View Full Version : Zenith 25EC58 won't shut off


Adam
01-02-2016, 12:02 AM
…whether I use the switch on the set or the remote. When I plug it in, it is off, but after turning it on there is no way to turn it off again. Changing to auto/manual, even disconnecting the on/off switch won't do it, so the problem must lie with the space command unit. Also the channel down function on the remote stopped working. But vol and channel up on the remote still work.

Given that this happened right after I swapped the CRT, I'm thinking that I just knocked something loose. The one thing I did notice was that electrolytic capacitor circled in the pic came loose on the + side (It's only connected - side to ground now, so it's not doing anything.) It's 100uf 50v. But I find no reference to it in any of my schematics, so I'm not sure to what it was connected.

Any guesses as to what's going on?

I'm looking at the schematic for the space command unit now. However this thing is supposed to cause the set to turn on/off is really not making too much sense to me. But I don't want to stay up super late two days in a row, so I'm going to bed.

rca2000
01-02-2016, 01:38 PM
The SC unit uses either a TRIAC or RELAY...for power switching. Likely a triac.
There also is a device called an optocoupler--that drives the triac and isolates it from the AC line. NOT like a modern one--this will look like a tubular or possible a little squarish "blob", with a pair of wires on each end. Sometimes..they short--and THEN...the chassis cannot be turned off. Other times...they will only partially open, and "clip" the signal top the triac..causing an impure output...and the power tranny will NOT like it..

There also may be a disc cap on the triac...if it shorts...the same result--no turn off.

zeno
01-02-2016, 02:27 PM
Set uses a triac for on/off. If you follow the black/blue/grey wires
to the tuning assy its there mounted in an odd position. That
is driven by either a photo optic isolator or small relay nearby.
PCB will be marked "PC" & "L". Both gave trouble.
There is a kit on e-bay now for the isolator. It comes
with instructions & a cable that goes is series with above wires.
It has a VDR on it for improved reliability.
There was one other lot that may have had a triac, it showed
IIRC three of the above cables.
BE SHURE you didnt knock one of the little plug in transistors
loose. You flip-flop may be flipping & not flopping :D

73 Zeno:smoke:

zeno
01-02-2016, 07:37 PM
http://www.ebay.com/itm/Zenith-Replacement-Kit-800-617-for-162-9-Photo-Optical-Isolator-Television-TV-/131690185606?hash=item1ea958c386:g:8XEAAOSwjVVVuOg z

I found the other lot but no triac kit, just cables. The triac is
the MOST likely problem.

Big IIRC's here...... When these came in with either triac or
isolator bad we did the following to prevent call backs.
Changed triac, isolator, manual on/off button & remote-manual
switch. The switches would go int & cause int on/off by itself
problems. List of symptoms, you can get one or more:

Dead set
Stuck on
Int off
Int on
Int breaker tripping
Int severe pix breaking up
Int turn off by itself
Int turn on by itself
Probably more but its been 35 yrs !!!

BTW this circuit is used in the flat chassis AND the CC2 upright
remote sets E & F line. G line switched to a normal on/off relay.

73 Zeno:smoke:

Adam
01-03-2016, 12:41 PM
This must be the most complicated on/off switch ever devised! I think I understand this circuit some now. In order for the AC current to be able to flow through the triac, the resistance needs to drop in the photo-optic isolator, and this only happens when the light is on. The light is controlled by the flip-flop, in which it is only possible for one one transistor to be conducting at a time. When current flows through one transistor the other one is cut off. The low side of the light is connected to the collector of one of the transistors. When that transistor conducts, the collector voltage drops and current flows through the light. Which transistor is conducting changes every time there is a triggering pulse. Now now how that 2 transistor triggering circuit works still confuses me, but it doesn't matter because I got it working now.

The problem really was just loose transistors. The channel down not working I fixed with touching up the alignment. (Why it all of a sudden needed realignment I don't know) And I just pulled that random extra electrolytic, I have no idea what it was doing in there.

The 2 further problems I still have are that the volume control now seems to have only 2 steps and not 4. And while I could get the convergence nearly right-on, the I could not get purity perfect. I think I can put this down to the fact that so much of the plastic of the purity ring unit disintegrated when I pulled it off the old CRT, so that when I tighten it around the new CRT neck, it isn't centered. But I have a new purity ring / blue lateral magnet unit on the way. I'll look into the volume control problem when I get to redoing the purity and convergence adjustments with the new unit in there (which might not be until summer).

Putting the CRT in was majorly difficult. There's this sort-of metal envelope which surrounds the back of the CRT and holds it in. But neither attaching that to the CRT first and then putting it in the cabinet, nor attaching it to the cabinet and then trying to slip the CRT in will work.

This is the procedure that eventually worked for me:

There's 2 straps which screw together which clasp the CRT in the metal envelope. Undo these. Attach the metal unit just on the bottom loosely to the cabinet. Then slip the CRT inside. (being very careful not to scratch the front of the CRT on the back edges of the thing - it's harder than it sounds) It won't go all the way in, at most I could get it about 1" from the front. Then attach the top of the metal unit to the cabinet, loosely. Now comes the hard part - you have to pull those straps close enough together that they will sort of lock in place. This took me several hours and I cut up my fingers on the straps doing it. Once this is done attach the metal CRT holder to the cabinet completely, and then CRT will be pushed all the way to the front by tightening the straps.

Is there an easier way to do this? There must be! :worried: Thanks for the help on this. And a pic of it working...

Electronic M
01-03-2016, 04:15 PM
I always put the CRT in first and put the shield on over it....The trick is pushing on the sides of the shield to convince it to fully seat in (and not stick on the edges of) it's groove....