QL Video issue - Red color missing

[Quickie]

Hi,

In my recent attempt to resurrect my QL I've been able to learn and fix it partially, but I still have some way to do, so just hope that someone with more experience and knowledge could help me a bit.
Here are the facts:

- Bought a non-working QL -> Did not even power up (No image on display, no power on led). Checked the IC35 -> 7805 Power regulator and found that was giving just 1V. Replaced. And replaced D25 and D26 and IC36 and IC37 with the coresponding 78L12 and 79L12 transistors.
- This fixed the supply problems and the QL was brought to life again, EUREKA! However, there was something strange (even for me that I wasn't used to the QL-color scheme...).
Someimes a picture is worth thousand words, so here it is:



So, by some reason the red color is not present.
My first culprit was the MC1377P and the oscilloscope proved that my idea could not be wrong at all: I could see activity on pins 4 and 5 but not on pin 3.
So, I ordered a couple of ICs on Aliexpress and waited until they arrived.

Finally they arrived and yesterday did the IC swap...but the problem persists
Turned to check resistor R48 (measured 8,2Kohm) and C23 (the capacitor gave me around 40uF so replaced it by another 22 uF but that did not make any difference).

So, arrived to this point:

- The MC1377 is discarded as it's been replaced and it measures 12,01V on pin 14
- I noticed some activity readings on the osc on the left pad of R57, but checked continuity on all pads (R57<->R48<->C23<->IC pin 3) and was fine.
- I tried a desesperate attempt to gain some "red" color and tried to connect a bypass wire from R57 to pin3 of MC1377. This did not bring back the RED color where it should be, but in the opposite, the black color had a reddish tonality.

- Could it be that I'm assuming that the problem is related to R signal and may be other problem laying around? (perhaps chroma / luma?)

Thanks for any help in advance!

[Dave]

8301:
Red pin 30
Green pin 31
Blue pin 32
CSync pin 12
VSync pin 11

Hope this helps.

[Quickie]

Hi Dave,

Thanks for pointing out the 8301 ULA.

Bad news: no activity on IC22 pin 30, while there is on pin 31 and 32.
Is it time to find a ULA replacement?

Cheers

[Dave]

Yes. Sometimes, shocks can travel from the unprotected RGB socket directly to the 8301 and blow an output.

5.6V zeners to ground from all five lines would be prudent, if you're comfortable to retrofit them.

[Quickie]

Thanks for the guidance.
An ULA replacement is on the way. I'll post the results here (hopefully with the fixed QL)

In regards of adding a zener diode to the lines it does make much sense for me after suffering this. I have some protoboards laying around home.
I can create a very simple circuit with direct 1->1 pins except the 5 mentioned lines that could sit between the IC socket and the ULA itself.
Appreciate the help!

[Dave]

You'd want the zeners as close to the RGB socket as possible. That way you're protecting the 1377 too. Just from R/G/B/C/V to GND. You could probably do it on the underside of the PCB on the back of the RGB socket.

[Quickie]

You'd want the zeners as close to the RGB socket as possible. That way you're protecting the 1377 too. Just from R/G/B/C/V to GND. You could probably do it on the underside of the PCB on the back of the RGB socket.

You're absolutely right. This would be a much safer solution. I'll plan to do it once I finish some other projects.
By the way I was able to acquire a working CLA2310 ULA, and have just replaced it on the QL. This is the result! :



Finally fixed! Thank you all for the valuable hints.

Kind regards,

Raúl

[jivrt]

You'd want the zeners as close to the RGB socket as possible. That way you're protecting the 1377 too. Just from R/G/B/C/V to GND. You could probably do it on the underside of the PCB on the back of the RGB socket.

You're absolutely right. This would be a much safer solution. I'll plan to do it once I finish some other projects.
By the way I was able to acquire a working CLA2310 ULA, and have just replaced it on the QL. This is the result! :



Finally fixed! Thank you all for the valuable hints.

Kind regards,

Raúl

Hi Raúl. Nice image!
How did you connect the ql to the monitor?
RGB to scart cable? Homemade?

Juan


Enviado desde mi iPhone utilizando Tapatalk

[Quickie]

Hi Juan,

Actually that lead is an antenna cable connected to my LCD TV which fortunately still has an analog tuner.
However I'm planning to do a home made RGB cable (which I'm now thinking that may have the zener diodes built-in)
In that case I would plan to do the upgrade protection based on this diagram:



Saludos!

[martyn_hill]

Hi Raul

In addition to the level-shifting resistors, you may well also need to convince the monitor/TV to switch to RGB mode via the SCART socket (SCART defaults to Composite Video instead of RGB.)

There are various ways to do this, but the simplest seems to be to tap-off the Composite-PAL signal from the QL (pin 1) and feed directly in to the BLANKING pin 16 on the SCART connection. The 1V pk-pk of the Comp-PAL signal from the QL proves sufficient to trigger RGB mode (BLANKING actually expects somewhere between 1V - 3V according to the SCART definition, but anything that averages to more than 1V seems to do the trick.)

This is for example how the QL SCART lead from Retrocomputershack achieves the RGB mode and many on this forum use this lead successfully.

EDIT: I see that your diagram also includes something similar, but by using Comp SYNC to BLANKING. Given that CSYNC from the QL spends most of its time at +5V, I can see how this too might work to drive RGB mode...

I regularly reference the following site for good info on both Spectrum and QL RGB through SCART, though their solution to generating the BLANKING signal/RGB mode is slightly more complicated:

http://www.fruitcake.plus.com/Sinclair/ ... TCable.htm

The QL specific page can be found in a link near the bottom of that intro page.

Good luck!