Display issue

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norm8332
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Re: Display issue

Postby norm8332 » Thu Jan 04, 2018 5:41 pm

I would still try it on a different display, but I must say that the video output circuit of the cost reduced (CR) version of the VIC is not as good as the older 2-pin model. The older 2-pin VIC usually has a better picture. Now that I see the pictures you posted, I'd say that your VIC chip is not bad and that problem is post VIC chip in the output circuitry or an incompatibility of the CR VIC-20 with that monitor. Also a faulty capacitor in the brick power supply can cause video noise. Changing all the electrolytic caps in the video section is also recommended.

a4000bear
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Re: Display issue

Postby a4000bear » Tue Jan 09, 2018 6:52 am

I concur with Radical Brad.

Your colour subcarrier is off frequency. In NTSC this would result in incorrect colour, but in PAL, the colour phase is reversed 180 degrees on each line, cancelling out the incorrect colour on each pair of lines. In extreme cases you will get 'Hannover bars' due to the large correction between each pair of lines becoming noticeable. Its still better than watching people suddenly turning green on your screen!

The red trimmer capacitor needs to be adjusted with a plastic screwdriver. If you only have a metal one, try tiny adjustments, removing the screwdriver each time to view results. If the capacitor is adjusted too far off the correct 4.4336 MHz frequency, the colour may drop out completely, but it will be restored by adjustment in the reverse direction. (Note that some VICs have an 8.866 MHz crystal, which is divided by two to give 4.4336 MHz)

Note: Best to perform this adjustment on a VIC that has been on for at least 15 minutes so that it is at operating temperature.

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MCes
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Re: Display issue

Postby MCes » Tue Jan 09, 2018 9:33 am

vicclk2.JPG


The aging slow down the Xtal quartz and sometimes the right frequency become out of range of C48 adjust, in this case you have to desolder a side of C50 (or substitute Xtal with a new not-old one) and then adjust the C48.

Remember that for adjusting C48 you have to use a not magnetic screwdriver: a plastic, fiberglass or ceramic one.
The target of adjustment could be the middle point where the TV lock the colours (try clockwise and anticlockwise and use the middle of the two middle points).

This procedure will match the VIC20 frequency with the frequency of Xtal inside the TV, so it's RELATIVE.

If you have a frequencymeter that reach 5MHz you can read the Xtal frequency on FB4 or FB5, if your frequencymeter can reach only a little bit more than 1MHz you can mesure 1/4 of the Xtal frequency: the clock line.

In both cases you can made an ABSOLUTE adjust of C48, but remember that it could be 4'433'618 Hz +/- 50..100ppm (100ppm = 0,01%).

This narrow tolerances explain the PAL decoding failures for Xtal aging, and remember that for a good ABSOLUTE adjust the frequency meter had to be a 10 times accurate respect the adjust target..... often the frequency meters reach their carchteristics after an hour of power on.
Also the VIC 20 oscillator have to thermally stabilize to be correctly tuned....
“Don’t argue with idiots because they will drag you down to their level and then beat you with experience” (Mark Twain ?)

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srowe
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Re: Display issue

Postby srowe » Tue Jan 09, 2018 2:04 pm

Thanks for this schematic, it confirms I'm adjusting the correct component. The joints looked a bit suspect, there was some flux that suggests someone may have replaced it. I've redone them but it doesn't change anything.
Should there be a physical stop on this? It seems to rotate in both directions without end. I only have a (metal) watchmakers screwdriver that fits which causes all sorts of distortion if I try to adjust while powered on.

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MCes
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Re: Display issue

Postby MCes » Wed Jan 10, 2018 2:08 am

On 4.43MHz VIC20 Xtal oscillator you have to find a 56pF on the rear of PCB
otherwise the oscillator will tuned with (and generate) the Xtal third harmonics (13,3MHz).

http://www.polytechnichub.com/construction-ceramic-trimmer-capacitor/

The variable capacitor has endless rotation, it has a couple of opposite points: a point of minimum and a point of maximum.
It mean that from minimum you can reach the maximum (exploring intermediate capacitance) by two way: clockwise and anticlockwise,
so the same intermediate capacitance can be reached in 2 different points.

Remember that C48 is a breakable component.

Using a metal screwdriver its parasitic capacitance (and your skin dispersion) will detune (or switch off) the oscillator and consequently the master clock of VIC20: it's not a good idea....


