Stymied Again
 

Since getting the B+ voltages back to normal, I ran the voltage chart again. V1 through V17 looked pretty good. But when I got to V18, pin 8, which previously showed the appropriate 380 volts, now overflows my 600-volt-max multimeter. I tried again with a 1000-max-volt multimeter, and that one overflows, too. In both meters, the resistance/capacitance sections are fried. But they still work for AC and DC voltage.

My first thought was to throw the set out the window. But when I calmed down, I rechecked every connection that I had recently worked on and over the next few days rechecked components, transformers, resistance everywhere, and I found nothing wrong. Nothing wrong except having over a kilovolt at V18.
resistance/capacitance section is dead. The second meter also acts funny in the resistance area.

Other observed changes are that I get no noise through the speaker and cannot hear the flyback anymore.

https://lh3.googleusercontent.com/-0...52520Volts.jpg

Here’s another observation: It’s not good to turn on the set when the HV rectifier cap is not connected.

https://lh3.googleusercontent.com/-S...%252520Off.JPG

I’m a bit frustrated, so I put a time limit on this project. If I don’t get a raster within the next ten years, than I’ll think about quitting.

Please, what in the world should I look for to resolve the too-high voltage issue.

That's probably 380 volts DC but there are some big AC spikes throwing off your meter.

Surprizing the meters didn't release a gob of magic smoke. The high level spike on the the damper cathode is normal, the product of the tube's operation. The spike can be well over 1KV in a BW set, and 3-4 KV or more in a color set. Something to bookmark.
Never put a meter on any point that's directly connected to a flyback winding.

Thanks. That's a really good thing to know. When I started working with this set five years ago I didn't have this problem--it just read 460 VDC. I would have thought that Sams would have noted this on the voltage data chart, but perhaps this wasn’t an issue with VTVMs of that era?

Does someone have time to advise me on two other issues?
1. Since I burned up the 7.5 ohm/5W resistor-fuse, should I add a 5W fuse in line with the 7.5ohm/5W replacement resistor?
2. When I discharge the CRT and electrolytics there seems to be no charge at all--no spark, no “pop!”
In previous years, I would get a healthy discharge. Should I look for a reason it’s not charging or for something that’s bleeding off the charge.

I'm not sure how you would calculate the CURRENT rating of a fuse that you could add to the 7.5 ohm resistor. About 0.8 A would produce 5 W in a 7.5 ohm resistor. But the fuse isn't meant to protect the resistor!

Sounds like either something is draining off the charge or the voltage isn't as high as it was or its you imagination. If whatever you are discharging with has a higher resistance, it won't pop or spark as much. It is actually better for the capacitors not to use a dead short, less of a current surge.

Do be careful of measuring the higher voltages. If your meter is only rated to 600 V, the probes may not be safe to hold above that.

I once saw a YouTube video made from an industrial accident investigation about a fatal electrical flash-over where a voltage too high was measured with a meter. The guy was trouble shooting a large motor controller, his meter was rated to 600 V, most of the controllers in the room had 440 V on them but the one he wanted to check was, unknown to him, 10,000 V. Someone saw him running out of the room after the flash-over, the only piece of clothes unburnt on him was his belt!

That would be the averaged-out (or RMS) value of the main B+ and the high-level spikes.
The editors mighta figgered a techie already knows not to put a meter there. Certainly it would risk arcing in the meter probe or selector switch.

Thank goodness that couldn't have happened to me--I wasn't wearing a belt.

Edit.. bad call, not funny

Of course we don't deal with high power. Your posts just reminded me of the story. It was only 2300 V, not 10KV. Finally found a video about it. This isn't the version I saw a couple of years ago. Gruesome descriptions, no views of the guy, just the equipment.

https://www.youtube.com/watch?v=hfnEuRA7-vo

Hey, Winky. I think that I have a NOS focus coil for that set. Will be at the storage area on Monday to verify. Charles

Hey, Winky. I have a NOS focus coil, a Stancor FC12, that should work fine in your Motorola. Price is $32, ppd. Resistance is 370 ohms. Please contact me if this will help. Broadcaster