Improving the power supply of an early solid state TV (regulation, isolation) - Page 1
Forum section: electronics / beginners

author: special_K
author_rank: Regular Contributor
timestamp: September 22, 2025, 09:58:19 pm
content: Hello all.So I've got myself a rather old (circa 1972) TV from germany, which I need to do some redesign to to get the best.Firstly the power supply sucks. Like a lot of TVs of the time it's hot chassis, with half wave rectification, couple of big-ish caps and a couple of dropper resistors aiming for 220VDC. There's no actual regulation going on.This is a big inconvenience to me because it's originally a German TV where wall power is usually around 220VAC, and I live in the UK, where most places (definitely in my apartment) it's 240V or higher. The TV does not at all like the higher input voltage and malfunctions. I'm running it right now on an autotransformer, which means another box lying around my living room, and I would like to eliminate this.I could just replace the dropper resistors with higher ohm ones, but this still wouldn't provide any regulation, still be a silly half wave load.Also, I'm not satisfied with the performance of RF modulators. I would rather just disconnect the IF section and inject luma and audio signals directly, which I've done on other TVs in the past. But this needs isolation.What would be ideal for me is some off the shelf SMPS which has a 220VDC output and about 100W capability (TV only uses 60W, but added an overhead), but I can't find something like that for sale anywhere.Alternatively, I could build a simple linear PSU, but I cannot find a jellybean linear regulator which can deal with anywhere near 220V output.I considered adding isolation at the AV inputs instead of at the PSU. For audio this is easy to do with a small transformer, but for video that kind of transformer doesn't have the bandwidth. I'm uneasy with trying to do a capacitive isolation because of the consequences of getting it wrong.I have attached my TV's circuit diagram and would be grateful for anyone's insight into the problem.

author: themadhippy
author_rank: Super Contributor
timestamp: September 22, 2025, 10:07:15 pm
content: 1 to 1 isolation transformer for the isolation,and 220v zener and suitable transistor for the regulation

author: special_K
author_rank: Regular Contributor
timestamp: September 22, 2025, 11:03:22 pm
content: I found some interesting regulator circuits using a zener like you recommend, but more in the 24v range than 220v.https://sound-au.com/articles/regulators-2.htmI figure these designs could just be scaled up, but what would be a good transistor to use? I've never built something in a voltage this high before.

author: special_K
author_rank: Regular Contributor
timestamp: September 23, 2025, 05:13:13 am
content: I'm finding that the price of a 1:1 isolation transformer is prohibitively expensive.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 23, 2025, 07:11:57 am
content: Video bandwidth is 6MHz. I don't think this is beyond the bandwidth capability of a transformer. This is by far the best solution to isolating an input signal into a hot chassis, especially for those with little to no high voltage experience.In a transistor pass regulator, the 220V zener can be made up of multiple zeners in series.This still leaves the problem of isolation, where ultimately you will need a transformer, the 1:1 being the easiest (not cheapest).

author: Analog Kid
author_rank: Super Contributor
timestamp: September 23, 2025, 08:28:57 am
content: Hmm; wouldn't capacitive isolation work just as well, if a guy just wanted to isolate the AV input?I'm thinking something like just a couple of 100nF caps or so. The 50Hz power won't be able to get through that but the signal will.

author: MariuszD
author_rank: Regular Contributor
timestamp: September 23, 2025, 09:33:43 am
content: Quote from: special_K on September 23, 2025, 05:13:13 amHave you looked for used ones?You can also use two identical transformers connected in opposite directions, for example, 240V > 110V > 240V.I bought insulation transformers very cheaply, for under $30.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 23, 2025, 09:53:19 am
content: You can wire a small 25VA toroid w 24V AC secondary as autotransformer and mount that in the tv-set. Should work.Back in late '80 Harris had an optocoupler out with high linearity (diff gain & phase) and acc to their appnote it was primary intended for analogue video. Did'nt remember the type - second symbol was an 'H'. I build that video-isolator and put it in my Trinitron chassis, cause it was unable to accept CVBS - only Rf-modulated video (no mains isolation). Worked!AvagoHP shows for their optos HCPL-4562 & HCNW4562 in Fig 4 an example using them as video isolator.https://docs.rs-online.com/7369/0900766b80779716.pdf

