Zx Spectrum french Peritel interface

[payty]

Hi,


I have this Peritel interface which is supposed to convert the YUV video signal to RGB.

https://hardware.speccy.org/hardware/Pe ... mas-i.html


I bought this on ebay from France. It does not include the video cable and I need to build one.

From the schematic, it should be an easy job, but the thing is that it does not appear to work.

In the schematic there are R, G, B, Sync signals and one that is +9V , connected to pin 8 of the SCART connector for function switching.

Shouldn't it have a blanking signal? Pin 6 from the interface is connected to +5V, but on that schematic it does not go anywhere to the TV.

For the Spectrum I have a modern PSU from a Sony video cam that gives 8.8-8.9V output. Should it be a problem? The function switching specs of SCART pin state that

5-8V switches on the 16:9 ratio and 9.5V-12V 4:3 ratio. So 8.8V is somewhere outside both options.

[Ast A. Moore]

Blanking is normally necessary, indeed. You need something in the range of 1–3V. The aspect ratio pin needs something around 12V.
Some TVs, however, allow you to manually switch the input source. Mine does, for example. It’s a 4:3 CRT TV, so the aspect ratio signal voltage is moot. However, I can manually select between composite and RGB inputs on the SCART connector, so I leave the blanking signal disconnected. (Well, I have wired it for the +2A, because it was easy; but for the Toastrack lead, I just left that signal out.)

If you need the blanking signal, you could derive it from either the 9 V or 5 V rail on the edge connector via a voltage divider. You can also grab +12 V from the edge connector for the Function Switching signal.

[1024MAK]

Shouldn't it have a blanking signal? Pin 6 from the interface is connected to +5V, but on that schematic it does not go anywhere to the TV.

Looking at those details, if there is a 140Ω resistor (R20) in the +5V line on pin 6 of the RGB DIN socket, you can connect this pin to pin 16 on the SCART plug, this should provide the required voltage to select RGB mode.

Mark

[payty]

Well, the schematic is not accurate (or there may be different versions of the interface). And the components do not have any numbers on the pcb (R1, R2, ...) and it is a little difficult to trace the signals.

I will check for that resistor.


If I connect the Sync signal that comes from the interface to any of the TV RGB inputs I get a black and white picture. But if I connect the Sync signal to Scart pin 20 (where it should be) and any of the color components (R, G or B) to the corresponding TV input, I do not get any image on the screen. The screen just flashes white for a second and goes black after that.



What is the voltage that should be present at the Sync output? I have ~4V. The Y output of the spectrum is ~2V (composite output works ok on this computer)
The Tv should be able to get the Sync signal directly from the Y output of the spectrum just by inverting it?

[1024MAK]

With the interface disconnected from the power and from any other devices, a multimeter should be able to tell if there is a resistor between the +5V supply and the DIN pin 6 connection.

The signal on SCART pin 20 is 1V whether it is being used for composite video or composite sync. But keep in mind that this is when the signal is terminated (with the 75Ω in the TV or monitor).

Mark

[RMartins]

You have not mentioned to which ZX Spectrum you are connecting this interface.

And as far as I remember, only the 16/48K provides the video signals required on the expansion bus.
https://faqwiki.zxnet.co.uk/wiki/ZX_Spe ... _connector

So if you are trying this with any other machine then a 16K/48K, it won't work, if it requires de video signals on the bus.

Note: There are some other interfaces that generate an image based on the bus data being read by the ULA, but it's clearly not the case for this interface.

[1024MAK]

On the subject of the edge-connector video signals, note that issue 1 and issue 2 16k/48k boards need the appropriate PCB link wires to be fitted in order for the video signals to be present at the edge-connector contacts. Not all boards had these links fitted as standard (marked VID, U, V and Y).

Mark

[payty]

Meanwhile I have managed to get this thing working (almost...). The trouble was because of the missing blanking signal (pin 6 of the DIN connector). Thanks 1024MAK for help!

The Spectrum is a rubber key computer (issue 3). The picture is pretty bad -> the Sinclair Research text is displayed with blue ink and magenta paper. Image not very sharp. TV is an older CRT Samsung.

Definitely there are problems somewhere.


I have an old CRT oscilloscope and I have tested the R G B signals. Only the Blue signal seems to be stable, the others display a lot of noise . All capacitors were replaced but no improvement.

There are two versions of this interface, probably the schematic is for second version (and I have the first one). The design is a rather ...uninspired one, with two boards linked together by a large connector. The first board is just a pass-through board and the second one is actually the interface. Probably because of the long tracks for the YUV signals, there is a lot of noise coming in right at the input of the interface.


I will post some pictures later with the image displayed and oscilloscope signals.

[payty]

...meanwhile I managed to burn the ULA. Probably the interface was badly aligned in the expansion port (there is no key on this interface so I have to insert it carefully.

Fortunately, the ULA is not the original one, just a replacement bought from retroleum. So, until I order another one, no further testing.