All posts by Mike

I keep an electronics test bench and I love repairing old radios or building other electronic or amateur radio projects, usually late at night listening to shortwave or talking with other hams. My Amateur call sign is KM5Z.

Keithley Megohmmeter

This was a find at the Antique Science and Retro-Tech Show a couple of years ago. The box was bent and dirty, and covered with various stickers. A ‘megohmmeter’ measures huge resistances, in this case (Model 500) will test from 107 ohms to 1013 ohms, or 10 Meg ohms to 10,000,000,000,000 (ten-million Meg) ohms.
If the internals were no good, at least it could make a good project box.

Front of the Keithley Megohmmeter
Front of the Keithley Megohmmeter, original and before inspection, dirty and covered with stickers.
Front of Keithley Megohmmeter, original and before inspection, dirty and covered with stickers.
Keithley Megohmmeter, original and before inspection, dirty and covered with stickers.

Turns out it was good and needed only some cleaning and, because it was designed for old mercury batteries, it had been on a shelf since mercury batteries were discontinues (about 1996). Fortunately, there’s a fix, which I’ll include, along with the somewhat rare schematic.

The original batteries for this were 1.3V (filament) and 8V (plate). These mercury batteries are no longer available. The 8V mount was fine to accept a standard 9V Battery. I replaced the 1.3V mount with a standard AA battery holder.

A new battery holder for a standard AA battery and a 9-Volt connector.
A new battery holder for a standard AA battery and a 9-Volt connector.

Now, to adjust the voltages to match the odd mercury battery values:
For the AA filament battery, I soldered a 1N4017 Schottky diode, which will drop the voltage about 0.3V to 0.45V. The 5886 vacuum tube used here only takes about 0.10A (10 milliamps) of current, so the Vf will be toward the low end of the 1N4017. In addition, the 5886 tube will accept up to 1.5V at the filament.

For the 9V Plate battery, the nominal (new battery) voltage is about 9.48V. To drop this to 8.2V, I’ve soldered in two 1N4001 diodes in series. Typical silicon battery Vf should be about 0.6V and this battery is not used at higher currents. Two (2) silicon diodes should drop the voltage to an acceptable 8.28V.
Although not needed, this add protection from incorrect battery insertion.

To accommodate modern batteries, I’ve added one crystal diode (0.3 voltage drop) and two silicone diodes for the 9V.
To accommodate modern batteries, I’ve added one crystal diode (0.3 voltage drop) and two silicone diodes for the 9V.

The active element within the instrument is a type 5886 vacuum tube. This is a subminiature tube, probably made by Raytheon, but I also find data from Sylvania, GE and TungSol. This tube is readily available new-old stock. The circuit is setup to be very sensitive and uses a very low voltage (8V) to test a very high resistance. Many modern Megohmmeters use a high voltage (100 Volts, or more) to test resistances.

After setting up the substitute batteries and some cleanup, I was able to successfully test a Heathkit 336 High-Voltage Probe (used for testing High Voltage TV Tubes) which has a 1090 Megohm resistor, which cannot be tested with a conventional DVM. I was able to read the approximate 1090 Megohms of the probe.

Component Tester 3D Enclosure

This inexpensive little component tester is a marvel, but it’s bounced around my desk for a while. Fortunately I found an enclosure for it that allows access to the test points and the activation button.

Cheap, AVR-based component tester, probably from China.
Cheap, AVR-based component tester, probably from China.

Fortunate, because testers of this type were all the fad about 2012, when I bought this one (probably from a Chinese parts website). So, when I found it on Thingiverse, I had to print it to see it. Unfortunately, the top involves an unsupported angle, and so the slicer has to add support underneath. Given that it’s printed face-down, part of the external surface ends up not being smooth.

The 3D Printed Enclosure for the Component Tester
The 3D Printed Enclosure for the Component Tester

The original 3D object is on Thingiverse.
My make and setup is also available, giving notes and settings for a Monoprice 3D Select Mini.

Recipe: Puffed Pancakes

Puffed Pancakes

  • Servings: 4-5
  • Difficulty: easy
  • Print


  • 2 tablespoons unsalted butter
  • 4 eggs
  • 1 cup milk
  • 1 cup all-purpose flour
  • 1/2 teaspoon salt
  • 1 can apple pie filling (optional)


Preheat oven to 475°. Add butter to a 12″ cast-iron skillet and put the skillet in the oven to heat the pan and the butter.

Mix the eggs, milk, flour and salt in a pan, until smooth. Pour the batter in the skillet. (Optional: spoon pie filling around in the batter.

Return the skillet to the oven. Cook until the pancake is well puffed and golden, about 12 minutes, or longer for larger pancakes or those with a thick layer of batter.

Serve at once.

Apple pie filling is optional.

Looseleaf Logging Sheets

Collins Radio Ham Station LogbookI’m continually logging contacts on postit notes, and I always omit something — date, time, or frequency —
so I decided I’d keep these arount to print off and use ad-hoc, since I don’t usually keep a laptop or computer
running with logging software started if I’m on HF at night.