author: special_K
author_rank: Regular Contributor
timestamp: September 23, 2025, 02:38:29 pm
content: Some interesting replies here, I'll get to them one by one.Quote from: Andy Chee on September 23, 2025, 07:11:57 amOh I've already tried using a transformer to pass video on another set, using a 1:1 audio transformer and the picture was incredibly blurry. The connection point on that TV had a 5V DC bias, so it might have saturated the TX?I've built/been around plenty of linear power supplies before, this is the first time I've considered building one with an output DC voltage that's in the hundred volts range though, so my only stumbling block was thinking "wait, there's no 78xx that goes this high!"Quote from: MariuszD on September 23, 2025, 09:33:43 amYeah there are none. Maybe in a few months someone will sell some 600+ VA monster in a sealed plastic enclosure about the size of a cinder block, and when they do it'll probably have a 55-0-55 output for british worksite tools. I need a bare transformer 60-100VA.Come to think of it a 1:1 TX is probably inappropriate. after rectification and smoothing that'd be something like 300VDC. dropping 80V with regulation seems excessive. Around 14 watts of heat.Quote from: Uunoctium on September 23, 2025, 09:53:19 amI already have such an improvised autotransformer to get the TV working, as mentioned in my first post.Quote from: Uunoctium on September 23, 2025, 09:53:19 amI'm looking at fig 4, and it shows 3 connections on the video input side: Vin, VCC1, and GND. What is VCC1 and where does it come from?

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 23, 2025, 03:18:42 pm
content: Yep, for the primary side of opto unfortunately an auxilary PSU is necessary. An old charger for cellphones can do that job.

author: special_K
author_rank: Regular Contributor
timestamp: September 23, 2025, 05:02:30 pm
content: A pity. I haven't checked to see if that opto is currently available, but if I were to use it, I'd prefer to put together a small PSU and fit it inside the case rather than have a wall wart cluttering up the place. At that point I think I might as well just stick to putting the isolation on the TV's power side.I have found a source of not cheap, but affordable (£33) 60VA 1:1 240v isolation transformers. I'm going to contact the seller and ask about it's height/width/length.I'll draw up a schematic of what I'm thinking to do and post it here when it's done.

author: inse
author_rank: Super Contributor
timestamp: September 23, 2025, 05:15:30 pm
content: A RF modulator should be by far the best solution to feed a video signal to old sets.They can be salvaged from old VCRs for example.

author: Analog Kid
author_rank: Super Contributor
timestamp: September 23, 2025, 05:48:29 pm
content: Nobody addressed my (speculative) solution, so I'll ask again:Wouldn't a couple of capacitors of the appropriate size at the signal input solve the problem?

author: Benta
author_rank: Super Contributor
timestamp: September 23, 2025, 07:04:12 pm
content: No one answered it, perhaps because it's obvious.The chassis is live, which means the cable shield can be 0 V if you're lucky, or 230 VAC if you're unlucky. 50:50.

author: special_K
author_rank: Regular Contributor
timestamp: September 23, 2025, 07:12:21 pm
content: Quote from: inse on September 23, 2025, 05:15:30 pmI already addressed RF modulators in the very first post.A consumer RF modulator is plug-and-play, but the picture quality is always atrocious, because they're so low powered, cheap and nasty. They're nothing like the performance of real over the air broadcast modulator.A VCR is also just about the worst place to get one from. They're highly integrated into the input tuner/IF strip/detector module and are also usually i2c controlled so you can use the remote to set output channel via an onscreen menu. There'd be no way to use it outside of the VCR.Installing AV inputs is a massively superior solution always. It's why after using TVs for things other than watching OTA broadcasts became established, TV makers responded by providing those connections.Quote from: Benta on September 23, 2025, 07:04:12 pmI thought for a moment it could work if both gnd and signal had a cap in their path, like the antenna connector does. But then I remembered CVBS has a DC component.DC could be restored afterwards but it's extra work. Though, if the TV doesn't like line level input I might have to build an op-amp stage anyway...but yeah I would rather just isolate the PSU and work out one problem at a time.

