Commodore 1701 monitor - bowing at the top of the screen - Page 1
Forum section: electronics / repair

author: aa7ae
author_rank: Contributor
timestamp: November 21, 2022, 05:40:31 am
content: I just completely replaced all the capacitors in this Commodore 1701 monitor. Before I replaced the capacitors, the image geometry was missing part of the bottom of the screen, but it looked fairly ok otherwise. After I recapped everything, now I have this bowing going on at the top of the screen which I don't know how this started. I have tried all the adjustments I can find, but this looks like a more complicated problem with the yoke. Thoughts anyone?Thanks

author: inse
author_rank: Super Contributor
timestamp: November 21, 2022, 06:02:03 am
content: There is also an issue at the bottom of the screen.Is the diagonal line and the ghost image at the top real or some camera exposure or reflection issue?

author: aa7ae
author_rank: Contributor
timestamp: November 21, 2022, 06:18:50 am
content: The diagonal line seems to be an artifact of taking the picture with my phone. The lines at the top where the bowing is occurring is real.

author: inse
author_rank: Super Contributor
timestamp: November 21, 2022, 09:44:28 am
content: Can you post or link the schematic of your set?

author: aa7ae
author_rank: Contributor
timestamp: November 21, 2022, 03:07:39 pm
content: https://wiki.console5.com/wiki/Commodore_1701#Schematic

author: inse
author_rank: Super Contributor
timestamp: November 21, 2022, 09:21:28 pm
content: Can you check the B+ voltage (110V) for stability?

author: m k
author_rank: Super Contributor
timestamp: November 22, 2022, 06:31:26 pm
content: "COMMODORE BASIC" and "BYTES FREE" have different postures.Have you adjusted h-lin?Is the shape equal if you turn the service switch?

author: aa7ae
author_rank: Contributor
timestamp: November 23, 2022, 03:47:14 pm
content: After Thanksgiving, and with the arrival of some more parts, I'm going to go through and verify all the caps I replaced and check things with a voltmeter. Will be using an isolation transformer.

author: aa7ae
author_rank: Contributor
timestamp: December 04, 2022, 04:57:31 am
content: I have adjusted the B1 voltage to be on spec, 110v. I have also changed out a few more capacitors around the IC responsible for horizontal and vertical sync as well as changed out the IC itself. No difference.I made further adjustments to vertical size, linearity, and so forth with limited success. It's a little better. There is also some odd black raster lines visible on the screen as well.

author: amyk
author_rank: Super Contributor
timestamp: December 04, 2022, 08:29:15 am
content: Is the distortion dependent upon the content being displayed? In your example image there are lines at the top where the duty cycle of the gun is reduced due to the black text, which seem to coincide with the wider portions.

author: aa7ae
author_rank: Contributor
timestamp: December 04, 2022, 06:27:45 pm
content: Yes, the distortion is still there regardless of what game I play or image is on the screen.

author: aa7ae
author_rank: Contributor
timestamp: December 04, 2022, 10:11:15 pm
content: I have discovered what the problem is. The schematics aren't accurate. In the pin circuit, the schematic shows two capacitors in parallel C517 and C518, however in real life, there is no C518 capacitor. Since I was going off the schematic, I installed a 47uF capacitor for C517, when it should have been 470uF as shown on my parts lists on the internet. I could not understand why these things did not jive until I saw there are no parallel capacitors. There must have been a change to just go with one capacitor of a larger value which was not reflected in the schematic.

author: inse
author_rank: Super Contributor
timestamp: December 04, 2022, 10:31:25 pm
content: Great, finally you fixed it

author: m k
author_rank: Super Contributor
timestamp: December 05, 2022, 06:40:33 pm
content: 1702 has 470/25 there.