Old TV RGB interference

[John Bennett]

Hi,
It seems the rather nice little 14 inch CRT I've found for a test monitor has a big drawback.
When using RGB, there is horrific noise coming from one of the other sources that puts fuzz all over the screen. I guess back in the day, it'd been tuned in to analogue stations and there's just be faint ghosting from the selected channel, but untuned channel white noise is much worse.You can make it a bit better by changing analogue channel to the 'satellite' RF channel, but it's still unuseable

. Composite mode doesn't have this issue.

Anyone got any ideas? I'm going to try putting a voltage on the SCART blanking pin to see if it needs cajoling, but after that it'll be bypassing stuff.
Looks like the teletext PCB can be bypassed (could that do it?) but other than that it looks like RGB and aerial all go into one big IC (TDAB361) and that presumably does settings-related stuff to RGB that can't be bypassed.

It's a Mitsubishi CT-14MS1BM.

Also need to find a universal remote...

 

[andrewsm]

Im guessing that the RGB mode should be normally fine as probably used for VCRS etc in the day. Its possible that although RGB is going in, the TV is not switched to that mode as needs a voltage on pin 8 I think.

If this is done and interference is still there then maybe an earthing issue going on with scart. Bypassing the teletext probably wont make much difference as the decoder chip will switch the inputs over.

 

[John Bennett]

Gah looks like the TV is probably knacked :-(. I hadn't realised earlier that it starts off fine, then the picture degrades with more black speckles appearing.

The teletext was a long-shot as it overlays onto the RGB, prior to passing into the main IC. Guess if it isn't that or some passive or connection before the main IC, then it's unfixable (as the RGB coming out of the main IC is fine as it works for RF and composite inputs).

Back off job then...

If I remember right, VCRs were usually composite - RGB was the obscure mode that only a few games consoles did if you were lucky enough to buy the right scart lead and your TV actually had the pins wired-up :-D (I remember the pain from my Megadrive).

 

[andrewsm]

Ah ok. Must say doesnt look too healthy so maybe a spares/ rainy day job

 

[penrhos]

Check what tube is in it - you might be able to run it on an arcade chassis.

 

[Brettster]

Could also just be a dry joint or bad cap if it gets worse as the TV warms up

 

[John Bennett]

'Fixed' it - yaay!

After pulling it apart 3 times and reading schematics and IC datasheets, it turned out that I needed to mod the Wii cable with a lower pull-up resistance to the blanking pin (16) as the TV was pulling the voltage down to near the threshold. Doh - should've just gone straight to a JAMMA/SCART setup or used a multimeter on the pin... I nearly binned the TV.

Interestingly, the blanking signal at the scart is a shared connection to the teletext(where present) and the OSD circuitry. The OSD pulses it on a pixel-by-pixel basis to tell the main IC to knock off it's signals to allow the OSD overlaying. So you need to pull it up hard enough to stop these devices toggling it.
So if you connect something external that only drags the signal to near the threshold, you get the undefined behaviour I was seeing.

Some devices were flapping around the joints were so dry, so not at total waste of time :-D. Plus I think I know a little bit more about old TVs now (and what bits I can rip out)John Bennett2018-01-01 10:31:25

 

[Brettster]

Good to know!
and great fix

 

[John Bennett]

Thank you, very kind!

For reference-sake, I should've added that the ON/OFF blanking thresholds for the internal control IC (TDA8361) are 0.4 and 0.9V, so you need to check you've pulled pin 16 comfortably above 0.9V (assuming all TVs are similar).

I ended up with about 300 ohms of resistance to 5V, which actually seems to be what most RGB scart leads have in them (or even down to 100 ohms).

My Wii RGB cable was fitted 470 ohms in it for some reason...