Unstable Autotransformer (230 > 110v)

Dear forum,

I have an issue trying to adapt Japanese appliance to the UK grid.

After prolonged use, the deck gets tempromental, i.e. not enough power is getting to to it. It switches off sometimes, when I switch it off / then on again, the display struggles to light up, then it dies again. The auto-trans gets pretty hot but nothing overly alarming. The DAT deck reads '32W 100v'. Not exactly power intensive class A stuff...
The sticker on the transformer reads '230v to 100v, 50VA'

There was an non-factory sticker on the deck which read "110 volts", naturally I got a transformer advertised 110v / Uk > Japanese.

Any idea's whats going on here?
I could return the auto-transformer, but I'll need a suggestion for a replacement.

I actually have a UK model of the same deck, but the transformer block on this design would've been PITA to swap over, the cables run through the chassis and are soldered to PCB. I thought auto-transformer was simpler. I'd like to get the UK deck working again too as it's a spare.

Thanks for the help where it ensues...
What is the input voltage that you require? 110Vac or 100Vac?

The UK is on a nominal 240Vac, but can regularly vary 5Vac either side.
The extreme limits for the UK are 216Vac to 253Vac.

Any transformers you buy easily (non custom) will be rated for 230Vac to make them compliant with the European harmonised supply.

That means that your 240Vac into a 230Vac transformer will give out more voltage than on the label. About 4.3% more.
If your normal voltage is 245Vac, you will be 6.5% higher than the 230Vac rating.

So a 230 to 115Vac autotransformer running on 245Vac will give out 115*1.065 = 122.5Vac when fully loaded. This will not be under voltage for any 110Vac equipment. It is a significant overvoltage.
For 100Vac equipment it may destroy the equipment.
Last edited:
Hi andrew,
Japanese DAT Deck, factory sticker reads 100v 32W.
Somebody added 110 volts to it.

Transfomer see link. UK > 100v, 50VA.

Deck un-modified, other than how I use a dummy tape in it... which I doubt causes this effect - I suspect the transfomer.

I use it in rec mode for longer than 120minutes, to use it as a permanent A/D D/A converter, so, unlimited by tape length. To do this I cut the ferric-tape but leave tape inside drawer to trick deck.
Off topic:
Theres seperate behaviour which is apparently 'normal operation to prevent head wear'. In normal operation, i.e. regardless if dummy tape is used or not, the rec button switches off auto every 10mins (there's an interupt at IC level somewhere). The tape isn't moving in rec mode, but the drum head spins at 2000rpm, really I think the tape is more subject to damage here than the drum.
So I have to press REC every now and again when I listen to music which is a PITA.


Paid Member
You need the 100VA transformer with an output voltage of 100V, the sticker is probably afterthought implying that operation on 110V is OK or possibly there is a tap for 110V. Note that U.S. line voltage is 120V nominal these days and Japanese voltage devices powered on U.S. mains voltages usually fry their power transformers quickly.

You don't know the power factor of your recorder, but the VA rating of your autotransformer is likely exceeded. Usual rule of the thumb for sizing auto transformers is at least 2X the expected power consumption. (I grew up in the EU and ran U.S. made electronics on auto-transformers without difficulty)


2012-02-23 7:02 pm
Hi andrew,
Transfomer see link. UK > 100v, 50VA.

I tried to use a very similar looking unit to drive a DAC and it kept cutting out. Unit was rated for 30watts continuous and my device was consuming 18watts, but after about 30-45mins the adapter would overheat and cut out. I pulled it apart and found it had a thermal fuse inside it which was cutting out ~65c. Short answer, go buy a bigger converter.
Watts and VA aren't the same. In order to estimate the VA needed to run a given appliance, use W/PF, where PF is the power factor (a number between 0 and 1), and W is the power requirement off the appliance label. A perfect resistive load will have a power factor of 1. Most appliances will use a power supply with a capacitive input filter (which trashes the power factor). A conservative estimate of PF for such a situation is 0.5 So, as a rough rule of thumb, select a transformer with a VA rating at least twice that of the watts on the appliance name plate. In this situation, a 100VA transformer is indicate, as mentioned before.
I tried to use a very similar looking unit to drive a DAC and it kept cutting out. Unit was rated for 30watts continuous and my device was consuming 18watts, but after about 30-45mins the adapter would overheat and cut out. I pulled it apart and found it had a thermal fuse inside it which was cutting out ~65c. Short answer, go buy a bigger converter.

