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


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
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