Is TV refresh frequency dependent on mains frequency?

[Richardcavell]

Hi, guys.

In the US, where the mains frequency is 60 Hz, the Commodore Amiga, my favourite computer, produced a 60 Hz video signal and so did all native TV systems. In the UK, where the mains was 50 Hz, all TV systems and computer systems ran at 50 Hz.

Did the TV refresh frequency ever actually sync to the mains power frequency?

To put that another way, would there ever have been a problem running a 50Hz TV system on US power, or running a 60 Hz system on UK power?

I realize that these days with hardware being habitually sold internationally and with everything going digital, it's not so much of an issue. But it causes real concern, because computer games would run too fast in the US and part of the image would be cut off.

Richard

 

[Electroenthusiast]

YES. TV refreshing rate is 30 for 60hz , and 25 for 50Hz mains frequency. I read this in a Book.

 

[Nigel Goodwin]

Hi, guys.

In the US, where the mains frequency is 60 Hz, the Commodore Amiga, my favourite computer, produced a 60 Hz video signal and so did all native TV systems. In the UK, where the mains was 50 Hz, all TV systems and computer systems ran at 50 Hz.

Did the TV refresh frequency ever actually sync to the mains power frequency?

To put that another way, would there ever have been a problem running a 50Hz TV system on US power, or running a 60 Hz system on UK power?

There 'were' TV's that did run the frame directly off the mains - but these LONG home pre-dated computers - so it wouldn't have been an issue.

Particularly as the UK and USA used completely different TV systems (405 line in the UK back then, 525 line in the USA).

The relationship between frame rate and mains frequency was done to prevent fast moving hum bars on the picture.

 

[schmitt trigger]

As Nigel mentions, this was done at the very early stages of TV broadcasting.
On those days, everything was vacuum tubes (valves). Mains hum would be everywhere...from powering the heaters with AC, to very crude power supplies.

If the TV's refresh frequency (called field rate) was slightly off, a hum bar could be seen scrolling slowly across the TV screen.
That was very annoying. It was best if you had a single, static bar, which you could compensate for. Therefore, the idea was to synchronize to the mains frequency.

Nowdays, with the very sophisticated, low ripple SMPS supplies, this is no longer a concern.

 

[dr pepper]

I have had hum bar issues, I have a couple of old tv sets that are part of my vintage 'radio' collection.
These days though as far as the receiver is concerned there isnt any sync to the mains, the set is synced to the transmitter using frame and line syncs on the video signal (for analogue).
Incidently I have come across issues with older tv's, including some transistor models from the 70's and 80's, the frame sync from some home computers doesnt meet the original spec and the tv doesnt lock very well to it, the frame sync is a pulse train which maintains overall dc level, some home computers simplify this pulse train and the resultant dc inbalance messes up video amplifiers in old telly's.
None of this is going to be a problem for you though, unless your using vintage tv's.

 

[crutschow]

The NTSC (US) color system changed the refresh rate to 59.94Hz, which helped reduce the visibility of interference between the chrominance signal and FM sound carrier. So, if there were hum bars in the receiver, they slowly moved through the field of view after the introduction of color TV transmission.

 

[RCinFLA]

Originally yes, many years ago. There were not power supply regulators when vacuum tube (valves) ruled. There was almost always a little line ripple (2x for full wave rectified power supplies on the DC supplies. Having a syncronous rate make the shading bumps stay put on the screen which is a lot less annoying then moving hum bars. When color (or colour) introduction the vertical scan rate and horz. scan rate drop slightly to allow the color subcarrier to spectrally fit beween peak in B/W spectrum and reduce beat dot crawl. Most early color sets just limited Y (luminus, B/W) bandwidth to spectrum below color subcarrier. It allowed a simpler or total elimination of delay line on B/W path (narrower filter on color channel had more time delay so B/W path had to be time delayed to match). Later on some TV used comb filter to rake the B/W spectrum.

When solid state, lower supply voltages, and regulated supplies became economically possible it doesn't really matter any more. Only issue that still shows up is strobing between room lighting (particularly fluorescent lights) and a TV screen.

 

[Nigel Goodwin]

Originally yes, many years ago. There were not power supply regulators when vacuum tube (valves) ruled. There was almost always a little line ripple (2x for full wave rectified power supplies on the DC supplies.

'Old' TV's NEVER used full wave rectifiers - they were all just half wave

 

[Mike odom]

you can still see hum bars strolling up a tv screen, just look at it through another camera, digital or analog... the refresh rates won't be in sync, so you will see the bars float up the screen...

 

[RCinFLA]

'Old' TV's NEVER used full wave rectifiers - they were all just half wave

I worked repairing TV's for about 7 years. You obviously have *NEVER* worked on one.

Just about all the color TV is the late sixties to late seventies used power transformers weighting about 10-15 lbs (400-500 VA rating). A transformer run with half wave rectification has to be larger then when run as fullwave, not to mention the extra filter capacitor size to cut down half wave rect. ripple. It would be just plain stupid design.

Many B/W sets used voltage multiplier without transformer, charging first cap on negative half cycle of mains, then bootstrapping to higher voltage cap on positive cycle of mains, so also full wave rectification.

 

[Nigel Goodwin]

I worked repairing TV's for about 7 years. You obviously have *NEVER* worked on one.

