10. Variac Repair & add V A LED meters
A known failure mode of older instruments with SMPSU supplies is exposure to elevated mains voltages.
I had not considered this affected me until the appearance of mains monitors in the UK Government drive to reduce power consumption. I was already aware of my consumption but bought one out of curiosity and was surprised to find the voltage hovering close to 250Vac, well above the nominal 230Vac although I believe it used to be 240Vac, but was lowered after collusion with Europe. I'm sure in the past I measured it much closer to nominal and it has risen over time, doubtless to compensate for the significant increase in housing estates that have come to dominate what used to be the small country town where I live. A relatively new substation built nearby probably had something to do with it. I read somewhere UK electricity supply companies are permitted a 10% variation allowing up to 253Vac as 'normal', so there was no opportunity to lodge a complaint and even if there was, nothing would change.
It often takes an event to elicit change and the spectacularly smelly failure of the mains IEC filter in an HP4952 Protocol Analyser, see [General Repairs] was my catalyst to prevent the failure of more critical instruments.
At first I selected and bought a dedicated ac mains stabiliser that switched in different transformer taps under µP control, but I found it dissipated a good 30 watts by itself even when the equipment was switched off, and got annoyingly hot. I came in the morning after I installed it and found its fan screaming away accompanied by a nasty smell I later realised was overheating transformer varnish that took 3 days to vent out of the lab. Inspection of the design revealed it appeared to be switching in primary taps as loads, I assume to save the expense of the extra copper that would be needed for secondary taps. I found a buyer who fortuitously needed to increase their voltage and thought again.
I ran a 12 hour mains measurement with the Tek THS720P and found the mains voltage did dip, but overall it was consistently high. It occurred to me a much simpler solution with minimal dissipation would be to simply buy a variac, reduce the output volts, and power the lab off that.
I trust the longevity of vintage UK-built Claude Lyons variacs far greater than the many new cheap red Chinese/Eastern European ones flooding eBay in the 2020s. Berco is an historical Claude Lyons product, and they also acquired Zenith in the 90s. Claude Lyons/Berco variac models over time:
[W1] Claude Lyons (also Berco) variac model history.pdf
The lab already had a Claude Lyons type RK13, 13A Regulac variac running off a separate mains outlet but it was in regular use. I was very fortunate to win it on eBay UK for a mere £45 as they command a high price, and the new Berco type 72C, 10A Regavolt variac was double that price, but it had a housing that was perfect for a couple of $4 eBay China mains voltage & current LED meters that I fitted next, although how long these will last remains to be seen. Their fitment is described below the next section.
Below, THS720P 249.5Vac avg over 12 hours Below and right, the 10A Variac on arrival, and its ratings
10A VARIAC REPAIR
Whilst testing the LED meters I noticed the new variac that had worked well up until then, occasionally turned off its output completely, resulting in its teardown and the repair of a previous owner's bodge:
The hex socket screw in the top of the knob (uppermost middle photo) tightens a collet within, that secures it to the 1/2" rod upon which the variac rotor turns.
Above, left to right, 1-4:
Once this screw is removed, (1) the Knob together with its dial can be removed. (2) The knob is secured to the dial by 3 countersunk screws on the underside of the dial which has a reverse direction scale and 3 long screws retained by nuts. These screws lodge in 3 dimples on the top of a round wooden spacer (3). (4) is a better view of the underside alternative reverse scale with these screws removed.
Below, left to right, 1-4:
(1) The wooden spacer is attached to the rotor below, by 2 screws tapped into it and secured into the wood with yellow wax. Between the spacer and the rotor is a square cardboard washer, the purpose of which I assume was to add a little more height to the wooden spacer that wasn't quite tall enough.
(2) I tried adding a spring on a washer to add a bit more tension to the bodge, but the voltage still cut out about halfway around its dial.
I then tried to figure out why the bodge was necessary. Side view (3) reveals the rotor arm pinch bolt is missing. At first I assumed it had broken off, but closer inspection revealed its thread extended fully across the rotor pinch assembly and it readily took an M6 bolt. But this was with the wooden spacer above it removed and this is important, because from the very last photo at the bottom of this repair section, it is evident that one of the wooden spacer screw mounting holes is tapped into the rotor and into the end of its M6 clamp bolt thread, rendering it useless. Perhaps they didn't realise this before they drilled the hole, and bodged their own bodge?
There is another missing part: the existing 13A lab variac (4) has a bracket screwed into its the rotor side that attaches an integral washer below it, but this is missing from the bodged variac (3).
I wondered if the wooden spacer had been added to force more pressure onto the rotor to counter this missing bracket washer, but once I had added the M6 bolt it seemed to work without issue.
The rotor is the return path to the variac terminals, and this bracketed washer makes the circuit to the metalwork below the rotor that connects to the wiper terminal.
Another puzzling detail is why both rotors are painted black although the underside is free of paint, revealing they are made of conductive brass which again begs the question: why was the bracketed washer there? It also seems odd painting them black as they are hidden deep within the variac metal boxes.
Below left, bodge disassembled.
There was another problem: the screw securing the phenolic insulating tube around the central metal rod was too short to engage the insulator and stop it moving independent of the rod. This is the same screw in the centre of the rotor side in photo 3 above, removed in the photo below left, that secures the missing bracketed washer visible on the 13A variac in photo 4 above.