author: Analog Kid
author_rank: Super Contributor
timestamp: September 23, 2025, 07:16:04 pm
content: Quote from: Benta on September 23, 2025, 07:04:12 pmYes, understood.But if you put capacitors onbothof the signal conductors, including the shield, wouldn't that be effective isolation?

author: Stray Electron
author_rank: Super Contributor
timestamp: September 23, 2025, 07:18:49 pm
content: Quote from: special_K on September 22, 2025, 09:58:19 pmWill a 1972 era TV still work (receive) in your location? Many years ago the US shut down all of the analog TV transmissions and forced everything to switch to digital and the old TVs are now completely useless except for connecting to the video modulator of a TRS-80 or a Commodore 64.There were so many short cuts and compromises and "sub-optimum" quality parts in the 1970s TV that, IMO, you're trying to make a silk purse out of a sow's ear.From a 1970s TV repair technician.

author: special_K
author_rank: Regular Contributor
timestamp: September 23, 2025, 07:21:46 pm
content: Quote from: Stray Electron on September 23, 2025, 07:18:49 pmWhy reply when you haven't bothered to read the thread?

author: Gyro
author_rank: Super Contributor
timestamp: September 23, 2025, 07:27:31 pm
content: Quote from: Analog Kid on September 23, 2025, 07:16:04 pmYou would need Class Y caps for safety, and paralleling enough to achieve the required capacitance would cause an unacceptably high leakage current (50% of the time anyway). It would also result in a very nasty case of hum, both on audio and video.

author: Benta
author_rank: Super Contributor
timestamp: September 23, 2025, 07:39:21 pm
content: Quote from: Analog Kid on September 23, 2025, 07:16:04 pmTry it out. Take a 100 nF cap, hold one lead in your fingers and connect the other to 230 VAC while your other hand is grabbing something grounded. See how high you jump (let your wife film it and hold a phone ready for the 911 call).

author: special_K
author_rank: Regular Contributor
timestamp: September 23, 2025, 09:12:50 pm
content: Okay, I've had a moment to sit down and draw out a first iteration of the power supply, using the zener diode recommendation. (see attached)I decided to keep the little autotransformer in circuit simply because it'll reduce the amount of dropping the regulator will have to do, and I already own it, so it's not costing me anything.I kept the 100u caps because that's what the TV already has to smooth half wave, I figure that will be overkill to smooth full wave. The rest of the passives I haven't bothered to assign values at the moment.This is all informed by elliott sound products' page on discrete linear regulators. As a hi-fi guy he was thinking about a tenth of the output voltage and ten times the output current, so the transistors I've picked out here are nothing more than me guessing ignorantly. I would appreciate it if someone took a look at the suitability of Q3 in particular.I know it's primitive because there's no feedback, but the standard TV has no regulation at all, so I figure that doesn't matter too much. Of course, if making it more sophisticated can be done without increasing cost too much then I'm all for it.

author: floobydust
author_rank: Super Contributor
timestamp: September 23, 2025, 09:32:17 pm
content: Most TV's ran around 130VDC (North America) , nothing like 220VDC- this set is hot.Common linear B+ regulator IC used in TV's was the Sanken hybrids like STR30130.datasheetthere's a whole series of them with different output voltages. No idea what colour TV's used in Europe.Really, it is just a power Darlington and another transistor + HV zener. Look at the schematics, nothing special.You need some idea of current drain on B+. Need a heatsink as well.I don't think theMJE13005is tough enough. Look at the SOA curve Fig. 11 getting 0.5A with it is pushing it.Inrush currents, need the big power resistor up front.

