Camputers Lynx 128K “No signal”

[rogjosh]

I’m getting a “No signal” message and blank screen when using my OSSC with a Camputers Lynx 128K. I’m assuming the OSSC is version 1.8 but it has a plain surface on top. I think I bought it from VGP a while ago. It is running firmware 0.90

The OSSC works nicely with the 48K and 96K versions of the Camputers Lynx. The video lead is, in all cases, an RGB to SCART (which, as part of its design, takes +5V from an adjacent socket).

 

I’m assuming that the “No signal” message may be caused by a sync problem. The sync line on the 48K & 96K versions is a standard design which takes the HSYNC and VSYNC  outputs from a 6845 CRCTC and combines them by passage through an XOR gate (74LS86). However, the 128K circuit is slightly different. The HSYNC signal goes straight to the XOR gate, but the VSYNC signal is passed to a multivibrator to set its pulse width to 160 usec and then that goes to the other XOR gate input.

The sync signals look OK on a scope (HSYNC=15.61 KHz & VSYNC=50.0 Hz). A different SCART to HDMI converter (unbranded) does produce the correct display on the 128K with the exception of the first column not being shown. That, at least, shows that there’s not too much wrong with the Lynx itself.

 

I’m therefore wondering if the altered Vsync pulse width is causing a problem and, if so, are there any parameters on the OSSC I should be looking at?

 

Thanks

Alan

 

[BuckoA51]

Have you tried adjusting AV Sync opt. -> Analog sync Vth at all?

[rogjosh]

I hadn’t tried that but I’ve just discovered that adjusting “Sync Opts->Vsync Threshold” down to 8.76us or lower (minimum 4us) gets everything working properly. Thanks very much for the advice.

 

Alan

 

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