Motor problem

It was just another day at the plant.

A colleague from the neighbouring department called to say he had a problem.A replacement three phase induction motor, drawn from the stores and wired, just refused to start.

Our exchange :-

Voltage okay? 
Yes, all 3 phases terminal voltage 415V okay.

Maybe an overload or single-phasing. Does the overload relay trip?
No.

In that case it has no winding!
You crazy?!

No!
Okay, I'll check.

A little later the telephone rang again.

How did you guess?!

It turned out that someone in the motor rewinding department had sent an unwound motor back to the stores instead of a rewound one!
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Some thoughts on shack earthing

The main purpose of mains-supply-earthing (third pin) in the shack is to blow the fuse in the event of a live-chassis fault. Protection against static build-up / lightning strike is another matter altogether.

I was a SWL in the mid-1970s and the antenna I used with a Philips transistor portable receiver was a long wire at a height of about 50 feet. One summer afternoon, the receiver front end (AF117) blew right after a loud crackle of static. 

Then again, my first tube homebrew CW rig had a 3-pin mains supply plug. The antenna was a straight dipole 50 feet high. Operating on a summer afternoon, I received a jolt through my Junker CW Key and survived to hear the crash of thunder from a nearby lightning strike. I immediately yanked the twin-line feeder and threw it on the floor (upper floor of my 2 storey house). After a few moments I was surprised to see the arc from the banana plugs to the cement floor.

Those days, in our sparsely populated area, my antenna was way above other surrounding structures. During thunderstorms the static build-up on my 2m ¼ λ ground plane antenna would cause a whine in my 2m receiver as it dissipated through the front end coil to ground.

Conditions are totally different now, with my 2 storey house surrounded by high rises, cell phone towers and other structures with lightning arrestors. Problems of static build-up and lightning are a distant memory. I have not heard that static whine for years, even though I still use that ¼ λ ground plane antenna.

And my shack earth is just mains-supply-earth (third pin)!

Of course, if you live in the countryside, the lightning arrestor at the feeder entry-point, with a separate earth bonded to the mains-supply-earth, is a must.

While on the subject of bonding – a friend’s shack and attached bath had a separate safety earth, not bonded to the mains-supply-earth. His 2m antenna was mounted on a metal mast which was also separately earthed.

For reasons not known, the shack earth was open when the water heater developed a ground fault. The path of the fault current was through the floating earth wire, 2m rig, coax feeder braid, antenna mast and finally to ground, resulting in a coax cable fire. A parallel path also caused considerable damage to a Drake AC4 power supply.

Fortunately he had the presence of mind to pull the main breaker before he doused the fire. 

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Rugged 807 & 1625 tubes

The good old 807 and 1625 tubes can take a real beating.

One night decades ago, when I was working DX on CW with the shack lights off, the room suddenly lit up. The cathode-keyed 807 final was white hot! I let go of the key and cut the power.

Since I was monitoring the exciter signal, I was unaware of a break in the drive and had continued to send.

After the rig had cooled and the fault rectified, it was back to normal operation again. I still have that tube.
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Lightning jolt

It was a typical summer afternoon in the year 1977.

I was working CW on 40m - my homebrew EL84 rig feeding a straight dipole antenna with telephone drop wire as feeder.

Meanwhile the sky became overcast and there was a continuous rumble as I continued to work.

All of a sudden I received a jolt through my Junker key followed by the crash of a thunderbolt.

Without a thought I yanked the feeder off its sockets and threw it on the floor. My heart missed a beat when I saw the arc that followed between the banana plugs and the cement concrete floor.

The following morning's local newspaper headline was 'Lightning stuns football players'. It was a direct hit at the nearby football field. Fortunately only a couple of the players had fainted and there were no casualties.
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Drake TR-4C - receive problem

The old workhorse seldom gave problems on transmit.

A frequent receive problem was the receiver going dead on switching back to receive from transmit.

The problem was solved by replacing custom C145 (4 electrolytic capacitors housed in a can) with individual capacitors.

