Oh what a relief it is...

Fixing the Ten-Tec Century/21 Sidetone

I get some strange enjoyment out of using my 1977 Ten-Tec Century/21 CW-only novice rig.  It is a primitive radio for QRP(ish) CW operations but it has a charm that keeps drawing me back.  Maybe I just like to take the road less traveled.

Foreground: Audio pre-amp board in the bottom half of the Ten-Tec Century/21

It has a charm but something had to change

If you've previously read my blog you'll recognize one of my nits about the Ten-Tec Century/21 concerned its sidetone.  The same quad op-amp LM3900 chip is used for both the audio filter and produce the buzzy, square-wave sidetone.  The side tone pitch is fixed at 475 Hz while the audio selectivity filtering is centered around 750 Hz .  The sidetone volume on the C21 is adjustable but the pitch is not.  The later model Ten-Tec Century/22 offered adjustable pitch for the sidetone.

I had learned to live with the obnoxious sidetone and somehow even appreciated its novelty until I built an Elecraft AF1  kit to tighten the audio-bandpass filtering for this radio.  The Elecraft AF1 does its job very well; so well in fact that when zero beating a CW signal it results in the audio peak pitched at 750 Hz (the natural center for the C21 audio filter), the external Elecraft audio filter in its tight filter mode then filters out the sidetone which is pitched a few hundred Hz below the CW audio signal.  That means that while my AF1 filtered the incoming CW wonderfully and provided much better selectivity than that provided by the C21's built-in filter, I couldn't hear my sidetone when sending.  I could hear the clacking of my straight key but when using a bug I was literally deaf to what I was sending. 

I got around this problem temporarily by using my HAM Keyer as my key interface to the radio and listening to the sidetone from its external speaker (keeping one side of my headphones ajar).  Well that quickly became bothersome; both to me for having to wear my headphones askew, and to family members within earshot of the external keyer's audio.  

The pitch of the sidetone needed to match the pitch of the received CW

So I decided I needed to change the pitch of the sidetone to match the 750 Hz audio center of the CW filtering built into the radio.  Doing so would allow my external bandpass audio filter to pass the sidetone at the same pitch I'm listening to received CW.

A kind ham (WA4FOB) sent an email suggesting that I modify the op-amp circuit generating the sidetone.  He told me that I needed to change the resistance of R1 in the 80356 audio preamp  schematic.  But apparently Ten-Tec made some running changes to the pre-amp audio board because quite a number of resistor values on my board didn't match the values listed in the schematic. For example, the schematic showed R1 as a 68K resistor but no such resistor with that value was on the board.  

A bit of detective work was required.. 

The 80356 preamp audio board is directly connected to the AF and Selectivity knobs on the face of the radio and removing the board turned out to be more difficult than it should be (longer story not recounted here).  So without the ability to trace the circuits from the bottom of the board I had to determine the correct resistor to modify by tracing it backward to the correct pin on LM3900 by measuring resistance between components.




Op-amp schematic for the TT-C21 sidetone

The result

In the end it took me a little over an hour to find the correct resistor and then, with some trial and error, determine the correct resistance to add in parallel to bring the pitch up to 750 Hz.

R1 on my radio turned out to be a 47K ohm resistor rather than the 68K value indicated in the schematic.  I knew that I needed to lower it's resistance to cause the tone generation to raise pitch so I tested a few resistor values before settling on a 33K ohm resistor to be used in parallel with R1.  That gave me a resulting resistance value of:

1 / (1/47000 + 1/33000) = 19387 ohm

A resistance of 19387 ohm results in the sidetone pitch generated by one of the LM3900 op-amps very near my goal of 750 Hz.  The sidetone pitch now lies within the bandbass of my external filter and matches the received CW natural volume peak at 750 Hz. Another benefit of raising the pitch  has been to smooth out the brassy square wave.  It is now quite pleasant to listen to.  I may miss that old buzz-saw sidetone...  Nah, I won't.

All-in-all this was a fun little bit of detective work for an electronics novice such as myself.  I am quite satisfied with the result of this simple modification.




33K ohm resistor soldered in parallel with R1 to raise the sidetone pitch to 750 Hz

Scared of electronics?

I must admit that even though HAM radio has historically had a strong focus on electronics my background is completely devoid of working with electronics.  I was simply an "appliance user" of radios. But purchasing this old, inexpensive radio has been a boon for learning about a subject I should have studied long ago.  When I bought this radio it had a number of quirks.  The radio arrived with previous owner(s) modifications for driving an external amplifier and other "features". Some of those modifications negatively impacted both the QSK and the drive adjustment.  I spent hours studying the schematic and probing around the radio before I determined what to remove to get it back to a more factory-standard state.  It has been a great learning opportunity.  Since the radio was relatively inexpensive I am not too concerned about breaking it and the circuitry doesn't get much simpler in a transceiver than this.

