Atari 600xl monitor mod by jeff popp

[Calab]

On 3/20/2019 at 8:34 PM, Greg Miller said:

I'm digging up an old thread here because I have a couple of 600XLs that I want to repair and mod to work with my XRGB Mini. I can't find any info about acquiring the right part for L12. I looked at the 600XL FSM, and it just says "ferrite bead"...

 

So would this be suitable: https://www.jameco.com/z/BL01RN1A1D2B-Murata-Ferrite-Beads-Axial-7A_1844580.html

I'm now in the same boat... Got a 600XL and need to add composite, SVideo and audio out.  What to use for L12? I usually order from Digikey.ca

[reifsnyderb]

Something like this should work:   https://www.digikey.com/en/products/detail/fair-rite-products-corp/2743005112/8594218

 

Optionally, for video, you can just jumper over them and leave them out.

[nwah]

Just got a 600XL and did this mod with a kit from Best Electronics. Getting fairly clean composite output but the color is extremely muted compared to RF. (I was fiddling with the pot in between so the hues are shifted a bit between the two pics)

 

Based on the Super Video 2 mod, I tried replacing the 100Ω resistor in R136 with a 68Ω (didn't have any 75Ω) and it got maybe 5% better, but seems like there's a bigger problem. Not sure if I just installed some other resistors in the wrong spot or if something else is the matter. Any ideas?

[Beeblebrox]

Not sure if a factor but is the pot itself working OK? Try spraying contact cleaner and working it back a forth. 

[mimo]

What kind of TV are you hooking up to? Do you have another you can try?

[nwah]

Yeah definitely worth trying some contact cleaner in the pot.

 

The above pictures are just a crappy LCD TV I have, but it looks way better on my stock 800XL using same TV and video cable. Also just tried hooking it up to a Commodore color monitor and color is basically invisible, even cranking it all the way up (stock 800XL again for comparison).

 

Thinking I'll probably just do the UAV on this machine anyway, but am still curious what the issue is, since as best I can tell the composite video circuit is basically the same between this and the 800XL's.

[Beeblebrox]

@nwah not sure but also might be worth looking at the transistors in the video circuit. (2N3904's IIRC)

 

Edit also see here: 

 

Edited July 18, 2023 by Beeblebrox

[DrVenkman]

3 minutes ago, Beeblebrox said:

@nwah not sure but also might be worth looking at the transistors in the video circuit. (2N3904's IIRC)

 

Yeah, and double-checking all resistor values - the signal levels are definitely not right. Do you have access to a scope to check the signal amplitudes? 

[nwah]

@Beeblebrox not sure I have any suitable replacement transistors on hand but could definitely check what's coming out of them. Interesting in that other thread that they just completely removed a few of the resistors.

 

@DrVenkman I do have a cheap scope. Any reference for what I should be seeing?

 

I know next to nothing about analog electronics, but I suppose if the luma amplitude is way too high relative to chroma when combined into the composite signal would that also cause the symptom I'm seeing? (In other words, maybe the chroma part is fine and it's actually the luma part that has the issue, e.g. a short)

Edited July 18, 2023 by nwah

[DrVenkman]

5 minutes ago, nwah said:

I do have a cheap scope. Any reference for what I should be seeing?

There’s an NTSC spec for it, actually, but I don’t recall off-hand what it is. If you dig around the forums and find @Bryan‘s original UAV thread (not the later UAV Rev D thread), I think he discusses it in some detail there. But in lieu of looking up the spec, you could compare the chroma and luma levels on a comparable machine like your 800XL (presuming you added chroma to the DIN jack of that one). 

[nwah]

Hooked the scope up to the composite out and it confirmed what we already pretty much knew: virtual non-existent color signal making it through on top of luminance. Screenshots below show two tests of this 600XL vs. with my stock 800XL, first with vertical color bars and second a blue screen with white text.

 

I poked around and compared voltages in a bunch of places on the machines, and generally the 600XL was close to my 800XL everywhere… except a transistor like you suggested @Beeblebrox. Voltages coming out of the last transistor in the color circuit (Q9 on 600XL, Q5 on 800XL) were significantly lower voltage than 800XL equivalent: ~0.69V vs. ~2.38V. Everywhere else was within 5-10%, so seems like that might be the issue. I'll see if I can find a replacement to test.

 

(That said though I did go ahead and order a UAV since I'm already doing some other mods/upgrades on this machine) 

[nwah]

Little update: I replaced the transistor but no change. Replaced the resistors leading to that transistor, and still no change. Out of frustration I used a jumper wire to start shorting around the other resistors to see if any others were bad. To my surprise/confusion, when I shorted the other side of R128 and R129 I got what looked basically perfect color image. I put a blob of solder on the underside for now to bridge them. All the voltages are still in the 1-4V range so I'm assuming this wouldn't hurt anything but please correct me if I'm wrong.

 

But again because I don't really know what I'm doing, I have no idea why this works...

[reifsnyderb]

Are you saying you shorted the base and collector of Q7?

[nwah]

1 hour ago, reifsnyderb said:

Are you saying you shorted the base and collector of Q7?

Yeah I believe so

[reifsnyderb]

I'd remove that solder jumper and replace Q7.