Barco CRT Monitor making noise - Page 1
Forum section: electronics / repair

author: decoder_82
author_rank: Regular Contributor
timestamp: June 04, 2022, 08:53:17 am
content: 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_ENThe 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

author: decoder_82
author_rank: Regular Contributor
timestamp: June 05, 2022, 05:38:13 am
content: 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 ?

author: andy3055
author_rank: Super Contributor
timestamp: June 05, 2022, 05:56:23 am
content: It is the latter. I think the discharge is from the EHT plug which is common. You will have to clean the tube surface.

author: decoder_82
author_rank: Regular Contributor
timestamp: June 05, 2022, 06:10:09 am
content: 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.

author: andy3055
author_rank: Super Contributor
timestamp: June 05, 2022, 06:47:08 am
content: Make sure to have it nicely dry around that area.

author: decoder_82
author_rank: Regular Contributor
timestamp: June 05, 2022, 09:19:05 am
content: 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 ?

author: strawberry
author_rank: Super Contributor
timestamp: June 05, 2022, 10:46:16 am
content: CRT socket focus (~6kV) spark gap (when moisture get inside) or insulation breakdown between focus and PCB common = some artifacts on imagecorroded tube focus pin = (rapid complete focus/sharpness loss)FBT arcing would affect picture quality the most = melted focus/screen potentiometers and/or blown transistorcould 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

author: decoder_82
author_rank: Regular Contributor
timestamp: June 05, 2022, 11:45:40 am
content: 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.

author: strawberry
author_rank: Super Contributor
timestamp: June 05, 2022, 02:27:08 pm
content: deflection protection jumpercould ruin tube burning line or dot in it without deflectioncould disconnect filament first

author: decoder_82
author_rank: Regular Contributor
timestamp: June 05, 2022, 03:26:02 pm
content: Yes I have read it is possible to bypass the protection system by connecting resistors with value similar of the deflection coils.

author: free_electron
author_rank: Super Contributor
timestamp: June 05, 2022, 06:13:31 pm
content: wow. that brings back memories.. i learned to work on Barco in school (VTI Kortrijk).

author: decoder_82
author_rank: Regular Contributor
timestamp: June 06, 2022, 04:44:24 am
content: 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 06, 2022, 10:58:37 pm
content: 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.)

author: decoder_82
author_rank: Regular Contributor
timestamp: June 07, 2022, 03:01:00 pm
content: 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?

author: andy3055
author_rank: Super Contributor
timestamp: June 07, 2022, 04:24:00 pm
content: It could be a problem with the vertical hold circuitry.

author: strawberry
author_rank: Super Contributor
timestamp: June 07, 2022, 05:29:18 pm
content: from image it doesnt look like synchronization pulse but more like exponent pulsecould be ~100-400Hz repetition rate

author: decoder_82
author_rank: Regular Contributor
timestamp: June 07, 2022, 05:48:39 pm
content: 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 08, 2022, 03:26:16 am
content: 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.)

author: decoder_82
author_rank: Regular Contributor
timestamp: June 08, 2022, 04:35:51 am
content: 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 08, 2022, 10:59:54 pm
content: 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.

author: decoder_82
author_rank: Regular Contributor
timestamp: June 09, 2022, 05:27:30 am
content: 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?

author: inse
author_rank: Super Contributor
timestamp: June 09, 2022, 05:47:42 am
content: Check whether the arcing is in the tube socket/board or inside the tube itself.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 09, 2022, 08:19:48 am
content: 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.

author: decoder_82
author_rank: Regular Contributor
timestamp: June 09, 2022, 10:22:55 am
content: 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 09, 2022, 10:41:37 am
content: 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.

author: inse
author_rank: Super Contributor
timestamp: June 09, 2022, 10:41:46 am
content: If you switch it on like this with the focus voltage connected, does the arcing reappear?

author: decoder_82
author_rank: Regular Contributor
timestamp: June 09, 2022, 10:58:45 am
content: I will see if I can open it without butcher it too much.Another interesting point : focus voltage I measure is 10 kV. It seems a bit high to me. Adjusting the potentiometer from the focus knob doesn't make the focus voltage vary.

author: decoder_82
author_rank: Regular Contributor
timestamp: June 09, 2022, 11:00:17 am
content: Quote from: inse on June 09, 2022, 10:41:46 amYes it does

author: strawberry
author_rank: Super Contributor
timestamp: June 09, 2022, 11:20:38 am
content: I told it would be spark gap. seeing corrosion around PCB it is quite likely

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 09, 2022, 12:58:35 pm
content: Quote from: decoder_82 on June 09, 2022, 10:58:45 amYes, seems high... I assume this is with CRT disconnected.Still you should be able to measure some variation. What is your probe resistance?To me this points towards an open resistor in the HV multiplier. The service manual gives a little detail on internal structure although for the Siemens version, not for the exact one you have.If you have a megohmeter you may be able to try measuring resistances between focus wire / high side of focus pot terminal to see if reads open-ish.You could also throw the megohmeter at the focus spark gap before the butchery.I may look in my spares see if I have a similar multiplier for reference.

author: decoder_82
author_rank: Regular Contributor
timestamp: June 09, 2022, 02:22:30 pm
content: I used a 600Mohm probe to measure the voltage. I was not able to see the variation.I just swapped the HV multiplier with a spare one I had and it solved the issue. No more noise and image is nice and stable.Now I am wondering if the failure of the HV multiplier was linked to some other components in bad shape...

author: strawberry
author_rank: Super Contributor
timestamp: June 09, 2022, 03:22:38 pm
content: strange

author: decoder_82
author_rank: Regular Contributor
timestamp: June 09, 2022, 03:33:56 pm
content: Quote from: strawberry on June 09, 2022, 03:22:38 pmWhy does it seem strange to you ?I looked a bit online and it looks plausible explanationhttps://www.repairfaq.org/samnew/tvfaq/tvaasggdt.htm

author: strawberry
author_rank: Super Contributor
timestamp: June 09, 2022, 06:10:41 pm
content: usually HV parts beaks down arking burning melting until open circuitor open circuit first

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 09, 2022, 10:54:23 pm
content: True that many HV failures lead to meltdown until something shorts.In this case the input resistance on voltage divider in the multiplier must have a very high value (dropping down from 30kV to 2kV with minimum power loss, little current is needed by the CRT focus grid anyway) even with a dead short on the focus output if resistor's power isn't exceeded there's no reason for smoke.What is a bit of a mystery though is why the lower side of the divider would fail rather than the upper one, it's against electrical expectations. Could be mechanical (more stress during thermal cycling).

author: decoder_82
author_rank: Regular Contributor
timestamp: June 10, 2022, 06:37:52 am
content: Yes that's really strange maybe something mechanical as the issue was intermittent.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 10, 2022, 07:15:34 am
content: It also means the defective HV multiplier could still be used, if I were doing the repair for myself I'd take this option and keep the working one as spare.Either splice the focus wire close to the multiplier or take it from CRT PCB, find appropriate resistor value (a couple of tests will allow to calculate the internal resistance) insulate it well enough, add the original focus potentiometer and it should be like new.

author: decoder_82
author_rank: Regular Contributor
timestamp: June 10, 2022, 07:25:58 am
content: That's indeed an option I will not throw it away. Might be useful one day.I was quite surprised to see that actually there is still replacement that can be found brand new (HRT227BP)

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 10, 2022, 08:32:57 am
content: I'm surprised about that too, my usual supplier who's meant to be a TV service one, has completely given up on "triplers".The substitute reference also gives you the needed resistor values.