These can be used as a temporary measure to log a few contacts and then type them into logging software
later, or you can punch holes in the gutter margins (left on the Portrait version, top on the Landscape) and
put them in a notebook.

It also saves you buying a commercial logbook. The MS Word formatted files are provided for you to customize.

The Sheets

  • Portrait Orientation – 20 Lines, Numbered – PDF DOC
  • Landscape Orientation – 25 Lines, Numbered – PDF DOC
  • Landscape Orientation – 25 Lines, No Numbers – PDF DOC
  • Landscape Orientation – 10 Lines, No Numbers, Large Print – PDF DOC

The last item is a high-contrast, large-print version for those with sight limitations. (TNX to Bruce, VE4BLB, for the suggestion!).

The Hoverman UHF Antenna

An older design re-purposed to Digital TV


First off, none of this is my original work; I’m re-posting the GPL’ed files here for convenience only, along with some links to the original

Hoverman Antenna
Hoverman Antenna, built from PVC, wire mesh, and some bare copper wire.

sources. The original antenna was designed by Doyt Hoverman and patented in 1959 (and a unidirectional version patented in 1964). The 1965 patent expired in 1984.

The antenna is broadband and althought it’s designed for UHF TV channels (with gain), it will also receive VHF channels.

The Links

The Files

These are just local copies of other files I’ve found on constructing a Hoverman Antenna. I’ve put them here just because I’m too lazy to chase them down when I want to look at them.

Sentinel 160BL Restoration – Part I

This little old radio was given to me by a good ham radio friend of mine. It was excess to his needs and he called it a challenge: what can you do to improve this ugly old radio. And it was an Ugly Duckling, in very rough shape. As a portable, how many beaches had it visited? It was used so much the handle is gone! How many vacations or garages had it provided radio?

It’s a Reflex

Some research turns up that this is a nice example of a very simple Reflex radio. The first Tube, a 1A7G, tunes the signal. This is fed to the 1N5G which amplifies the IF signal. Note that the Volume Control wiper sends the incoming IF back to the input of the 1N5G.

Eventually a signal exits the 2nd IF Transformer to the Diode Plate of the 1N6G where the AM is Detected. The rest of the tube is an Amplifier for the Speaker. Each stage does double-duty!

Also, a simple AVC is provided via the Audio Transformer which, using the Screen Grid of the 1N6G, controls volume.

Tuning dial is yellowed but clear.
Tuning dial is yellowed but clear.

Together, with the three tubes (and the 1N6G is basically 1A5G with an added diode), the current draw is only 150 ma for filament and just under 7 ma at 90 volts plate.

The Sentinel 160BL was also sold as the Aria, in the Allied Stores in New York.
The Sentinel 160BL was also sold as the Aria, in the Allied Stores in New York.

This radio was marketed under Sentinel’s own brand, and also as an “Aria” branded radio, which was a department store brand. “Aria” was sold in Allied Stores of New York (ref: from ‘’, “ALLIED STORES (private brand Aria), 1440 Broadway, New York, N. Y.”).

The 1939 components were very nearly all out of tolerance. There are only one or two 'original' parts left underneath.
The 1939 components were very nearly all out of tolerance. There are only one or two ‘original’ parts left underneath.
Detail of the Tuning Capacitor, oscillator coil, and 1A7G tube.
Detail of the Tuning Capacitor, oscillator coil, and 1A7G tube.

Completed So Far

  • Inspected and cleaned.
  • Some of the wicker-covering was peeling; some glue repaired this.
  • Cabinet blocking inside was loose – glued to reinforce.
  • Missing Handle – Replaced with a not-quite-Repro quality (but functional) leather strap.
  • Replaced resistors and capacitors as needed, testing for significant out-of-tolerance.
  • Aligned, per the “Rider’s Perpetual, Volume 11 (1940 and before)”.

It’s a swell little AM Broadcast receiver now, and runs under test power.

But what to do about portable power options?

Hallicrafters S-120 Gets a Refresh

The S-120 resided in my office at work for almost a year until it developed an annoying 60-cycle hum that finally wouldn’t go away once it warmed up. Time to replace the electrolytics.

The Hallicrafters S-120 is essentially the same circuit as the S-38 series: an All-American 5 tube, minus the rectifier — replaced with a selenium rectifier. I bought this swell little radio from a fellow who thought it was dead, except for a few AM Broadcast stations. I brought it home and after a quick spray of Deoxid on the bandswitch and 3-in-1 oil on the tuning shafts (they were almost frozen) within 10-minutes it was working on all bands. Total cost: $10.

Antenna is a ferrite rod, built-in. The rear panel has clips to hold an extending rod antenna — mine is missing. Variations of this model include the SW-500 (same, but a sort-of slate-blue metal cabinet), and the WR-600 (same, but a wood cabinet). It’s a swell consumer-level general coverage receiver, and works great when provided a proper 75-foot long-wire plus a solid ground.

