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Hi All,

Well, I am my own worst enemy.? I have been usiong my DE machine for months with no issues.? I run multiple modes but when I'm not doing anthing particular, my machine is running WSJT-X.

The other day I saw that WSJTZ was available so I thought I'd give it a try.? Rather than being smart (and at least performing a backup or using a seperate SD card), I blundered into loading the software without.? Needless to say, it did not go well.

The software overlay appeared to work but when I tested it, I did not receive any stations and I was not transmitting even though the software appeared as if it was.? To make a long story short, I have unsuccussflly attempted to unload and reload the WSJTX and WSJTZ software and somehow in the process lost the audio interface between the radio and RPi.? As I continued my blunder of installing/reinstalling software and generally making a mess of things, the little radio said enough and one of the PA Mosfets failed.? I replaced the devices and attempted to start over thinking, I'll get the basic machine back to normal and then jump into the WSJT stuff later.? So I downloaded and installed the V2.1 image on a new sd card.? In doing so, I got the audio interface working.? Unfortunately, when I attempt to set PA Bias (full CCW, USB, MIC 0 & Drive 1) upon pressing PTT, I get 300mA of increase immediately before touching the pot and zero output on my scope connected to a dummy load.? From what I remember, this behaviour is uncharacteristic of previous experience where the PA_Bias adjustment although touchy was fairly simple to set.

I really don't fully understand how this works.? I read with interest message thread?/g/BITX20/topic/100042851#103639?in hopes that I to may be missing the proper sequence of transmiter settings should be.? I tend to think that setting the PA bias first is important so before I continue my blunder, I am hopeful that I can get hints on where to proceed.

So to summerize:

• Started with a good working machine.
• Attempted to load WSJTZ.? No joy.
• Attempted to unload WSJTZ and reload WSJTX?
• Audio interface disappeared
• Downloaded and installed sBitx V2 image and loaded.
• Regained audio control
• Good receive function
• PA_Bias does not behave normally. At CCW, PTT jumps 300mA before attempting to adjust.
• Q9 seems to operate normally.

Could my problem simple be bad settings (HW_Settings, TXCAL, etc.)?

Thanks in advance, Scott??

?

Scott,
you have set the hardware and software correctly in the PA_Bias setting.
What you need to rethink is that there is a current draw during reception and there is a current draw during transmission in the minimally uncontrolled state.
These two currents are not the same! Compared to the latter, you have to adjust the current consumption, which is used to pre-tension the PA fet.
--
Gyula HA3HZ

Scott,

When I use the mic PTT with Drive set to 1 and Mic set to 0 in one of the SSB modes, my current does jump up.? After that current stabilizes (should be less than 1 amp), you adjust the bias pot to get a 250ma increase.

Your power supply will need to deliver up to 1.5amps.? If the supply has current limiting (good to use when working on the sbitx), I would set it to 2 amps until the bias is set.? Then up it to 10 amps for normal operation.

Note that a drooping power supply can cause corruption of the microSD card if the supply goes below 5.4 volts.? This happened to me when I forgot to turn up the current limit.? A corrupt microSD card can cause damage to the finals IF the transmitter is used after corrupted.? Not always, but it can happen.? I would recommend having a backup with your latest configuration on another card.? If the finals fail, use the backup to create another backup and then test the repaired sbitx.

73
Evan
AC9TU

ps.: In the meantime, don't forget to have an artificial load on the antenna connector.
The reception current consumption of the radio (in any state) =/= during transmission is not the same as the current consumption at rest (without a signal).
--
Gyula HA3HZ

Scott - it's normal to see some increase in current when switching to transmit - even with the PA Bias pot fully counterclockwise. The driver stages draw some current when you switch to transmit. The 300 mA increase may be normal.

With the PA Bias pot fully counterclockwise, measure the gate voltage at Q16 and Q17 when you switch to transmit. It should be zero. If it's not, then the PA FET failure damaged the bias circuit, which is quite possible.

If you do see zero volts at those gates, then slowly turn the PA Bias pot clockwise and confirm that the voltage slowly increases. Don't exceed a Volt or so unless you are monitoring the current as well. Drain current increases rapidly as the FETs start to conduct.

73; Steve, N3SB


73; Steve, N3SB

Hi Guys,

Thanks for the feedback.? Here are some more notes.

