Sunday, February 26, 2017

Zeroing in on issue with transmit operation of HW-100

A pretty busy day as I had forecast, so only a bit of time I could devote to the debugging. 


I set up the scope with probes on the first transmitter mixer V5A, cathode and control grid. The cathode is the VFO signal and the control grid has the carrier oscillator signal from the balanced modulator by way of the first IF amplifier V3 . I can see the VFO signal running about 700 mV peak to peak. The carrier oscillator with the gain all the way up runs at about 2V peak to peak. 

Assuming the two signals should be about the same amplitude for mixing, this means I need to add an amplifier stage inside my DDS box, to boost its output by about 3X before injection into the VFO cathode follower V20, so that its output is now 2V PP. This is a tiny design and construction task, with the resulting board mounted inside my DDS VFO box.

Missing signal

At this point, I should see a good output signal at the output of the bandpass filter between first transmitter mixer V5A and second transmitter mixer V6. However, all I see on that side is some 12395 KHz signal which is the output of the Heterodyne Oscillator for the 80 meter band setting. There should be a 8395 to 8895 KHz signal from the first transmitter mixer.

I set up a probe on the plate of the first mixer, V5A, to look for the sum and difference frequencies that are input to the bandpass filter. Nothing, not an oscillation to be seen. The tube is not amplifying at all. Time to look at the bias voltages and components surrounding it.

When I look into the circuitry, I see that the tube V5A is biased to cut off while the transceiver is in receive mode, but that should cut off when the unit switches to transmit. This occurs by relay contacts that ground out diode D301. The line to the control grids pass through that diode to a resistor to -130V bias voltage. The relay will ground the diode, which allows the tube to get to a small positive bias and begin conducting.

Grid bias to cut off V5A, V6 and V7 during receive operation
I need to use my VTVM to watch the voltage on the control grid of V5A, I should see it heavily negative when in receive and jump up to a small positive value in transmit mode. What I saw instead was the large negative voltage drop to a low negative voltage, not to a somewhat positive voltage. 

I should see almost ground level, except for the voltage drop across D301. What I find interesting is that there is a user modification I made to this circuit, dropping out a 10K resistor between coil L101 and the control grid of V5A. I will replace the resistor and see what happens tomorrow.

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