Category Archives: Test Equipment

Test Gear, including new or antique test equipment, homebrew, restoration, or modifications.

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.

Repairing VTVM Pilot Light

Lurid Red of the new Pilot Lamp
Lurid Red of the New Pilot Lamp

I found a great solution to old tape used in the Heathkit VTVMs for the RED pilot lamp. The lamp is just a #47 bulb, shining through a hole in the top of the meter face. The tape isn’t mentioned in the instructions, so perhaps it was pre-installed in the back of the meter face.



A Cheap Fix
Also Useful for Repairing Antique Pilot Lamp Film


In any case, this tape sometimes falls off or is nearly falling off after 50 years. It also fades and loses it’s reddish glow. An excellent solution was found in the form of a red-neck repair from the auto parts aisle of Walmart: Tail Light Repair Tape, US$2.00.

Tape replaces old, crinkly, peeling tape from this spot.




Above: Below the roll of red, translucent tape, the old pilot lamp film and the new piece cut to replace it.

Left: Adhesive is sticky and the new piece goes over the hole in the meter through which the pilot lamp shines.

ATX Desktop Supply Adapter

Built one of the ubiquitous ‘Desktop Power Supply’ from a recycled, ATX-form, PC power Supply. Actually used it occasionally to power a 12V charger in the garage. Unfortunately, it fizzled an electrolytic capacitor and it’s really not worth reparing.

So, I rescued all my hardware and built a reusable adapter for ANY standard ATX power supply.

Parts List:
1 ATX Power extender – this is an extension to the wide plug that goes to the motherboard, usually about 9 inches or so (like this one:
All of the other hardware bits needed to alter an ATX supply for desktop use (binding posts, a switch, a 10 ohm / 15 watt resistor (I used a Dale, metal-cased).
And an enclosure – a wide, thin, Radio Shack enclosure I had on hand.

The 10 ohm 15 watt resistor goes from the +5V rail to Ground. This is needed so that the supply can sense a load – otherwise, it will not start.

Excellent how-to’s everywhere on the net, but this one is great:

Great current ratings here: The +5 and +12 rails can supply a good deal of power and these ATX switcher supplies are very efficient.

This adapter will provide desktop power supply access for a recycled ATX Supply.
ATX Power Supply Adapter

A $10 Scope

My B+K 1435 Oscilloscope went down over Thanksgiving weekend. Really, it went down – it fell against my chair (I shouldn’t keep it propped up like that, I guess). However – the “A” channel input went bad. I took it all apart – screws everywhere on the carpet.

I was so close to parting it out then W5AAN (Ginger) urged me to keep trying on fixing it. Turns out these old scopes are old-school. The traces are nice & wide and the parts aren’t surface mount (SMT). I wound up fixing a total of three problems with it.

The pot for the “A” channel vertical positioning had two broken traces right near the pot. I was able to easily scrape and short ’em with a bit of component lead wire. Soldered those on and it worked just great.

Got it all back together and found that now the “B” channel didn’t work. Well by this time, I know where everything is – opened it back up and the lead to the center conductor on the BNC connector had broken. Soldered THAT back into place and now the scope works great.

It works so well I’m considering buying some new Gel batteries for it – it’s a portable model and while it was all apart, I tested the charge circuit to be working.

Perseverance. My $10 scope soldiers on…

Heathkit IG-102, Solid State Edition

I’d recently heard about a conversion of a tube Signal Generator to Solid State. I found the original article in a great old book called ’99 Test Equipment Projects You Can Build’, by 73 Magazine. I snagged the book from eBay for only $2.00 (plus shipping). My copy is a small hardback, red cover. Print’s kinda small.

Same article mentions adding a three-range (10khz, 1mhz, 10mhz) crystal calibrator on-board (since you now have boatloads of room inside without the tubes and the transformer).

Each ‘half’ of both tubes is replaced with a FET Pin numbers are mentioned, so you go underneath (unfortunately sticking the leads in the tube socket holes won’t work…) and solder a FET lead to a Socket Hole. I think there’s one socket hole (besides the filaments) that remains unconnected.

I’ve written on the schematic which FETs appear to work. I settled on MPF-102s, although I tested a couple of versions. The book project specifies four 2N5951’s.

An IG-102 FET Conversion Schematic

There are two resistor changes (actually mods) due to the lowered voltages:
Solder a 75 ohm resistor across the existing 33k (see left of ‘BF Front View’ switch.
Solder a 90 ohm resistor across the existing 4.7k (see above V2A 1/2 6AN8). On some models the existing resistor may be a 10k.

There are no other changes other than what’s marked at bottom of the schematic (removing the cord and power supply. Replace w/9V battery. I also did the fancy LED thing. Nice to know if it’s ON so you don’t run down the battery.

I also added a ‘wall wart’ plug for outside power. Fancy.