The knobs are plastic, the chassis is separated from the outer cabinet by p

Hallicrafters S-120, Refreshed
Hallicrafters S-120, Refreshed

lastic spacers and screw attachment points — this is a hot-chassis radio. The ‘ground’ in the schematic is actually B-; chassis is separated from B- by C29 and R18 (in parallel). Polarizing the power cord forces B- to always be the ‘neutral’.

C31 – A, B, C, and D – the 4-in-1 electrolytic capacitor sits above the chassis, the same side as the tubes. Unfortunately, there’s no good place to put a terminal strip; plus the leads to the 4-in-1 would have to be spliced to extend above the chassis. So, I relented and put the 4 individual caps near their + connection point; B- is available at several points, which makes this a convenient way to go. An alternative would be to measure this 4-in-1 electrolytic and contact Hayseed Hamfest for a modern replacement. At the time of this writing, they have an exact replacement for the ‘can’ in the S-120.

Updates as of 2008:

  • Replaced the 4 Electrolytics, leaving the old 4-in-1 in place, but not electrically connected.
  • Replaced all the tubulars, basically any by-pass capacitors.
  • Reception is extremely strong on all four bands. Didn’t do an alignment.
  • Polarized the power plug.
  • Added a safety cap from ‘after the power switch’ to B-

After rotating the radio home, and an extended stay playing there, the selenium rectifier gave up it’s ghost, with the accompanying stink. The selenium rectifier was replaced with a modern 1N4007 diode and a series 200 ohm, wire-wound resistor. The modern diode has quite a bit less voltage drop than the old selenium, and the resistor compensates to keep all the internal voltages ‘pretty close’ to what’s on the schematic.


Here’s what $2.89 will buy…

If your’e going to own a Beofeng radio, you might as well have a cheap knockoff microphone to go with it. So, off to eBay we go.

And, surprisingly, for a Chinese purchase, it arrives in about 2 weeks.

This microphone cost $2.89 with free shipping.
After all, what else do you need, but an electret capsule, a tactile button and some plastic.

Works perfectly. It’s light – there’s nothing to it but the essentials. I have no idea if the Beofeng logo is ‘official’ or pirate. Cost: $2.89, free shipping.

This is great. Now I’d like another one to hack up, for other little radios I’ve built. My second order was placed during Chinese New Year, and it takes an additional week to arrive.

This one doesn’t work – no audio. And it has a small rattle inside. Well, whaddaya expect for $2.89? Out with a T9 Torx screwdriver. Turns out the electret capsule is soldered, but… somehow there’s no connection. Fixed it by soldering from the electret capsule to another exposed point.

But look at the PCB: the solder-mask is bubbly. It’s just poor quality. Heck, I don’t know how they can provide the plastic shell, the wire in the cable, the dual plug, the electret capsule, a tiny right-angle tactile button, and the screws — for $2.89, shipped.

Recipe: Sunset Jello

Sunset Jello

  • Servings: 4-6
  • Difficulty: easy
  • Print


• 1 3 oz package of Orange Jello
• 1 cup boiling water
• 1 9-ounce can crushed pineapple with juice *
• 1 tsp lemon juice
• pinch of salt
• 1 cup grated carrots

Empty gelatin into a small mixing bowl. Stir in the cup of boiling water.
Stir thoroughly until the gelatin has dissolved.
Add the pineapple (including juice), lemon juice, salt, and carrots.
Chill ’til thickened.

This recipe seems to be a variation of a one found in a 1920s Jello Recipe booklet The Greater Jell-o Recipe Book, called “Golden Glow Salad”, page 18, but with the substitution of Orange Jello for Lemon, and lemon juice for vinegar.

* Nope, can’t substitute fresh pineapple – fresh pineapple has an enzyme that will prevent Jello from jelling.

Recipe: Crawfish Étouffée

Crawfish Étouffée

  • Servings: 4-6
  • Difficulty: easy
  • Print


• 1/2 cup butter, cubed
• 1/2 cup plus 2 tablespoons all-purpose flour
• 1 cups chopped celery
• 1 cup chopped green pepper
• 1 cup diced yellow onion
• 1/2 cup chopped green onions
• 1 can (14-1/2 ounces) chicken broth
• 1 cup water
• 1/4 cup minced fresh parsley
• 1 tablespoon tomato paste
• 1 bay leaf
• 1/2 teaspoon salt
• 1/4 teaspoon pepper
• 1/4 teaspoon cayenne pepper
• 2 pounds frozen cooked craw fish tail meat, thawed

In a large heavy skillet, melt butter; stir in flour. Cook and stir over low heat for about 20 minutes until mixture is a caramel-colored paste. Add the celery, pepper and onions; stir until coated. Add the broth, water, parsley, tomato paste, bay leaf, salt, pepper and cayenne pepper. Bring to a boil.

Reduce heat; cover and simmer for 45 minutes, stirring occasionally. Discard bay leaf. Add crawfish and heat through. Serve with rice.

Originally from a reddit post /u/JohnnyBrillcream