Vic 20 with almost no color

Geronimo · Post by **Geronimo** » Sun Dec 11, 2016 1:49 pm

Hi everyone,

I have got a working Vic 20 which has almost no color, nearly black and white.

I can see someone before me has done some soldering on two transistors.

Someone told me that it might me solved by replacing the capacitors in the video signal path.

Can it be caused by the VIC chip?

Does anyone have some tips and/or solutions?

Boray · Post by **Boray** » Sun Dec 11, 2016 2:42 pm

Have you tried adjusting the adjustable components?

Schema · Post by **Schema** » Mon Dec 12, 2016 11:51 am

There are a couple of adjustable potentiometers that affect the video signal, under the metal RF cover. Try tweaking those!

I found I had to have quite different levels between a TV and monitor.

Geronimo · Post by **Geronimo** » Mon Dec 12, 2016 2:21 pm

Yes, I already tried that, I should have mentioned that...sorry for that..

I also checked Ray Carlsen's website (http://personalpages.tds.net/~rcarlsen/), and it says that it might be the VIC chip or the master clock oscillator.

I hope of course that the VIC chip is ok. I do have another VIC 20 which is fine but I don't want swap the VIC chips because of the risk of damaging it.. I want to keep my working VIC as it is unless i really run out of options... I am hoping someone on this forum knows how the troubleshoot it to find the problem...

Cheers,

Geronimo.

Gyro Gearloose · Post by **Gyro Gearloose** » Mon Dec 12, 2016 7:43 pm

Please, before looking for complex problems, what is the power supply voltage?

MCes · Post by **MCes** » Tue Dec 13, 2016 3:57 am

the clock oscillator produce the color carrier, but the the quartz frequency is lowered by age....
try to tune the trimcap (C48) with special screwdriver for radiofrequency (non-magnetic material like: http://www.hanlymtool.co.kr/upload/good ... K-90_4.jpg
Put the trimmer at middle point when "colour ok".
If it doesn't work try to cut one foot of capacitor C50 and then try to tune C48 another time.

Mike · Post by **Mike** » Tue Dec 13, 2016 7:39 am

One other question: are you going directly with the composite signal to a monitor, or are you (still) using the RF-modulator with a TV set? In the latter case, the RF modulator also could be the culprit.

MCes wrote:the clock oscillator produce the color carrier, but the the quartz frequency is lowered by age....
try to tune the trimcap (C48) with special screwdriver for radiofrequency (non-magnetic material like: http://www.hanlymtool.co.kr/upload/good ... K-90_4.jpg
Put the trimmer at middle point when "colour ok".
If it doesn't work try to cut one foot of capacitor C50 and then try to tune C48 another time.

Note however, VIC tends to cease video generation, when the capacity of the trimmer is altered during operation, resulting in a black screen. Then, a power cycle is required to get a picture again.

That also means a non-metallic screwdriver is not strictly necessary, if the changes are done with the VIC-20 switched off. Only one quarter turn in each direction from the original position should be tried out, anyway.

See also the thread 'ViC20 strange behaviour at power up.'.

eslapion · Post by **eslapion** » Tue Dec 13, 2016 9:08 am

Geronimo wrote:Hi everyone,

I have got a working Vic 20 which has almost no color, nearly black and white.

I can see someone before me has done some soldering on two transistors.

Someone told me that it might me solved by replacing the capacitors in the video signal path.

Can it be caused by the VIC chip?

Does anyone have some tips and/or solutions?

I can easily see half a dozen reasons why you get fading colors and the VIC-I being faulty is not among them.

What type of VIC-20 mainboard do you have and what type of connection do you use to connect the VIC to your TV/monitor ?

Most importantly, do you use the modulator ?

Geronimo · Post by **Geronimo** » Tue Dec 13, 2016 2:49 pm

Thanks for all the help and questions!

I am using a direct composite connection to a Commodore 1702 monitor, so no using an RF modulator.
The power supply should be fine as my C64 and other VIC 20 also runs fine on it.

The board is a cost reduced version using a round DIN connector for the power supply connection. The VIC 20 is made in England with a pretty high serial number.

I have tried to adjust both adjustable pots, with no change in color unfortunately...

I also did cut one of the legs of C50, but didn't notice any difference at all

Like I mentioned above, someone before me seems to have done some soldering on the two transistors inside the metal box with the VIC chip in it.

Help!

Geronimo...

Schema · Post by **Schema** » Tue Dec 13, 2016 2:59 pm

On the 1702, using the front connectors?

Geronimo · Post by **Geronimo** » Tue Dec 13, 2016 3:02 pm

More info:

assy 250403
fab no. 251040-01
REV D

All the IC's have a date code somewhere in 1983

I need to find the schematic as I can see now that the person before me has indeed done some soldering on the two transistor. The left one of the two seems to have its middle leg not soldered on the bottom of the PCB. Maybe the trace runs on on the top of the PCB.

Geronimo..

Mike · Post by **Mike** » Tue Dec 13, 2016 3:03 pm

One other possible reason that comes to my mind is a cold solder joint in the path of the chroma signal before it is combined with the luma signal to form the unbuffered composite signal.

Please provide photos of the RF box contents and of the soldering side in a quality compared to those shown here.

Then it might be possible to judge what is wrong by your description and what is necessary to revert the circuitry to working state.

Geronimo wrote:I need to find the schematic[...]

It's here for the RF box of the CR version: ftp://ftp.zimmers.net/pub/cbm/schematic ... 1027r1.gif.

Geronimo · Post by **Geronimo** » Wed Dec 14, 2016 11:52 am

Hi All,

I have done some more checking and I can see now that the transistor Q2 is not a 2SC1959 as per schematic.

There is currently a C639 transistor installed for Q2.

It is clear that someone has done some soldering on Q1 and Q2 and I guess that person has replaced the 2SC1959 transistor with a C639 transistor for Q2.

I guess there was a problem and the person before me has tried to fix it by replacing the transistor and couldn't find a 2SC1959 transistor so used a C639 instead.

I don't know if the new C639 transistor is compatible with the original 2SC1959.

It could also be that Commodore installed the C639 originally. All I can see is that some soldering has been done, so I am just guessing.

The Q1 transistor is a C945 by the way (instead of the 2SC1815 as per schematic).

eslapion · Post by **eslapion** » Wed Dec 14, 2016 12:02 pm

Q1 is used for the sound so a lot of different transistors could do the job just fine so that's no concern.

The 2SC639 used instead of the 2SC1959 seems like a valid replacement but the order of the pins may be different (Base, collector, emitter), yes, there are circumstances where inverting emitter and collector will give a functional or semi functional result.

Capacitor C13 may be at fault.

Check R9, one one pin you should get 5V and close to 3.3V on the other. If not then other components may be damaged. C17 is electrolytic so it may be faulty (aged liquid inside).

Geronimo · Post by **Geronimo** » Wed Dec 14, 2016 12:21 pm

Yeah I know Q1 is for sound and I did check the pin order, seems ok.

I measured and got 2.976 Volts and 5.15 Volts across R9. Is guess this is normal?

Denial

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