Central Electronics 20A follow up

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 Well, It's been a while since I gave this blog or Youtube any attention, but that does not mean things have been at a idle in the basement, on the contrary, quite a lot of things have gotten finished, new projects have moved in, some things have been given up on, and some have been stowed away for future endeavors :)


The Central Electronics 20A



It got it's recap, which went as planned. No major flaws were detected, and powering up was a breeze. I replaced the power cord with a 3 conductor cord, and found a good source here in SM for US type plugs and installed a step down transformer in the shack. (Dont want to mix things up using 115v in our 230v outlets).

The 20A needs an outboard VFO. Initially I used my HP8640B (which also was repaired, should probably do a post about that as well), and that worked just fine for alignment and the occasional qso's, but a more permanent solution was needed.

Alignment

I tried to follow the alignment instructions in the manual, but soon gave up. The manual calls for adjusting the audio phase network balance pots, and the rf phase transformers, trying to get a scope response similar to the pictures in the manual. I ended up doing it by ear, using my K3, switching between upper and lower sideband, while adjusting the 20A sideband generator for best unwanted sideband rejection. I eventually got close to 40db sideband rejection (according to the K3 s-meter) which I was happy about. These adjustments were made with 1Khz injected to the mic input. It would have been better using a sweep generator at the audio frequencies, and a spectrum analyzer (or sdr) for viewing the sideband suppression. 





Making RF

The 20A makes approx 20W on the low bands (160-80) and above that the power out declines, except for 20, which also gives about 20w. On 10 and 15 I barely got 3W.  I noticed that the driver tube, 12BH7, which is a cathode follower, driving the two 6AG7 finals, was weak. Replacing that yielded a couple more watts, now giving around 7-10w on 10 and 15. A mixer tube (6U8) died in the tuning process and was also replaced.  



VFO

The 20A was designed for an outboard VFO, mainly by converting a, back then, plentiful ARC-5 transmitter, providing a VFO frequency of around 5MHz. I did not want to convert my ARC-5 transmitters, plus, getting on any band beside 80 and 20 is a bit troublesome that way.

 The IF of the transmitter is 9MHz, generated by an internal crystal oscillator. By calculating the sum and differences of the vfo injection and the IF 

9+5.2=14.2 or 9-5.2=3.8 

we can see that 80 and 20 is covered with ~5MHz. 



The issue I saw with this, aside from accessing bands outside of 80 and 20, was that it was not user friendly regarding sideband choice. Depending on whether you are doing high side or low side injection, the sideband choice would get inverted. The sideband selection knob on the front panel simply says "Sideband 1" and "Sideband 2". Lets look at it again:

For 80m, we can inject 5.2MHz for 9-5.2=3.8MHz (low side injection). To transmit on LSB, the knob needs to be in Sideband 2. The other option for 80m is to inject high side, 12.8MHz, 12.8-9=3.8MHz. This time, Sideband 1 needs to be selected for LSB. Back in the day, when this was your main rig, you were used to it and it was the way to get on sideband, it was fine. Today, using my HP8640 injecting ~5MHz, I got so confused as how to set it up between uses, not knowing which sideband I was on, having to calculate the output frequecy, so I decided to build a VFO using modern stuff, with a constant high side injection.

I built WA2FZW's excellent "NJAD vfo"




WA2FZW/It-s-Not-Just-Another-Digital-VFO

It has an almost analog feel to it which suits this transmitter well.

NJAD (Not Just Another Digital Vfo) is composed of an ESP32, an SI5351A and a nice little display.

I'm not going to go into detail on that build, since it's well documented already over WA2FZW's github.

As noted earlier, I'm using high side injection on all frequencies, which makes Sideband 1 LSB, and Sideband 2 USB, regardless of frequency. 

Vfo coverage is 160-80-40-20-15-10. Other bands (warc) is possible in the vfo, but tests have shown that the tuned circuits in the 20A is not really suited for that. Stick with the "old bands".

