Using a variac when testing always wise?

[CambridgeWorks]

My reason for starting this thread was mainly focused on electronics with solid state circuits behaviour when using a variac. I appreciate that valve gear will generally tolerate a variac better.
There is a lot of solid state circuitry discussed on here all the time, it is not exclusive to valves due to the term "vintage".
I imagine that a vintage valve radio that has been in a garden shed will be far more tolerant of testing with a variac provided an understanding of what is actually being done is appreciated
I think that with a lot of comments regarding modern circuits, it confirms that is where the majority of problem circumstances could arise by using a variac.
After all, that is what most of my original post was about.
That is what I don't fully understand given the wide variety of circuits possibly being tested.
I wonder what percentage of members own a thermal imaging camera, rather than have work access to one?
I certainly don't!
Rob

[SiriusHardware]

Years ago I was given a 'night scope' which looked as though it was a pretty serious low light detector, possibly ex-military, with an incongruous cheap plastic lens stuck on the end. It was marketed as something to watch wildlife with, but at the time I never thought of using it to look for hot components. I should try to find it, if it is any good for that purpose I could probably use it.

[winston_1]

Originally Posted by high_vacuum_house

There are certain “smart” power supplies that could see a low mains voltage and switch to working at 110V ac. Then the mains voltage increases above this and damages the power supply.

Christopher Capener

Never known a power supply that does this. There are power supplies that can work from around 90 to 260v but they work on anything in this range.

[murphyv310]

Originally Posted by G6Tanuki

I never bother with a Variac or lamp-limiter; it seems to me to be perverse to introduce a new fault [low power-supply voltage] if you're expecting there to be other faults.

It's not just SMPS that dislike being fed fron current- or voltage-limited supplies; it can be very bad for some directly-heated valves to operate them for any time in a condition where the emission is constrained by insufficient heater-temperature, and may result in a thing sometimes called 'cathode stripping'.

With insufficient supply volts, regulators can't regulate properly, oscillators may not oscillate, PLLs may not lock, bias-supplies may not produce enough bias to protect high-power valves etc...

Getting the power-supply lines up to their intended voltage is, for me, one of the first steps in any fault-finding.

Only places I _have_ u sed a Variac are when checking the performance of a piece of newly designed/built gear against supply-voltage variations, and one of my field-techies was famous for finding elusive 'intermittent' faults in transmitters by feeding them from a Variac, winding it up to 300V, and running the TX 'key down' into a dummy-load while he went to lunch.

Hi.
I'd like to see scientific proof of cathode stripping at low voltages. Some valve equipment was run at 30to 50% of rated heater volts and no HT for quick start conditions. No cathode stripping or poisoning occurs. Some TVs had instavision, this effectively ran the CRT heaters constantly with low volts on standby and again I never replaced a CRT in those sets even after many years of use.
I only avoid using a Variac on SMPS. A Variac is one of my most used pieces of equipment.

PS I use an EZ80 in a early post war Pilot radio that had an obscure HT rectifier that has a 4v heater. The EZ80 has now run for over 10 years under run by 2.6v and has never missed a beat with the HT constant of many hundreds of hours, it's the old story of the proof of the pudding is in the eating. The longer heating time is also very kind to the rest of the circuitry.

[Al (astral highway)]

I think there might be a distinction between low power valves of the type you mention and eg high power valves like 811, 572 transmitting / modulator tubes where the cathode is the filament. These aren’t supposed to be run under temperature/ filament voltage.

Variacs. Looks to me that those who have them use them and those who don’t, don’t! Simples. I think they and lamp limiter do entirely different things. The visual with a lamp limiter is brilliant and enough protection for firing up loft finds and restoration projects.

I do have a variac which is specifically used on a homemade high power inverter where I need more volts in than mains but not doubled mains. So I wind it up to 270V and that’s it. I acquired it from a scrap yard when that kind of thing was negotiable. I don’t think they’re essential bits of kit at all but nicely engineered, the old lab grade ones at least.

[stuarth]

I used to find my Variac useful for old style transistor audio stuff. Saved blowing the new transistors if I failed to notice a blown resistor in the quiescent current setting circuit for example.

