More Serial Woes

[t0nyt]

Thanks for the info Martyn

I might try swapping IC28 too

Thanks

[martyn_hill]

Yes, in a SCART cable configuration, it might be the Composite PAL that is being used like I described for the (mono) Comp Video and IC28's sole purpose in life is to generate that PAL signal.

In fact, in most of my QLs, I remove IC28 altogether as they all have some form of SCART or VGA connection which has no need for the PAL signal... It also removes the load on the +12v line (the only other load being the 148x line driver IC.)

And - perhaps - therein lies the connection between the original SERial problem and this latest video signal issue. Yes, remove IC28 and see what you get - it'll do no harm to test booting-up without it inserted...

[t0nyt]

Yes, in a SCART cable configuration, it might be the Composite PAL that is being used like I described for the (mono) Comp Video and IC28's sole purpose in life is to generate that PAL signal.

In fact, in most of my QLs, I remove IC28 altogether as they all have some form of SCART or VGA connection which has no need for the PAL signal... It also removes the load on the +12v line (the only other load being the 148x line driver IC.)

And - perhaps - therein lies the connection between the original SERial problem and this latest video signal issue. Yes, remove IC28 and see what you get - it'll do no harm to test booting-up without it inserted...

It just got weirder

I tried with IC28 removed and I get NO output on SCART

So I swapped in a known good IC28 and I get the same problem as before BUT the blank section is now scrolling UP the screen instead of down...

<twilight music playing quietly in the background>

[Martin_Head]

Anything that moves up or down the screen slowly, makes me suspect a 50Hz ripple on one of the supply rails. Or a missing/bad ground return probably between the QL and the monitor.

When you tried another QL and it was OK, did you use the same power supply unit?

I seem to remember that once I had a problem with the video DIN socket on the QL, having loose connections, so that when you plugged something in. You did not get a good contact between the plug and socket.

[t0nyt]

Anything that moves up or down the screen slowly, makes me suspect a 50Hz ripple on one of the supply rails. Or a missing/bad ground return probably between the QL and the monitor.

When you tried another QL and it was OK, did you use the same power supply unit?

I seem to remember that once I had a problem with the video DIN socket on the QL, having loose connections, so that when you plugged something in. You did not get a good contact between the plug and socket.

Hi Martin,

I used the same PSU for both QL (and also have tried a different PSU)

Will try to test the DIN connections

Many thanks

[t0nyt]

Can’t find any issue with the DIN, doesn’t mean for sure there isn’t though

Also tried changing the regulator, just in case and as it was simple

It’s possible I’ve damaged the mobo when I was soldering in the IC sockets for the serial problem I guess

[martyn_hill]

Hi t0nyt!

Given those apparently diverse symptoms, I'm wondering whether your 12v regulator IC is defective (it's one of those two 3-pin transistor-like devices on the QL mobo.)

The fact that no picture is available via a SCART connection without the PAL encoder IC inserted, indicates that your SCART cable is using the PAL signal in one way or another and you were inadvertently seeing the Comp Video output in place of RGB, or else, your SCART display device is not being triggered to switch to RGB from this QL.

To ensure that RGB is selected, the SCART Banking pin needs between 1-3v applied, which is sometimes taken from the PAL or else the (mono) Comp Video pin on the QL. Whilst neither of these are DC signals, they both average out around 1v and are often used in QL SCART cables to drive the Blanking pin effectively to switch the display to RGB.

If you felt up to it, you could try dismantling your SCART cable (easiest at the SCART end), disconnecting whatever is currently attached to Blanking (pin 16) and manually apply a suitable DC voltage - e.g. from a single AA battery.

All this with IC28 removed.

If a stable picture results, it seems reasonable to conclude that IC28/PAL encoder is not getting a decent 12v supply (which would also impact the SERial line driver.)

Good luck!

[t0nyt]

Hi Martyn

I don’t really want to mess with my SCART lead

I thought maybe I could just change the 2 12v ICs you mention but looking at the QL maintenance manual they appear to be in the PSU rather than in the QL? Or am I getting confused please?

(I’ve tried 3 x PSU with the same SCART lead on 3 x QL and only the problem QL has the issue)

Many thanks

[Pr0f]

The QL power supply provides about 9v on 1 pin, ground and a 15V ac signal on the other pin - this 15V ac is fed to 2 diodes - one rectifies the positive side, and one the negative side - and then the regulator ic's that Martyn spoke of regulate these 2 voltages to +12V and -12V respectively. If the problem stays with the QL and not the supply it's likely that one of these chips (the 12V 78L12 regulator) is not working correctly.

[martyn_hill]

Hi t0nyt!

No, the 2 linear voltage regulators are both on the QL mobo - IC37 (a 78L12, +12v 100mA regulator) is the one that supplies both the PAL encoder (IC28 - MC1377) and the '232 Line Driver (IC25 - 1488).

[EDIT] I'd forgotten that that 12v power-line is also fed out both SERial sockets, albeit with a series 680ohm resistor to limit the current-draw. You'd have to be quite unlucky to pop the 78L12 by mis-connecting that 12v line at the SERial port (even shorted to GND, the series resistor would limit current to 12/680 < 20mA.)