Strange vertical display problem..... - Page 1
Forum section: electronics / repair

author: Good2know
author_rank: Newbie
timestamp: November 05, 2021, 08:27:33 pm
content: Hi,Am in the process of rebuilding my TV Typewriter from 1976 (TV Typewriter - Wikipedia) and needed an old TV to see if the RF modulator still worked. I was given a 1986 RCA ColorTrak TV, model F20517WNA01, chassis CTC146. The TV mostly works including the OSD text and menus, but am getting four duplicate text messages along with four cursors on the screen instead of one text message and one cursor. The text messages look fine and appear to be vertically and horizontally in sync. I tried another RF source from a vintage colorbar generator on the same TV and got the same multiple images instead of one.Attached is a partial schematic of the circuit in question. The U1001 chip is a A2302-1101-01A. I have verified the various waveforms on my scope and voltages appear to be in spec. I’m wondering if the A2303 chip has partially failed. Have tried to find more than a block diagram for the A2303 chip, but no luck. I also tried placing a 100k pot between pins 56 and 58 of the A2303 chip and could mess up the vertical sync, but the duplicates remained. I also tried varying the AC voltage between 90-130 VAC, but the duplicate images were unaffected. Hate to take the TV to the dump, but running out of options.Thanks for the help - Don

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 07, 2021, 08:16:43 pm
content: A picture of the display may be helpful.

author: Good2know
author_rank: Newbie
timestamp: November 08, 2021, 02:49:08 am
content: Thanks for the suggestion. Have add a short video. From the video you can see the multiple help messages, though only one should be showing as well as possible character slots.

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 09, 2021, 01:14:25 pm
content: Are you saying waveforms match service manual in amplitude and frequency yet display is bad?How does the TV behave with a composite video input if you have one of those around? (Should be possible to pick it before the RF modulator on your typewriter.)Unless it was a specific order from RCA their A2302-1101-01A may be a commonly used Japanese made jungle IC with a RCA part number slapped on it. I checked against the Toshiba TA8659 but pinouts don't match. One of the Sanyo LA76xx series maybe...

author: Good2know
author_rank: Newbie
timestamp: November 12, 2021, 02:10:09 am
content: Thanks shakalnokturn for the reply. I have a 2 channel scope and synced channel 1 using the composite signal from pin 57 of the A2302 chip and had channel 2 on pins 54 or 55 of the same chip showing just the vertical signal. When I compared the channels, channel 2 was not in sync with the composite signal though the vertical frequency appeared to be normal. The TV does not have a video in jack, but the composite signal on pin 57 of the A2302 matches the composite single from the TV Typewriter. I have tried feeding the composite video from the TV Typewriter to a newer TV with Video input and have no vertical sync problems. Will take a look at the Sanyo LA76xx series. Thanks again - Don

author: james_s
author_rank: Super Contributor
timestamp: November 12, 2021, 04:05:35 am
content: Is there a horizontal hold user control? That looks a lot like what you get if the horizontal isn't locked.

author: Good2know
author_rank: Newbie
timestamp: November 12, 2021, 05:15:21 am
content: Thanks James for the reply. I did not find a horizontal user control. Did find a vertical size control. Added the full schematic for the set. Wondering if the horizontal and vertical sync are controlled by the TV remote ? Don't have the remote however.

author: james_s
author_rank: Super Contributor
timestamp: November 12, 2021, 05:45:22 am
content: I've never seen a TV that had the H and V hold controls on the remote. The earlier sets all had a knob, usually a row of them under the screen. Once they moved to the microprocessor era those analog controls went away and they got a hidden service menu.

author: Good2know
author_rank: Newbie
timestamp: November 12, 2021, 09:34:32 pm
content: Hi James, agree no V and H controls on physical remote. Was talking about a service menu accessed through the remote. Just wasn't sure TV mfg provided menus for V or H hold via the remote. Have not found remote manual to confirm or deny.

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 12, 2021, 10:03:59 pm
content: If there were a service menu I think it would be in the alignment section of the service manual, would be odd if Sams left that out.From PDF page 4, check capacitors C4416 and C4417 in the horizontal area, and C4501, C4503, C4504, C4505, C4510, and C4511 in the vertical.Should be noted for safety that this is a "hot chassis" set and not at all isolated from the line, do not connect an oscilloscope unless the TV is powered through an isolation transformer.

author: Good2know
author_rank: Newbie
timestamp: November 13, 2021, 02:18:43 am
content: Thanks Broken Yugo for the suggestions. I have been thinking sync problems and only recently thought more of a fold over problem that page 4 of the service manual talks about. Will check out the caps and see what I discover. What is a little odd is the text from the OSD in not folded over and not repeating unlike how the the composite video signal is displayed.

author: james_s
author_rank: Super Contributor
timestamp: November 13, 2021, 03:49:39 am
content: The picture is too small to really see what's going on, but if there is foldover at the top of the screen that is almost always a bad capacitor near the vertical output IC.