Old Tube TV Keeps Changing Channel to Channel 2: Hitachi CT-1955 NP85XA Chassis / 19" Color-1984 Purchase

Back in February or March of 2010 the TV began changing from any channel to channel 2 intermittently. Sometimes very frequent. Other times more inte rmittent but it would not go more than a few hours without the problem popp ing 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?

Reply to
jeff.panasuk
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Spurious interference from modern light source? Some cheap ones produce a lot of garbage besides light.

Reply to
Sjouke Burry

to channel 2 intermittently. Sometimes very frequent. Other times more in termittent but it would not go more than a few hours without the problem po pping up. Replaced IC0102 (MN14834HH) Main IC for the Channel Selection Ck t 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). Ha ve schematics and service manual but can't isolate the problem. Nice pictu re and everything else works fine. Unit in storage but every 4 months or s o, when I see it and think about it, I fire it up. Hate to trash this old t v. Kind of part of the family now. Has anyone ever heard of this before?

Did you try disabling the keyboard?

Reply to
John-Del

Wow, haven't seen one of these in decades.

If this model has a manual on/off switch, check the reset circuit for the micro.

Reply to
John-Del

snipped-for-privacy@cfl.rr.com wrote on 6/29/2017 5:02 PM:

A friend had one of those big console TVs that would work, but there was something wrong with the controls, I don't recall just what exactly. I suggested he unplug the set for a while to let the caps drain off and the processor reset. It worked sure enough! I expect a power spike of some sort glitched the processor and it was working, but some variable was hosed messing up the operation. A clean reset got everything working right again.

I guess that's not your problem.

--

Rick C
Reply to
rickman

Does it have channel up and channel down buttons on the panel? Since channel 2 is the lowest channel, if the channel down button was occasionally acting like it was pressed, it might end up at channel 2. I had a computer LCD monitor that had a soda splashed on the panel, and it got into the front panel button board, and it would work for months or days and then start flashing through the menus. Severe cleaning of the board with the buttons fixed it.

Jon

Reply to
Jon Elson

to channel 2 intermittently. Sometimes very frequent. Other times more in termittent but it would not go more than a few hours without the problem po pping up. Replaced IC0102 (MN14834HH) Main IC for the Channel Selection Ck t 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). Ha ve schematics and service manual but can't isolate the problem. Nice pictu re and everything else works fine. Unit in storage but every 4 months or s o, when I see it and think about it, I fire it up. Hate to trash this old t v. Kind of part of the family now. Has anyone ever heard of this before?

Yes from neon channel selector sets, touch selectors & older pushbutton bar tuning sets. Show us the circuit.

NT

Reply to
tabbypurr

l 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 i t. Over time have replaced some bad unrelated hw (burnt resistor, Caps). Have schematics and service manual but can't isolate the problem. Nice pic ture 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 ?

ar tuning sets. Show us the circuit.

Hmmm... you're drudging up long forgotten memories. Back in the late 70s Z enith used a varactor tuned electronic tuner that used neons as indicators for the pre-set push button selector, and sometimes those neons were the ca use of the TV jumping channels or refusing to engage a selected channel.

By 1984 I would think that all manufacturers would be using a true digital selector system for anything no using a mechanical tuner.

Unfortunately, OP didn't say if the TV would scan downward to channel 2 or jump to it. He also didn't say if the TV normally defaulted to channel 2 w hen the TV was powered off or when the AC was lost.

There were a lot of TVs then that used a digital keypad, but were not remot e control and used a mechanical AC switch and volume control. In this case , it's possible that the uPc is only used for channel functions and his pro blem could be on the micro's reset line.

Reply to
John-Del

nel to channel 2 intermittently. Sometimes very frequent. Other times mor e intermittent but it would not go more than a few hours without the proble m popping up. Replaced IC0102 (MN14834HH) Main IC for the Channel Selectio n 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 p icture 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 o ld tv. Kind of part of the family now. Has anyone ever heard of this befo re?

bar tuning sets. Show us the circuit.

Zenith used a varactor tuned electronic tuner that used neons as indicator s for the pre-set push button selector, and sometimes those neons were the cause of the TV jumping channels or refusing to engage a selected channel.

l selector system for anything no using a mechanical tuner.

r jump to it. He also didn't say if the TV normally defaulted to channel 2 when the TV was powered off or when the AC was lost.

ote control and used a mechanical AC switch and volume control. In this ca se, it's possible that the uPc is only used for channel functions and his p roblem could be on the micro's reset line.

Until the OP tells us what the technology is we have really no way to advis e usefully. 1984 could be a few things, switched linear pots with varicap i s likely, or touch sensors maybe. There were also old technologies used in some sets, so can't completely rule out a mechanical selector device operat ing a tuning bar.

NT

Reply to
tabbypurr

formatting link

Might be something usable here.

My S.W.A.G. guess based on similar-vintage technology failing-to-default is a weak capacitor that does not hold the charge necessary to 'latch' to the chosen channel.

Peter Wieck Melrose Park, PA

Reply to
pfjw

Most likely it is going into reset. Does it hold your settings, like color, brightness and whatever ? Probably not. They are likely all going to facto ry reset.

That would mean you are losing power to the micro or maybe the memory IC, o r the memory IC is defective. Ir it has a memory IC it is probably an eight pin EEPROM. Look at the board and see if it has a supercapacitor or whatev er, or a battery. Those could well be failing. That is actually more likely than an IC failing.

Keeping this old shit running is a good thing because all the new stuff is junk. My Ma watches an old CRT based Toshiba which works just fine. I do no t have a TV hooked up because the programing is all junk. The scene changes so fast you cannot see anything. PUT THE IMAC DOWN ! Anything I want to wa tch is old.

