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.

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:smoke:

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.

So let me make sure I get this correct. With the HOT removed I will be able to connect my 10x probe to the collector lead of the HOT socket?

Will the waveform I get be enough to see the ringing if it exists?