Composite mod voltages

[angus]

My modded Zeddy has ceased functioning. Voltages around the board seem OK but I have no image.

The PD between earth and the pin on the composite output seems 2.6V DC, is that normal? Any chance I've fried the transistor?

[Moggy]

Hi Angus

Try measuring the DC voltage at the tip of the phono plug that goes into the TV and it's outer shield.
I get get just over 2.5 volts on three of My converted Zeddies using the composite mod with the 100OHM resistor and transistor, no voltage would suggest transistor blown, an excess voltage might suggest composite input blown on TV
( I'm assuming the fault is no picture?).
I seem to recall that TVs need about a volt for composite but most decent models can handle about 3 Volts using its own circuitry to knock it down to required voltage or something on those lines, though I may be wrong.

Regards
Moggy

[angus]

I'm getting 2.6V between the outer case of the plug and the tip. Also 2.6V between the modulator casing and the tip of the plug. But a blank display. Such a shame as it worked REALLY well for a while.

[angus]

Took the Zeddy into work and it's functioning fine on a CRT television here...so it's the TV's at home which are the problem. Odd since I had this exact machine working fine long term on my pocket TV and it would run for around half an hour on the main flat screen. Very odd.

[zx80nut]

To be honest, that video mod schematic (it it's the same one as I think - there are no references in the posts to the schematic) looks rather "dangerous", as the transistor (if the ULA pin goes high for some reason) has a very low impedance to the 5V supply rail. Most (all?) video outs that I've seen elsewhere has a 75R (approx) resistor in series with the output, and the transistor stage suitably voltage doubled to take account of the drop across the resistor. As a result, if driven to the supply rail, the max voltage to an input pin on a monitor/TV would be a bit less than 2.5V.
The 75R series output impedance also reduces reflection/ghosting that would normally result from an impedance mismatch.

...personally, I wouldn't feel happy connecting a low impedance drive to the inputs of my TV !

Grant.

[zx80nut]

Additionally, the 2.5V "permanent" DC on the output is having a constant 33mA current to the TV input.
Again, this is not good - if DC is present then it should be blocked with a big capacitor (I think 220uF is recommended for a video input).

... the above is not a "told you so", but is intended as a warning in case you do it.

Grant

[Moggy]

Hi Grant

Thanks for the warning the mod I use is this one from the German ZX team.

Not being the most technically gifted could You please show me what You would add to make this safer.

Thanks again for he heads up.

Regards
Moggy

[zx80nut]

Ah, that is the one I thought it was

The simplest way (and probably sufficient) is to add a 220uF electrolytic capacitor in series with the output.
Positive to the transistor/resistor junction, and negative to the TV/monitor.
That would eliminate the DC, and should be large enough to show no brightness issues (gradually getting darker across the screen). I haven't tried this myself, but it is the same value as suggested in the AD724 datasheets to block the DC.
You may be able to go smaller, but that needs a bit of experimentation.

This then reduces the standing current to zero (well, very nearly!) and if the transistor output is pulled high then the video output would only be high for a short time.
As an additional safeguard, a zener diode (maybe 2V to 2.5V ?) could be connected across the output, between the capacitor negative and to ground preventing excessive voltage, even for short periods, if a fault occurs.

Regards.


Grant

[sirmorris]

I use this design which came from a spectrum site.

C

[zx80nut]

Hee hee - same as what I proposed

The zener is for extra protection if required.

Grant