Oh so close, but now what (new problem)

[DaveWM]

good read thanks for posting Zeno.

[TinCanAlley]

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.

[TinCanAlley]

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.

[TinCanAlley]

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.

[zeno]

Originally Posted by TinCanAlley

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.

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

[DaveWM]

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...

[TinCanAlley]

Originally Posted by DaveWM

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.

[TinCanAlley]

Originally Posted by zeno

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

The waveform will be different, but it will be there and should be enough to make sure the horizontal board is working properly?

[zeno]

Originally Posted by TinCanAlley

The waveform will be different, but it will be there and should be enough to make sure the horizontal board is working properly?

The hoz drive will be OK up to the C of the hoz drive transistor.
IIRC the HOT base will look more like a shark tooth than square.
It will be there.

73 Zeno

[TinCanAlley]

Originally Posted by zeno

The hoz drive will be OK up to the C of the hoz drive transistor.
IIRC the HOT base will look more like a shark tooth than square.
It will be there.

73 Zeno

I thought I wasn't supposed to go near the HOT w/out a hi voltage probe? Also, whose idea was it to make a transistor with the shell being the collector? Wouldn't it have been easier to make it a three lead unit and then you wouldn't have to worry about isolating it so as not to short to ground? I looked over the thing carefully and found the case is being energized by a lead on the screw hole.

[old_tv_nut]

Originally Posted by TinCanAlley

I thought I wasn't supposed to go near the HOT w/out a hi voltage probe? Also, whose idea was it to make a transistor with the shell being the collector? Wouldn't it have been easier to make it a three lead unit and then you wouldn't have to worry about isolating it so as not to short to ground? I looked over the thing carefully and found the case is being energized by a lead on the screw hole.

The high voltage retrace pulse is developed at the collector of the HO transistor, so if the transistor is removed, it does not occur, and you will be able to read the base waveform with an ordinary 10x probe.

The transistor collector is the substrate of the die, which has to be soldered to the case to get good thermal conductivity to the heatsink. Hence, the case will be connected to the collector. There will be an insulating washer and some thermal-conducting (but electrically insulating) grease.

[TinCanAlley]

Okay, the dim bulb tester is done. She's not the prettiest, but she'll get the job done. I was going to wire in a switch, but decided to put a inline rotary on the cord. That'll make it less cluttered.

[DaveWM]

that is a common bulb limiter by a lot of folks for checking major shorts in a set, but I think zeno was looking more for shorts of the HOtransistor

quoting here

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.........

stop quote.

the prob with just using a dim bulb as you have it (at the primary of the power trans) is if there is a prob in the circuit around the HOT it may still blow even with the dim bulb. I could be wrong though, as I thought these chassis have loosely coupled power trans that should limit the current to some degree. Zeno needs to chime in to clear up.

[TinCanAlley]

Originally Posted by DaveWM

that is a common bulb limiter by a lot of folks for checking major shorts in a set, but I think zeno was looking more for shorts of the HOtransistor

quoting here

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.........

stop quote.

the prob with just using a dim bulb as you have it (at the primary of the power trans) is if there is a prob in the circuit around the HOT it may still blow even with the dim bulb. I could be wrong though, as I thought these chassis have loosely coupled power trans that should limit the current to some degree. Zeno needs to chime in to clear up.

Well I can jumper the hot and neutral of the outlet and put the hot and neutral of the plug end in-line with pin 4 of T207. That way don't have to undo all whole DBT and rewire it.