Can isolation transformer damage device? - Page 1
Forum section: electronics / beginners

author: littlebike
author_rank: Newbie
timestamp: January 26, 2021, 05:20:31 am
content: I have a Triad Magnetics N-92md isolation transformer and beginning to believe it is the source of my pain.First:Recently I had a Hammond 270AX transformer burn up. I naturally assumed it was my circuit, kicked myself for not having a fuse on the hammond, and wrote it off as a lesson.Second:While waiting for the replacement transformer to come I started troubleshooting my Peavey CS800X power amp - one channel was kicking into clipping protection even at low volume. It was doing anything really horrible like kicking breakers, etc.I pulled it apart looking for obvious stuff and posted questions on forums looking for a place to start. Key pieces of advice I received is not not connect a speaker to the thing and check resistance across the speaker outs.I reassembled - I would say its impossible to do wrong, everything is keyed connections - connected to isolation transformer, powered on. After 30 seconds the Isolation transformers circuit breaker kicked in and the peavey let the smoke out; pretty sure the output transistors cooked. What was also odd, the circuit breaker on the Peavey did not kick in.This puzzled me BUT, I knew it had a problem. Possibly my digging around did something. Again, I accept the amp already had an issue.Third:New 270AX transformer arrives, this time I have a .5amp fast blow fuse in line. The .5amp fuse functioned to test power to rectifier tube, though the output of the tube did not look good on the scope - one phase had higher output than other.I connect to power supply circuit and the .5amp fuse blows.Move to 1amp fuse.After 15 seconds I'm not liking the output of the power supply circuit, getting nowhere near what I should. Also, the transformer is quite warm.Disconnect power supply circuit and replace tube - thinking it may be damaged - with diodes feeding my scope, one phase had more output than the other, definitely not the tube.Took a break for a few hours to let things cool off.Hours later I check the 270AX on my mains and it is already ruined. After less than one minute of power it is already warm, I check the output:One Red to Red/Yellow is 197v.Other red to red/yellow is 10v.Green to Green is under 6v.Either Red to red/yellow should be 274vGreen to Green should be 7+ voltsIn this second go round, it was powered on for maybe a minute and terribly hot; Another $60 down the drain.Isolation Transformer:N92MD has its output to a hardwired powerstrip.Connect meter:Live+Neutral = 130v (I regularly get 125v+ at my wall outlet).Live+Ground = 25.8vNeutral+Ground = 25.9vLive to mains ground = 25.6vLive to mains neutral = 26vI was not aware the powerstrip ground was connected to house ground, I thought it was floating.This is the only isolation transformer I've ever used and I have powered other items in the past without issue, like a 65watt tube amplifier. At the moment, I do not believe I can trust it.Any thoughts - aside from me being a newb?Are there tests I should/could run to have peace of mind?

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 26, 2021, 05:54:25 am
content: In the USA, every outlet should have a ground unless things predate ww2. Power strips are just outlet extensions/socket multipliers so yeah, they should be grounded.The wider blade of the outlet is neutral at zero voltage, the narrow blade is 120v, the round pin or U shaped hole is ground.Transformers fail when overloaded. You overloaded them. Or mis-wired them. The over-heating is evidence of that. You either put an incandescent light bulb in series or the better way is to use a varic/powerstat and watch the current as you increase the voltage. It’s very easy to see an overloaded transformer that way and a common way to bench test things, especially tube based devices.EditThe captain obvious comment: You should’ve have stopped when the proper rating fuse blew. With the higher rating fuse, you cooked the transformers. Something in the amp is shorted and is overloading the transformer(s). The higher fuse let the smoke out of the transformers, literally. Current = heat. Too much current is never a good thing.

author: james_s
author_rank: Super Contributor
timestamp: January 26, 2021, 07:00:53 am
content: I don't think the isolation transformer is the issue here. I'm not sure why your're using it in this application though, isolation transformers are used when you want to test something like a "hot chassis" TV or SMPS that does not have isolation. A device that is powered by an iron transformer is inherently isolated by the power transformer in the device and doesn't need a separate isolation transformer.

