The DSB80 Part 2 And A New Addition To The Shack

I did make some progress on the DSB80 a couple of weeks ago and then put the project aside.  I became a bit sidetracked by other things, some of them non-radio, so want to provide an update in case the DSB80 project gets shelved for a while.  I received a wonderful e-mail from Frank Ogden G4JST, co-designer of the DSB80. He sent me a picture of his prototype DSB80 with my letter to him alongside. What a thrill! He said that he powered up this fine little rig, and it was still putting out 4W of DSB on 80 with fine audio and a stable VFO:

Frank told me that when he designed the DSB80, he scratched the circuit out on a piece of paper and sent it straight to Tony G3WPO, who came up with the PCB design. It was never bread-boarded.  Amazing!  Frank has many happy memories of making many QSO’s with southern England and northern France from his brother’s boat on this rig.  As I’ve mentioned before, the DSB80 I built from the kit supplied by WPO Communications worked fine.  How I wish I still had it 🙂

If you recall from the last post, I had been having some problems with the buffer amp in the VFO not buffering too well.  Jim K4AHO suggested that I capacitively couple the output of the VFO to the input of the buffer instead of coupling it resisitively. He also suggested a high-value resistor from the gate of the buffer JFET to ground to act as a self-biasing resistor. This cured the problem nicely – on terminating the output of the buffer with a 51 ohm resistor and touching it with a screwdriver, there was no noticeable shift in VFO frequency (unlike before). The output from the VFO transistor was a nice sine wave:

Here’s the output from the buffer transistor when terminated with a 50 ohm resistor:

The above waveform is about 0.9V pk-pk which, by my calculations, across a 51 ohm resistor, translates into around 2mW – just +3dBM, which is a little low for a level 7 diode ring mixer. Also, that waveform – I don’t know how important it is that the VFO buffer outputs a clean sine wave into a 50 ohm load, but I’d sure like to see it.

I decided to press on and see if I could finish the transmitter. This is where I got:

If you saw the earlier iteration of this board from a previous post, you’ll see that I have removed the on-board VFO.  In it’s place is the mic amp for the TX section. I also built the TX driver and TX final. The TX seemed to be generating a pretty nice-sounding DSB signal from the driver using a 2N3904 in place of the originally-specified BC238 device. G4JST very kindly offered me a VN66AF (the device originally specified for the final) for my experimenting, but I wanted to adapt the circuit to use the commonly available IRF510. Heck – you can even get ’em at Radio Shack.

I’ve had a couple of problems with the IRF510 final. The first was solved with the help of NT7S. Jason did a bit of brainstorming with me and we found that there is a choke in the collector circuit of the driver transistor that is designed to resonate with the input capacitance of the MOSFET final.  It turns out that the input capacitance (Ciss) of the IRF510 is in the range of 135 – 180pF, while that of the VN66AF (the final in the original DSB80 design) is around 50pF. Changing the choke from 33uH to 12uH increased the output from the final from 3/4W to something like 2 – 2  1/2W (I didn’t write it down). However, I also noticed that the rig was putting out significant carrier in the absence of modulation. I don’t think this was due to poor carrier suppression – rather due to a spurious response somewhere in my circuit layout.

This is where I’m at with my version of the DSB80. The receiver sounds good and has given me fresh inspiration to build some more direct conversion receivers. I almost wish that I had not built the TX section, as I don’t do a lot of transmitting on phone anyway, and this is the only part of the circuit I’ve had a few problems with! I’m sure that it’s my layout, or my use of different active devices, as my original kit back in 1983 worked well.

So my DSB80 board is sitting up on the shelf for the time being while I give it a rest.

By the way, there’s a new addition to the shack, which you may have noticed from the pictures above. It’s a Tektronix 465 oscilloscope. A very generous local ham gave me his old Tek 465.  I’m not sure whether he wants to remain anonymous or not but he knows who he is, and to say that this was a generous gift is a huge understatement. Thank you very much – you know who you are!

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