Running 120V AC main radio with 240V AC - Page 1
Forum section: electronics / rf-microwave

author: vinlove
author_rank: Frequent Contributor
timestamp: May 16, 2020, 03:38:46 pm
content: Hi AllJust wondered, what would happen, if you plug in a 120V AC mains operating radio into 240VAC Mains wall socket?Would it always blow off and get serious damage in the radio? Why? Or would it work, but with some problems? Why?What do you think? cheers.

author: greenpossum
author_rank: Frequent Contributor
timestamp: May 16, 2020, 04:10:34 pm
content: Please make a video and post its URL.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: May 16, 2020, 04:27:28 pm
content: I agree about the video and linking it in.If it’s a transformer power supply, the output would be double so everything after the transformer would be seeing a much higher voltage. The transformer is also getting a higher input, so that may cook itself too (depending on fuses and load)The transformer may have taps that you can change the input voltage, but being cheap consumer goods, they would have dropped that expense in the first design meeting.If it’s a switchmode power supply with a single range input, it’s going to blow the caps after the rectifier. If it has a wide input voltage, they would have marked it as such.

author: chris_leyson
author_rank: Super Contributor
timestamp: May 16, 2020, 04:59:46 pm
content: If you are lucky then 240VAC will blow the primary side fuse, assuming it has the correct fuse fitted. If it's an AA5 then it will, maybe, fail safe. You will have turned the tube heaters into little flash bulbs and the HV rectifier won't be supplying any DC to the smoothing cap. Expect exploding X caps, if fitted, wear eye protection. Edit: If it's a transformer supply then the primary winding flux density will double and the transformer will saturate blowing the primary side fuse, if fitted, or the primary winding. Finally, who in their right minds would power a radio from a switching power supply !!! Will it work ? NO.

author: bob91343
author_rank: Super Contributor
timestamp: May 16, 2020, 05:47:17 pm
content: If you have two identical radios, put them in series and it will work. You can pretend it's stereo.

author: vinlove
author_rank: Frequent Contributor
timestamp: May 16, 2020, 06:54:35 pm
content: The situation with this thread is that, I bought an old radio from Germany. The seller posted it quickly but without any document or manual or instructions.So, I thought that the radio was from Germany, just assumed that it would operate on 220V AC. And it could be plugged into our (UK) domestic mains 237V AC without any modification or voltage adaptor (+- 10% tolerance on AC input voltage with the appliances are norm), and powered it on. It worked. It didn't blow off. But the radio was really noisy. I had to reduce the RF gain right down to hear anything.I thought this is strange, so I plugged the radio into my variable AC power supply, and reduced the input power to the radio to 110V, and it worked fine. It is now operating OK with normal sensitivity.This old radio from 1970s had no labels and nothing to say that it is to be operated on 110V, and I am not sure how the original owner of this radio in Germany used it. I think the seller was just house clearing seller who had no info and knowledge about his items for sale.Anyhow, I thought it was really strange that it didn't blow off when powered on with 237V AC. It worked like normal, but it blew the small light bulb for the S meter. It has been working with 237V AC for 2-3 days without getting fried, but with very noisy reception.Now running normally and quietly with 110V AC from my variable AC PSU.I just wondered if this is normal or very weird case. And also I was wondering if it had damaged any internal parts while being operated with 237V AC, even if it is not showing now, which will show off later on.The radio has an internal transformer which converts AC to DC 12V, and it says 110-120V only. I cannot see anywhere to tap or change the input AC voltages.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 16, 2020, 08:24:20 pm
content: If it is in fact a 120V primary transformer, I'm surprised it survived that!Normally, when you put too much voltage into a transformer it will saturate the core (too much magnetic flux) causing the primary winding to draw excessive current and cooking itself in pretty short order.It is possible the transformer was built with a much larger core than necessary and doubling the voltage did not draw enough current to overheat the primary. That would be the exception rather than the rule but you know, they don't build them like they used to.If all the circuitry powered by the secondary has sufficiently high voltage ratings (particularly capacitors and any ICs) it could indeed survive double the voltage, or if there is a regulator, but the improper functioning of the radio when powered from 240V would suggest there may not be a regulator, or at least not all of the circuitry is powered from regulated DC.I'd say you got lucky, but I certainly wouldn't continue operating it from 240VAC. Many parts could be pushed very close to their maximum ratings and greatly affect long term reliability.

