Chinese 5.5 inch B&W TVs

[Karen O]

Firstly, apologies to the moderators if I have chosen the wrong category for this posting.

I am something of a hoarder. I cannot go to a car boot and pass a stall with one of those dinky little 5.5 inch TVs on display. I must have ten or twelve! They make great little monitors.

Are they vintage? Probably not but they work on a standard that will be obsolete in a couple of months

I have for some time pondered the possibility of converting one of these for 405 line operation. Is that feasible? (I think Neon Indicator mentioned the possibility)

I'm pretty sure it will involve more than just increasing a time constant in an oscillator. And if my understanding of line output circuits is correct, lowering the pulse drive repetition frequency would result in higher EHT output, over-scan etc. and probably breakdown of some component (hence monitors catching fire whe driven with an incorrect timebase)

I'd be happy for someone to put this idea from my mind for good. Or maybe confirm that it can be done...?

[Danny]

Hi, nothing is impossible but there would be low EHT at 405 and the LOPT might not like being run at the slower rate. Plus other issue with the video 1 not impossible though the only way to find out would be to run the line oscillator at 405 to see what happens.

Or you could build a separate line driver stage and use the sets LOPT just as an EHT source. You would need an inductane to replace the scan coils and some form of line output transformer for the 405 drive to the coils!, similar to a Ferguson TX10.

Danny.

[Steve_P]

You need to do the following -

Invert the video. (Easy).
Convert the timebase. (Might be in a chip, if so then it's tricky.)
Line Output needs to be modified and EHT may be low. (Tricky.)
Shift the sound carrier detection from 6Mhz above to 3.5 Mhz below. (Yeuch)
Sound again - AM rather than FM. (Tricky)
It may have a Band 1 tuner, but does it go as low as Channel 1?

So yes it can be done, but why would you want to??

Cheers,

Steve P.

[Jim_746]

They make great little monitors.

Karen O, what do you use your little sets for?

Regards,

Jim

[Karen O]

I wasn't thinking in terms of RF path adaptation - that sounds like a complete redesign. These sets have A/V inputs on the back and I was planning on feeding 405 lines in that way (though I could use my UHF modulator so it uses 625 RF practise but 405 line timing, if you get my drift)

I am also interested in narrow band television (spinning disks etc.) I have made a couple of PIC standards converters that allow me to see 32 lines on a 625 line monitor. Those little tellies are really handy for that sort of thing. Then there's my PIC SC/MP emulator running NIBL. But that's another story.

Truth be known: I really hoard these little tellies because they're cute!

So the EHT would be on the low side as result of driving the LOPT with a lower frequency?

[Jim_746]

Originally Posted by Karen O

I am also interested in narrow band television (spinning disks etc.) I have made a couple of PIC standards converters that allow me to see 32 lines on a 625 line monitor. Those little tellies are really handy for that sort of thing. Then there's my PIC SC/MP emulator running NIBL. But that's another story.

I am also interested in NBTV and was hoping to display the 32 lines on a 625 line monitor. Could you send me some details on how you did it please.

Regards,

Jim

[Steve_P]

I hoard big TV sets for the same reason! Anyway, the sound will be OK, but the Line Output may be a problem. It's probably a Diode Split LOPT, so it's all in one little can. You could try feeding the LOPT with a 405 line drive signal (Probably a square wave) and see what it does. The tube heater may come off it too so you'll know if the tube heater still lights properly whether you've got enough of anything.

Also, with it being Chinese junk, how long will it last!!

Interesting to find out.

Cheers,

Steve P.

[murphyv310]

Hi.
Actually the width and EHT will rise due to the longer time the timebase has to ramp up. Think back to the dual standard days. The Lopt would be designed around 625 and a bucking coil or damping resistors would be used to absorb the power dissipated on 405.
It still can be done though.

[Karen O]

Trevor, that was exactly my thinking - the current in the LOPT/scan coils is built up fairly linearly for most of the line time, then it's abruptly cut off with a resulting voltage spike that delivers the EHT and collapses the current in the scan coils in a matter of microseconds.

If that analysis is correct then extending the line beyond the design time will allow the current to grow to an excessive level. I did think of lowering the (12V) supply rail to the LOPT/LOPT transitor to compensate for this.

Jim, I have two designs: one with and one without a frame store. The one without was featured in the NBTVA newsletter. It generates a non-interlaced 625 line signal (312 lines?) but, because it has no frame store to speak of, it only displays eight NBTV lines in each frame. As a consequence there's a lot of flicker because the full picture is spread over four frames - but no more flicker than you'd get with a mechanical monitor.