Hi Gyula,

Yes sir.? The reception current draw is approximately 680mA @ 13.8VDC.? When I key the mic (PA Bias pot fully counter clockwise) current jumps to approximately 1.05A.? It's as if there is a partial short in the bias circuit.? I'm just not finding it.

Yes, I've got a 50ohm dummy load connected.

Hi Evan,

Right.? I expected an increase and if memory serves. when I did this the last time, the increase was 10's of mAs, not 100's.??

Power supply is a 20A linear source.? It's pretty darn solid.

Hi Steve,

Actually, this is a DE machine where Q16 and Q17 are isolated and the circuit is bypassed (reference?).
If I'm reading this right, removing C30, C32, C100 & C101 isolate the original driver circuit.? The only component that has not been removed is R50.? That's the 12VDC source for the now removed Drive Bias circuit. I've not removed it because I see no voltage drop at Q9, R10, R50 or R77 when transmit is asserted.? That leads me to think there is something wrong after the power amplifier.? Perhaps a short in the output filter somewhere.? A sticking relay?? My SWR bridge?? I kind of think it's not in the bridge because it's a 100W device and if it were shorted, I'd not get reception.

What's weird is the adjustability of the PA Bias doesn't work as it has.? At PTT activation it jumps 300mA.? Is that right?? It doesn't seem so.? Starting at CCW, I'd expect the current to only increase 10's of mAs and after a few degrees of rotation, it would start to climb.? It doesn't react until about 50-60 degree of rotation and then is climbs very quickly above the 300mA initial jump.

Maybe I'm doing this wrong.? I know the bias is to be approximately 200mA above the CCW level but is this to be 20mA above the initial PTT increase?? Maybe it is and I'm just confused.

Anyway, thanks for your help, Scott??

?

Hi Scott;

You should definitely see a voltage drop across R77 when in transmit. When transmitting, one side of R77 is connected to +12 Volts, and the other side is connected to D6, a 5.6 Volt Zener diode. Please check again. It's possible that the Zener diode has opened up, or that some other part in the PA bias circuit got fried when the FET failed.

Adjusting the PA Bias pot will have no effect on current until you get close to the point where the FETs begin to turn on - then current will begin increasing rapidly. Your description of the PA Bias pot behavior sounds correct.

Transistors Q12 - Q16 are still in the transmit chain on a fully modified DE. They are known to get toasty hot when in transmit. I can see them accounting for the 300 mA current increase. (I haven't bypassed Q16 and Q17 driver FETs in my DE, so I can't compare results with your DE.) You can measure the voltage drop across R19, R20, R45, and R48, and determine the current being drawn by each transistor using ohm's law. Don't forget that these four resistors got replaced with new resistor with a lower value (The V2 has 4.7 Ohm resistors in these four locations.) Gordon KX4Z and others have provided voltage drop measurements at these four resistors here on the BITX20 forum, so you should be able to compare your readings to those previously stated here.

73; Steve, N3SB

I hope you guys are not wasting too many brain cells on this one.? I figured it out.

I had two things, no, three things wrong.

First, I was confused where to set PA Bias.? It is set after the initial PTT power indication.? Then you increase it 200mA more. Next, you perform metercal and txcal to set the power settings.? Finally, and most importantly, you have to make sure your power connection is solid.? I was fooled thinking I had killed it again when in reality, at high power demand, the power connector fails.? When it cools off, it makes contact again and you think you broke stuff.? Tomorrow, I will replace the connector with a XT-60 (or similar current device).? Note to self: Next time Allison makes a suggestion... take it!?

Thanks guys.? Sorry to waste your time.

Cheers, Scott

This is probably the case with others.
Until you need to fix or adjust something yourself, you can read about these things.
You feel that yes, it is useful and I will save it.
When it is needed, the only thing that comes to mind is yes, I read this somewhere, but what needs to be done.
These are all human traits.
We are dealing with too many different things and our brain is dividing what is important and what is not. We forget in one word.

Then there is one more thing. Many people want to combine many good things.
In our country, they say that it is good if the goose is fat.
I think that many of us also like precise things and want to produce the best.
--
Gyula HA3HZ

Hi Gyula,

You are spot on.? I went down a rabbit hole and thought I was lost.? The act of typing to the group and reading everyone's response helped me reconsider and get back on track.

I now have a better machine and a better understanding how it works.? It's funny how we get ourselves into these situations.

Thank you Gyula, Evan and Steve.

Cheers, Scott??