Pick up an old clunker to mess around with

My little investigations under the covers of this rig have encouraged me to consider building something more complicated than the regen-radio and audio filter kits that I've built to-date.  Without the Ten-Tec C21 I don't think I would have the confidence to move forward in my nascent journey into the world of electronics.

If you are an amateur radio operator who, like me, is a total novice at electronics consider picking up an older solid-state radio such as this one just to learn about electronics and experiment on.  I think it will encourage you to learn more and become a better ham.  

But as with all things that run on and generate voltages that can maim or kill, go slow in your learning process and take precautions to keep yourself and bystanders from injury.

That's all for now.

Lower your power and raise your expectations

Richard, N4PBQ

Finer than a frog's hair split twice

Elecraft AF-1 Audio Filter Kit

I've been working a lot of SKCC CW stations during the holidays and adjacent stations really interfere with my ability to use the vintage Ten-Tec Century/21.  Its built-in audio filter is relatively effective if the band isn't too crowded but if I'm working a station and others pop up within a 1kHz on either side I have a real hard time keeping track of which QSO to listen to.  I wanted some relief from the relatively porous audio filtering provided by the old girl.

A kit from Elecraft seemed to be the ticket to better signal isolation for my old radio.

Elecraft AF-1 Audio Filter Kit

Building the kit

This kit is about $60 and is pretty easy to build  There are a couple of ICs to solder so it will be easier if you have a temperature controlled, fine tipped soldering iron. Santa brought me a nice soldering iron for Christmas so this was my first chance to make some use of it. Everything about the kit is very straightforward and Elecraft has wonderful build instructions along with a well laid out board. Their instructions list components in the order that you will be installing them on the board, left to right and even include the color coding or numbering for resistors and capacitors right there in the instructions so you don't have to keep going back to look up the coding.  I've only built one other kit previously and the Elecraft instructions are better.

My only gripe is that this kit has been out for quite some time, but for some reason they shipped me a Version "A" board that required a trace to be cut and a couple of jumpers installed to correct a board error.  I would hope that they would ship new versions of the board but apparently you can get an old version.  Next time I order a kit from them I may specify that I want the latest revision of the kit.

Performance

The board powered up and worked right as the last of the solder smoke was wafting away.  I connected it to the Ten-Tec and tried to find some adjacent station operations to test against but the bands were not terribly busy tonight. I did manage to get a decent audio test on a calling station and created a video to demonstrate the board's capabilities.

Summary

This is the first kit I've built from Elecraft.  It was a simple one.  The instructions were excellent, the silkscreen layout on the board was straightforward and all the parts were in the bag.  The kit does not come with an enclosure so it looks a bit unfinished and the knobs are a bit wobbly on their tall, plastic shafts.  That is my only negative concern regarding the finished kit.  Putting it in an enclosure would also require the battery holder to be moved to the bottom of the board to clear the shafts exiting the top of an enclosure.  Certainly not a big deal but I would be willing to pay an extra $10 for a ready made enclosure because when I've tried to make them they look like junk.

But the bottom line is that if you have an older radio that lacks good filtering or you've built a homebrew radio that you want to be more usable on the air, the Elecraft AF-1 is an excellent addition.  I think it is going to serve me well with my vintage radio.  Now to find a project box to fit it.

That's all for now

So lower your power and raise your expectations

73/72

Richard, N4PBQ

Which antenna is better?

Real world antenna comparisons

I don't have many choices for my antenna at my residential QTH.  I have an 80m OCF Dipole (aka Windom) hung from the peak of my roof at about 25ft and a ladder line fed 40m Doublet folded around in my attic at about the same height.

Generally I've been using my 80m Windom for most contacts because it is resonant on 7 bands and I don't have to tune it.  I've had a number of on-air ragchews where I switch antennas and ask for signal reports but it is often a hassle and I don't like to interrupt a ragchew to ask for that sort of comparison. 

So how do I know which antenna is better for certain bands and directions?

The RBN

The Reverse Beacon Network offers an objective and patient antenna tester.  I just recently learned about this resource and it's a fabulous tool.  When my CQ isn't being answered I'll often bring up the RBN and check my signal reports and then switch antennas to see how they compare.

It has been very interesting to learn that the Doublet in the attic often outperforms my Windom usually by 12db and sometimes by as much as 20db especially on the higher bands.  I'm guessing that part of it is the greater efficiency in the ladder line but that doesn't tell the whole story because the Windom offers a low SWR on most bands and shouldn't be eating up much power in the coax through SWR losses.  Antennas are magic.