I've been repairing TV's professionally for over 40 years, and still do - I think I've a LOT more experience than you

As I said, OLD TV's all had half wave rectifiers - only 'modern' sets used full wave ones - you then choose to list modern TV's that used full wave ones?.

Try reading the title of the thread, we're discussing TVs so old that they used the mains frequency for the frame timebase - LONG, LONG pre-dating colour (even in the US).

 

[tvtech]

I am supposed to be ought of here.....

Well I am not

Just had to chirp here....Don't F... with Nigel. Why??

Because he is the reason why ETO exists anyway. End of story.

Nigel Goodwin and I never got off to a good start. We had a disagreement with my very first post here....a Posistor versus PTC thing.

That was around 3 Years ago.

Subsequently, though, I have both learned to respect and admire his way of both handling this Forum and his way of doing things.

Calls a Spade a Spade. And rightly so. And bears no grudges.....ever. Otherwise tvtech would have been history with his very first post.

Good man, good Forum.

Thank's
tvtech

 

[Nigel Goodwin]

Because he is the reason why ETO exists anyway. End of story.

No, not me!!.

Otherwise tvtech would have been history with his very first post.

Many people have been!

 

[dr pepper]

I'm with Nige on this one too.

I have a small collection of valve radios and tv's, only one has a full wave rectifier in it, and thats an odd ball.

I think the reason for half wave is that in the valve era copper and iron were cheaper than valves, so a transformer not fully utilised was cheaper than using 2 valves, 2 sockets, extra wiring and heater currents.

I'm not much past 40 so I've got a bit of learning to do to catch up to you Nige!

 

[Nigel Goodwin]

I'm with Nige on this one too.

I have a small collection of valve radios and tv's, only one has a full wave rectifier in it, and thats an odd ball.

I think the reason for half wave is that in the valve era copper and iron were cheaper than valves, so a transformer not fully utilised was cheaper than using 2 valves, 2 sockets, extra wiring and heater currents.

Metal rectifiers were more common than valve ones - but again, not cheap - live chassis sets were common, mains transformers (particularly in TV's) were fairly rare (again, down to cost).

In my 'early days' a common procedure was replacing metal rectifiers (in record players, radios and TV's) with silicon ones, along with a wirewound resistor to create the higher impedance of the metal rectifier.

 

[tvtech]

The first TV with a Bridge Rectifier and chopper PSU I ever saw was what we called the Thorn 4000.

These arrived at our shores maybe around 1974. Not sure. When I started working for the biggest TV repair and service organization in SA @ the time in 1992....these sets were already 18 Years old. On service contract. With us.

Man, experienced Tech's were scared of them. And I had to go out on a Service call one day to fix one of them....as a rookie.

I will never forget the LOPTX. When you touched it it wobbled like a Jellyfish. Strange. Tripler right next to it, but hard to change.

Anyway, luckily for me the Tripler had a bubble. Eventually got it changed and fired the set up...and it worked. And then I got the hell out of there before something else decided to fail. That would be another Service call booked for another day. And not me the Rookie

LOL, those were the days

Regards,
tvtech

 

[Nigel Goodwin]

The first TV with a Bridge Rectifier and chopper PSU I ever saw was what we called the Thorn 4000.

The 4000 was a bit of a flop for Thorn, it was over complicated and unreliable, we never sold very many of them.

However, the previous chassis the 3000/3500 chassis was a great set, and we sold large numbers of them - it was the first domestic use ever of a switch mode PSU. I actually built a jig for repairing panels for them, as it was a completely modular set.

The 3000 was Thorns second colour set, the previous 2000 series was dual-standard, was again rather complicated - and was the worlds first ever all solid state colour TV - Thorn never made a valve colour TV, they were many years ahead of all other manufacturers.

 

[tvtech]

Hi Nigel

Great to speak to a person that knows the TV business/history/repairs etc.

Technical TV stuff is at times hard to comprehend.....for others. We both know through experience what we deal with on a daily basis..

I feel sad for Derstrom though. He on behalf of another Member is sacrificing himself. Not fair.

Nobody can reliably build what the "host" needs. Please look at this thread:https://www.electro-tech-online.com/threads/competition-high-voltage-power-supply.132493/.

Stay well Nigel. Really time to go for me.

Best regards,
tvtech

 

[dr pepper]

I only have one set (made in yugoslavia) with metal rectifiers, aka selenium diodes, they were popular in the 60's, allthough I do have a wartime receiver that has one as the am detector of all places to have one.
A lot of older british radios tend to have mains tx's, american tabletops like to go without, some even have resistive mains leads that drop the extra 120v, later bush sets dont have a mains tx, for some odd reason they have a reversible mains connector, dangerous in its own right, but if you plug it in one way, the whole chassis (and the pot shafts out the front) is live.

 

[Nigel Goodwin]

A lot of older british radios tend to have mains tx's

And a LOT didn't

As you say, it was before the days of 13A plugs, most used two pin plugs only, and simply putting the plug in the wrong way round connected the live wire directly to the chassis - as did almost all TV's.

Quick professional tip - ALWAYS stroke the chassis gently with the back of your fingers to see if it's live - if it's live it feels 'rough', then you swap the plug round so it's connected to the neutral instead as it should be.

Not that this really applies any more?.