I had to modify some of the code to suit my needs. In short, I used the I/O expander option, I set the offsets for the IF, and I turned off alot of things, not needed for this transmitter.

Band limits are set like this. It might look strange, but if we break it down, it makes sense.

Looking at 80m, beginning at the left side, we can see the number 3725xx, which is the starting frequency (3725KHz) of VFO A. 3668xx is VFO B startup frequency. 3500xx is the  reference frequency (3500KHz) (displayed frequency) followed by 12500xx which is the injection frequency of 12.5MHz. The following 3500xx and 4000xx is the bottom and top band limits. INC_100 tells us the step size when tuning. The BS_160 through BS_10 is a pointer to the bandswitch reading; if the bandswitch is at the 80M position, the program will look up the needed band limits, offsets etc, from this table. 


   band_data bandData[] =
{
//	|    vfoA   |    vfoB   |  refFreq  |  vfoRef  |  lowLimit  |  topLimit  |  incr |   bSW |dir| mode
    {  1936000UL,  1825000UL,  1800000UL,  10800000UL,  1800000UL,   2000000UL, INC_100, BS_160, +1, MODE_LSB },
    {  3725000UL,  3668000UL,  3500000UL,  12500000UL,  3500000UL,   4000000UL, INC_100, BS_80, +1, MODE_LSB },
    {  7100000UL,  7040000UL,  7000000UL,  16000000UL,  6000000UL,   7300000UL, INC_100, BS_40, +1, MODE_USB },
    { 14200000UL, 14060000UL, 14000000UL,  23000000UL, 14000000UL,  14350000UL, INC_100, BS_20, +1, MODE_USB },
    { 21150000UL, 21150000UL, 21000000UL,  30000000UL, 21000000UL,  21450000UL, INC_100, BS_15, +1, MODE_USB },
    { 28250000UL, 28250000UL, 28000000UL,  37000000UL, 28000000UL,  28500000UL, INC_100, BS_10, +1, MODE_USB },
}


I also ended up using the SI5351A to generate the 9MHz IF, since my 9MHz crystal was drifting pretty bad. 

The code snippet i modified to add a constant 9MHz output on SI5351A CLK1 is as follows: 

mode_data modeData[] =

{

{ 9000000UL, C_OSC_CLK1, MS_LSB, 1,     0, "LSB"     }, // PCF8574 Pin 0

{ 9000000UL, C_OSC_CLK1, MS_USB, 2,     0, "USB"     }, // Pin 1

{9000000UL, C_OSC_CLK1, MS_CW,  3,     0, "CW"      }, // Pin 2

{ 9000000UL, C_OSC_CLK1, MS_AM,  5,     0, "AM"      }, // Pin 3

{ 9000000UL, C_OSC_CLK1, MS_DIG, 8,     0, "DIGITAL" } // Pin 4

};


Buffer amplifier

The SI5351A output was not enough to drive the grids of the tubes directly.  A buffer amp was needed. I built a W7IUV 2N3866 amp, and after a few tries got it working satisfactory. Had to include a 2db pad on the amp output, to get proper drive levels to the tube.



An identical IF amp was added a bit later, but that was a "professional" pcb I designed, in made by JLC pcb. Will perhaps replace the dead bug style vfo amp for another "permanent" amp, or maybe, most probably not ;)


      The breadboarded VFO buffer amplifier


IF oscillator modifications.

To go from crystal oscillator, to buffer amp, a  capacitor was shorted, to create decoupling for the screen grid. A thin coax was routed from the back of the transmitter, to the grid pin of the tube, via a small coupling cap.

With this, and a SI5351 calibration, the 20A is spot on frequency



Exterior.

The front panel is a heay, plastic decal, laid out in inkscape. A local company that specializes in custom decals made this from my sketches, and I'm reasonable happy with it. I could have done a better job of lining up screws for the display, since those can be seen, buckling the decal, but overall, I'm happy.








 Thanks, CU DE SA2CLC



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