Oh so close, but now what (new problem)
 

Okay, I was getting so close to finding the jail bar problem, but it wasn't meant to be. I went to turn the set back on to scope a couple more points and all I got as a very loud buzzing through the speaker (even with volume all the way down) and then the circuit breaker went. I reset and tried it again, but turned it off before the circuit breaker went again.

I looked over all the boards and connections and found nothing out of place. Nothing is shorting and all the transistors are seated. The only thing I did was earlier when I put in a new 695 transistor in the ABL circuit. It was working for quite some time after that swap, so unless it was a delayed reaction, it doesn't seem to be that. And just in case, I put the old one back and still have the same issue.

So anyone know what could make the 25EC58 buzz and die?

Thanks!

did you ever replace the large oval metal cap?

You mean that oil filled capacitor next to the large transformer? The 3.5uf @440V?

yes that one.

shorted power diode, horiz output transistor, shorted filter cap in power supply.

Nope, it's the original.

So if it were your set, where would you look first?

ok the usual suspects will be that plus the items the other items mentioned.

does the 3.3uf cap get hot before the circuit breaker kicks off? if it does get hot its prob bad.

If not then I would try disconnecting the horz out transistor (remove it from the socket). if it still kicks on then you need to start checking things like filter caps and diodes in the power supply.

Okay, I ran it until the breaker went. The oval canister didn't change temp (at least not that my hand could tell). Also, the HOT didn't change temp, either.

Okay, I took out the HOT from its socket and the set had a very low volume buzz and the breaker never tripped.

So what does this mean? Is the HOT bad, or is there something in between that needs to be looked at?

Well no matter what connections I do with the diode checking mode, the darn thing tones and shows a 9. Those were for negative on B and positive on E or C, Positive on B and Negative on E or C and negative on E and Positive on C. So I guess this thing is toast. Can only find an NTE replacement. Are those up to the job? Also, should I assume this thing went belly up due to something else in the chain?

I'll probably order a few of them since it might an issue elsewhere and kill the first one I put in.

Well I looked up the Zenith part number and got an NTE238. In the Sams there are 4 other suppliers of the HOTs, so I decided to look them up. What I found only confused me more. It seems that each part number comes up with a different NTE HOT, each with slightly different specs. So now the question is, which is closer to the original?

Here are the numbers I got: NTE238, 163A, 164 and 165. Those correspond to: TI, Motorola, International Rectifier, Sylvania.

What's your original Zenith part number?? - the transistor may only be marked with a suffix, but the prefix will be 121- . Zenith made some Horizontal Output Transistor replacements as kits, with the skinny clear tube of Monkey-Snot heat sink compound and mica insulator. I remember the 4-lead safety cap CCII sets had a subbed horizontal transistor that didn't come as a kit - most others came simply by their 121- number. I've got some used-but-good Horizontal Output Trannys, but a part number is what would help right now.

Cheers,

What's installed is a 121-831 and is made by TI.

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:smoke:

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.

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. :thmbsp:

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.

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.

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.

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.

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.

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 :D ).

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.

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

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?

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:smoke:

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

Some pics of the HOT diode, HOT cap and resistor and ABL cap and resistors. The ABL is the one with the bright blue cap that has 021 K0 written on it).

I like that bulb technique Zeno is describing, current limiting the HOT sounds smart.

This is worth reading.
http://www.repairfaq.org/sam/reppic/horiz-tv.pdf

If its a simple variac add an AC ampmeter to the set-up.
The set will usually start running about 75V but the regulator
will kick in soon & bring the 125V up almost instantly. Keep your
amps under 1 amp. A normal running set will draw abt
.75 amps at 120 VAC, .5 for the HV. With this type regulation you may
have to do the bulb trick. IIRC they start to work but
pulsate then as you go up kick in 100% quickly.

BTW the variac we used was a Sencore power right or the B&K
equivalent. They have a variac, isolation, metering for volts
& amps & leakage test. A must have for any newer sets or
working on hot chassii.

73 Zeno:smoke:

good read thanks for posting Zeno.

Is it okay to run the set without the HOT? I want to scope the points on the horizontal board to make sure it's sending the pulses. I read that no signal from the driver can kill the HOT. I also want to measure the LV section, etc.

Is this resistor burned or just a defect in appearance? It's in the ABL circuit and runs off one of the terminals of the brightness limiter pot.

Okay, I feel stupid. I think I might have caused the HOT to die. When I was trying to check test point 115 on the horizontal board I couldn't see the number on the board that notes connection 115. There was a good size capacitor in the way. So I pulled it towards me a bit until I could see the number marking on the board. I then connected the probe ground to "U" and touched the probe to "15" and then turned on the set. That's when it groaned and died.

I just removed the board to test the transistors and I realized I had pulled the lead of the capacitor to the point where it looked like it made contact with the emitter of the wave shaper transistor. This must have done something to the horizontal pulse that the HOT didn't like.

I'm attaching a pic of the area, but moved the capacitor before I thought about taking a picture.

Its OK to run it out. The base waveform will not look the
same though.
Newer sets are more complex & wont run. They have start, shutdown
& closed loops that require different techniques. Thats when
the real fun begins...........

73 Zeno:smoke:

Don't beat yourself up too much, you learned a good lesson, make sure everything is correct before power up. Just work the issues one at a time, don't change too many parts at once, you don't want to introduce problems. The thing about zenith sets with the hand wired chassis is you have to be VERY careful when handling the chassis. With all the parts stuffed onto the bottom of the chassis it gets pretty easy to push something and create a short.

Using a targeted approach to a problem is the way to go, the scope will help but it can also mislead, esp if the scope patterns are not clearly shown like the sams often are.

Stick with what zeno advised on replacing that HOT with the current limit light.

Some problems can be very elusive, and sometimes very easy to fix but hard to find. a poor dag connection, a missing screw that grounds something, all can turn into tough dogs to find.

My toughest was a poor ground connection caused by a loose screw on a pcb from a black and white maggie. It created a hula that took a while to find.
I ended up spending a lot of time, finally turned up by using DC on the filaments and slowly putting AC back one pcb at a time.

a simple 1/4 turn of a pcb mount screw would have fixed it. this is where experience comes into play. I had used a scope and tried to find it but it was just not something that I could see. So you have to use tools and field exp both sometimes. Take it for granted the 1st thing I do when I get a set that has pcb grounds thru screws is to tighten them all up. Same with ground stakes on RCA's, term grounds on zeniths, dag grounds etc...

I ordered two replacement HOTs. One is the sacrificial lamb and will go straight in and turned on. If it was the short I created that killed it, all will be fine. If not, I'll have the second one an use a dim bulb, etc.