My second design, which I must confess is unfinished because I switched attention to the CF card player idea, has a frame store. The picture's flicker-free and has wider bandwidth than the first design. But it's more complicated. The design without a frame store is a single PIC! No other silicon except the voltage regulator. A nice fun project!

So you have a choice: I can give you details of the simple flickery one now, or you can wait until I've finished the one with the frame store (I have to get the sync working better)

[davegsm82]

As Trevor says, the timebase may well work, however because you are feeding it more energy you may have to increase the energy storage within the LC circuit (LOPT and tuning cap).

This means you may have to strip the cores out of the LOPT and gap them slightly further apart to increase the inductance.

I've got one of these funky little TV's too, and i've opened them up in the past. From what I remember there is a lot of discrete transistor stuff in there, but the majority of it is built into one IC (which I think also handles the frame output).

I had also considered converting to 405 or multi-standard, but got no further than basic consideration.

Dave.

[Jim_746]

I'll go for your first design with out the frame store if the flicker is similar to mechanical monitors as it is a single PIC.

Cheers,

Jim

[neon indicator]

Going for Video in on A/V you likely only need to parallel a couple of capacitors as there is usually a huge amount of manual adjustment possible.

If it has separate line and frame hold presets and height and width controls it will work. It's very unlikely you need to do anything to LOPT other than increase any capacitors. These are not as critical as a Colour set or even a 14" B&W.

[dominicbeesley]

Hi Karen,

I've messed around with a few of these (well a colour one) - with help and advice from here...mainly to get NBTV going but also I've fed them with 405 lines.

I ended up lifting the LOPT, and adding an inductance instead of the scan coils and running it seperate from the scanning circuitry to allow for multi-standard operation with no messing. It took me a while to figure out all the connections but in the end ended up with a useable setup. I also managed to get it working down to about 8kHz with a lower supply.

I then knocked up some adjustable frame and line driver circuits and connected it all up...some of it here: https://www.vintage-radio.net/forum/...&highlight=crt I started going through getting Harry up and running on the NBTV forum too http://www.taswegian.com/NBTV/forum/...t=crt&start=15


If you're interested I've probably got more info. The Transistor Line Timebase articles posted by KeithsTV in the first link are well worth a read before embarking on this....it took me a whole weekend to get my head round what was going on...I still don't "get" inductors!

Dom

[murphyv310]

Hi.
You can reduce the HT to the line output stage, it will work but is not elegant. Tuning caps will reduce the EHT but will cause havoc with width, you would be able to fill a 33 inch widescreen!
The best idea is an inductor across the lopt but as you wont have much info on the tranny it's trial and error time, stick with it though and it will work.
Most line oscillators whether discreet or IC based will run fine at 10,125 hz.
I have an ex 9" CCTV monitor that I modded to 405/625 a few years ago that works very well, it has only 6 extra components.

I think some of the previous posters are forgetting their theory regarding flyback driven line output stages!

[neon indicator]

The width may be adjustable enough and I can't see how it would go more than 9" equivalent.

[murphyv310]

Neon.
When the frequency of the oscillator is lowered from 15,625 to 10,125hz there is a much longer time for the output stage to attain energy, this results in two things about 50% more EHT and often more than 50% increase in width. Altering the tuning of the Circuit can help to reduce the EHT but the width will increase even more.
The transformer has to be damped, this is what happened in dual standard days or the transformer had different tappings that were selected between the two standards.
I always remember wiring up a 1400 lopt wrongly and having low EHT and lack of width on 625 but on 405 massive width, fizzing and arcing all over the place then the multiplier failed!

The Circuit for the Murphy V659X illustrates the practice of damping the lopt on 405 very clearly and I will post it here if anyone is interested.

[Tazman1966]

Yes Trevor, that'd be very informative.

[neon indicator]

50% increase on 6" is 9"

Yes, you need to add parts, damp LOPT stage etc, but no need to take apart LOPT on such a small set.

[murphyv310]

Hi.
Here is the V659X circuit, as you can see there are two main things that enable the set to supply the same EHT between both standards and maintain width.
Winding "h" is shorted on 405, this effectivly damps the lopt so EHT is the same between both standards, the winding is damped on 625 by R45 to eliminate striations.
S6B & S7A switches the scan coils to a different tapping on the lopt as you can see S7A is closed on 405 which is lower down the lopt for width on 405.

NOTE this diagram has an error in that S8B is CLOSED on 405

[Tazman1966]

Thanks Trevor.

I had to study the line output stage very hard on the Sobell ST195 convertible (in name only and cut-outs in the side of the cabinet!) that I bought at Wooten Bassett. It had been hard wired converted for 625 lines neg video VHF at some time I think in the 70s and similar aspects were present but that's for another thread! Initially I wound down the line frequency to 405 lines (on the 625 setting) but the stage didn't like it too much!