author: vinlove
author_rank: Frequent Contributor
timestamp: May 16, 2020, 09:30:32 pm
content: Yes, I think I was extremely lucky.I got my old step down transformer out and running it with 110V, and the radio seems happy. It works well especially on the amateur bands CW and SSB reception.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 16, 2020, 11:20:08 pm
content: Quote from: bob91343 on May 16, 2020, 05:47:17 pmor if there's space, put in a second similar/same transformer in the radio,and wire both transformers' inputs in series (for 240v)and the outputs paralleled going to the electronicsDon't mess up and check that both transformers polarity are the same, check operating temperatures,and wack in some fuses in case the radio complains about hurried oopsie DIY wiringi.e. don't do it, just use a stepdown tran or variac like I do, and enjoy the radio

author: bob91343
author_rank: Super Contributor
timestamp: May 17, 2020, 12:23:27 am
content: What is the model of this radio? Very few consumer radios have an S meter.

author: vinlove
author_rank: Frequent Contributor
timestamp: May 17, 2020, 08:26:32 am
content: Yeah, there is not much spare room inside the rig anyway. And I am not into modifying radios if there are other easier and simpler option to get it working safer ways.The radio is a very rare HF receiver from 1970s, MINIX MR-73B. It works great. I think it is very similar performance to Yaesu FRG-7, but without the WadleyLoop Tuning method. I prefer this MR-73B.

author: richard.cs
author_rank: Super Contributor
timestamp: May 17, 2020, 03:42:43 pm
content: This one?https://www.radiomuseum.org/r/minix_mr_73mr7.htmlHaving looked up that radio it has a conventional transformer-rectifier power supply and it looks like it was made for 12V, 120V and 220V, though it's not clear if those latter two are switchable taps or different models. Most likely the transformer was saturating and limiting the energy out of the secondary. The output voltage might then have not been too far above normal. Usually the transformer might be expected to overheat within small numbers of minutes, but it does look quite small, and on very small transformers the primary resistance can be enough to protect them from most abuse.Most likely you had the transformer running hot, but not quite to the point of insulation failure. I wouldn't be entirely surprised if the transformer fails in the near future though.

author: vinlove
author_rank: Frequent Contributor
timestamp: May 17, 2020, 06:35:50 pm
content: There is another weird fact with this radio. It runs with 110V AC, but as soon as the voltage drops below that, it does not run. It goes silent. I would have thought that if it was rated to run at 110V, then it would run with ±10% of fluctuation of input voltage for nominal, but in practice, it would even run well below 90V to 80V?The only reason I believed that it was rated at 110V AC was the quietness of the reception, but that couldn't be logical criteria for the proper input voltage could it?Using my trusty VARIAC I changed the input voltage into the radio, and the lowest it will run is 110V AC, and it runs all the way to 237V.But below 110V, it goes silent = no reception.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 17, 2020, 10:15:45 pm
content: Some of the transformer wiring cooked and or zapped rectification components?

author: TheMG
author_rank: Frequent Contributor
timestamp: May 17, 2020, 11:57:10 pm
content: Quote from: vinlove on May 17, 2020, 06:35:50 pmDid you measure the secondary voltage while doing so? From the schematic, one of the secondary windings should be 12VAC, and the other secondary (which powers the bulbs) should be 6.3V.That would be a quick easy way to determine proper voltage. If you put 120VAC in, and you only get 6V and 3.15V out, then it should be running on 240VAC. Simple.

author: vinlove
author_rank: Frequent Contributor
timestamp: May 18, 2020, 07:05:15 am
content: Quote from: TheMG on May 17, 2020, 11:57:10 pmI will open it up, and check it out.But last night, the radio was running on 220V AC from the variac, and it was the best performance.So I am convinced now, it is wired for 220V. If it were 120 AC, then it would have blown off as soon as powered on 240V input.But it sometimes sounds better on the lower voltage when the bands get very noisy and busy. At that time, reducing the input voltage to 120V - 150V AC makes the reception very quiet and more listenable.But below 120V AC input, the radio simply doesn't operate.If it were wired for 120V, then I am sure it would operate on 70-80V AC input albeit very quiet and with much reduced sensitivity.And the input voltage where the radio goes total silence would be somewhere about 60 - 65V AC.