One question here - does it return to channel 2 as you are watching it ? If so, that means there is a temporary interruption in the power supply, prob ably the standby supply. That would make me suspect a bad connection, or ma ybe one inside of a device like a regulator or something.

It can also be caused by low supply voltage to whatever holds the memory. I would be headed for the power supply.

Reply to
jurb6006

I ran the television service department in the corporate headquarters for the largest seller of Hitachi sets at that time in the U.S. so I might be able to help you. First disconnect pin 17 and 18 of IC0102 and see if the problem disappears. If so, then it is likely the channel tact switches. If not, take freeze spray and a hair dryer to the tuner. The two main failures in this model, when there were tuner problems, were the ic you replaced and the tuner itself. The band switch IC IC0104 occasionally would fail and any 2SC458s in the circuitry can cause problems. If these checks don't work, post again and we'll go deeper in depth. Chuck

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Reply to
Chuck

Also replace C703 and R703 (off the flyback) if the previous hints don't work.

Reply to
Chuck

Ignore my last post; I followed a wire incorrectly. These parts are in the XRay shutdown circuit and do not pertain to your problem.

Reply to
Chuck

to channel 2 intermittently. Sometimes very frequent. Other times more in termittent but it would not go more than a few hours without the problem po pping up. Replaced IC0102 (MN14834HH) Main IC for the Channel Selection Ck t 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). Ha ve schematics and service manual but can't isolate the problem. Nice pictu re and everything else works fine. Unit in storage but every 4 months or s o, when I see it and think about it, I fire it up. Hate to trash this old t v. Kind of part of the family now. Has anyone ever heard of this before?

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I want to thank all of you for your time and comments. Here is some feedba ck to you:

Ultimately I would be fine if the only two channels I can get are 3 or 4 fo r 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 (befor e I disabled the remote). As you watch your chosen channel it will instant ly 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. Pr oblem happens in any room. The most exotic rf source I have are is my comp uter, monitor and microwave. No ?new? technology stuff and problem happens with computer and monitors off. No supercapacitor or batt ery for backup and not listed in parts. I don?t see any central mi croprocessor in the schematics or parts list that applies to this model and chassis. The CT1957 model has a Channel Selection memory IC IC0103 (MN122

8) 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/vid eo controls are adjusted using potentiometers located behind a flip down pa nel 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 mo nths 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 th ink this is normal. ? Power on and set to CH5 / Vol=15. ? While not having issue I pulled power cord without depressing pow er 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. Wit hin 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 st ayed at 15. It did not change to 21 as what happens when ac is removed fro m the unit.

S1 = ?Normal? (VHF/UHF) : Problem does not happen. Swi tching 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 i n ?Normal? mode and powered on for hour or days until I hav e confidence in . . . . ?.. something.

Tried freezing and heating the tuner and components but no impact. I thoug ht I ruled out the keypad but can?t swear to it. Will check. No n eon?s.

From my last troubleshooting notes:

6/9/16 Per T/S flow chart ordered and replaced items below including an in coming power electrolytic 22uF/250v. Not related to problem but has a dome d 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 rep lace

? 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.

Reply to
jeff.panasuk

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

Reply to
Chuck H.

to channel 2 intermittently. Sometimes very frequent. Other times more in termittent but it would not go more than a few hours without the problem po pping up. Replaced IC0102 (MN14834HH) Main IC for the Channel Selection Ck t 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). Ha ve schematics and service manual but can't isolate the problem. Nice pictu re and everything else works fine. Unit in storage but every 4 months or s o, when I see it and think about it, I fire it up. Hate to trash this old t v. Kind of part of the family now. Has anyone ever heard of this before?

No STD, IRC or HRC position switch or other type of such options. Just the single 3-position switch: CATV1, CATV2 or Normal. TV has been running for more than 24 straight hours in the ?Normal? switch positio n mode and has not changed to channel 2.

Moving 3-way switch to position:

CATV1 TV switched to channel 2 in a few seconds CATV2 Waited a few minutes but it did not change this time Normal Steady

When switch is moved to one of the three positions the tv defaults to CH2/V ol=21. I cleaned the switch and all is stable while I type these notes. Will monitor and report out.

Reply to
jeff.panasuk

MN14834HH is processor, check oscillator (pins 36/37) and reset voltage (pin 39). Replace resistor RO125 and DO110 (on pin 6).

Reply to
megamusic

to channel 2 intermittently. Sometimes very frequent. Other times more in termittent but it would not go more than a few hours without the problem po pping up. Replaced IC0102 (MN14834HH) Main IC for the Channel Selection Ck t 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). Ha ve schematics and service manual but can't isolate the problem. Nice pictu re and everything else works fine. Unit in storage but every 4 months or s o, when I see it and think about it, I fire it up. Hate to trash this old t v. Kind of part of the family now. Has anyone ever heard of this before?

Thanks for the info. I'll add it to my troubleshooting short list. I clea ned the switch S0125 on 7/3 and the TV has been running solid through today 7/7. This has been the longest period without the problem since it first popped up in 2010. The TV is now back in action on its shelf in the kitche n area.

Reply to
jeff.panasuk

to channel 2 intermittently. Sometimes very frequent. Other times more in termittent but it would not go more than a few hours without the problem po pping up. Replaced IC0102 (MN14834HH) Main IC for the Channel Selection Ck t 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). Ha ve schematics and service manual but can't isolate the problem. Nice pictu re and everything else works fine. Unit in storage but every 4 months or s o, when I see it and think about it, I fire it up. Hate to trash this old t v. Kind of part of the family now. Has anyone ever heard of this before?

8/26/2017 One final note. It's been over 6 weeks now and the tv has worked fine since cleaning the 3-way switch. I guess it's fixed. Thanks for all your help guys.
Reply to
jeff.panasuk

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