Anyone can help me with my BitX20?

Started by gil, July 19, 2015, 05:56:29 pm

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gil

Hello,

I've tried everything I could think of, can't find out why I get no modulation.. Anyone here with an oscilloscope willing to have a look? I'll ship it to you with a bit of money...

Thanks,

Gil.

12Footer

Did you just complete it?
I live in SW Florida, so to ship it might be tricky. What is it doing/not doing, other than lack of modulation?

gil

Hello and welcome to the forum.

I recently finished it, yes. It is the BitX20A from qrpkits.com. The manual is on their site.

The VFO works. The receiver seems to work, though the volume seems extremely low. There seems to be a carrier because the carrier suppression variable capacitor does have a slight effect on the RF output. I have biased the finals, and voltages are correct. The problem might be around Q15 or further up the pipe. Hard to say without an oscilloscope. I am waiting for a DSO138 mini oscilloscope kit but that is only good for audio frequencies. I connected a dynamic microphone but noise does not generate any RF output.

Where are you in SW FL?

Gil.

12Footer

August 12, 2015, 11:57:13 am #3 Last Edit: August 12, 2015, 01:18:18 pm by 12Footer
My apologies for the delayed reply, Gil. Was on vacation and had not yet bookmarked RP forums up in my favorites. My name is Sean, N4ZAW, and the QTH is Lehigh Acres, a township East of Fort Myers. I hang out on 7.188MHz when I'm hamming, which isn't often this time of year due the lightning.
I just got done reviewing the schematic and agree with your theory that the issue likely lies before Q-15. And since it has poor amplification of RX and mike audio in-common, I would suspect D14,D15 in the balanced mod network, or a shorted winding in T-6. Enameled #26 wire is a BEAR to wind without it's coating failing. I've been there/done that before.
And even though I was gifted with almost superhuman near vision from birth, winding that small gauge of coated wire would not go well. I recently underwent lens replacement surgery. This resulted in the loss of that gifted vision, almost to the point of avoiding the workshop in total lately. Hopefully, in about five months, my eyes will have "progressed" enough for the final laser-zapping, and I can then be fitted with prescription reading glasses. In the meantime, it's dollar-store specials, which aint cutting it out there in the shop. Also, I discovered that by using a refocused $30 endoscope, I can get-by out there -- though one-handed.  ::)

gil

Ah, I'm in Sarasota.. Well, I got the DSO-138 mini oscilloscope now to use for troubleshooting the BitX. That will allow me to follow from the microphone up the pipe... It's only good for audio frequencies but allows visualizing the waveform. I need to finish my MTR-5B however before diving back in to the BitX.. Stay tuned! Thanks for the suggestions :)

Gil.

cockpitbob

Gil, make yourself an RF probe for debugging the RF stuff.  Here's an example I like.  The 0.01u and 1K mean it will have a BW of 16KHz so you can see the audio envelope of modulated RF.  Using a 1N34 or schottky diode would make it a little more sensitive, but a 1N4148 should work just fine.

gil

Yep, I have one Bob, from building a K2, it was included in the kit..

Gil.