Two highlighted records show the difference when I switched antennas and moved the transmit frequency slightly to cause a new report to be sent

Process

Ok so here's what I do... The RBN stations will report only when they see a new frequency or it's been 15 minutes since the last report for a given station, so when I switch antennas I will move frequency slightly to trigger the new report.  Now admittedly QSB or other conditions may change between the reports so you'll need to do this a bunch of times over the course of days and compile some records to get a clear idea of how each antenna is performing in certain directions.  This isn't hard science but it's more real-world than a smith chart or the occasional signal report from a station or two.

If you make use of the RBN I encourage you to donate on their site.

That's all for now...

So lower your power and raise your expectations

72/73

Richard N4PBQ

Vertical Space

Taking my station to new heights

My burgeoning QRP station was getting messy, having stuff strewn all over the desk, so I made a shelf from some junk laying around to give me some vertical space. It is much more organized now.

A shelf helps

Left to right top shelf:
Ozark Patrol Regen Receiver Kit, MFJ Versa Tuner providing both tuning and antenna switching duties, Two Position Rig switch on top (blue box), Ham Keyer (circa '78) electronic keyer

Left to right under shelf:

Yaesu HT connected to home-brew J-Pole in attic, two switching power supplies, VHF/UHF meter, Elecraft KX3, Ten-Tec Century/21 (circa '77), Homebrew frequency counter on top of TT)

Three keys:
Bencher paddles, Vibroplex Bug, Kent Hand Key
That's all for now.

So lower your power and raise your expectations

73/72

Richard N4PBQ

QRP fun and games in the December SKCC WES

Sprinting with a Straight Key - QRP Style

So far in my CW/QRP trek I had not entered a contest nor tried to work so-called "sprints" other than making a few casual contacts.  My copy skills and knowledge of what was going on just was not up to the task.  But after getting my SKCC Centurion certificate last week I was motivated to accelerate my timeline for making the SKCC Tribune level and for that I needed 50 new Centurion contacts.  This weekend was the December SKCC WES (weekend sprintathon) and I determined to make an effort to see how this sprint stuff worked.

I operated 5 watts QRP and used my Vibroplex Bug for most contacts but switched to my Kent Straight key for stations that were sending slower than 13 wpm.

SKCC operators only use manual keys; straight keys, bugs and cooties during SKCC contacts.  So, in general, the operating speed is quite sedate compared to other sprints or contests.  I'd guess most exchanges were below 20 wpm.  That is a good thing for a new CW operator.  The flip side to that is that the operators are all using manual keys and thus the precision of the Morse Code that would be lent by an electronic keyer is let's say, missing.  While most stations I worked had great sounding FISTs I was challenged on a few occasions to copy some very non-standard sounding code so, as they say; YMMV.

For the most part I called CQ rather than tuning around for contacts. There were a few times where multiple stations answered at once and I admit that I couldn't make heads or tales of what I heard and just sent AGN? until I could hear part of one call separated from the others.  I have a greater respect now for contest operators who can pick a call out of the cacophony of multiple stations calling on the same frequency.

My "Weekend" Sprintathon was actually only 3 hours

I only had the opportunity to operate for about 45 minutes Saturday morning when there were a lot of stations looking for contacts, then I had to break until around noon and the bands were not as lively.  I then had another break until late afternoon before I had a Christmas party to attend, so in total I only had about 3 hours.   The WES is actually still running but you can only operate for 24 hours of the 36 hour sprint so my 24 hour window is over.

In my 3 hours had a rather poor showing of 41 contacts but I recognize that if I could have operated longer during the morning and some Saturday evening I certainly could have logged more contacts.  Nonetheless, it was a good experience.  I realize I need to work on copying call signs.  I'm used to listening to them at least a couple of times to copy them but often in a sprint or contest they are only sent once so you need to be listening carefully.   After maybe a dozen more such sprints I might think about entering an actual contest.

Log Snippet

SKCC Logger

Summary

My goal was to get 50 new Centurion contacts but as you can see from my log summary above there were only 3 "Cs" logged.  Centurions appear to be the rarest of the breed so getting to Tribune is going to take longer than I thought.

Correction: I was contacted by a couple of SKCC members to tell me that any new contacts since my Centurion award with Cs, Ts or Ss count toward the Tribune.  Also they told me the 24 hours is operating time rather than a window... so I should have hung in there but I already submitted my log so I'll know better next month.

So if you are a new(ish) CW operator and want a low stress, slow speed introduction to a contest "type" event,  I can highly recommend the SKCC WES.  I think it's also ideal for QRP operators because these don't seem to be zillowatt station operators or big-gun contester types working these events and your modest power should be sufficient.  One suggestion is that if you're calling CQ rather than chasing stations you will only be getting called by stations who can hear you well enough to copy and likely their signal to you will be better than yours to them so that makes it even easier.

