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

[rubber_jonnie]

Arrrgh!!

Still no VIC chip and I can't track it either!! Maybe tomorrow...

[rubber_jonnie]

Still no VIC chip, so I decided to have a go at the 600XL problem again.

Now there are 2 crystals in the video circuit, Y1 which is 3.546894 Mhz and Y2 which is a 4.33618 Mhz.

Y2 is only present AFAIK in PAL only models, which @ijor mentioned a few pages back, and is related to the colour carrier.

Nevertheless, I checked both out and compared to my 800XL which ran the VBXL flawlessly before.

Y2 on both machine match, but on the 600XL, Y1 does this:


https://www.youtube.com/shorts/RPAnuxHE1I8


Y2 on the 800XL is completely stable, so considering that I might have a bad crystal and/or bad caps in the circuit.

Since I definitely don't have a replacement crystal, I'll get one on order and see what caps I need versus what I have in stock.

I don't know for sure if it's a problem, but it has to be worth looking at to try and get rid of this damn shimmer.

@stephen_usher How are you getting along with the jailbars on the C64? Or have you gone off for Easter break?

[stephen_usher]

The caps arrive at work tomorrow but I’m now away from home for a fortnight.

That looks like the rate of the shimmer doesn’t it? The trouble with the caps is that they need to be high precision ones, which are less common.

[rubber_jonnie]

The caps arrive at work tomorrow but I’m now away from home for a fortnight.

That looks like the rate of the shimmer doesn’t it? The trouble with the caps is that they need to be high precision ones, which are less common.

Damn that's annoying re your caps, but I hope you have a good holiday at least.

I thought the same in terms of the shimmer rate too, seems too close to be unrelated.

Looking at the circuit:

Screenshot 2022-04-10 at 18.13.13.png (148.7 KiB) Viewed 1563 times

There's just the one capacitor involved, C48, which according to the parts list is a .1 uF ceramic with a +80 -20% rating, which seems a little more flexible than yours.

Of course there could be an issue with the Q2 and/or Q3. I'm thinking the resistors are likely ok, but I will check them anyway. I have a replacement crystal on the way.

Will keep this thread updated as I go.

[stephen_usher]

That’s a very complex oscillation circuit.

[rubber_jonnie]

That’s a very complex oscillation circuit.

I've seen simpler!!

Going to have to have to refer to the schematics for resistor/capacitor locations as it really isn't clear n the board.

[rubber_jonnie]

Looking at the resistors in the oscillator circuit, they all check out fine, as does C48, so it's looking like the culprit is the crystal.

I'm tempted to swap the one in my 800XL out with the 600XL, see if it makes a difference.

[rubber_jonnie]

Ok, so a crystal swap with the 800XL made no difference and the 800XL composite out is the same as before.

I guess that leaves only the transistors that could be bad

Best order some.

[stephen_usher]

You could try swapping the two transistors over and see if the behaviour changes as Q3 seems to be the one giving the most feedback and Q2 doing the amplification, I think.

[rubber_jonnie]

You could try swapping the two transistors over and see if the behaviour changes as Q3 seems to be the one giving the most feedback and Q2 doing the amplification, I think.

I'm going to order some transistors anyway, but will try swapping as soon as I have a chance, thanks.


EDIT: Transistors now on order, and also found the 600XL service manual has a typo. It states the 2N3906 is NPN, when it's actually PNP. Its brother, the 2N3904 is NPN and is also used in the 600XL.