Discussion in '"Boat Anchor" & Classic Equipment' started by KA5SDC, May 18, 2019.
Can't believe no one has stressed the use of an isolation transformer yet. GET ONE and USE IT
An isolation transformer is not necessary if the radio has a power transformer and a grounded (U-Ground) to chassis. Isolation tx is needed for those AC/DC radios where the low side of the line is connected to chassis.
A Variac does not isolate your radio from the AC source.
BSU is definitely correct! An isolation transformer is not needed with transformer powered units but should be used with AC/DC units.
I have only one AC source on the bench,
And its powered by a 1 KVA Isolation transformer.
It is a prehistoric design, and much too expensive to purchase new, but the BK Precision 1655A is an isolated, metered variac with a few other doo-dads. Yes, the variable output is indeed isolated. They sometimes pop up at hamfests.
FWIW, I use both of the methods described above. IOW, in older gear I typically replace the most suspect capacitors if they're original especially electrolytic caps in higher voltage supplies. But I also do a quick test with a variac especially if I received the rig with the fuse missing or the fuse blown. That's based on a number of old rigs I've worked on over the years that had transformer shorts that might have been caused by something like shorted filter caps or perhaps something else but it's frustrating and time consuming to repeatedly blow fuses when you have a major supply short like the transformer itself and a variac is a pretty safe way to figure those kinds of problems out.
It also depends on what I'm working on and how old it is. There's a lot of posts on these boards talking about things like Kenwood or Yaesu hybrids that are primarily low voltage solid state circuitry as Boat Anchors. I'd rarely pull out the variac for one of those but on an all tube design with several relatively high voltage supplies and components that are 50+ years old I'm more likely to do an overall supply test especially if the rig has sat for a long time and is an unknown condition.
The only thing I ever use a variac for is a plate supply on a class C final.
Years ago I tried "reforming" a number of old capacitors and concluded that only kicks a necessary replacement down the road.
Bringing up vacuum tubes on a variac extremely slowly is not a good idea. You just need to bring the line voltage up gradually but immediately, otherwise you will cause cathode stripping on the tubes. Think of it like a step-start. Whether a capacitor see a rise in voltage in a few seconds or a few hours makes no difference, if the capacitor is going to fail at a given voltage it going to fail whether a few seconds in or a few hours. If it needs to be reformed it should be replaced regardless. Period.
Using a variac extremely slowly can do more harm than good, and removing the tubes during the process does not present actual conditions for the caps regardless. Using the variac slowly over hours with the tubes in place will cause cathode stripping and possible filament failure. If something smokes, well ... it was going to do that anyway, variac use timing notwithstanding.
If you do not check the condition of the transformers before applying power, you're putting the cart before the horse anyway in your methodology.
I'm not sure what this means. Are you guys saying units where the neutral of the line is connected to the chassis? What is meant by U-ground? I'm sure that was done (mostly in the tube era) but on more modern gear? I just can't put my finger on an example of this. And is not any AC power supply, whether internal or external, the same as "transformer powered"? Are you taking about DC only gear with no power supply where the negative can sometimes be connected to the chassis? In that case why use a transformer? I'm sorry if I am misunderstanding the example cited for isolation transformers use.
I have always thought an isolation transformer was a simple 1:1 transformer across the hot and neutral on an AC line with the ground being handled separately. The normal purpose being to isolate the hot and neutral from ground (as well as slowing transients and overshoot). Either side of the secondary can then be connected to earth ground with it not creating a problem, unlike the primary where the neutral is connected to earth. The ground lug is still part of safety ground on either side of the transformer. The nature of the power supply is independent of the use of an isolation transformer and its intended purpose. Am I incorrect?
Basically, any radio that powers directly from the AC mains (no transformer inside, etc.) needs to be at least tested first using an isolation transformer. A Variac is NOT an isolation transformer. Such unit just varies the voltage using 1-side of the AC mains.
Some of the old transformer-less units did have one side of the power cord tied directly to the chassis while other units had a "buss bar" arrangement where the "buss bar" was "floating" and tied to the chassis with 1, or more, capacitors. It is not unusual for these capacitors to short which then puts a direct connection from the AC mains to the chassis.
These units had either non-conductive housings (wood, plastic, Bakelite, etc.) or metal housings with insulators (rubber grommets, fiber, etc.) isolating the housing from the chassis. More often than not, these insulators have now failed and the housing is now connected to the chassis directly.
Even as late as the 1950s, there were still a few DC electrical systems providing 110 VDC to the customer. These systems were, primarily, in large eastern cities like New York City, Boston, Philadelphia, and so forth. AC / DC radios were designed to be used on both AC systems and DC systems. When plugged into a DC outlet, if the radio wouldn't play, the line cord plug would have to be "turned over" so that the voltage would be passed through the rectifier. Of course, since these units did not have a transformer, etc., they were much less expensive to produce. As such, AC / DC radios sold like hotcakes because they could be used on both types of power mains.
An isolation transformer is needed to protect from things like bad insulators between cabinet and chassis as well as to protect from failed capacitors between buss bars and chassis. This is because virtually all test equipment has cabinets, test probes, etc., directly grounded and that produces a shock hazard when working on these types of units. Also, it is certainly possible to directly ground the "hot" side of the AC circuit and that can result in blown circuit breakers, fuses, and even severe consequences to the person working on the unit.
Isolation transformers are cheap and, in a "pinch", it is possible to wire 2-filament transformers with the low voltage secondary connected together as an isolation transformer. One primary is connected to the AC mains and then the other provides the isolated AC voltage to operate the unit.