Below right, top view is of the good 13A variac, bottom view is of the newly repaired 10A variac.
Above, you can never have too many metric hex screws, another eBay bargain.
Also visible in these photos, are the eBay China voltage & current LED meters I added, see description and more photos in next section.
This worked, even without the missing bracketed washer, but I was concerned it would not be reliable, otherwise the 13A lab variac would not have one, so I set about making my own from a piece of aluminium sheet, and fitted a longer screw through it to secure the insulator:
Top 3 photos: making and adding the bracketed washer.
Far left: bracketed washer dimensions.
Near left: a metal ring from a dismembered HDD into which I've drilled and tapped an M3 hex screw (fitted). To its upper right, a template to add 2 more screws.
Bottom left: the bodged variac's spacer lock screw on the rotor was functionless and a small metal ring at the base
crudely stopped the phenolic insulator falling out.
I replaced it with my HDD ring secured to the insulator to stop it moving, and sealed the 3 hex screws with RTV. The new M6 clamp screw secures the insulator to the rod.
Initially I felt some resentment I had paid a high price for a bodged and faulty piece of equipment, but I'm glad I was able to repair it. I wish I knew more about its history.
SO MANY QUESTIONS
I'm puzzled why the rotor clamp bolt isn't located near the end of the clamp. From the previous side views of both rotors, clearly this is because the rotor thickness sharply reduces immediately after the bolt and the clamp end is too thin to support an M6 bolt, but there are no smaller holes there either.
If the end of the clamp is unused, why is it there?
The sides of the rotor itself have a rough surface, almost as if it has been manually cut from metal and dressed with a rough file, unlike the smooth surfaces on the 13A lab variac. The clamp is also a different design.
It took some skill to tap holes into the rotor and melt yellow wax into a near perfectly cut round wooden spacer (what were the 2 holes in its side for?), and to machine the metal ring with the grub screw that was a perfect fit on the bottom of the shaft. All of this surely could not have been due to a missing M6 bolt and/or bracketed washer. Furthermore I can't think of a reason why this washer is missing if it's essential for reliable wiper connectivity.
It's also strange the rating label has been removed from the rotor arm (all that remains is 4 small mounting holes in each corner).
It has crossed my mind this might have been an early prototype. Perhaps discarded afterwards, the washer and label moved onto a sellable model and this one thrown in the bin, only for someone else to recover and reconstruct it for themselves using whatever was to hand?
LED VOLTAGE & CURRENT METERS ADDED
Backtrack:
When the 10A variac arrived, the first thing I did was open it and discover it was wired with its output on the left and input on the right, which is counter to Industrial norms and confusing without a label. I immediately
swapped them, numbering them in the process.
I also terminated the bare wires in ring crimps, removing the possibility of wires working loose over time, far safer for high voltages and currents.
Left, I/P on right Right, I/P on left
To be able to fine tune the variac without exposure to high voltages, I bought a selection of coloured LED mains supply dual 50-500Vac / 0-100Aac meters from eBay China for ~$4 each.
I selected red for the input voltage and current, and green for the output.
Current is measured via small toroid transformers slipped over the wires.
Initially I fitted both but soon realised this was pointless and left just the output toroid in place, which also made the wiring less cramped.
Although only CHECK 34mm deep, the meters were too long to fit in the cavity so I added the bottom section of a small ABS box as a spacer, screwing it to the metal top via its lid holes. The meters have ADD DIA mm round bodies. I used a 22mm hole saw to cut two holes equidistant on the variac metal shroud so they would appear side by side on the plastic box, then ran the hole saw 5mm guiding drill bit through the metal holes to create centering holes in the plastic box.
Below left to right: drilling the metalwork to take the LED meters.
Bottom left: as hole saws have a rough edge, I then ran the hole saw through the outer surface of the plastic box via its pre-drilled 5mm holes. Next I dressed the rough holes with a metal file, screwed in the LED meters and sealed their nuts with RTV silicone rubber. I did the same for their wires, which I had pre-measured for the variac, and terminated in ring crimps:
All that remained was to pop the plastic safety covers on the backs of the LED meters, wire them into the variac and click the transformer leads into their mating small black connectors.
One last touch was to pop off the input meter's clear plastic cover and add some black electrician's tape over its now redundant current display that was permanently showing 0.0 (amps), changing it to display IN instead. Finally I added three 0.5ND plastic filters to the insides of the coloured LED covers which were otherwise far too bright; the filters also hide the black tape from view.
Over the course of a few days I fine-tuned the variac output and eventually settled on a compromise of ~15Vac less than the incoming mains: based on observation and being not far from a substation, it seems highly unlikely my mains will fall to 240Vac, let alone to its nominal 220Vac,
I checked the LED meters against the HP3468A DMM and found them to be pretty accurate at around ±3Vac. The current is less accurate, but not too far out although between the four meters I bought, current accuracy varies more than voltage. Mains current is only a concern for me if it rises near the the 10A carry ability of the variac, but that is highly unlikely to occur. Nonetheless it's beneficial to have the current displayed should I ever approach 2kW of mains power in the lab, in which case I'll offload some of it to the other, 13A variac instead.
Having successfully added LED meters to the 10A variac I decided to add an orange output LED meter to the 13A variac as well.
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