There was a Sony Trinitron TV which has an odd device in the PSU and line output stages (Silicon Controlled Switch?). When these blew, they took other stuff with them. There was a Sony bulletin on testing after repairs to ensure you’d caught all the faulty bits by powering the line oscillator from a DC supply (19V?), and slowly bringing the rest of the set, including the line output stage, on a Variac whilst watching various nodes with a ‘scope. From memory, on first applying power, the line oscillator supply set off towards full mains, and was only controlled when the oscillator started up and turned off the SCS. Shows the issues which might occur if powering an SMPS from a Variac.

Stuart

[HamishBoxer]

Never would think of a Variac on a SMPS , as it upsets the mark space ratio and says, goodbye silicon!

Other than that they are essential really.

[Maarten]

Originally Posted by winston_1

Never known a power supply that does this. There are power supplies that can work from around 90 to 260v but they work on anything in this range.

Some circuits use an automatic doubler on their input. Some of those can apparently pop the capacitor or worse when you start out low and turn it up.

[Maarten]

Originally Posted by murphyv310

PS I use an EZ80 in a early post war Pilot radio that had an obscure HT rectifier that has a 4v heater. The EZ80 has now run for over 10 years under run by 2.6v and has never missed a beat with the HT constant of many hundreds of hours, it's the old story of the proof of the pudding is in the eating. The longer heating time is also very kind to the rest of the circuitry.

That one isn't directy heated though. Different constructions can react differently. While no scientific evidence, Philips states in their databooks that heaters should be run within 10%, otherwise consult with them.

[kalee20]

Originally Posted by winston_1

Never known a power supply that does this. There are power supplies that can work from around 90 to 260v but they work on anything in this range.

Dedicated IC's to do this did exist! Years ago, switching power supplies used a pair of large electrolytics after the bridge rectifier, and a link to make it a voltage-doubler. The IC drove a triac to do the linking.

It's a scheme that didn't last very long in the industry, because wide-range power supplies were developed, as you say. I've just tried using Google to find such an IC, and failed.

[Maarten]

ST made IC's to drive a triac, Sanken had hybrid circuits (STRxxxx) that did all of the work. Some were more clever than others.

[Hartley118]

It's useful to think just why one is bringing up a piece of kit on a variac. There's the psychological view of treating the kit like a sleeping animal and not wishing to wake it too suddenly, but does that have any technical justification in practice? Probably not.

The most obvious components that need special gentle treatment (perhaps the only ones) are electrolytic capacitors, but if the set uses a valve rectifier, the variac isn't going to be a very suitable reforming approach because the rectifier cathode will be doing little at low heater voltages. My preference with valve equipment is to connect a variable HT supply directly across the reservoir capacitor and bring up the volts gradually to reform it and the successive decouplers progressively whilst monitoring the current. In the absence of a mains HT supply, a string of PP3 batteries can work, bringing in more batteries step by step as the current drops whilst the capacitor reforms: I've also successfully used batteries to reform lower voltage smoothing and decoupling capacitors in solid state equipment.

Once the capacitors seem OK, I reckon it's safe to try applying full power whilst keeping a close eye on things. If a fuse blows or magic smoke gets released, that for me indicates the need for a lamp limiter. For example a shorted solid state rectifier diode can cost a lot of fuses without some form of current limiting and would also likely cause a smoking variac.

Martin

[GrimJosef]

I'm with Martin (post #32) on this. His point about the ineffectiveness of a Variac when the HT rectifier is a thermionic one is important. Likewise a lamp limiter doesn't help much with electrolytics either. If the electrolytic has lost its high-voltage capability then the sequence will be

1. Switch on
2. Capacitor starts to charge as normal. Lamp limiter does not prevent this as, at this point, there is no fault.
3. Capacitor reaches breakdown voltage, arcs internally and will never work again. It is destroyed.
4. Lamp limiter detects overcurrent and limits it, but now the rectifier is driving a short circuit in the capacitor, so the lamp-limit may not be low enough to save the rectifier. The mains transformer may survive though.
5. Operator notices bright lamp glow and powers the unit off.

