Oh so close, but now what (new problem)

[zeno]

Originally Posted by TinCanAlley

How likely is it that it died on its own and plugging in a replacement is safe? If it's not that common, what should I look for before installing it and risk damaging it?

Tough question 50-50 ?.
Anyhows the 121-831 was used for years, even in 9-160 sets IIRC.
Newest # is 921-500 IIRC. The ECG sub works fine, cant speak
for others. If you are real lucky the cause of the jail bars
caused it & you kill 2 birds.........
After replacing since you prob dont have a variac you should use
a "rubber fuse" to protect it. You have many of them, a 75W
lamp. Remove the wire from pin 4 of the FBT. Hang in the lamp in
one end to pin 4 & the other end to the wire. If it lights dim
& you get a small dim pix things are OK for now. If it glows
bright you have problems.........
If things look good reattatch the wire, turn set on a few seconds
then off & fell the HOT. It should be cool. keep doing it extending the
on time say 5sec, 10sec, 20sec, 1mn 5mn etc. The HOT will
get warm but not hot. You may also want to put an ampmeter
in place of the lamp, should be abt .5 amps tops.
When changing clean & replace the compound with the white,
pink or blue stuff. Inspect the mica. Check R353 & C264 at HDT.

73 Zeno

[Zenith26kc20]

Check the horizontal output transistor emitter to collector on ohms. If it is bad it will read near or at zero ohms. Diode checking does no good as some of these have damper diodes in the transistor itself. If it is bad, once replaced, before you turn it on, disconnect the tripler from the flyback (at the tripler). The lead is soldered on to the tripler but you will have to scrape away the silicone on the tripler post to get at the solder. Some (few) of the EC chassis had the lead come out of the tripleer and solder to the flyback. If so, it will have to be CAREFULLY unsoldered from the flyback. A picture of the flyback/tripler would be very helpful. These sets were known to have triplers fail. Also, follow the lead from the transistor to the horizontal driver transformer. There is a 1.5 ohm resistor with a small electrolytic across it. You will know it because the positive of the electrolytic is on ground. This pair fail and can also kill the horizontal output transistor. Also, from the collector of the horizontal output transistor, there are a number of capacitors (hopefully red/orange, or white, if original). Check these. If they are white change them. Zenith had a lot of problems with the white ones. Pictures of the horizontal area would be helpful as well as the horizontal area unter the chassis.

[TinCanAlley]

Originally Posted by zeno

Tough question 50-50 ?.
Anyhows the 121-831 was used for years, even in 9-160 sets IIRC.
Newest # is 921-500 IIRC. The ECG sub works fine, cant speak
for others. If you are real lucky the cause of the jail bars
caused it & you kill 2 birds.........
After replacing since you prob dont have a variac you should use
a "rubber fuse" to protect it. You have many of them, a 75W
lamp. Remove the wire from pin 4 of the FBT. Hang in the lamp in
one end to pin 4 & the other end to the wire. If it lights dim
& you get a small dim pix things are OK for now. If it glows
bright you have problems.........
If things look good reattatch the wire, turn set on a few seconds
then off & fell the HOT. It should be cool. keep doing it extending the
on time say 5sec, 10sec, 20sec, 1mn 5mn etc. The HOT will
get warm but not hot. You may also want to put an ampmeter
in place of the lamp, should be abt .5 amps tops.
When changing clean & replace the compound with the white,
pink or blue stuff. Inspect the mica. Check R353 & C264 at HDT.

73 Zeno

I ordered two NTE238s today. They should be here on Monday. I've also got a new damper diode for the HOT, a new LV transistor and all the resistors that go around those components. I will do the diode, test the voltage regulator and then put in one HOT as a sacrifice. If it goes, I've got a few other things to check and one more HOT to test again. I'm hoping that one will be enough and leave me with a spare.

C264 was replaced during the recap, but I didn't change the resistor. I'll put it on my list for inspection/testing.

Is the HOT supposed to have a mica? This one didn't. It was just sitting on a bed of thermal compound. Even the legs had the stuff on them. A real mess.

I'm tired of being on the floor most of the time, so I'm going to pull the chassis for testing/replacement.

[TinCanAlley]

Originally Posted by Zenith26kc20

Check the horizontal output transistor emitter to collector on ohms. If it is bad it will read near or at zero ohms. Diode checking does no good as some of these have damper diodes in the transistor itself. If it is bad, once replaced, before you turn it on, disconnect the tripler from the flyback (at the tripler). The lead is soldered on to the tripler but you will have to scrape away the silicone on the tripler post to get at the solder. Some (few) of the EC chassis had the lead come out of the tripleer and solder to the flyback. If so, it will have to be CAREFULLY unsoldered from the flyback. A picture of the flyback/tripler would be very helpful. These sets were known to have triplers fail. Also, follow the lead from the transistor to the horizontal driver transformer. There is a 1.5 ohm resistor with a small electrolytic across it. You will know it because the positive of the electrolytic is on ground. This pair fail and can also kill the horizontal output transistor. Also, from the collector of the horizontal output transistor, there are a number of capacitors (hopefully red/orange, or white, if original). Check these. If they are white change them. Zenith had a lot of problems with the white ones. Pictures of the horizontal area would be helpful as well as the horizontal area unter the chassis.

Okay, I did the ohm test and got 8.9 collector to base, 2.3 collector to emitter and 9.5 emitter to base.

I'll check the 1.5 resistor. The capacitor with it was replace over a month ago when I recapped the chassis. All electrolytic caps were replaced.

