A Blast From The Past. 

 

By Tony Boddy VK6DQ February 2019

I know this is not radio stuff but it is a bit electroniky. I spent so much of my electrical career as a trouble shooter. Just had the knack of being to see into the black magic innards of electronics that were creeping more and more into things electrical. Not always. I remember back in my apprenticeship days when we used to come across any electronic stuff that had let the smoke out, we would throw our hands up in horror and send for the spooks. I can tell you we really believed that these jokers came straight out of the twilight zone. We, I mainly, kept out of that for years even though I used to dabble in the electronic side of things with a bit of interest in radio. That all changed as I ventured further onto my job.

Years later I was camp maintenance electrician of a mining camp at Shay Gap in North Western Australia. The camp commandant, I used to call him that, was Bob De-Laurie, helleuva nice bloke A Canadian. I had much to do with him because he was the camp administrator, we talked a lot. He came to me after hours one night “The camp movie projector has died”. “What do you mean, Its died?” Two nights a week there were movies, I usually did not go. There were two projectors, one dud, and the second with a bit of lightfooting by the projectionist to keep continuity when changing reels was OK but now number two projector had lost sync. Didn’t know much about projectors so had to learn pretty quickly about the synchronous shutter in the light output line that enabled the picture to be seen. Without that shutter the picture was just a blur. Now both machines were dead.

There was a bit of a fuss and a whole angry mob of mine workers without their entertainment. Had a quick Bo peep at the innards and found that the centrifugal clutch on the synchronous shutter drive had thrown the friction pad. What the hell am I going to do with that? The end of my trusty leather tool belt was about the right thickness so I chomped a bit of that off, trimmed it up and riveted it in into place with some rivets made from a bit of copper wire from my toolbox. Bingo we had a projector again. The grumbling from the quite mad audience died down and Bob sorta cajoled me into fixing the other projector as well. Fortunately, it was only the same sync problem, another piece off the end of my tool belt saw to that. With two projectors on line there was no delay in changing reels and continuity was seamless. The anger from the crew dissipated pretty rapidly when both projectors came into play. My camp electrician status went straight up to guru level. Coulda had free beer for life after that but I don’t drink. Played projectionist for a while because I was interested and became quite a good friend to Bob De-Laurie mainly, I think, because I saved his bacon.

Bob said to me one day. “Got an old radio that you may like to have for spare parts if you want”. Now radio parts did not turn me on much because all of my stuff was electric and heavy duty. “Thanks Bob”, mainly because I had no wish to offend him and he really thought he owed me. The radio would have been a pretty good unit in its day but the removalists had really knocked it around. Modern transistor job with a bundle of bands and I expected good audio because it had a nice speaker. Pulled it all to bits, the loopstick was in a dozen bits and there were about 20 or so wires all broken away from the connection points across the PCB and band switch. Those were the days when I could really see so I was able to match up the wires by looking very carefully at the ends and the points where I guessed they may have come from. That all looked OK but what about the loopstick. The ferrite was really in a mess and there was no way I could get spares 2000 km up in the bush. I figured the loopstick was a magnetic circuit anyway and it would work just as well as a series connected job. It was a job too. Borrowed some PVA glue from the camp carpenter and glued all the bits together with one layer of paper to hold it in place.

It was indeed a magic radio. Couldn’t take it from Bob so I gave it back to him. He would not take it. We came to an agreement that we would share it and whoever went home first would have it. Both of us were due to ship out at about the same time so it sounded a bit fair. I did go before Bob; the radio became mine and served me well for many years. He came to Perth on his way back to the States and I arranged a billet for him at my mother’s place till he went home. Still could not get Bob to take his radio back, it was mine for sure. That episode reawakened my interest in Radio. Still, life gets in the way and I just did not have the time to follow up my dream. We did move to New Zealand where I had the time to study and become an Amateur Radio Operator. In the meantime, I had begun to specialise in troubleshooting. I was the bloke they called when everyone else failed. I never gave up and always there was a good result. Amateur radio opened up so much for me, electronics in industry was everywhere and without knowledge of it in some way you were stuck being just an ordinary wire jerker. Not for me. It was good. I learned much and gained a good reputation for fixing the impossible. All this story so far is to lead you into one of those magic fixes. Magic because none of it scared me anymore.

The Kiwis are a pretty economical lot, they need to get the best out of everything. So, they brought some pretty sophisticated machinery into NZ from all over the world. In the end I could read English, German, French and Italian drawings. Just could not hack it with the Chinese or Japanese though. Chinglish and Japlish just never sat right with me and the Russians really gave me some gyp. Getting back to NZ economy, they used to mill timber, some was pretty rough, too many knots and shakes to make decent lengths of anything. They were cutting the knots out of really shonky pine and finger jointing 150mm long bits together to get longer timber. But the best was the laminated beams. They did use longer timber for that and some of the bigger beams were indeed magnificent. I got involved with it because the company I worked for became agents for a gigantic RF laminator which I had to install and commission.