I'm afraid a lamp limiter isn't really a complete protective device. It is, in part, an informative (diagnostic) one. It lets the fault happen, destroying some number of precious components, and then it informs you that the fault has happened. It might also save some other upstream components and in this sense it is probably better than nothing.

If the electrolytic capacitors need to be reformed then there is a correct tool for doing that. It is a capacitor reformer. They are really valuable things and I strongly recommend them. Using alternative tools might work (sometimes) but only after a fashion - a bit like using a hammer and cold chisel to take bolt heads off.

Cheers,

GJ

[Bazz4CQJ]

Originally Posted by Hartley118

In the absence of a mains HT supply, a string of PP3 batteries can work,

There's something I've never tried.

My Variac has been at hand for about 40 years now and I am pretty sure that nothing it has been used on as ever died as a consequence of its use. I will concede that it probably hasn't been used very often with switched mode stuff, and there are clearly some who believe there's appreciable risk there, which is noted for such time as SM equipment may come my way.

I'm currently using the Variac to help get the right resistors in place on a small HT supply with a main line of 250V on load and a stabilised line of 100V on load, using a transformer that is closer to the limits on the current available than is ideal. Quite helpful; avoids unnecessary 'warming' of the transformer or making the OB2 stabiliser go incandescent .

B

[stevehertz]

If an amount of 'pre-testing' is carried out, there is little or no need to 'limit' the mains supply to a vintage radio in my opinion. As I touched on earlier, I always test, reform or change as appropriate main smoothing caps. I change all waxies on sight. I clean all switches, pots and sockets (including valve sockets/bases), remove and test valves, check for shorts across the mains, HT to chassis etc. And then, carefully apply full mains using one's eyes, ears and nose to monitor the situation. Workwise, it's probably not a lot different to switching on using a variac or a lamp limiter as the aforementioned issues will have to be tackled at some point anyway. But doing it this way, they're already eliminated as a source of trouble. We're all different, I get that, but personally I see little point in gradually applying mains to a set that in your heart you know is probably riddled with faults anyway. Just sort them out, eliminate them and that 'worrying' switch on is likely to be a more of a damp squid that an indoor fireworks display.

[trsomian]

I also would not use a variac on a device that contains a mains motor, such as older record players, tape recorders etc, because the motors can seriously get annoyed by under voltage

[Hybrid tellies]

Using a variac can cause problems for valved or hybrid televisions as with two low a voltage the line oscillator may not start up causing the line output valves to overheat.

You can get around this by disconnecting the screen feed resistor to the line output valve.

[broadgage]

Originally Posted by SiriusHardware

Years ago I was given a 'night scope' which looked as though it was a pretty serious low light detector, possibly ex-military, with an incongruous cheap plastic lens stuck on the end. It was marketed as something to watch wildlife with, but at the time I never thought of using it to look for hot components. I should try to find it, if it is any good for that purpose I could probably use it.

I do not believe that this idea will work.
Night vision equipment for military use or for wildlife observation uses normal visible light but at levels too low for the human eye. Starlight is enough.

To detect over hot components, one needs an infrared camera, these show a false colour image and can highlight hot parts. Much less expensive than in the past.

[G6Tanuki]

The cameras built into phones have a significant response to IR [that's why you can use them to check the operation of IR remote-controls].

I wonder if anyone's tried using a phone for thermal-imaging of a radio/TV/amplifier chassis to detect hot-spots? Maybe do it in the dark so the phone's camera winds-up the sensitivity to max??

[G6fylneil]

I was using an automatic PAT tester for production testing, when I was working late I started to have some units fail (just) on high leakage current, due to the filter capacitors. The tester used an unregulated HV and when everybody went home the mains voltage went up and with it the test voltage. I used a variac to supply the tester with 240V and they all passed. Later another engineer was doing the testing, and had the same problem, I told him about the variac trick, but he wound it up from zero, unfortunately the tester had an automatic mains voltage selector, it must have selected the 120V range and the tester didn't survive.