That's all for now

So lower your power and raise your expectations

73/72 

Richard N4PBQ

SKCC Centurion Achievement

I set forth my hand on key...

Only manual keys like straight keys and bugs count on SKCC contacts

I joined the SKCC when I started operating CW in late August this year.

SKCC is an abbreviation for "Straight Key Century Club" (not to be confused with "Silent Key Century Club").  

SKCC is an organization similar to FISTS, dedicated to promoting Morse Code / CW operations, but in particular, fostering the continued use of manual keys (straight keys, bugs and side-swipers) because of the historical significance of manual keys in Morse Code.  The SKCC group has some nice tools on their website for looking up member information as well as a special link to the K3UK Sked site so that you can arrange a QSO whenever you like.  You don't have to call CQ till you're blue in the knuckles, just post your frequency to the Sked page and within a couple of minutes you'll likely be in a QSO... the only caveat is  you should be using a manual key (no electronic keyer).

Awards and activities

SKCC members are an active and chatty group of operators who exchange their member numbers even when not in a contest.   They offer a number of achievement awards; some awards are for logging contacts with active SKCC members over time.  The first achievement level is the Centurion Achievement which is given for 100 unique SKCC member contacts.  I hadn't been keeping close track of my count of unique SKCC contacts but I figured I likely had over a 100 unique. I finally went through my log and found that I had over 150 unique SKCC contacts since I became a SKCC member.  So I submitted my log to the SKCC and within a couple of hours it was verified and I received this amusing notification via email of my achievement:

Hear Ye! Hear Ye! Gather 'round and pay heed to the news of this day !

Let there be celebration to recognize a fellow SKCC member who has advanced to the Centurion level of SKCC achievement !!!

Having submitted a log and sworn statement, documenting the completion of QSOs with 100 other SKCC members, Richard Carpenter, N4PBQ, SKCC #14276, is duly proclaimed to be SKCC Centurion Number 1261 effective at 0000Z on 9 Dec 2015. SKCC Centurions are encouraged to proclaim their accomplishment by appending a C to their SKCC Number.

The Master Centurion List has been duly scribed where all SKCC Members may reflect upon and pay tribute to this momentous achievement.

Duly approved, recorded, and published.

I set forth my hand and key.

8 Dec 2015

Ron Bower, AC2C
SKCC 2748S
SKCC Centurion Administrator

As a new CW operator little things like this are nice incentives to keep moving forward with my on-air activities.  This isn't contesting but I feel like I am achieving something as I reach little milestones.

Update 12-19-2015

Well going from Centurion to Tribune is pretty easy when you spend some time in the weekend sprint.  My Tribune level was confirmed today, but going from Tribune to Senator is quite a long road so I expect it to take more than 11 days haha.

That's all for now

So lower your power and raise your expectations

72/73

Richard N4PBQ

Junkbox eardrum saver

Volume control for my volume control

So my lovely 1977 Ten-Tec Century/21 transceiver has not-so-lovely CW sidetone.  The sidetone has a pronounced thump or pop on the first key-down of each letter. When listening through the external speaker it's not too noticeable but when wearing headphones it becomes really obnoxious and even painful during a long ragchew.

There is a variable resistor on the C21's audio board for the side-tone but it doesn't eliminate the pop, in fact it makes it more pronounced as the sidetone volume is reduced and it is least pronounced when the volume is increased but then the sidetone is really, really loud, especially when using headphones.

Besides that, the headphone AF output is quite hot anyway even at the lowest AF level with my comfy Sennheiser headphones so I figured I could benefit from an in-line volume control.  

I looked at ordering one but I've used in-line volume controls built-into headphones before and they always seem to go bad within a year so I scrounged around in my junkbox and found some variable resistors and volume pots.  I tested them and found one that goes from 10 ohm - 475 ohm so it seemed to fit the bill.  I fished around some more and found a 1970s era Switchcraft 1/4" phono plug and some more old lamp cord and I was in business.

I used the end cap from an old USB thumb drive to lend some mechanical strength to the attachment of the variable resistor to the cord.  Wallah, in-line volume control with a hip see-through enclosure.

Junkbox, in-line headphone volume control

It works great.  Now I can tone down the killer sidetone audio at the headphone jack from my old beauty and as a bonus, the old Switchcraft plug is a historical match for the C21. The variable resistor let's me tailor it to my ear fatigue.  I know this is a simple little 5 minute project but sometimes those are the ones that you appreciate the most.

Free in-line volume control
might buy me a couple of months of hearing in the future

That's all for now.

So lower your volume and raise your hearing longevity

73/72

Richard - N4PBQ