A visually impaired duo - VIC-20 and Atari 600XL

[rubber_jonnie]

Well, it's a bank holiday weekend so I wouldn't expect any deliveries of VIC chips or anything else.

Talking about clocks... Have you checked the clock inputs on the VIC-20's VIC chip socket?

C35 is used to tune the oscillation, then it uses UB9 (7402) for the main clock,plus UB10 (74LS74) as a clock divider.

R7 and R32 control the bias on the composite video output.

Actually, the postie did come round, but sadly nothing for me other than one of those letters from your credit card company trying to make you use your credit card some more.

I haven't looked at the clock inputs, but it's a damn good call, so in lieu of parts, which I have emailed them about today as it's been over 2 weeks since they shipped it, I can look at that.

I have tried tweaking the two pots, but whilst the picture does improve a little, the colours are still very muddy, either through my Retrotink (Oddly the colours are slightly better this way) or directly via composite into a TV.

Might be worth getting the modulator sorted and see what the picture looks like on my period colour TV

[stephen_usher]

R7 should control the intensity of the video signal. R32 controls the zero level of the signal.

Can you measure to see if R7 is varying it’s value if you move it?

You should be able to see how these affect the signal if you put an oscilloscope on the composite signal.

[ijor]

Certainly is, and having left it for 30 odd minutes there is no sign of the shimmer back at all. I will try replacing the transistors to see if the circuit can be fixed.

The output from the 3.5Mhz oscillator circuit feeds the GTIA - this creates the TV picture, and since its output is no longer used as it is provided by the VBXL, it may well be safe to leave it disabled.
...
he VBXL takes a 3.5ish Mhz clock from pin 3 of U24, a 74LS74.

Glad you fixed the issue. But may be just for the record, the 3.5MHz feed to GTIA it is still very much needed. Even if you don't use the normal video output, GTIA functionality is still needed for other purposes and that is the only GTIA clock. So actually VBXL doesn't take the 3.5 MHz clock from U24, it injects its own clock there.

[rubber_jonnie]

Glad you fixed the issue. But may be just for the record, the 3.5MHz feed to GTIA it is still very much needed. Even if you don't use the normal video output, GTIA functionality is still needed for other purposes and that is the only GTIA clock. So actually VBXL doesn't take the 3.5 MHz clock from U24, it injects its own clock there.

Yes, you're right, I suspect it was just a mis-read of the instructions by me, the 3.5Mhz clock is from the VBXL.

By leaving the original 3.5Mhz oscillator enabled, I was getting a clash between the original and the VBXL, which caused the shimmer, either because of harmonics or the fact the 2 clocks were fighting against one another..

Main thing I'm saying is that for the 600XL, the parts to remove are Q2 and R49, the rest of the install should be as per the 800XL instructions. Q8 is totally different on the 600XL.

[stephen_usher]

Check out the last few minutes of this, re adjusting VIC=20 video…

[rubber_jonnie]

Check out the last few minutes of this, re adjusting VIC=20 video…

Lol, I have this on my watch list for this weekend

[rubber_jonnie]

So, I got in touch with the people selling the new VIC chip I ordered, and they are saying a small package deliveries to the UK are taking 3-4 weeks, and we are just over two, so I will hang on for now before declaring it lost.

@stephen_usher I've had a look at C35, and the clock at the VIC is matching the output of the oscillator circuit, though the frequency is anywhere between 8.851Mhz (ish) to 8.8Mhz ish, so not super stable. The divided clock is super stable with no variance at half the value of the main clock.

C35 does adjust the oscillator to a point where you loses signal, but no amount of tweaking seems to help with the dark and murky video, same for R7 & R32, there is an image change on screen when adjusting these two as well.

I guess I need to wait for the new VIC to arrive. Pretty shocked at how hot the VIC actually gets, even after a few minutes.

[rubber_jonnie]

So, it seems that this odyssey is finally over!!

This arrived:

PXL_20220421_075750855.jpg (191.46 KiB) Viewed 1381 times

Yep it's a NOS VIC chip!!

Quick test with Super Lander and the colours are bright and there is no on screen flickering

PXL_20220421_080010259.jpg (245.05 KiB) Viewed 1381 times

And although the basic prompt looked just fine before, I'm pretty sure it's got a little more colour and is sharper now.

PXL_20220421_080352833.jpg (212.01 KiB) Viewed 1381 times

I may give the colour pots a little tweak, but overall very happy. Interestingly the original VIC ran really hot after just a couple of minutes, the new one just gets warm.

[rubber_jonnie]

And for bonus points, my Omega Race cartridge now works too

PXL_20220421_104202612.jpg (205.21 KiB) Viewed 1373 times

Though for some reason my SD2IEC seems to have stopped working, on the VIC-20 and C64, so I guess I'll have to figure that out next.

[rubber_jonnie]

SD2IEC working again, seems to have been a power problem, so it's time.

Before:

Old_doom.jpeg (314.41 KiB) Viewed 1359 times

After:

PXL_20220421_105452241.jpg (274.54 KiB) Viewed 1372 times

Ready to play!!

PXL_20220421_105541981.jpg (195.76 KiB) Viewed 1372 times

So nice to have resolved the whole problem of the muddy colours!