Monacor CDM-1203 B/W CRT video monitor - Page 1
Forum section: electronics / repair

author: anton74
author_rank: Contributor
timestamp: March 02, 2021, 03:04:25 pm
content: Hello guys,I am trying to repair an old CRT monitor. The screen is divided by half, the upper part of is somewhat black (please see the photo "no V-sync"). An attempt to adjust the "V-hold" potetiometer on the front panel results in rolling up of the picture. As I understand, there is a problem with the verical deflection. The vertical deflection IC is AN5763. Unfortunately, I have no service manual or the diagram for this monitor. Nevertheless, I attached the typical circuit and the pinout of the AN5763 ("AN5763 circuit" and "AN5763 pinout"). The synchronizing signal at the pin# 1 of the IC is present (photo "pin1 AN5763"). The same signal before the input filter is shown at the picture "v-synch". The output of the IC at the pin#8 is shown at the picture "pin8 AN5763". Does the output signal look OK? I checked all the electrolytic capacitors near the IC, their ESR is OK. Is there a chance that the IC is defected?Thanks!

author: CaptDon
author_rank: Super Contributor
timestamp: March 02, 2021, 04:46:16 pm
content: Very much looks like a bad I.C., Seems like half the output amp is failed to open circuit.This was common with Zenith, the top half or the bottom half of the picture was collapsed.

author: anton74
author_rank: Contributor
timestamp: March 02, 2021, 07:04:43 pm
content: Quote from: CaptDon on March 02, 2021, 04:46:16 pmThank you. It seems that these ICs are not easy to find. I did not find them on Digikey, for example. Is there any substitution for this IC?

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 03, 2021, 06:12:33 am
content: Looks like ebay has them. Keep in mind most will likely be salvaged by Chinese ewaste recyclers and untested, regardless of how they are represented.

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2021, 07:13:57 am
content: You won't find obsolete specialized ICs like this from the normal component vendors, they will be surplus.First thing to try is replace the electrolytic capacitors around the IC, these typically use a charge pump and a bad capacitor can cause vertical collapse, I saw that a lot, though usually it was just a partial collapse when I worked on it.

author: anton74
author_rank: Contributor
timestamp: March 03, 2021, 12:07:50 pm
content: Quote from: BrokenYugo on March 03, 2021, 06:12:33 amOrdered from eBay from the US seller; the item was labelled as a new original one, probably new old stock. Thanks!

author: anton74
author_rank: Contributor
timestamp: March 03, 2021, 12:09:19 pm
content: Quote from: james_s on March 03, 2021, 07:13:57 amI tested all the caps with the ESR meter; all of them are OK. Will try to replace the IC. Thanks!

author: anton74
author_rank: Contributor
timestamp: April 04, 2021, 10:27:34 am
content: Well, I replaced the AN5763 IC. I think the IC replacement somewhat improved the situation: now I can adjust the vertical synchronization, with the help of "V-hold" potentiometer, and I can get the full screen image. However, it seems that the refrashing rate is still incorrect - the picture is flickering. Also, without any video signal applied to the monitor, I can see a bright wide line on the bottom of the screen. Tried to check and replace the electrolytic capacitors near the IC, their ESR is OK and the replacement of thece caps did not really help. What else could I check? I know that it probably unlikely, but could it be the MC1391P horizontal processor?

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 04, 2021, 02:35:16 pm
content: What about the big electrolytic that couples the signal into the vertical deflection coil? Also check the power supply?Doubt this has anything to do with the horizontal side.

author: anton74
author_rank: Contributor
timestamp: April 04, 2021, 04:05:09 pm
content: It turned out the small 0.33 uF electrolytic capacitor (encircled in red on the attached schematics). Replacement of this cap solved the problem. Thanks!

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2021, 06:05:52 pm
content: That was exactly what I suggested, it's very common for bad electrolytic capacitors around the vertical deflection IC to cause a vertical collapse. Usually in the beginning it causes the top of the image to fold over and then it gets progressively worse until it collapses completely.