Video noise on VIC-20

[sean_sk]

Hi all,

I have an issue with video noise on my PAL VIC-20. It manifests itself as 2 horizontal bars of noise that slowly scroll down the screen. I have replaced both the electrolytic and tantalum capacitors in the video circuit hoping that perhaps they were contributing to the problem. Unfortunately, that didn't fix the issue. I'm using a new composite video cable I bought off eBay. The same problem occurs when using the RF adapter. One thing I noted when using the composite cable; the noise seem to get a bit worse if I disconnect the cable's left and right audio plugs. Could it be a grounding issue? All things considered, the picture is actually quite sharp when the video noise isn't scrolling over it.

The board type is: 250403 251040-01 REV. D
I have attached a picture to show what is going on. Hopefully someone might have an idea what might be causing the noise. Thanks guys.

[groepaz]

My first suspect would be a ground loop, indeed

[mathop]

Is this the board that needs external 5VDC (as opposed to 9VAC)?
Are you using the original PSU or a modern replacement?
I am asking because I have a C64, so not really a VIC-20, but comparable, with one of these Meanwell PSUs and it would have the exact same phenomenon on the video output (rolling noise).

[sean_sk]

Hi guys,
Thanks very much for your replies.

@mathop: I believe this board takes 9vAC (according to the serial label underneath) and also 5vDC. It is a smaller late revision board that takes the 7 pin DIN power connector. The PSU I am using is a new one from the following link:

https://www.ebay.com/itm/114806245028?h ... Swz8BejB97
The main difference between the one I got from Electroware and the one shown in the Ebay link is that mine is a bit older and delivers 2.0A on the 5VDC output instead of the 4.0A advertised in that listing.

When I got this VIC a couple of years ago, the original PSU was dead so I wasn't able to test to see if it had the same issue with picture noise.

[groepaz]

Another possibility is that the filter cap (C39 in this schematic: http://www.zimmers.net/anonftp/pub/cbm/ ... 1027r2.gif) is dry and needs replacement

[sean_sk]

Another possibility is that the filter cap (C39 in this schematic: http://www.zimmers.net/anonftp/pub/cbm/ ... 1027r2.gif) is dry and needs replacement

Thanks groepaz. Yeah that was my thought too, so I replaced that as well as all the rest of the electrolytics but, unfortunately still no difference.

[gordonf]

I have a very similar problem, even in S-Video.



I'm using an after-market power supply too, as I didn't trust the epoxy brick that came with this machine. I also re-capped it using a Console5 recap kit.

If it's the after-market PSU doing this, any suggestions? I suppose I could try the original brick as long as it doesn't have an over-voltage problem.

This system had an odd modification done to it prior to me getting it, and was also missing a couple of ceramic caps. I kept the ceramic cap and ferrite bead I removed - maybe I could use these?



You are also welcome to watch the video where I did the S-Video modification (20 minutes).

[beamrider]

I would be inclined to test with the original PSU briefly after checking the voltage with a meter of course.

[gordonf]

I would be inclined to test with the original PSU briefly after checking the voltage with a meter of course.

I just did, and I get a very clean picture this time. I haven't done the re-cap R10 adjustments yet, and was about to.

So what is this after-market brick doing, I wonder.

[gordonf]

I would be inclined to test with the original PSU briefly after checking the voltage with a meter of course.

Just did the re-cap R10 adjustments and tested the stock power supply. The PSU is part number is 251052-01, which wasn't on the Wiki for some reason. Fortunately the voltages checked out - they would only check out if I connected the PSU to the VIC 20, and then I could check the voltages before power-on and after. +5V is steady.

The stock PSU improved the video quality a lot! No more interference. I'll need to ask the vendor for the aftermarket PSU to see what's going on.

[sean_sk]

So it seems then that it could be the new power supply causing the video noise. I opened mine up and it has a Meanwell 9v transformer and a 5v switch-mode PSU. I wonder if it's the switch-mode causing the noise? Other than that there doesn't seem to be much more to them. If it is the power supply causing the issue, then it's a shame since I've had this thing 4 years and can't really go back to the seller and it's not really serviceable by the looks of it either. The original PSU is long gone as that was truly busted.

[beamrider]

The stock PSU improved the video quality a lot! No more interference. I'll need to ask the vendor for the aftermarket PSU to see what's going on.

Be interested to hear what they say. I was actually considering getting one of those PSUs from Poland, but will wait to see if this gets resolved now.

[Gyro Gearloose]

Is this after-market power supply using a two prong AC connection? These ungrounded supplies, or more accurately, un-earthed, supplies have a lot of (legally acceptable) current leakage between the AC and DC side. Guess what the frequency of this leakage current is.

[gordonf]

The power supply I got had a ground pin on it, but it might not be connected to anything at the VIC 20 end

[Gyro Gearloose]

Sorry, I was too lazy to click on your eBay link at first because it sounded just like a "switch mode power supply that uses no Earth connection noise injection" issue.

But you said "power supply I got had a ground pin on it" so I clicked the link and saw this

https://i.ebayimg.com/images/g/jWQAAOSw ... -l1600.jpg

I am confused. I am also highly hung over and drinking again so who knows ...