| jeff.panasuk@cfl.rr.com: Jul 03 02:54PM -0700 > Back in February or March of 2010 the TV began changing from any channel to channel 2 intermittently. Sometimes very frequent. Other times more intermittent but it would not go more than a few hours without the problem popping up. Replaced IC0102 (MN14834HH) Main IC for the Channel Selection Ckt but no help. Remote control IR circuit not the problem as I disabled it. Over time have replaced some bad unrelated hw (burnt resistor, Caps). Have schematics and service manual but can't isolate the problem. Nice picture and everything else works fine. Unit in storage but every 4 months or so, when I see it and think about it, I fire it up. Hate to trash this old tv. Kind of part of the family now. Has anyone ever heard of this before? ---------------------------------------- I want to thank all of you for your time and comments. Here is some feedback to you: Ultimately I would be fine if the only two channels I can get are 3 or 4 for my direct tv cable. Can't live with channel 2 though. The tv will go to the channel you select using the key pad or remote (before I disabled the remote). As you watch your chosen channel it will instantly change to channel 2 and not scroll to it. I can't imagine a light source being the problem. The IR receiver works but was disabled to rule this out. While it was disabled the tv would still change to channel 2. No suspect rf sources in vicinity either. Problem happens in any room. The most exotic rf source I have are is my computer, monitor and microwave. No "new" technology stuff and problem happens with computer and monitors off. No supercapacitor or battery for backup and not listed in parts. I don't see any central microprocessor in the schematics or parts list that applies to this model and chassis. The CT1957 model has a Channel Selection memory IC IC0103 (MN1228) but this model CT1955 does not. The circuit board has an empty spot for this 18 pin IC. Unit has buttons for channel/volume up/down and power. All the picture/video controls are adjusted using potentiometers located behind a flip down panel at the bottom center of the front of the unit. No mechanical selectors other than the video pots. Your feedback was excellent so I power up the unit for the first time in months and it defaulted to channel 2. I set up some tests. Test #1 Sequences: • Power on change to channel 5 and set volume to 15 • Power off button when not having problem. Power back on channel is at 5 and volume at 15 • Power off button, when not having problem, and remove power cord from ac: Plug back in and power up: Channel = 2 and volume = 21. I think this is normal. • Power on and set to CH5 / Vol=15. • While not having issue I pulled power cord without depressing power off button: Plug back in and power up: Channel = 2 and volume = 21 Test #2 Rear Switch Settings (S1 3-Way Switch): My default settings are CH=5 / Vol=15 S1= Either CATV1 or CATV2: Monitoring TV unit CH=5 / Vol = 15. Within 2 hours of power on the problem occurred and then it started occurring frequently. The tv Changed to channel 2 from 5 instantly and the volume stayed at 15. It did not change to 21 as what happens when ac is removed from the unit. S1 = "Normal" (VHF/UHF) : Problem does not happen. Switching back to CATV1 or 2 problem happens within minutes. Put Switch in "Normal" mode. No problem for next hour+, up to the point of posting these notes. If this holds I'm good. I will leave S1 in "Normal" mode and powered on for hour or days until I have confidence in . . . . ….. something. Tried freezing and heating the tuner and components but no impact. I thought I ruled out the keypad but can't swear to it. Will check. No neon's. From my last troubleshooting notes: 6/9/16 Per T/S flow chart ordered and replaced items below including an incoming power electrolytic 22uF/250v. Not related to problem but has a domed top. : • IC0102 MN14834HH High hopes…. Big letdown…… • Socket for above • Q102 2sc458 B/C/d 2sc1740Q/R • Q0103 2sk105 E/F • Q0104 2sc1213A • IC0104 LA7910 Band Drive Ordered and have part but did not replace • ZD0101 Measured OK: RD33EB1 ZD HZ6(C)1 If problem happens with S1 in "Normal" mode I will replace IC0104 (Band Drive) and get to the remaining 2SC458's later. There is a 5v Regulator powering IC0102 I'll try to get that monitored also Just looked at original receipt. Purchased in 1986 not 1984 $356. |
| Chuck H. <chuckh@hotmail.com>: Jul 03 10:57PM -0500 On Mon, 3 Jul 2017 14:54:30 -0700 (PDT), jeff.panasuk@cfl.rr.com wrote: >• ZD0101 Measured OK: RD33EB1 ZD HZ6(C)1 >If problem happens with S1 in "Normal" mode I will replace IC0104 (Band Drive) and get to the remaining 2SC458's later. There is a 5v Regulator powering IC0102 I'll try to get that monitored also >Just looked at original receipt. Purchased in 1986 not 1984 $356. The set didn't come on the market in the U.S. until 1985. If all you need is channel 3 and it doesn't do it with the switch in the normal mode, I'd just leave it be unless you are just curious to know the solution. I don't have the schematic in front of me because I am on vacation but doesn't that normal - cable switch also have a HRC position? I'd be curious if it goes to 2 in that position. If it doesn't I'd clean the switch, it possibly could be the problem. Chuck |
| whit3rd <whit3rd@gmail.com>: Jul 03 01:41PM -0700 On Monday, July 3, 2017 at 3:01:28 AM UTC-7, frank wrote: > (striking at about 200V) and I've found the following behaviour: > https://www.youtube.com/watch?v=DeKPkG5T1xc > Basicly it seems to heavily ring on the turn-off curve? The impedance of a gas discharge, especially at low current after a discharge event, is negative. So, it can oscillate. Geiger and proportional counter tubes have a gas fill that includes quench gas (CO, or methane, or somesuch) that is intended to reduce the negative-resistance behavior. Those gasses may be omitted from your device, or they may have been used up (depends on how much 'test' discharge the tube has seen). So, this behavior might not happen on an as-delivered device. |
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