Topics

#bitx40 Q8/Q9 dead - no TX or RX #bitx40


Ryan Flowers
 

Hi everyone, it's been quite a while since I posted here! 

The short version: I have no receive, and Q8/Q9 show now voltage on the base or emitter. Collector has the correct voltage, about 9v. I replaced them (and Q7) with 2n3904's, and the symptom is the same. I checked all resistors in the DDS preamp circuit and none of them were open. I'd think that R92 would be suspect because it biases Q9, but it tests to 10K resistance. C91 and C92 are removed. R91 is fine, as is R80 and R81. So, I'm at a bit of a loss. Here's a refresher on the circuit:



The longer version: It quit transmitting one day, so I took it out of service until I had time to work on it. As I was working on it, receive stopped working. I think my little 5v regulator for the arduino bumped something, and something that might not have been able to get 12v nominal got a dose of 12v nominal. The little power supply lives right above that area on a standoff, but it was floating free while I was working on it. That was a Bad Idea. For now I'd just like to get this thing going again. 

What confuses me is that replacing the transistors didn't do anything to resolve it. I did check capacitors, and some of them showed about 3k-5k resistance, but that was in circuit, so kind of expected. 

If anyone can assist, I'd be grateful for the help to get my beloved BITX back on the air. 

Oh, and as for the dead PA, I think I have that figured out. Guess who didn't have diode protection on Q13? <--- this guy.

--
Ryan Flowers - W7RLF
MiscDotGeek.com
Multi Band BITX40
The BITX40 FAQ


Evan Hand
 

Ryan,

The only way that I can see a 0 voltage on the base of Q9 is if the DDS1 signal is shorted to ground.  You said that the 5v regulator for the Arduino bumped something.  Maybe the DDS source is not working.

I would remove the DDS1 connection and check Q8 and Q9 voltages again.  If there is now bias, then the DDS1 signal is the problem.

73
Evan
AC9TU


Jerry Gaffke
 

Ryan,

Well, I haven't posted in some time as well.

A possible quick fix:  remove Q7, Q8, Q9.
Drive the mixer exactly as Farhan now does in the uBitx with a 4 resistor network at the transformer.
The only reason Q7,Q8,Q9 are in there is because the Bitx40 used to have an analog VFO.

Longer answer, if you feel compelled to keep your rig unmodified:
I assume L4 has been removed, it must be if this rig is used with a Raduino.
And as you say, C91 and C92 have been removed, mostly so it works with a high side VFO under Allard's firmware.
Unplug the two pin cable from the DDS1 connector, just to make sure that is not fouling things up.
.Now when you power up, R92 and R93 should be putting 4.5v on the base of Q9,
and the emitter of Q9 should be about 0.6v lower than the base, so about 3.9v.
WIth no Raduino connected, those should all be DC voltages and easy to check with a DVM.

If the emitter is not 0.6v lower than the base, then Q9 is suspect.
If the emitter still appears to be shorted to ground, check it out with an ohmmeter with power off.

Moving on, the base of Q8 should be the same DC voltage as the emitter of Q9.
And the emitter should again be 0.6 volts below the base.

Q7 is AC coupled by C72, but if you jump across C72 with a wire you should see
the emitter of Q7 at 0.6v DC below the emitter of Q8.

Checking things out at DC does not guarantee proper AC operation,
but may be the best route to figuring out this particular problem.
Maybe put a separate voltmeter full time on the 9v rail, my best guess
is that it looked fine at first but for some reason went to 0v later.

Having 12v DC go to some random point into the radio might be very bad news,
you could have a significant learning experience ahead of you.

Jerry, KE7ER


On Thu, Sep 17, 2020 at 06:09 PM, Ryan Flowers wrote:
Hi everyone, it's been quite a while since I posted here! 

