Atari 600XL VBXL installation gotchas

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rubber_jonnie
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Atari 600XL VBXL installation gotchas

Post by rubber_jonnie »

Having finally come to a conclusion as to why my visually impaired 600XL had a shimmer to the screen (https://www.exxosforum.co.uk/forum/viewt ... =18&t=5265) and realising that I'd made a mistake because there are no specific 600XL install instructions for the VBXE and I'd figured it out wrong, I thought the best thing I could do was to point them out so others who find themselves in the same boat can at least hopefully find this or my other article and get around these problems too.

So what are the gotchas?

Firstly, there are differences in IC numbers. This is relatively minor, since the install guide does refer to the chip type, so even if the U number doesn't match directly, identifying the ICs is still quite easy, but just be aware of it.

The second gotcha is what happened to me. Shown below is the 3.5Mhz oscillator circuit on the Atari 800XL:

PXL_20220414_212544707.jpg
PXL_20220414_212544707.jpg (366.09 KiB) Viewed 1107 times

You can clearly identify the crystal, Y1 and the rest of the oscillator circuit components, and the instructions state to remove Q8 and R110, which disables the oscillator by removing a key component, Q8.

However, on the 600XL, Q8 is a completely different transistor and not part of the oscillator circuit, though still part of the video circuit.

Looking at the 600XL circuit below, we can see that physically it looks much the same as the 800XL 3.5Mhz circuit, and schematically it is:

PXL_20220414_211751111.jpg
PXL_20220414_211751111.jpg (325.38 KiB) Viewed 1107 times

However the transistors in the circuit are labelled Q2 and Q3 and not Q8 and Q9.

Looking at the photos, it's kind of obvious now, but the upshot is, if you don't remove Q2 on the 600XL, the oscillator circuit remains live, and you end up with a harmonic between that oscillator and the 3.5Mhz clock picked off the 74LS74, which gives you a shimmery screen as per this video: https://www.youtube.com/shorts/5LpZocVyeRI

So in summary, when installing VBXL on an Atari 600XL, remove Q2 instead of Q8, and remove R49 instead of R110.

Then complete the wiring as described in the original install guide.

Hope this helps somebody else who may have misinterpreted things the way I did.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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