Broadcast - 7 July 2024
First up this week is a correction to the Ben Lomond repeater site update made last broadcast. Yes the change of call sign to VK7RBL is correct. But I must apologise to Barry Hill for not using his correct callsign, which should have been VK7BE. Thank you to all that brought the error to my attention. Thank you also to Barry for recording the new Repeater ID, in recognition of his original ownership of the repeater.
NTARC live Clubrooms presentation was by Lionel VK7ZLB, covering Megger and AVO test instrument beginnings plus the famous AVO valve testers in particular. There is no way I can comprehensively cover below the surface of this expansive two hour coverage, but if you missed the presentation, sorry!
Lionel brought a selection of his Megger and AVO portable test instruments to help refresh our memories. The scene was set with a brief introduction covering the early history Megger Group Limited and AVO.
Megger started when Sydney Evershed and Ernest Vignoles bought the instrument section of Golden Trotter and formed Evershed & Vignoles Limited on 5 February 1895.
Sydney Evershed applied for several patents one of which was for a "hand dynamo", this allowed the generation of high voltages from the hand turning of a crank handle. These high voltages could be used to measure insulation resistance in the higher megohm range. This was combined into a portable instrument and was named the Megger, this trademark name was registered in 1903.
The term “Megger” now days is common language for checking insulation resistance. However there would be a hundred manufacturers or rebranded models on the market now. Like the English expression “to hoover” the floor, it has taken on the original manufacturers name for the actual function being performed. To Megger something has passed into our language lexicon. Lionel’s pristine condition “Wee Megger” is an excellent example of one of the earliest compact Bakelite constructed units manufactured.
On the AVO side of things Donald Macradie is credited for the combining of multiple instruments and inventing what we now take for granted, the multimeter, which he patented in 1923.
He even came up with the brilliant name for such a device…. AVO standing for Amps Volts Ohms.
Just think, at the time, there was no combination or multifunction meters available.
According to Wikipedia, “The AVO-Meter, the world’s first multi-meter, was launched in 1923. AVO multi-meters were to remain in continuous production for more than 85 years. Every product range eventually comes to the end of its life and late in 2008, production of the iconic AVO 8 Mk7 analogue multi-meter finally ceased. Production did not cease due to lack of demand but came about due to the impossibility of obtaining components needed to build it.”
This whole presentation came about because of Lionel’s need to test a few valves for the club.
Having an AVO - VCM 2 valve tester, physically in excellent condition, complete with “Valve Data Manual 2” it would seem a relatively easy process. There was one known resistor that smoked rather heavily a few years earlier but it should only be a five minute repair… Deep down I have a feeling that Lionel knew that these were Famous Last Words! Hihi
Seeing the AVO – VCM 1 was brought out in 1948 and this model 2 was manufactured in 1951 it had 73 years under its belt. That’s a long time especially where precision component longevity was needed for measurement accuracy and repeatability. Fixing the blown resistor was just the start. Lionel walked us through the steps required to repair and re calibrate the AVO valve tester starting at step 1: Check the main meter. The magnet has probably lost some of its mojo or magnetism. Well that’s one that wasn’t uppermost in my mind and proved to be a very important place to start. When Lionel tested his meter for Full Scale Deflection or FSD, current had increased from 440 uA when manufactured to about 600 uA when tested, that’s a large variation. Adjusting R24 for FSD fixes where the needle is but upsets the entire meters
calibration. Just think of the number of component divider chains……
Lionel continued through his component testing and replacement and we were finally taken through the testing of an actual valve provided by Peter VK7KPC, from his WW2 Wireless Set 62. The valve passed!
Thank you Lionel, this was an amazingly interesting journey, some things just aren’t as easy as they look initially to an outsider!
To be carried over till next week: we have a battery powered temperature controlled soldering station and a Ten-Tec Jupiter transceiver. Plus have a look at the picture of the mystery device, possibly a single valve something? Any ideas?
As always, pictures will be available on the NTARC Web site under “Blogs” for this broadcast. NTARC Blogs
NTARC QSL Cards - Andrew VK7DW, our QSL manager has advised me that there has been a recent delivery of QSL cards to the club rooms for the following. VK7’s AT, AU, DON, GC, HBR, JFD, KW, MAT, MX, ZPE and ZR. If you just heard your call-sign then there are cards waiting at the NTARC club rooms for collection on the next Coffee morning or Technical Night. Thank you Herman for your continued support regarding QSL deliveries and thank you Andrew for the update.
UPCOMING EVENTS
Mid-year Club get-together and meal – This coming Wednesday night, the 10th of July. The venue is the Iron Horse Bar and Grill located at 468 Westbury Rd, Prospect Vale, Launceston. We will be arriving at 6pm for a 6:30 meal. If you have made a last minute decision to attend, please e-mail or notify the Secretary by Wednesday mid-afternoon so catering can be adjusted. See you there.
Finally - A reminder to all members that if you have any items of news you would like added to our weekly roundup, no matter how trivial, then please email them to the Secretary at the following address news@ntarc.net all items to be received no later than 5 pm on the Friday prior to the Broadcast.
That’s all folks,
73, Stefan VK7ZSB, Secretary NTARC Inc.