The short version: I have no receive, and Q8/Q9 show now voltage on the base or emitter. Collector has the correct voltage, about 9v. I replaced them (and Q7) with 2n3904's, and the symptom is the same. I checked all resistors in the DDS preamp circuit and none of them were open. I'd think that R92 would be suspect because it biases Q9, but it tests to 10K resistance. C91 and C92 are removed. R91 is fine, as is R80 and R81. So, I'm at a bit of a loss. Here's a refresher on the circuit:



The longer version: It quit transmitting one day, so I took it out of service until I had time to work on it. As I was working on it, receive stopped working. I think my little 5v regulator for the arduino bumped something, and something that might not have been able to get 12v nominal got a dose of 12v nominal. The little power supply lives right above that area on a standoff, but it was floating free while I was working on it. That was a Bad Idea. For now I'd just like to get this thing going again. 

What confuses me is that replacing the transistors didn't do anything to resolve it. I did check capacitors, and some of them showed about 3k-5k resistance, but that was in circuit, so kind of expected. 

If anyone can assist, I'd be grateful for the help to get my beloved BITX back on the air. 

Oh, and as for the dead PA, I think I have that figured out. Guess who didn't have diode protection on Q13? <--- this guy.

--
Ryan Flowers - W7RLF
MiscDotGeek.com
Multi Band BITX40
The BITX40 FAQ


Jerry Gaffke
 

Actually, not much need to remove Q7,Q9,Q9 if doing the quick fix.

Remove R70, short across C71 to ground T2-1,
cut the trace from Q7 to T2-6,
then add Farhan's 4 resistor network between Raduino and T2-6.

Removing 12v from the LM78L09 at U2 might save a few milliamps.



But again, you may have a bunch of other issues to chase once this is cleared up.


Evan is right, a grounded DDS1 would give the symptoms you describe.
Base goes to ground, and so does everything else around Q8 and Q9.
But DDS1 has a series cap on the Raduino, so a short on the DDS1 signal
would almost have to be a problem with the DDS1 wiring harness.

Jerry



On Thu, Sep 17, 2020 at 07:00 PM, Jerry Gaffke wrote:
A possible quick fix:  remove Q7, Q8, Q9.
Drive the mixer exactly as Farhan now does in the uBitx with a 4 resistor network at the transformer.
The only reason Q7,Q8,Q9 are in there is because the Bitx40 used to have an analog VFO.


Evan Hand
 

Syd,
Building off what Jerry said, and know remembering the Raduino connector pinout, I think you are one pin off on the Raduino to DDS1 connection.  On the Raduino every other pin is ground around the Clock signals.

73
Evan
AC9TU


Ryan Flowers
 

Thank you for the answers, gents! It's possible that I messed up my DDS in the process- that would definitely explain it. It's a QRP Labs Arduino dev board and SI5351 board, and a regular Arduino running my own custom (read: horrific) hack of the uBITX code. Once I get it fixed, I'll be redoing the code.

I'll try these things as I have time.
--
Ryan Flowers - W7RLF
MiscDotGeek.com
Multi Band BITX40
The BITX40 FAQ


Ryan Flowers
 

Evan, you called it! I didn't think to involve the DDS. The output of the SI5341 (clk0) goes straight to ground. Poof! I have another one, I'll swap it in later. I'll also consider feeding the mixer with a network as you mentioned, Jerry. That's a neat idea! I plan on homebrewing my own BITX40 this winter, so that'll be something I'll do with that one. It'll be my first scratch built transceiver :) 

--
Ryan Flowers - W7RLF
MiscDotGeek.com
Multi Band BITX40
The BITX40 FAQ


Jerry Gaffke
 

Ryan,

When directly driving a Bitx40 or uBitx mixer, you definitely want the series blocking cap that Farhan has on the Raduino, one on each output.
Many (most?) of the Si5351 boards don't have this, as they are meant to drive digital pins, not low impedance mixers at reasonably high frequencies.

If you don't have a Si5351 board on hand and want to see something work,
could bring back the old analog VFO by adding L4, C91, C92, and a 10k linear pot at TUNING1.
Usable but rather unstable, there's a reason Farhan eventually went with the Raduino instead.