Apologies for long return,

What did you settle for in the end?

In regards to what andrew said about over voltage i'm not keen on buying yet another item at 40quid only to get it wrong. I spotted a 75VA and ponder if that is sufficient as opposed to 100VA... It is a little easier on the purse.. ( see link ).

I manage to get into the transformer block in hope to just swap them out but they are cemented in iron casing.

Thanks again peeps


  • pics_v2.php.jpg
    42.4 KB · Views: 100


Paid Member
2015-12-31 4:57 pm
All the 230/240v to 100/110V "wall wart type" autotrasformers I tried in the past were almost unusable for audio equipment. Extremely bad iron quality, low efficiency and high losses: the output voltage was distorted and started to sag after 20 minutes; plastic case had a burnt smell after a short while. They seems to be geared towards occasional "travel" use for small appliances.

I then discovered that high quality toroidal transformers with dual 55v output windings are cheap and plentiful at any shop that does industrial automation. They also are on stock at most online distributors such as RS components. With series wiring, the output is stable at 105-110v and the transformer is quiet and cold. They are real transformers, with insulation between windings.
Last edited:
^^^^^^^^^^^^^^ what russc said.
That or a custom wound 230/100V autotransformer.

Just as an example, I am not hawking my products, I custom wind autotransformers with 220/127/110/100V taps for sale in Brazil, to attend real needs there.

To begin with, Brazil is an officially dual voltage Country: roughly 50% 220V and 50% nominal 110V .
So much so that you can find both in the same city, often switching just by crossing a street, or inside certain buildings, so most equipment sold there includes a very visible 220/110V switch.

BIG problem is that national system is being converted to 220/127V (three phase 220V with 127V phase to phase) for economy reasons, which is already overvoltage for old 110V stuff and *deadly* for 100V amplifiers.

WHICH 100V amplifiers you may think?:

Brazil has World´s largest Japanese population outside Japan, about 1.5million people, and it´s traditional for ~18/20 y.o. kids fresh out of high school to travel to Japan for 1 year, both to get in touch with Culture, and to earn some U$ dollars working at a factory (think cars or electronics) .
And many use their hard earned cash to buy desirable Amplifiers (mostly Marshall which are very expensive in Brazil) , of course they get the 100V version , which works fine until they go back to Brazil.

Transformers such as I suggest are the only reliable answer.
I adhered the suggestions but £33 excluding VAT for the 100VA one was too much, the original one I had, causing the problem was purchased for £35, I only managed to sell it on ebay, at a 25 pound loss...

Luckily later suggestions of a twin primary transformer inside the case with a switch is are more elegant. There's only so much room inside the case and I suspect a custom wound will be marginally smaller than a twin primary but not by much.

JMFahey any particular coil you suggest?
Best voltage transformers and converters seem to be by ACUPWR, which are made in the US. They come loaded with safety features. I'm actually using an old Scott mono amp in the UK as a guitar amplifier. The ACUPWR step down transformer is quiet and never gets beyond lightly warm.
safest line/mains voltage converter i found

I had a dilemma with using an old and sentimental 117V Scott hifi amp in a 220 and 50 Hz country and found a brand on Amazon called ACUPWR. It is american made and has many safety features. Working very good so far. No heat and low noise. Lots of iron.
Unstable Autotransformer (230 >110v)

Hello giro1991
The conversion / power changing from 230/240 to 100 volts can be accomplished several ways. You need to know the power required and verify it. Then plan 2x power for the long run. You should get or borrow a Kill O Watt type meter. They are about $30.00 investment. 120 volt and 240 volt types. Several ways to get the 100 volts are to use a BOOST / BUCK transformer configuration. By using a filament transformer 6.3V, 12.6V with a current rating of 2-4 amps the voltage can be adjusted to 100 volts ±10% or better. This transformer can be placed in either Primary or Secondary. Make sure that the transformers are rated for 50Hz operation. Japan products are 100 volts 50/60Hz.

No need for 2X the power

From my experience, the only mains voltage converters were the Chinese crap. So it was refre3shing to find that brand. It has features that eliminate the need for buying 2 x the wattage that you actually need. And I talked to their technician who mentioned that because of the quality and amount of iron used I didn't have to worry about the frequency. I know this--the transformer runs only slightly warm and the amp sounds great. No red plating or anything like that on the tubes.