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SANDRO - PY2PE
It seems like the BFO mixer is causing you some problems.
There are two slightly different schematics available for the BITX20 design. How we troubleshoot your particular circuit is somewhat dependent on which schematic version you used to build your transceiver. The major difference is that one has a resistive pi-network attenuator at the output of the BFO Mixer, and the other does not have this attenuator. Maybe Farhan could tell us which one is the correct schematic? Without knowing which diagram you used, I will assume that you have the one with the resistive attenuator after the BFO Mixer. If you have the other, you can ignore step 5 below.
Listed below is a set of things to check:
- BFO Injection Level: The unloaded output of Q8 should be at minimum about 6 volts AC. If you disconnect the 1K going to the base of Q9 and using a simple half-wave RF detector you should find in excess of 5 V AC on the emitter of Q8. Reconnect the 1K to the base of Q9 and disconnect the 0.1 between the emitter of Q9 and the mixer transformer. You should measure in excess of 2.5 V AC as the unloaded output of Q9. Re-connect the 0.1 between the emitter of Q9 and the mixer transformer.
- Audio Levels: Disconnect the 1 MFD cap from the collector of Q10 (microphone amplifier). With no load on the output of the microphone amplifier you can expect to see something near 6V DC on it's collector and about 2 V AC with a loud whistle into the microphone. If you have an oscilloscope, this waveform should be close to a good sine-wave. Reconnect the 1 MFD to the collector of Q10. Now, if you look at the output of the microphone amp with an oscilloscope, the waveform may be distorted at higher audio levels. This is due to the loading factor caused by the base-emitter conduction of Q4, and it is normal.
- Mixer Transformer: Proper phase relationships between windings of the mixer transformer is critical. The two secondary windings must be connected as a push-pull center-tapped winding. If they are erroneously connected in-phase, you may be able to receive, but you will not be able to transmit. Farhan's schematic shows exactly how this connection should be accomplished from both a physical and electrical viewpoint.
- Mixer Diodes and Mixer Balancing Capacitors: If the polarity of one of the BFO mixer diodes is not correct, you may be able to receive, but you will not be able to transmit SSB. With one of the diodes reversed, you will not get a good dip in carrier output when trying to balance out the carrier. If you disconnect the two mixer balancing capacitors (the 10 pf fixed and 22 pf variable units) you will still get a dip in un-modulated carrier output when adjusting the 100 ohm carrier balancing potentiometer. However, it may not be as definite as can be obtained with the capacitive balancing components connected. Remember to re-connect any capacitors or diodes that you disconnected for testing. NOTE: The carrier balancing potentiometer must be a non-inductive (not wire-wound) type device.
- Mixer Attenuator/Diplexer: Check the attenuator at the SSB side of the BFO mixer to make sure that you did not put the 22 ohm to ground and one of the 220 ohm resistors in-line with the signal path. This attenuator should have a 220 ohm to ground, then a 22 ohm in series with the signal path, and another 220 ohm to ground. If you accidentally swapped the 22 ohm with one of the 220 ohm resistors, this could result in the extremely low mixer output that you are experiencing.
Beyond the tests indicated above, you may need to check the voltages on Q11 to insure that it is working properly. Also, maybe change out Q11 on the chance that this device may have an Ft that is not high enough to provide sufficient amplification at 10 MHz.
I hope this helps. Please let the forum know what you found, as others may have similar problems.
Arv - K7HKL
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On Mon, 2004-09-20 at 19:29, py1pe@... wrote:
my bitx20 is with problems in the production of DSB (1st mixer), I already did of everything and he/she didn't find a way, it exists but it is extremely small, there is not apparent cause for the problem (mysteries of the electronics).
I should set up an entire new block.
the receiver works very well.
forced to all that tried me to help!!
SANDRO - PY2PE