Barco CRT Monitor making noise

[decoder_82]

Hi,

I am working an old Barco DCD-2740 monitor. The monitor delivers the image correctly. High voltage is present (+-30KV).
Service manual can be found here: https://archive.org/details/manual_DCD2740PAL_SM_BARCO_EN

The monitor makes some noise and I cannot determine where is comes from. Attached audio recording.

I have seen bad flyback making noise but usually the is no image.

Any advice on where I could look to further troubleshoot ?

Many thanks

[decoder_82]

Some update - when looking at the back of the tube in the dark I see some arcing between deflection coils and tube surface.
Does it mean there is a grounding issue or high voltage leaking ?

[andy3055]

It is the latter. I think the discharge is from the EHT plug which is common. You will have to clean the tube surface.

[decoder_82]

I will clean the tube and add fresh dielectric grease to the anode cap. Will see if this arcing issue is linked to the noise I hear from the monitor.

[andy3055]

Make sure to have it nicely dry around that area.

[decoder_82]

Cleaning the area and adding grease removed the arcs around the tube area which is already a good thing.
Unfortunately this was not connected with the noise I was trying to fix.
Which parts in a monitor can make such a noise ? FBT ?

[strawberry]

CRT socket focus (~6kV) spark gap (when moisture get inside) or insulation breakdown between focus and PCB common = some artifacts on image
corroded tube focus pin = (rapid complete focus/sharpness loss)
FBT arcing would affect picture quality the most = melted focus/screen potentiometers and/or blown transistor
could be tube grounding but that would effect picture brightness (tube glass wont form good capacitor)
could be between tube arking under deflection coil but that would cause deflection to malfunction and bad tube most likely

[decoder_82]

Using a piece of cardboard between the chassis and the tube, I have been able to identify that noise comes from the CRT neck / deflection coil area. Not the flyback transformer.
I then disconnected the deflection coil to see if it has an impact on the noise but once disconnected the monitor doesn't start.

[strawberry]

deflection protection jumper
could ruin tube burning line or dot in it without deflection

could disconnect filament first

[decoder_82]

Yes I have read it is possible to bypass the protection system by connecting resistors with value similar of the deflection coils.

[free_electron]

wow. that brings back memories.. i learned to work on Barco in school (VTI Kortrijk).

[decoder_82]

The Barco monitors were quite popular in Belgium in the early 90s. Unfortunately they there is less service documentation compared to more popular brands such as Sony.

[shakalnokturn]

Re-check your CRT graphite coating is correctly grounded. (Usually spring+braid leading to chassis ground by a single wire)
A leakage from anode cap should only show between anode and graphite.
Arcing between graphite and deflection yoke isn't normal.
On rare occasions the H deflection coils can have bad insulation, or the HV regulation went bad and broke through the CRT neck under the yoke. (In either case should come with picture symptoms.)

[decoder_82]

When I connect a video source to the monitor, I get some dark bars moving from top to bottom and the image in black and white (image is stable not rolling). The issue goes away (then no bar, no noise can be heard and colors there) from time to time and then reappear. Maybe a bad cap?

[andy3055]

It could be a problem with the vertical hold circuitry.

[strawberry]

from image it doesnt look like synchronization pulse but more like exponent pulse
could be ~100-400Hz repetition rate

[decoder_82]

Yes does not seem to be sync issue as image is stable. What is strange is that the issue is intermittent.

I will start with reflowing solder joints in the area of vertical hold and check / replace caps in this area. Unfortunately most caps are axial so it needs a little more work to fit radial ones.

[shakalnokturn]

You're out to replace elecyrolytics for fun?
I still think it's poor CRT grounding leading to arcing. You can even see the discharge in the horizontal bars. (Suddden dip in brightness with slow recovery.)

[decoder_82]

I checked for continuity between the chassis ground and the braid wire around the tube at different locations and didn't find any area without continuity.

[shakalnokturn]

Next most likely would be focus (G3) arcing in CRT socket spark gap. (As previously mentioned by strawberry.)
This could be due to dust, smoke accumulation and/or dampness in the spark gap itself or a defect in the HV multiplier assembly (blue Siemens?) or focus adjustment.
Try playing with the focus adjustment see if you can trigger or stall the problem or see if it has any effect on the frequency of the noise and bars.

Edit: If problem does seem to come from focus you could completely disconnect the focus wire to confirm.

[decoder_82]

Thanks ! Indeed I disconnected the focus wire and this removes the bars and the noise.
Does it mean the unit with the focus adjustment (in green on the picture) needs to be replaced or are there things that could be repaired?

[inse]

Check whether the arcing is in the tube socket/board or inside the tube itself.

[shakalnokturn]

It's a little delicate, the CRT socket can usually open in 2-3 parts by un-clipping. Once you have access to the focus SG you can check if the arcing is happening there or in CRT. I doubt it is in CRT given picture symptoms and sound made, could be wrong, worth checking at least you get to see if SG is dirty.

If arc is in SG and SG clean, then focus voltage is probably too high or unstable.
I haven't often seen the focus potentiometers fail, most often they'll get dust contamination when not sealed.
You can't easily check the potentiometer with a DMM they're often in the 30-70M Ohm range and won't necessarily show the same behavior at lower voltages. Disassembly and cleaning is an option, beware not to loose the small parts...

The voltage multipliers do fail, sometimes you can see cracks in the body near the narrow part, the internal resistors that drop the voltage from the anode to focus voltage or potting around them can fail allowing the focus voltage to ramp-up until SG breakover.

 

[decoder_82]

Unfortunately the CRT neck adapter seems sealed and cannot be open in a non destructive way. There is no gas discharge tube visible.

I also checked the potentiometer of the focus knob it has values between 0.5 to 28 MOhm depending on its position.

[shakalnokturn]

The CRT sockets aren't usually sealed in the way that things are melted together or potted, they're factory clipped together, most can be undone but it isn't easy.
The part you want to pry off to look inside is the dusty one that says "Hosiden" on. Luckily yours seems to have a separate cover for the focus spark gap that may make access easier but by now my bets are already on a defective voltage multiplier assembly.