author: CaptDon
author_rank: Super Contributor
timestamp: January 26, 2021, 02:15:40 pm
content: The CS800X probably has issues. I have to look this up, rectifier tube?I though CS800 was a fully solid state amplifier? As far as fixing the peavey you will have to pull every single power output transistor and measure it for both shorts and leakage. Usually one side of the output stack has a transistor go dead short which generally saves the others in parallel from damage, but now the amplifier has a huge voltage offset and the opposite polarity side of the channel (NOT the opposite channel) tries to pull the offset back to zero volts and multiple transistors on that side overheat and several blow up instead of just one. I have repaired so many Peavey amplifiers which cost the customer about 5 times what it would have if they would have never tried to 'fix it themselves'. High power amplifiers are a bad place for a clueless newbie to think they can fix it without turning it into expensive trash. Also, isn't a CS800 400 watts per channel at 4 ohms with a total wall socket power consumption close to 1000 watts at full load and around 9 amps? And your little transformer has a .5 amp fuse?? Of course your dinky fuse would blow before the CS800 circuit breaker trips. Now that you have fixed it to death good luck getting that CS800 to ever live again.

author: littlebike
author_rank: Newbie
timestamp: January 26, 2021, 03:02:31 pm
content: Two different projects.One is a tube phono preamp using a 270ax for high voltage side.Other is Peavey.I agree with you in regard to Peavey, I did nothing aside from opening it up, looking over boards front and rear for obvious damaged components. Swapped Chanel A and B boards to see if the issue followed the boards - did not. This led me to believe the issue was on output board and that's where I stopped.I have assembled a stereo tube amplifier a few years ago. In mid 2020 I completely rewired that amp to clean up my previous work - make is a bit nicer on the inside; chalk up to covid boredom.anyway, I have something horrible with Peavey that was on isolations trans at the time. A 270ax that was on isolation at the time. And voltages I dont understand coming off the isolation trans.My greater concern at the moment is the isolation trans.Live+Neutral = 130v (I regularly get 125v+ at my wall outlet).Live+Ground = 25.8vNeutral+Ground = 25.9vLive to mains ground = 25.6vLive to mains neutral = 26vIf the other measurements were 65v I would not be as puzzled.

author: CaptDon
author_rank: Super Contributor
timestamp: January 26, 2021, 03:14:49 pm
content: Those lower voltages are caused by a slight bit of capacitive leakage within the transformer and are actually somewhat normal. Your meter has a very high input resistance/impedance. Probably 10 megaohms. Capacitance or capacitors have Xc capacitive reactance which can be thought of simply as 'resistance' in this case. So if the transformer had some slight capacitive reactance (as all transformers will) what your are really 'measuring' is a few microamps of leakage across the input resistance of your meter. In fact you could use ohms law to calculate that leakage. I = E divided by R, you know both R and E and your leakage should be in the microamps range. This is very normal to see those voltages you are seeing even with 'isolation'. The higher the frequency the more a capacitor acts like a short circuit to A.C. while still blocking D.C. current flow. Inductors are the opposite, the higher the frequency the more they look like an open circuit to A.C. but will indeed still pass D.C. current flow. Cheers mate, best wishes!!

author: Jwillis
author_rank: Super Contributor
timestamp: January 26, 2021, 04:06:43 pm
content: When ever you work on or diagnose any tube amp you should have a current limiter . If a fault occurs any surplus current is "Burned off " in the light bulb instead of burning up your circuit .This guy has some videos that are very informative for the novice.

author: littlebike
author_rank: Newbie
timestamp: January 26, 2021, 05:20:59 pm
content: Quote from: Jwillis on January 26, 2021, 04:06:43 pmThanks, I'll likely build one of these today.Very helpful

author: MathWizard
author_rank: Super Contributor
timestamp: January 26, 2021, 10:26:01 pm
content: Hi OP, I think my IF has a fuse, but I better check. It's also some 500W or so Hammond. It gets pretty hot just plugged in, I always try and remember to unhook it. I've never needed more than a few dozen Watt's from it so far.I need to put it in a box w/ a light bulb and plug/switch.