Can’t for the life of me remember the name now, it was over 40 years ago. The machine was very similar to a sandwich maker with the exception that it was a press that worked in a downward direction with a secondary press that squeezed in from the sides. If you can imagine a sandwich maker three meters long and one and a half meters wide that would squeeze up to one hundred and fifty millimetres thick. The top and bottom plates were the capacitor in a gigantic tank circuit. They were tough as old boot stainless and their main job was to hold the loose bits of timber flat while the side press came in to hold them all together. It was quite a process. There were roller tables on the output side of the press and flat tables on the input side. The operators would juggle shorter bits of timber to maintain the right width and make sure there were not too many ends lined up to create a weak spot. In other words, the joins in any of the lines had to be well staggered. There were about four operators feeding timber in keeping it all lined up and also coating the sides of the timber with glue

The big 27 MHz oscillator was powered by a fifteen kilowatt tube the like of which I had never seen before nor probably will ever see again. I reckoned you could fit two sumo wrestlers inside; it was that big. Once the timber was coated with urea–formaldehyde glue, rolled into the press then squeezed flat and together it was treated to a fairly heavy dose of RF to set the off the glue to hold it all together strongly enough to be moved through the press and be stacked. Beams could be made to almost any length as long as timber was inserted and as long as there was space on the output rollers to take it. The beam would be left to cure to full strength, normally for 24 hours and then it could be used. It was possible to make unlimited length beams and they did make some thirty meters long. The oscillator had many safety features built in and the one that was causing us trouble was the auto overload setup.

If perchance there was a glue build up in the RF path between the top and bottom plates of the press, excessive RF would flow through the glue line, the oscillator would sense this and give the whole thing a big burst of energy. Normally the glue line would be burnt away but if it was not, the burst of energy circuit would repeat four more times then shut down for a manual inspection and clearing. The circuit was quite simple, each clearing burst had a relay that would lock out the RF cycle for a few seconds. If the fault was cleared it would reset and resume operation. If the fault remained it would cycle the burst of energy circuit till it was cleared or it reached the fifth shot at which time it would lock out the press until the fault was cleared and the lockout manually reset.

Our problem was that there was no lockout, the press kept cycling after the fifth shot. This thing sounded like a slow firing 50 calibre machine gun. A bit scary when you have never seen a machine like this before. The worry here was the damage that could be done to the oscillator with a continuing run, in safety mode. The circuitry was available, new machine benefits hey, so delved into that and found that each burst cycle was locked out by a tiny relay coupled with an LED indicator to let the operator see what was happening. Why was the fifth cycle not locking out? Lots of measuring, static values of each component, checking voltages and comparing the five stages of lockout gave me consistent results. That looked OK but what about the function, how did it work?

Each burst shot energised a relay which was operated by a pulse triggered SCR. The relay would pull in, the LED would light up and the combined currents of them both was sufficient to hold the SCR in its conducting mode and the relay would lock out the machine. A timer would fire the oscillator again until the fault was cleared automatically or burst five relay locked everything out. Burst five relay was not working! Well, it was but not properly. Every component checked OK. What was holding number five relay on or not on in this case, the holding current through the SCR. Checked the data on the little SCR and found it to be in the low milliamp range. Was the current of both the relay and the LED combined enough to keep the SCR conducting? Easy job here just swap the relay or the LED out to find out. It was the weekend so could not buy a thing, I was also a thousand km from home and my personal supplies. The relay checked out resistance wise so flagged that away, pulled the LED, that looked OK but when I pulled one of the others from the circuit and ran a load test on both I found the current of the suspect led was only 10mA, the other was 20mA. Did it make a difference? Sure did. The problem I had was no spare LEDs.

Ten lousy milliamps difference in a 20 cent LED current was stopping a million-dollar machine operating. That ten milliamps was essential to maintaining the hold on current in an SCR circuit. I had no wish to make changes to the manufacturer’s circuit lest it become none standard. I bridged out shot five and reconnected shot four to lock out the machine just for running up trials and we were able to finalize the installation and testing for a Monday start-up. After Monday a green LED was purchased and installed. The company for whom we installed the press was happy and so was I to be able to return it to the original circuitry. All this fault finding stuff is not as difficult as it seems, systematic, methodical, checking, is the best way to go. Correct diagnosis of symptoms will find the cause of a problem. That applies to most situations and the best is to treat the cause, fixing the symptoms does nothing. Having circuit diagrams is a big bonus because it gives one somewhere to theorise hairy problems. It can be done with no circuitry but the path to success in that case is longer and slower. Having data books on hand to look at device specifications is also valuable. You can’t hold it all in your head. Sorry it’s not really radio related but the 27 MHz made me chuckle when I thought about the RFI that the NZ chicken banders would have encountered when this great beast was running. It was not a clean signal.