[Central-Electronics] updates, musings and questions...

[Eugene Hertz]

Ok, Gents! This is a REALLY long email. So if you are inclined to read it, you may wish to print it out and read it on the train or bus!

Good news – for those of you following along at home, my most recent trouble with the CE 100V was that I blew a 6AU4 tube.  I still plan to replace this with solid state, but before I did that I replaced the tube with another 6AU4 I purchased. I wanted to see if the tube failure was the cause or the symptom. And the good news is the transmitter is A-OK. So the tube seems to have been the culprit, nothing more insidious! So now I feel comfortable to proceed with the solid-stating. Sure will save a bunch of heat. 

On to some older news. Some of you may recall a previous problem I had that the transmitter output frequency was very unstable. It was not uncommon for the CW frequency to move around 100-300Hz between key up and key down and mostly during key-down.  Some folks suggested this is about what I might expect from a 1950’s transmitter even though the manual suggested about 25Hz after a 5 minute warm up.  The real problem was using RTTY, I was all over the place. I simply could not conduct a QSO.

Seemed I tracked some of this down to the house voltage changing when the well pump kicked in.  Also, when transmitting the voltage in the shack could change 3-5VAC (I will be running new power into the shack once I can save up some shekels).

So to combat this problem, I did several things. For one, I replaced the 0A2 regulator using a hand-selected tube that gave the best output regulation under the worst input conditions.  This helped somewhat.  I then borrowed an "automatic variac" made by GenRad that allows you to set a constant output voltage. Using a motorized variac it will adjust +/- 10% to keep the target output voltage (set to 115 as per the 100V manual).  Slew rate is surprisingly fast. This also helped. 

Now my fluctuations are about 30-40Hz.  So I was somewhat satisfied. I wonder what the manual means when it says 25Hz after 5 minute warm up. Does that mean over a period of hours? Minutes? Seconds? For me this fluctuation is within 30 seconds.  Of course, the variac is pounding away with every dit and dah, but I’m happy to be on the air. RTTY is still a problem, because the fluctuations are frequent. 30-40Hz every 3-10 seconds. Especially when I stop keying for receive and then begin keying again for transmit, I need to constantly adjust the vfo to stay in the passband of the TU (I use MMTTYY for monitoring my own transmit signal).

So I decided to dig more deeply to find where the instability was coming from – even though I may not be able to improve it. Was it from the VFO or maybe one or more of the crystals and/or the HF oscillator?

I decided to look at VFO-out from the rear apron of the transmitter. I used an RCA “T” so that I could monitor the VFO at rest and at play (transmitting).  What I found was even at rest, the VFO is moving all around.  I use a racal-dana 1992. Seems that with the key up, the VFO likes to wander around in the neighborhood of 30-40Hz. This is true on all bands. With the key down (with variac regulator off), the VFO out wanders around in the neighborhood of 100-110Hz within a 5-10 seconds. If I were to wait longer, the variation might be 200+Hz. When the key is released, the frequency comes back up to around where it was (+/- 20Hz that it normally wanders) but it may take 30-40 seconds to come back after transmitting.

Here’s what I am wondering about.  The 150V regulated voltage is very well regulated at this point despite any fluctuations in mains input voltage.  However, is it possible for fluctuations in filament voltage and/or unregulated B+ to cause the VFO to wander? In particular I wonder about B+.  If the electrolytics are not up to snuff, could there be enough ripple on B+ to cause this? The input mains are pretty well regulated (with the auto-variac), the 0A2 output is well regulated. So could some load to either A or B voltages be causing them to sag even though the mains input is constant?

Or, could simply replacing a tube in the VFO possibly improve things? 

So I was wondering if someone might be willing to take some measurements for me. Would anyone on the list be willing to measure their VFO out (using a T of sorts) to see how/if it wanders around in a resting state and to see what it does when transmitting? For me, watching the frequency counter for less than 30 to 60 seconds seems to give a good indication of its ranges of wandering.  To make a good apples to apples comparison, use a counter with 1Hz resolution.

Also, would someone be willing to measure the filament and B+ voltages at rest and during transmit and also measure the ripple? I will make the same measurements too so we can compare notes.  I believe there are two filament windings, one for most of the tubes and a separate one for the rectifiers. I will attempt to measure them both.

So here’s an interesting side note: Just for kicks, I disconnected the VFO-IN from the VFO-OUT and instead, I substituted the output of my HP 8657A RF generator as the VFO set to 5.5Mhz.  And you know what? ROCK SOLID ULTRA STABLE TRANSMISSIONS! I guess that modern technology should be more stable after 30 years! But what this really told me was that the problem was not from any of the crystal oscillators, only the VFO. This was so stable in fact, that I saw approximately +/- 5Hz fluctuations! and these looked more like phase noise type stuff than the wandering experienced with the VFO. With the VFO the more time you were transmitting the further away from the desired frequency you would get (up to about 200 Hz). With the external VFO, you were always +/- 5Hz from the moment you pressed the key until however long after.  Really neat experiment! 

So this has led me to think about making my own external VFO for this. But first, I am interested in other people’s experience with drift from the VFO out connector.  Please let me know what you find.  

Thanks for your time (and patience!)
Eugene