Jerry, KE7ER


On Thu, Sep 17, 2020 at 07:43 PM, Ryan Flowers wrote:
Evan, you called it! I didn't think to involve the DDS. The output of the SI5341 (clk0) goes straight to ground. Poof! I have another one, I'll swap it in later. I'll also consider feeding the mixer with a network as you mentioned, Jerry. That's a neat idea! I plan on homebrewing my own BITX40 this winter, so that'll be something I'll do with that one. It'll be my first scratch built transceiver :) 


Evan Hand
 

Ryan,
Glad you found it!  I would echo what Jerry suggested to add the blocking capacitors if they are not already on the board (since it is an AdaFruit board, those are often designed for logic circuits and may not have the DC blocking caps).

Have fun with the new build!
73
Evan
AC9TU


Ryan Flowers
 

Jerry, Evan thanks again! I'll check out the blocking cap. But, this is a QRP Labs board made for radio, not the adafruit board. As for the drifty varactor VFO... No thanks! Been there done that. This thing spends most of its time on digital modes ;) It has been hacked to death and back more than once, and pretty soon it'll get hacked yet more :)
--
Ryan Flowers - W7RLF
MiscDotGeek.com
Multi Band BITX40
The BITX40 FAQ


Ryan Flowers
 

Had a 10 minute work break (working from home is great!) and a new Si5351 is installed with a 100nf decoupling capacitor between the clock output and the board. I'm now getting DDS sounds again, but still no reception. More troubleshooting to do. I'll update later when I've had time to play with it. For now, back to work! 

--
Ryan Flowers - W7RLF
MiscDotGeek.com
Multi Band BITX40
The BITX40 FAQ


Jerry Gaffke
 

The QRP Labs synthesizer kit does not have series caps on the output.
This is typical of Si5351 boards, if you want series caps to level shift such that the signal is
centered on ground, then those caps can be included in your target circuit.
You typically don't want the caps when driving digital loads.

Not an issue for driving a stock Bitx40, but the caps are needed if driving the transformers directly as the uBitx does.

Jerry, KE7ER



On Thu, Sep 17, 2020 at 10:51 PM, Ryan Flowers wrote:
I'll check out the blocking cap. But, this is a QRP Labs board made for radio, not the adafruit board.


Ryan Flowers
 

The Plot Thickens!

It ate another SI5351 board :-/ I had assumed that I had killed the board, but I think the death goes deeper somewhere, as now that board is dead. And, the arduino might be dead too. I have some parts to order and I'll be trying this again after replacing the Arduino Uno, si5351 board, and checking all voltages on the BITX40 according to K7AGE's PDF.

A scratch build is looking better and better...

--
Ryan Flowers - W7RLF
MiscDotGeek.com
Multi Band BITX40
The BITX40 FAQ


Jerry Gaffke
 

The stock Bitx40 has few wires between Raduino and the main Bitx40 board,
just 12v DC to power the Raduino's 5v regulator, that Si5351 out through a series cap,
and a couple grounds.
The series cap helps avoid danger to the Si5351.
Very little danger of destroying the Raduino if you're careful with where 12v might touch.
Especially problematic has been the extra leads from the Bitx40 Raduino wiring cable,
many in the forum have had the tip of an unused lead hit 12v and blow up the Nano.

If you did Allard's mods, make sure his additional wires do not have anything
that exceeds 5v going from the radio back into the Arduino..

Jerry, KE7ER


On Fri, Sep 18, 2020 at 08:27 AM, Ryan Flowers wrote:
It ate another SI5351 board :-/ I had assumed that I had killed the board, but I think the death goes deeper somewhere, as now that board is dead. And, the arduino might be dead too. I have some parts to order and I'll be trying this again after replacing the Arduino Uno, si5351 board, and checking all voltages on the BITX40 according to K7AGE's PDF.


Ryan Flowers
 

Yep. I agree 100%. There's something else that's hinky, and I don't know what it is. I need to do more testing before I power this thing up again. I think 12V got somewhere it shouldn't have. It might have been isolated to the arduino and the si5351 or it might not have. I've been working on it in its enclosure, which is a custom 3d printed job, and the close quarters is making things hard.:Something is damaged though, and it killed the 5351 again and I need to track down why.

Like I said, this does have a couple of Allard's mods, but is my own firmware and built with very different parts. There's no Raduino here. Further digging will commence when I can. I need to order more parts.

--
Ryan Flowers - W7RLF
MiscDotGeek.com
Multi Band BITX40
The BITX40 FAQ