Capacitors drying out was the root cause.
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Yaesu FT-747GX / FT-757GX - failure to power up

Every Yaesu FT-747GX / FT-757GX user would have been flummoxed by the rig refusing to power up only to discover later that the MOX button had been left actuated.

It's a safety interlock to prevent the rig from going to transmit on power-up.
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Yaesu FT-747GX - PTH problems

A batch of Yaesu FT-747GX transceivers, produced in the 1990s, is plagued with PTH issues.

They show up as intermittent problems in receive and transmit and finally lead to total failure.

The faulty boards are the filter selection board (for receive) and the power module (for transmit).

In the filter selection board, faulty PTH connections result in the switching diodes failing to connect the required filter.

Similarly, in the power module, the result is a break in the drive to the bases of the driver transistors.

The solution is to solder jumper wires across the faulty PTH junctions.
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Injection moulding machine problem

It was a crisis call for a table-top injection moulding machine. Clear acrylic meter faces were being moulded and the output quality had deteriorated overnight.

The controls were fairly simple, the key ones being those for temperature control of 3 zones. Temperature setting was through potentiometers with graduated dial plates. The instruction manual had a chart listing the 3 settings for various plastics.

The easiest way to check whether the temperature controllers were working was to connect bulbs across the heating elements and observe the change in the switching while changing the settings. All 3 tested okay.

When I was mulling over the problem the owner excused himself for a while.

Then 'Eureka'! I turned the knobs fully anticlockwise and the pointers were not at 'zero'.

All I had to do then was to loosen the knobs to rectify that.

After changing back to the settings for acrylic, the machine was back in business!
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Speaker vibration/distortion

A Radio Cassette Recorder was received for repair.

The problem: Speaker vibration/distortion within days of its purchase in another city.

On opening the set, a cockroach was found perched on the back of the voice cone. It appeared to enjoy the vibration, for it wouldn't budge even with the volume turned high.

Without getting into details, out went the cockroach and along with it the vibration/distortion.

It was my fastest repair job, ever!
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My first satellite QSO

My first satellite QSO was with my good friend OM Bindu - VU2IR.

It was around 6.00pm, one evening in the early 1990s, when Bindu and I were chewing the rag on 2m and he excused himself to work through a satellite.

I was curious when he said he would work SSB via RS-12 with a 21MHz uplink and 145MHz downlink and would I care to join?

With hardly 10 minutes to go, I hurriedly set up a Yaesu FT-757GX (just serviced and ready to be delivered) with my 40m inverted 'V' dipole antenna for the uplink. My homebrew 2m converter and ¼ λ ground plane antenna would serve for the downlink, with my RCA BC312-D receiver as the tunable IF stage (see My homebrew 2m FM rig for more details). And my first satellite contact was thus established.

Fortunately, it was just the two of us and we continued our ragchew for the duration of the pass. With OM Bindu just 5km away, it was once again proved that the long way round is the short way!
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Remembering radio pioneer J.C.Bose

Here's a comprehensive article on radio pioneer J.C.Bose.

J.C. Bose: 60 GHz in the 1890s

 Interesting read.

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Crystal Radio Circuits - Alternative Concepts

 1. Parallel-tuned, series-fed configuration:

Parallel-tuned, series-fed Crystal Radio circuit

The diode-clipped parallel-resonance voltage is output to the phones. 

High-impedance diodes and phones are required to match the high impedance of the parallel-resonant 

circuit. 

This configuration is suitable for reception of distant, weaker stations.

   
2. Series-tuned, shunt-fed configuration:

Series-tuned, shunt fed Crystal Radio circuit

The diode-clipped series-resonance current is output to the phones. 

Low-impedance diodes and phones are required to match the low impedance of the series-resonant circuit. 

This configuration is suitable for reception of nearby, more powerful stations.

3. Series-tuned, shunt-fed variant:

Series-tuned, shunt-fed Crystal Radio variant

This is a variant of the above series-tuned, shunt-fed configuration. 

Using low-impedance diodes and phones, it delivers identical performance.

Related post: Parallel-tuned Compact Loudspeaker Crystal Radio

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