Need an Isolation Transformer for CTX-S (Philips 14CT2006)

[generec]

I have this old Philips 14ct2006/05 television that uses the CTX-S chassis.

I’m planing to add a scart connector to it for video and rgb input.

I’m using a schematic from an old article to build a scart board.
This Tv uses a very similar chassis and the same TDA2540 and TDA3650 chips described in the article.

This unit has a live chassis and needs a isolation transformer installed. The article doesn’t go into specifics much but says a 120va toroidal should be adequate for sets like this.
I’ve looked around at some transformers but I’m not sure what type I need regarding primary and secondary values.

Can anyone help determine a specific transformer to use?


Also the only part of the scart schematic I don’t understand is the “AV time-constant” output.
The article mentions connecting it to “the line generator chips VCR select input” and “the line generator chips time-constant is changed by applying 12v to the selector pin” but doesn’t make any other specific mentions of this.

Any idea where the “AV TC” output of the circuit goes?


I’ve linked the full article here: Pages 20, 22, 23.
https://worldradiohistory.com/UK/Pra...on-1991-10.pdf
I can't link or post the full ctx-s service manual here and I'm not sure what parts would be most useful to help determine a compatible transformer, so I've just included a crop of the bridge rectifier for what its worth.

[nigelr2000]

I believe it applies 12 volts to switch the time constant on the line stage so the top of the picture doesn't flap about on pre recorded tapes when using a vhs machine. The equivalent function was on the tuner buttons, normally channel 8 or 0 on most tv's for use with a VCR. Had a customer bring back a VCR in a taxi with his TV as he said they were faulty. I asked him if he had it on the VCR channel and he said yes. OK I said if it's a fault in the VCR I will pay your taxi fare. He turned up. Bought his telly in with the vcr showed me it misbehaving and had it tuned in to button 5 I flicked it over to 8 and a miraculous cure no flapping picture and a sheepish customer having to pay his own fare . As for the transformer one with 220-240v volts in and 220-240 volts out will be suitable rated at 120 VA. Partridge transformers have one listed for £50 but its a standard transformer so you would need to have it externally mounted http://www.partridgeelectronics.co.uk/16Cprokit.htm
A quick google found no isolation transformers in toroidal though

[KeithsTV]

That article brings back memories as it's around 30 years since I wrote it! At the time toroidal isolation transformers were readily available but are few and far between these days. The reason for using a toroidal transformer is the low external magnetic field as I mounted the transformer inside the TV. The stray magnetic field from a conventional transformer mounted internally would affect the CRT too much. If you can't get a toroidal transformer it would be best to mount the isolation transformer, such as the one suggested, externally. You could try asking Ed Dinning or Mike Barker if they could wind a suitable transformer although I don't think they can wind toroidal transformers.

If you are only using it as a video input then most of the circuit relating to video and audio out can be omitted. The components around TR5 and IC2c & d can be omitted and IC2 replaced by a dual op amp such as a TL072.

Incidentally I still have the TV I used to design the interface which gets used occasionally.

Keith

[generec]

nigelr2000
I'm glad you explained that as I recently used this with an rf modulator, had the top 1/4 or so of the screen distorted only with vhs and I couldn't figure out the problem. I'll give that another go.

KeithsTV
Wow I didn't expect a reply from the author, I actually tried to search your name to see if I could find you to ask about it. small world I guess.
As you both mentioned, mounting the transformer externally seems like the way to go, I'll look into picking one up or getting one wound.

I'm looking at the manual for the cvc800 and it looks like the tc6 you mentioned goes to pin 11 of the TDA2593, described as "v.c.r switching" and "coincidence detector" although pin 12 is labelled "time constant switching".
The closest thing I can find on the ctx-s is the TDA2577 in the "Synchronisation" part of the schematic with "coincidence detector" on pin 18.
So I'm still a bit stumped by this one. I still have a lot to lean about crts.

Indeed I don't need the audio part so I was assuming I could omit everything below IF disable as I thought that was all for audio. I don't suppose you could modify the schematic to show me what needs to change?

Thanks for the advice guys, your help is greatly appreciated.

[ben]

It seems like a lot of work/expense to go to just to add an AV input to a 14" set. If you want something 'period' I'd look for an ICC3 which had SCART as standard. Otherwise there are any number of sets around, often given away, from late 80s onwards which have AV inputs and can be used safely.

[Refugee]

The Thorn TX10 has an isolated chassis and there was also an official AV kit for composite only.
I might even have some detail of the kit.

[KeithsTV]

For just a video input delete all the components highlighted in red on the attached.

I noticed there were a few errors that crept in as D6 and D9 are the wrong way round on the published circuit and I don't know what the connections X & Y are for.

If you are not going to connect it to a VCR then D8 could be also deleted.

Keith

[generec]

ben refugee
I can't see any of those available but I'll keep an eye out.
however I'd like to get this one working with video inputs either way.

KeithsTV
Thanks, that's great. I thought D7 and R21 could also be omitted because they seem to be for the audio chip.

I do intend to use it with a VCR, I'm just trying to decipher where D8 would connect on a CTX.

[KeithsTV]

The TDA2577 data sheet shows that connecting pin 13 to 12v via a 12k resistor changes the time constant for use with a VCR. Therefore try connecting D8 to pin 13 via a 12k resistor.

If you are going to use it with a VCR I'd keep the audio input but delete all the circuit associated with the audio output ( IC2b, IC2c and IC2d and the associated components) I'll post a modified circuit later.

Keith

[generec]

Oh I see that now, thanks for pointing that out.
I'll be able to get this done once my parts arrive.

[KeithsTV]

Here's the modified circuit which includes the audio input which I think would be essential if you intend using it with a VCR.

As only one op amp is needed the quad op amp IC2 can be replaced by a single op amp such as a 741 or TL071. The pinout shown is OK for a 741 or TL071. If you use a different op amp check the pinout.

The unused inputs to the 4053 switch, pins 1,3,2 and 5 must be grounded.

I've also corrected the orientation of D6 & D9 and added a 12k resistor in series with D8 in the time constant switch control line.

Keith

[generec]

Thats great, thanks for taking the time to do that.