author: special_K
author_rank: Regular Contributor
timestamp: September 23, 2025, 10:16:54 pm
content: Thanks for pointing me to fig 11. I picked out transistors last night at 5am or so, which isn't the easiest time to read a sheet. I guess I'm on the lookout for a transistor that's even higher voltage.I've already measured current drawn by the TV on the U1 rail, it's only 0.18A.

author: special_K
author_rank: Regular Contributor
timestamp: September 23, 2025, 11:20:32 pm
content: I find myself looking at the BUF420AWhttps://4donline.ihs.com/images/VipMasterIC/IC/SGST/SGSTS29182/SGSTS29182-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 24, 2025, 03:41:54 am
content: The video opto costs about $4.50 at Mouser / Digikey in THT! Suggestion: If the hole in your toroidal transformer is not filled with epoxy resin, you can wind about 20...30 additional turns of almost any diameter to obtain an auxiliary supply and omit the isolation transformer. (I assume 3 to 4 turns per volt.)

author: floobydust
author_rank: Super Contributor
timestamp: September 24, 2025, 03:42:07 am
content: That's a trap - HV switching transistors are not great in the linear region. The datasheet sneaks in FBSOA (ICPK) but not for long.I couldn't find any HV Darlingtons around 250-300V at 0.5A or so. Then Digi-Key bot security locked me out for some reason.

author: special_K
author_rank: Regular Contributor
timestamp: September 24, 2025, 03:37:04 pm
content: Is it possible that a suitable transistor simply does not exist?

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 25, 2025, 11:29:06 am
content: Quote500VA SMPS, Input switchable: 110V±15%/220V ±15% ACacc to dropdown selector: 0-220VDC out, but listing says only 200VDC, adjustable up to 203VDCPrice: 23,19€https://de.aliexpress.com/item/1005009894808353.html?

author: richard.cs
author_rank: Super Contributor
timestamp: September 25, 2025, 12:49:40 pm
content: Quote from: special_K on September 23, 2025, 02:38:29 pmI wouldn't have expected anaudiotransformer to work at all. Nonetheless there are RF transformers that will happily pass 10 MHz or more, I am less sure what is available with sufficient isolation. This is video baseband so it will need to go down to fairly low frequencies depending if you pass the syncs separately or not.

author: floobydust
author_rank: Super Contributor
timestamp: September 25, 2025, 04:11:33 pm
content: Quote from: special_K on September 24, 2025, 03:37:04 pmMy reply yesterday vanishedDigi-Key's anti-bot thing is way too hostile lately.I was saying I didn't see anything in a Darlington. say 250V. It's just that the TV has big capacitors so inrush currents will be high.You can look at older TV schematics and see how they did it - sometimes used a NTC, power resistor across C-E of the transistor to help it out.I'm worried this TV is literally space-age for its day, with the SCR deflection. Sony and RCA didn't go there until the 1980's.So B+ coming up too slow I'm not sure if the set has coverage for that.Consider some audio power amp transistors likeNJW0302.MJE15034 not much SOA 100V 0.2A but experts can suggest something.

author: special_K
author_rank: Regular Contributor
timestamp: September 26, 2025, 02:33:36 am
content: Quote from: Uunoctium on September 25, 2025, 11:29:06 amWhoa there. I'm surprised you found it but I didn't, I was looking all over aliexpress. Only finding 24VDC LED light supplies, that sort of thing. Now we have a strong contender, and for less than just the price of a 60VA isolating transformer on it's own.Quote from: richard.cs on September 25, 2025, 12:49:40 pmThere are isolation transformers for baseband video sold for use with analogue CCTV systems, but they're absurdly expensive, nearly £200.Quote from: floobydust on September 25, 2025, 04:11:33 pmYou say space age but look closer. DC coupled video path to stop the picture from washing out, very nice... but there is no feedback regulation of anything. Bright scene makes picture expand, dark scene makes picture shrink. It's very silly.