If the problem of your VIC20 is the wrong 4.43 MHz frequency then I suggest to buy an not-magnetic screwdriver (1,5€ from Cina, less than 6€ from EU) or to make it from a splinter of plastic or fiberglass....

NOTE:
normally a TV that don't lock the subcarrier color will kill the color and present a black & white image....
If your TV is multistandard don't set it in "AUTO" but set it in "PAL".
“Don’t argue with idiots because they will drag you down to their level and then beat you with experience” (Mark Twain ?)

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srowe
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Re: Display issue

Postby srowe » Wed Jan 10, 2018 2:29 am

There's no capacitor attached to the rear of the PCB, perhaps someone has removed it? Or the board revision already includes it? These are the numbers on the rear

Code: Select all

FAB NO. 251040-1
ASSY NO. 250403    REV.D KU-14194HB


I'll try and find the proper tool, I've tried making one out of wood and plastic but the slot it goes into is so shallow they don't work.

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MCes
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Re: Display issue

Postby MCes » Wed Jan 10, 2018 3:01 am

Post some pics of front and rear of PCB (the shielded part).

Read on Xtal surface: is it writed "4.43......" or "8.86....."?

The 56pF capacitor has to be soldered electrically in parallel to R5 (@ 4.43 MHz Xtal)

IMG_20180110_100401.JPG
“Don’t argue with idiots because they will drag you down to their level and then beat you with experience” (Mark Twain ?)

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srowe
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Re: Display issue

Postby srowe » Wed Jan 10, 2018 2:42 pm

Here are pictures of the front and rear of the PCB

2018-01-10_19-55-17.jpg


2018-01-10_19-54-32.jpg


The crystal is 4.433618, there's no sign of a capacitor being soldered to the rear. Shall I try adding one, it's just a standard ceramic?

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srowe
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Re: Display issue

Postby srowe » Wed Jan 10, 2018 2:58 pm

Actually looking at R5 under a magnifying glass it looks like the green capacitor next to it is in the same holes as the resistor. I can't see any markings on the capacitor.

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MCes
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Re: Display issue

Postby MCes » Thu Jan 11, 2018 2:40 am

The point of situation:
We are speaking inside hipotesis that 6561 is well working ( are you able to test your 6561 into another working VIC20? ) and that problem is only the color subcarrier frequency misalignment.

Without serious instruments I suggest to:
A) using of Cathode Ray Tube TV
B) set the TV on "PAL" (if it is multistandard)
C) get a proper screwdriver ( buying or making one)
D) switch on the VIC 20 and slowly spin for 1 turn C48

If you don't find a point of C48 that show coloured picture on TV you had to try the desoldering a side of C50 (only a side: the capacitor will be preserved for a future reinsertion in case of Xtal substitution), and then repeat the D) point.

If problem was the incorrect frequency your VIC20 will work well, otherwise you have to find the other problem waiting an Black/White screen and after you must re-tune the 4.43 MHz frequency to restore the TV colours decoding
“Don’t argue with idiots because they will drag you down to their level and then beat you with experience” (Mark Twain ?)

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srowe
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Re: Display issue

Postby srowe » Thu Jan 11, 2018 12:46 pm

I finally had the courage to swap the 6561 with my two-pin system. It's definitely a faulty VIC, I have some on order.

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toby405
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Re: Display issue

Postby toby405 » Thu Jan 11, 2018 3:44 pm

Are VICs in short supply? Where did you order from?

norm8332
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Re: Display issue

Postby norm8332 » Thu Jan 11, 2018 4:30 pm

They are in very short supply and thus expensive...this is NTSC https://www.ebay.com/itm/1PCS-MOS6560-101-MOS-6560-101-6560-101-DIP-40-/252958104913

I paid $12 each 5 months ago and when the guy realized they were in short supply he doubled the price.

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MCes
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Re: Display issue

Postby MCes » Fri Jan 12, 2018 1:10 am

If I remember well I have 6561 (PAL) spare....
In case of interest PM me
“Don’t argue with idiots because they will drag you down to their level and then beat you with experience” (Mark Twain ?)

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srowe
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Re: Display issue

Postby srowe » Fri Jan 12, 2018 4:08 am

toby405 wrote:Are VICs in short supply? Where did you order from?


PAL VICs are available on AliExpress from a couple of traders. Prices are higher than they were but should work out at about £15 each if I avoid paying VAT.


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