CTM640 - Unstable voltage

[jelcynek]

Hello,
Lately I try to repair my CTM640 and need advice from some more experienced users. So, the case is that in general it works flawlessly. Colors are vivid, picture is crisp. The only problem is that it gets random voltage drops which are visible as short snaps on screen and drops also occurs on 5V what cause computer to hang up and show garbage on the screen. It happens once very few or dozen seconds. 

Here are videos from oscilloscope showing short voltage drops on 5V (around 40 second):
https://share.icloud.com/photos/038JU4iCuNlgOC0j0iGoToUCQ

and the display itself:
https://share.icloud.com/photos/0ceHkV7FwlW8QJTzo5ft_p4GQ

I opened display, discharged it and went through all of the boards and electronics. Judging by the look everything looks fine. No puffed or leaked capacitors, no burn marks. So right now, my further actions would be to measure voltages on the board with service manual but I'm a little afraid of that or to desolder element by element and check with tester if everything is right. 
v
But first I decided to write here. For my limited electronic knowledge, it looks like maybe some capacitor responsible for filtering voltage is broken. Any ideas?
 

[pelrun]

Just because the electrolytic capacitors aren't visibly damaged doesn't mean they're in good shape. It's almost certain that most of them are dead or on their last legs, simply from age.

[jelcynek]

Yes. I'm aware of that. But before I will start replacing every capacitor, I thought that maybe there are the ones which may be more responsible for that behavior than other. Thus, why I asked here. You know, like a hint where on the schematics looks closer and what to check before I start replacing everything.

[Bryce]

Hi,
    is there also an audible "snap" when the voltage drops?

Bryce.

[jelcynek]

is there also an audible "snap" when the voltage drops?

Yes. Right now, I use power supply for 5V for computer and everytime when there is a voltage drop on the display there is an audible "pop" or "snap" and on the display it can be seen as well. Video also makes snap. 

[darkhalf]

Just because the electrolytic capacitors aren't visibly damaged doesn't mean they're in good shape. It's almost certain that most of them are dead or on their last legs, simply from age

Sort of agree with this one. It depends on the capacitors. I've certainly had some devices with bad capacitors (including 90's motherboards, STB boxes, and Toyota ECUs) which have expanded or leaked and need replacing

However there are also other brands which are still showing good ESRs almost 40 years later. So it depends on the brand and quality of the capacitor. Using an ESR meter to check is the best way to know for sure. 

There is nothing worse than sourcing and then replacing parts, later knowing that it was for nothing

What I do know with going through the pain of repairing two Amstrad monitors is I've checked all the caps ESRs and they were fine. Also with this type of problem, we are seeing an intermittent spike/drop in voltage associated with crackling. That doesn't really appear capacitor related IMHO

everytime when there is a voltage drop on the display there is an audible "pop" or "snap"

ATM I'm calling the flyback on this one or there is a dry joint which is causing the spark (check and reflow joints)

If you search for CTM640 or look through the threads in this section, you will see I've made a few posts on both CTM640 and CTM644 monitors which died on me. 

So there is a STK voltage regulator which provides regulated power to the rest of the monitor. There are two power rails which come out of this. What is more interesting is there is power from the flyback which goes into the 5V regulator. So since you are seeing the intermittent voltage drop from the regulator, but also hearing a crack, it leads to that crack coming from something that is starting to fail (transformer, regulator, flyback)

With flybacks there is a place Tulli Electronics which has these for a decent price. These are the last (in the world!) that I know exist and there appears to be 3 remaining
https://www.tullielettronica.net/prodotti/hr6355-145961.aspx
So my money is on flyback, given I've had to replace 3 in monitors in the last year. The coils vibrate, enamel flakes and they short out

[MoteroV4]

I recently repaired an intermittent fault in a CRT where old solders made bad contact. As they reach temperature, certain components (especially power components) tend to expand microfractures not visibles.