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Tech Chat Bench Testing RRi Teletext Board with TIFAX

General Technical Discussion

Last Post by crustytv 1 week ago

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crustytv

(@crustytv)

Posts: 12234

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Topic starter

Hi guys,

I've been trying to fathom a way of supplying the entire teletext interface board from the T12A, on my bench. However, there are 11 x 7xxxx ic's off board from the TIFAX, so I need to also power those two regulators that supply the main board.

One regulator is straight forward, a 7812, so I put 18V in on pin 1, and have 12V out on pin 3. I have the right side of the board running, 18V on the 4700uF cap and just under 5V on one of the ic's on that side.

The other left side of the board is more difficult as I cannot find data for the other regulator, this is what is printed on it, any ideas? This side is needed as it supplies teh majority of the 7xxxx chips but importantly the 5V to TIFAX. The TIFAX module expects 5V on PL1/10, PL2/20 & GND is PL1/12, PL2/13 via the two white wires as indicated.

What's the purpose of all this? The idea is simply to go all around the ic's and see if anything is off from what is expected on the data, something high when it should be low or vice versa. This was certainly one of the fault conditions I had where the truth table was not set correctly for picture, resulting in the teletext side always loading in "mixed". This may be a foolish, but I thought it might be an interesting side project. I did think about put a supply to pin 1 of the unknown reg, and just see what I get out of 3 and see when it peaks.

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Posted : 27/09/2024 12:08 pm

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Testing Tifax

Nuvistor

(@nuvistor)

Posts: 4650

Famed Member Registered

@crustytv This any help,

https://findcomponents.net/SFC%plus%plus2805%plusRC%plus%3D%plus7805%plusKC

Appears at first glance to be a 5 volt regulator but not sure what output current.The 78xx series I saw them upto I think 1.5 or 2 amps, standard ones are 0.5 amp.

Frank

Posted : 27/09/2024 12:35 pm

Cathovisor

(@cathovisor)

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Just a quickie, Chris - TTL is very fussy about supply voltage. Anything below 4.75V and it can get very erratic indeed.

The '7713' on that TO-220 device is its date code; it's an SFC2805, which apparently was a Thomson-manufactured 5V regulator.

Posted : 27/09/2024 1:08 pm

crustytv

(@crustytv)

Posts: 12234

Vrat Founder Admin

Topic starter

Thanks for the top tip Mike re voltage and for clarity on the id of the SFC2805 👍

So into the SFC2805 regulator I fed 12V 1A, that side of the cct is now drawing 880mA. The 18V 500mA fed into the 7812, is feeding 12V into the cct, but I've no current draw. Since powering the SFC2805 the TIFAX module is now powered with 4.8V the chips are warm. Most of all the other 7xxxx chips on the left side are also powered.

My main concern is the right side, although being fed with 12V, the cct appears to be not drawing current. I do have 4.3V on the two ic's on that side. The fact there's no current draw might be down to the fact it just won't in this detached mode from the TV. To be honest, although It appears to be powered, I've no idea if what I'm doing is even feasible. Still, I shall plod onwards.