhar Farhan has stated that these are not needed with the new mounting method for the finals.? They do not hurt, so cheap insurance helps with heat dissipation.
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73
Evan
AC9TU
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Hello Evan,
I have verified that bias circuitry works properly. I have even checked that the finals have full power suply voltage on their? drains (13.3V) and that its DC value drops to ca 1V when I press the key.
So it looks like I need to check the path via the LPF filter.
I have found that the HV voltage measured on R213 remains 12.6 V even when I start to transmit. Is the voltage multiplier broken then?
73, Wojtek
Hello Wojtek,
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I had similar problems with my DE.? That turned out to be overheating drivers in one instance and a failure in the Q9 and Q101 equivalent circuits (the DE documentation did not have Q101 listed).? Verify that there are 12 volts on the drain of Q9 (the tab) when the PTT is pressed.? Also, verify that Q12 to Q15 are on with a 0.6-volt drop from Base to Emitter.? This may be difficult to determine if one has failed since the two are connected in parallel.? The other test is to verify the voltage on the emitter resistors to check for the transistor being on.? They should all be close to the same.
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Another possibility is that the PA_Bias1 circuit has failed. Verify that D6 has 5.6 volts on the banded side when the PTT is pressed.
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Here is the relevant circuitry:
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I agree with Gordon that a shorted IRF510 is unlikely. The first step is to check for proper soldering on the devices.??
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I also strongly agree with Gordon about installing the AlN insulators instead of the mica.? Do this while you add the external fan. As Gordon said, the AlN will make the heat sink even hotter.
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The sbitx uses a bar to hold down the transistor, so you do not need a hole and screw insulator.? If you have other applications, buying the insulators with the hole may be a better choice.
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Post on 开云体育:
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Note: Ashhar Farhan has stated that these are not needed with the new mounting method for the finals.? They do not hurt, so cheap insurance helps with heat dissipation.
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73
Evan
AC9TU
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-- Wojciech M. Zabo?otny
My GPG/PGP keys:
standard: 8192R/7E17F00A (B53C 1F09 BEA9 ED46 51B6 ?2C2E 4344 F292 7E17 F00A)
confidential: 16384R/1FEDB03C (3412 2330 E225 D57D 70C3 ?2340 B294 9408 1FED B03C)
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I have checked signals with the scope (measured with 1:10 probe with DC coupling, 500 mv/div so 5V/div considering probe).
Ground level:
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DC level on drains of finals without RF:
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Voltage with RF on the drain of the first final transistor:
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Voltage with RF on the drain of the second final transistor:
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At the same time, the voltage on the secondary coil of the output transformer looks like below (100mV/div - 1V/div considering probe)
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Is it possible that the transformer got broken due to overheat, or that something in the LPF filter shortens the signal?
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73,
Wojtek
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 Do you have 1V here when you transmit?
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Wojtek,
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Is your power supply voltage dropping?? Strangely, the drain voltages drop when you key the mic.? This could suggest that T7 has an issue.? Confirm that you are seeing the drain voltage drop on the IRF510s.
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Does the bias control work?? Can you adjust the radio's current when following the set bias process?? From sbitx.net:
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How to set the bias
Siegfried Jackstien, DG9BFC (modified by me for v3 circuits)
- Set both the POTS to the minimum current
- Select Mode as USB. Reduce the MIC to zero.
- Set the Drive Level to 1
- Press the PTT.
- Without speaking into the microphone, Note the current.
- Keep the PTT on the microphone pressed, and increase the PA_BIAS POT until the current increases by another 200 mA.
- Release the PTT.
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The above is for the V3 or radios that have been upgraded, not the prior versions that have not been modified.? The Drive_Bias pot is either not there or has been disconnected in the upgrade to the V3 PA design.
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73
Evan
AC9TU
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Wojtek,
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Is your power supply voltage dropping?? Strangely, the drain voltages drop when you key the mic.? This could suggest that T7 has an issue.? Confirm that you are seeing the drain voltage drop on the IRF510s.
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As shown in the scope images, the DC of the drain voltages remain at ca 12V. It was the DC multimeter that got crazy when measuring the voltage with RF.
73,
Wojtek
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Wojtek, the scope photos were very helpful.
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That transformer is supposed to Step UP the voltage, not down.? ?So something is terribly wrong, and it appears tht bothof your FETs are doing fine.
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You can divide the system in half and help to figure out where the real problem is, by lifting the Orange wire that goes to the CAPACITORS and send it instead to a 50 ohm dummy load of suitable power handling capacity.? ?Then measure the voltage or power in that line.? ?Of course it will have sizeable HARMONICS so you dont' want to send it to an antenna, but sending it to a dummy load will allow you to check that the transformer is still working properly.? ?You should get PLENTY of power out of the orange wire.? ??
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Assuming you DO get power out of the Orange wire -- then the problem is in the LPF system; something is indeed shorting out your power.
If you do NOT get power out of the Orange wire -- then there is something wrong with the transformer.? ?I didn't see enough in your tofigure it out.? ?It should have two brass rods that are probably soldered together on one side and not on the other, but I'm not able to see mine and not exactly sure how they are assembled.
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Splitting the system into? ?MOSFETS+TRANSFORMER? one one side and? LO PASS FILTERS out of the question will help you figure otu where the problem lies.
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73
Gordon KX4Z
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Is it possible that the software got crazy and didn't enable any section in the LPF filter?
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73,
Wojtek
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As shown in the scope image, I have ca 4.5V peak-to-peak there when transmitting.
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The below message was an answer to sa7buj.
As shown in the scope image, I have ca 4.5V peak-to-peak there when transmitting.
-- Wojciech M. Zabo?otny
My GPG/PGP keys:
standard: 8192R/7E17F00A (B53C 1F09 BEA9 ED46 51B6 ?2C2E 4344 F292 7E17 F00A)
confidential: 16384R/1FEDB03C (3412 2330 E225 D57D 70C3 ?2340 B294 9408 1FED B03C)
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“Is it possible that the software got crazy and didn't enable any section in the LPF filter?
73,
奥辞箩迟别办”
It’s unlikely that the software is doing anything wrong. It’s more likely that some piece of hardware has a problem in order to figure that out. We have to divide this somewhere and decide whether the problem is on the left side or the right side. With your voltmeter, unable to measure DC voltage properly in the face of RF, this is going to be a little more difficult and may require the scope.
But the first problem is probably to split this into modules and figure out where the issue is : with the transformer or beyond that.
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It should be possible to test the LPF selection process with a DMM.? Use USB, set the drive to 1 and the mic to 0.? Then, measure the voltages on the banded sections of the selection diodes.? Only one pair should be close to zero; the others should be close to the input voltage (12 volts) without the RF voltage doubler.
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73
Evan
AC9TU
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Good point Evan!
But I can’t figure out is how disconnecting a Lowpass filter would result in a lower voltage. It should result in a higher voltage. ?
I tried to look for what things might be shorting out. There’s a chance that the high-voltage system is shorting out but it’s only connected by 47 pf?
?and that’s not much. ?
Gordon?
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On Feb 28, 2025, at 16:35, Evan Hand via groups.io <elhandjr@...> wrote:
? It should be possible to test the LPF selection process with a DMM.? Use USB, set the drive to 1 and the mic to 0.? Then, measure the voltages on the banded sections of the selection diodes.? Only one pair should be close to zero; the others should be close to the input voltage (12 volts) without the RF voltage doubler.
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73
Evan
AC9TU
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