Tuesday, January 16, 2018

Closing Loops

A while back, I walked picked up a couple of deals at the Pomona swap meet -- a Heathkit HG-18 sine and square wave generator that looked like it was fire-bombed and an AN/URM-25D signal generator that also looked like it was at death's door. The HG-18 was preliminarily inspected and posted here and the AN/URM-25D here. I wanted to close the loop on these, so here goes.

Actually, you don't "pick up" an AN/URN-25D at a swap. They're magnetic and..well, as I explained to the folks on the Antique Radio Forum:

Anyway, these things are not nearly as bad as they look. Military gear is virtually hermetically sealed and the innards are usually pristine. I used to collect military radio gear and some of the World War II and Korean War stuff -- particularly that which was to serve in tropical environments were water and fungus-proofed to beat the band. So, you can imagine how clean the insides of test equipment that dwelt in a service shop must have been babied was like.

Bottom line on this was that there was a cold solder joint on the frequency turret and, once repaired, the little beast was dead bang on. All that was left to do was get out the can of elbow grease and get 'er done.

..rock solid stable as well. Not bad for some old gal you pick up at a swap meet, eh?

Thursday, January 11, 2018

Proposed Winter Field Day Site

This is at Mile Square Park in Fountain Valley. Looks a lot gloomier than it really is. This is in the midst of Winter after two days of rain took down a lot of leaves. This is a large park bounded by Warner Avenue on the South, Brookhurst Avenue on the west, Euclid Avenue on the East, and McFadden Avenue on thr North. It's one mile by one mile ~~ hence the name Mile Square Park. (Stultifyingly obvious.)

Dick, WB6JDH, and I wandered over there yesterday to snap some shots of a prime location within a short walk from my QTH. While there are a lot of trees -- many very high oak or eucalyptus -- there is this one spot bounding what is called "The Glen" that features a very promising stand of trees for antennas.

The shots below are rather gloomy and stark but this was on a cold day after two days of rain and a lot of clouds. With any kind of sun, the site is much more pleasant with an abundance of benches and an oft-disregarded sign advising against the intake of spirits. (This can be circumvented with the judicious use of brown paper bags should one desire a can of suds with his or her tuna sandwich.)

If the hike in is too much, there is parking very close by this spot. Also, apres contest (around 1600L), there is a nice commodious pizza joint a half mile down the road.

Map of park showing "The Grove" location circled in red.

Prime spot with stand of trees in "The Grove"
Looking into "The Grove"

Benches near the prime stand of trees in "The Grove".

These be the trees..certainly big enough for decent antennae.

Dick accosting a squirrel.

More squirrel action with Dick playing hard-to-get.

Some other trees.

Pond across the street.

Benches near the prime spot. Note proximity of parking in background.

More benches.

Disregard; just bring brown paper bags.

Pond across the street.

Additional sites across the street. Typical of locations throughout the park.

Monday, January 1, 2018

Looking for the perfect match..

After a delay for the holidays and my son coming into town from his USAF training at Sheppard AFB in Wichita Falls, Texas, I am finally resuming the NorCal 40A Anniversary project started by Chuck Adams, K7QO. This little juncture covers the crystal matching phase of the effort. And, typical of me, it's a start-to-prepare-begin-to-plan-to-organize-to-commence affair where I divide the two piles of crystals I got off of eBay into matched sets of six for the radios I am contemplating. (I will be assembling a 40-meter and 30-meter version; more about that later.)

In order to do this, I ordered the crystal Colpitt's oscillator checker boards from ALLPCB.COM per the project instructions to be found here. Here is the video on this sub-project:

Incidentally, as shown in the video, you order ten but the All PCB folks will reward you with any over-runs they have when manufacturing your order. In my case, I received 13 boards which I will use to do some experimenting with on Colpitt's oscillators. (I am not exactly satisfied with the results returned with the first two I built and want to determine how to optimize or augment the circuit. If nothing else, you can build up three or four of them with different crystal values and use them as sources of RF. But be advised that the Colpitt's oscillator -- at least this design -- has problems oscillating above 15 MHz.)

And the schematic for the little Colpitt's oscillator board can be found here. Chuck's thought was to use this connected to a Morse code frequency enunciator to determine a crystal's frequency. As each is measured, it is sorted -- matched -- with other crystals of similar frequencies. This is essential to determine the band pass frequency of the filter and the BFO of the radio we are building.

Chuck likes to match them to the nearest 10 Hz and does so as he shows below:

So, following this process, I did make a "grid" to help facilitate this matching process using Excel. It differs from Chucks but is useful in accomplishing the deed.

Note that the gross frequency for a batch was determined by measuring about 10 or 15 of them and generating the grid of one kilohertz spread by 10 Hz increments as Chuck did.

The sheet is laid over some 8x11 Styrofoam I have and as crystals are measured, they are "punched" into the appropriate slot. However, as I started measuring the crystals, I noted that the frequencies tended to drift (mostly upwards) and it was not convenient to determine the actual frequency. At first, I thought about experimenting with a higher value for R0 as Chuck mentions in the first video but, not wanting to go any further down the rabbit hole, I opted for the "just get 'er done" philosophy and opted for a "timed measurement" by letting the crystal "cook" for approximately 15 seconds to determine the frequency. Besides, I had just scored a couple of really, really neat stop watches manufactured in the USSR in the late eighties -- byproducts of the Soviet space program -- and thought it neat to put them to work.

Also, I felt a little more comfortable using a frequency counter instead of the CW enunciator. So, below is the setup halfway through my measuring/matching the 4.9152 MHz batch I got for the 40 meter version:

 The stop watch was just started and a crystal inserted and, after 15-20 seconds whizzed by, the crystal was "posted" to the Styrofoam in the correct slot. The stop watch just kept running so I could approximate the 15-20 seconds; better than reading the tiny second hand on my wristwatch and around 15 seconds was adequate for a crystal to stabilize.

This made fairly quick work of a pile of these little beauties but it was interesting to note that the 8.000 MHz crystals were more "spread out" than the 4.9152 MHz crystals.

4.9152 MHz Initial Batch

4.9152 MHz "Overflow" Batch

8.000 MHz Batch
While the 4.9152 MHz batch will not be hard to gather into six crystal groups, the 8.000 MHz batch will be a little problematic because of their distribution. But that's for another day.