HELP NEEDED: 1084S-D1 (PAL) monitor with fading color and 'jumping' white images

[LuKeJerry]

Hello,

I've been trying (unsuccessfully) to fix a Commodore 1084S-D1 monitor that:

1. Works correctly when cold, for the first 3 or 5 minutes after being turned on.
2. After 5 min, starts to lose color, gradually, toward the center of the screen. I'm not 100% sure if the color disappears or if it's 'overlapped' by an horizontal white band. It seems that a white-band is visible even when there is no input signal.

See linked pictures:
https://imagizer.imageshack.com/img921/596/pW4a4u.jpg
https://imagizer.imageshack.com/img923/6809/E9vLEy.jpg
https://imagizer.imageshack.com/img921/194/gFZ8tP.jpg
https://imagizer.imageshack.com/img924/2299/MLzE1f.jpg



3. When a "very white" images is on screen, it starts to "jump", with black lines and distortion (some lines bended to the right) in the top part of the screen. It's easy to reproduce increasing a bit brightness or contrast, but often it does it with defaults.


Video here:
[ Show youtube player ]


- All problems above are shown when using CVSB (composite) or S-VIDEO inputs (C64). With RGB (Amiga) there is no problem of fading color, but remains the problem of screen 'jumping' with white images.

* Already done:

>> All electrolytic capacitors have been replaced already (with Panasonic or other good brands)
>> C613 and X601 (variable capacitor and crystal oscillator) replaced
>> C613 'tuning' attempted, of course...

but all lead to no improvements :-(


Also I have tried the same monitor with different inputs both CVBS/S-VIDEO or RGB, but the result is always the same - just to rule out that my C= computers are faulty


Thank you in advance for any help!


Cheers

// LuKe

[LuKeJerry]

*BUMP* / UPDATE:

Thanks to excellent video series from GadgetUK164 on fixing a 1084S-D1 monitor (exactly same model as mine): here [ Show youtube player ]

I've got the idea of buying some freezing spray and try to locate the component that warms up and creates the color fading failure on my monitor.


Good news: I think I've found it, freezing the voltage regulator STK73410II (on the main power supply circuit) restores the color (data sheet here: https://www.promelec.ru/pdf/STK73410%20II.pdf)

Bad news: I really doubt this component is in fault, rather something is sucking too much current from it, causing it to heat up and eventually shutdown for protection when it reaches too much current/too hot temperature (the behaviour on my YT video from previous post).

Now: what could be the main culprit for such a high current usage? Dying flyback? the tube? other components?

I've already checked the current on the 12V line and it's stable around 230mA - seems in specs with the ICs connected to it. I've replaced also all tantalum caps around PAL decoding circuit (TDA4510) and RGB control (TDA3507), plus I've replaced the TDA4510 itself (before hunting the STK with freeze spray).

I think next would be to measure current on different voltage lines / components to find who is stressing so much the STK.

Any advice from experienced technicians would be veeeeeery appreciated


TIA

// LuKe

[Misclegend]

if the monitor starts ok then when get hot issues appears then is obvious a capacitor problem
check the biggest caps and replace them

[LuKeJerry]

Originally Posted by Misclegend

if the monitor starts ok then when get hot issues appears then is obvious a capacitor problem
check the biggest caps and replace them

Thanks for the tip, but (from first post):

>> All electrolytic capacitors have been replaced already (with Panasonic or other good brands)

Or do you suspect another type of cap? Ceramic one shorting somewhere, after some time?


Also in my experience caps have bad behaviour when "cold" and eventually behave after a while (except perhaps tantalum ones?)


// LuKe

[Hewitson]

Regarding the screen "jumping" when predominately white, try reducing your screen voltage and increasing your brightness/contrast.

[Misclegend]

Originally Posted by LuKeJerry

Thanks for the tip, but (from first post):

>> All electrolytic capacitors have been replaced already (with Panasonic or other good brands)

Or do you suspect another type of cap? Ceramic one shorting somewhere, after some time?


Also in my experience caps have bad behaviour when "cold" and eventually behave after a while (except perhaps tantalum ones?)


// LuKe

surely you have bad replaced some capacitor with a higher capacitance than needed
then when the monitor is cold such capacitor work with the capacitance needed by the monitor and show the color correct

when the monitor get hot, capacitance increase out of monitor needs, and you get the fail

my advice: use a hair dryer in cold mode to refresh and cool to detect which capacitor is
or
put back the original capacitors if they are not bulged

[LuKeJerry]

Well I’m pretty sure I have replaced all capacitors with exactly the same values in capacitance, they are not hard to buy. Perhaps some are rated for higher voltage, but that’s not a problem.

Also the defect didn’t start after I’ve replaced the capacitors, but it was exactly the same, before and after.

On using a hair-dryer, I already have a freezing spray can, and I’ve already found that the main voltage regulator is the one that needs to get cooler, by having less load. Now I’m hunting for the component(s) that drive such high load.


// LuKe

[Misclegend]

Originally Posted by LuKeJerry

Well I’m pretty sure I have replaced all capacitors with exactly the same values in capacitance, they are not hard to buy. Perhaps some are rated for higher voltage, but that’s not a problem.

Also the defect didn’t start after I’ve replaced the capacitors, but it was exactly the same, before and after.

On using a hair-dryer, I already have a freezing spray can, and I’ve already found that the main voltage regulator is the one that needs to get cooler, by having less load. Now I’m hunting for the component(s) that drive such high load.


// LuKe

also you can try freezing the voltage regulator and all the important transistors to detect if one of those is causing the problem

The voltage regulator is essential,ie in a sony trinitron monitor if the voltage regulator is out of specs: the monitor do not start, in old monitors you can have bad regulator ( bad voltage in one of the pins, but correct voltage in others ) , yet the monitor starts normally

[LuKeJerry]

Originally Posted by Misclegend

also you can try freezing the voltage regulator and all the important transistors to detect if one of those is causing the problem

The voltage regulator is essential,ie in a sony trinitron monitor if the voltage regulator is out of specs: the monitor do not start, in old monitors you can have bad regulator ( bad voltage in one of the pins, but correct voltage in others ) , yet the monitor starts normally

I'm more persuaded now that the problem is only in the power supply. It seems that the voltage regulator STK73410II is overloaded, I suspect by the primary of the SMPS transformer or by something else on the primary side.

I've noticed that the voltage drop after few minutes of usage is on both secondary spires of the SMPS transformer, and to me this tells that there is less current coming from the primary.

Will re-study the power supply circuit and eventually try to find a replacement for the SMPT trafo.

// LuKe