I replaced all the white safety caps a couple of months ago (5 of them).

I'll get myself some sensor safe RTV so I can remove the tripler input lead and insulate it again when I put it back.

[TinCanAlley]

What's the point of the mica inserts? I understand that sometimes you want to insulate from electrical contact, but from what I see in a couple of examples in my set, that can't be the reason.

The HOT doesn't have one, but even if it did, it would still conduct to the chassis via the screws. The same for the voltage regulator. The regulator is sitting on a mica insulator, but is fastened to the chassis via a metal screw through the metal tab on it. This makes a connection.

If it's just heat conduction they want to improve, then a good, thin coat of thermal compound should fill all the gaps and do the job. Much better than the mica and large globs of thermal compound used.

[DaveWM]

I would not recommend re-engineering the use of the mica sheets. Make sure you examine parts closely to make sure where mica is present or not present.

[TinCanAlley]

Originally Posted by DaveWM

I would not recommend re-engineering the use of the mica sheets. Make sure you examine parts closely to make sure where mica is present or not present.

Have the chassis out and found that there are three pieces of mica on the HOT and the screws never touch the chassis. The go through a bigger hole and the threads are actually on the isolated board.

One of the mica pieces came apart trying to remove, but the other two are in good shape. I'm actually going to try the NTE Thermopads. They take the place of the mica and compound.

[DaveWM]

that ringing is coming from the tripler ref post, did you ever get a chance to check the damper diode? not sure how well a reg diode check would work (unless its completely open) since its a pretty high power diode).

[TinCanAlley]

Originally Posted by DaveWM

that ringing is coming from the tripler ref post, did you ever get a chance to check the damper diode? not sure how well a reg diode check would work (unless its completely open) since its a pretty high power diode).

I have an NOS replacement for the damper diode. I can test the original and compare it to the new. Should just put the new one in since it's an important component.

What is the tripler "ref post"?

Speaking of triplers....a bad tripler wouldn't cause the problem I'm having, could it? If so, I do have the new tripler with built-in focus. Was keeping it more as a spare since I have the new focus divider installed.

[DaveWM]

I assume its just a low voltage tap off the tripler.

I don't like to replace a lot of parts an one time, unless its clearly bad. So if the diode checks ok then I would leave it for now. You have spent a lot of time on this set, I want you to find the EXACT problem. With the scope hopefully you can narrow down the issue (lets hope that ringing in the brightness is it) and then checking/replacing one part at a time in the effected area and checking the scope each time will get it fixed. I know you would prob rather just get it done once to avoid all the chassis pulling, but for your own education process I think its better to see the effects of each part (I don't know for sure if the damper has this effect, so its not just for your education ).

good luck, your doing well. The scope is a real asset, but only to a point.

[TinCanAlley]

Originally Posted by DaveWM

I assume its just a low voltage tap off the tripler.

I don't like to replace a lot of parts an one time, unless its clearly bad. So if the diode checks ok then I would leave it for now. You have spent a lot of time on this set, I want you to find the EXACT problem. With the scope hopefully you can narrow down the issue (lets hope that ringing in the brightness is it) and then checking/replacing one part at a time in the effected area and checking the scope each time will get it fixed. I know you would prob rather just get it done once to avoid all the chassis pulling, but for your own education process I think its better to see the effects of each part (I don't know for sure if the damper has this effect, so its not just for your education ).

good luck, your doing well. The scope is a real asset, but only to a point.

The diode checked out okay and was in the same range as the new one and the other two that are installed in the set. So I'll leave it.

I'm still going to take out and test the low voltage regulator as it is one of the pieces that can kill the HOT. It will allow me to clean up the transistor, mount and socket. I really hate it when they put a ton of the thermal compound on and it oozes out all over the place. Not only does it look bad, it really doesn't help much in the thermal coupling.

[TinCanAlley]

Originally Posted by DaveWM

I assume its just a low voltage tap off the tripler.

This tripler has the input, ground and anode. No other connections/taps on it.

[DaveWM]

interesting, my zenith manual show input, ref, HV anode. I just assumed it was a low volt tap. the triplers I am used to generally have a input, focus, and the HV lead.

the ref goes thru a couple resistors, the pot to ground and has a zener diode. Perhaps that is the same as the ground lead on yours?

[zeno]

Originally Posted by TinCanAlley

I have an NOS replacement for the damper diode. I can test the original and compare it to the new. Should just put the new one in since it's an important component.

What is the tripler "ref post"?

Speaking of triplers....a bad tripler wouldn't cause the problem I'm having, could it? If so, I do have the new tripler with built-in focus. Was keeping it more as a spare since I have the new focus divider installed.

A tripler will cause it. So can yoke, HOT bias, the white caps, FBT or any
heavy load or mismatch on the FBT. The ref. is the ABL terminal. If you
dont do the bulb at least do the heat check. But if its really loaded
down the HOT will INSTANT death. We used the variac to bring
them up slowly & watch the current but ive used the bulb also.
It will protect the HOT & save $$.

73 Zeno

[TinCanAlley]

Originally Posted by zeno

A tripler will cause it. So can yoke, HOT bias, the white caps, FBT or any
heavy load or mismatch on the FBT. The ref. is the ABL terminal. If you
dont do the bulb at least do the heat check. But if its really loaded
down the HOT will INSTANT death. We used the variac to bring
them up slowly & watch the current but ive used the bulb also.
It will protect the HOT & save $$.

73 Zeno

My brother has a variac. Guess I should borrow it.